Adding upstream version 115.7.0esr.upstream/115.7.0esrupstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

6 files changed, 2917 insertions, 0 deletions

diff --git a/third_party/rust/base64/src/engine/general_purpose/decode.rs b/third_party/rust/base64/src/engine/general_purpose/decode.rs
new file mode 100644
index 0000000000..e9fd78877b
--- /dev/null
+++ b/third_party/rust/base64/src/engine/general_purpose/decode.rs
@@ -0,0 +1,348 @@
+use crate::{
+    engine::{general_purpose::INVALID_VALUE, DecodeEstimate, DecodePaddingMode},
+    DecodeError, PAD_BYTE,
+};
+
+// decode logic operates on chunks of 8 input bytes without padding
+const INPUT_CHUNK_LEN: usize = 8;
+const DECODED_CHUNK_LEN: usize = 6;
+
+// we read a u64 and write a u64, but a u64 of input only yields 6 bytes of output, so the last
+// 2 bytes of any output u64 should not be counted as written to (but must be available in a
+// slice).
+const DECODED_CHUNK_SUFFIX: usize = 2;
+
+// how many u64's of input to handle at a time
+const CHUNKS_PER_FAST_LOOP_BLOCK: usize = 4;
+
+const INPUT_BLOCK_LEN: usize = CHUNKS_PER_FAST_LOOP_BLOCK * INPUT_CHUNK_LEN;
+
+// includes the trailing 2 bytes for the final u64 write
+const DECODED_BLOCK_LEN: usize =
+    CHUNKS_PER_FAST_LOOP_BLOCK * DECODED_CHUNK_LEN + DECODED_CHUNK_SUFFIX;
+
+#[doc(hidden)]
+pub struct GeneralPurposeEstimate {
+    /// Total number of decode chunks, including a possibly partial last chunk
+    num_chunks: usize,
+    decoded_len_estimate: usize,
+}
+
+impl GeneralPurposeEstimate {
+    pub(crate) fn new(encoded_len: usize) -> Self {
+        Self {
+            num_chunks: encoded_len
+                .checked_add(INPUT_CHUNK_LEN - 1)
+                .expect("Overflow when calculating number of chunks in input")
+                / INPUT_CHUNK_LEN,
+            decoded_len_estimate: encoded_len
+                .checked_add(3)
+                .expect("Overflow when calculating decoded len estimate")
+                / 4
+                * 3,
+        }
+    }
+}
+
+impl DecodeEstimate for GeneralPurposeEstimate {
+    fn decoded_len_estimate(&self) -> usize {
+        self.decoded_len_estimate
+    }
+}
+
+/// Helper to avoid duplicating num_chunks calculation, which is costly on short inputs.
+/// Returns the number of bytes written, or an error.
+// We're on the fragile edge of compiler heuristics here. If this is not inlined, slow. If this is
+// inlined(always), a different slow. plain ol' inline makes the benchmarks happiest at the moment,
+// but this is fragile and the best setting changes with only minor code modifications.
+#[inline]
+pub(crate) fn decode_helper(
+    input: &[u8],
+    estimate: GeneralPurposeEstimate,
+    output: &mut [u8],
+    decode_table: &[u8; 256],
+    decode_allow_trailing_bits: bool,
+    padding_mode: DecodePaddingMode,
+) -> Result<usize, DecodeError> {
+    let remainder_len = input.len() % INPUT_CHUNK_LEN;
+
+    // Because the fast decode loop writes in groups of 8 bytes (unrolled to
+    // CHUNKS_PER_FAST_LOOP_BLOCK times 8 bytes, where possible) and outputs 8 bytes at a time (of
+    // which only 6 are valid data), we need to be sure that we stop using the fast decode loop
+    // soon enough that there will always be 2 more bytes of valid data written after that loop.
+    let trailing_bytes_to_skip = match remainder_len {
+        // if input is a multiple of the chunk size, ignore the last chunk as it may have padding,
+        // and the fast decode logic cannot handle padding
+        0 => INPUT_CHUNK_LEN,
+        // 1 and 5 trailing bytes are illegal: can't decode 6 bits of input into a byte
+        1 | 5 => {
+            // trailing whitespace is so common that it's worth it to check the last byte to
+            // possibly return a better error message
+            if let Some(b) = input.last() {
+                if *b != PAD_BYTE && decode_table[*b as usize] == INVALID_VALUE {
+                    return Err(DecodeError::InvalidByte(input.len() - 1, *b));
+                }
+            }
+
+            return Err(DecodeError::InvalidLength);
+        }
+        // This will decode to one output byte, which isn't enough to overwrite the 2 extra bytes
+        // written by the fast decode loop. So, we have to ignore both these 2 bytes and the
+        // previous chunk.
+        2 => INPUT_CHUNK_LEN + 2,
+        // If this is 3 un-padded chars, then it would actually decode to 2 bytes. However, if this
+        // is an erroneous 2 chars + 1 pad char that would decode to 1 byte, then it should fail
+        // with an error, not panic from going past the bounds of the output slice, so we let it
+        // use stage 3 + 4.
+        3 => INPUT_CHUNK_LEN + 3,
+        // This can also decode to one output byte because it may be 2 input chars + 2 padding
+        // chars, which would decode to 1 byte.
+        4 => INPUT_CHUNK_LEN + 4,
+        // Everything else is a legal decode len (given that we don't require padding), and will
+        // decode to at least 2 bytes of output.
+        _ => remainder_len,
+    };
+
+    // rounded up to include partial chunks
+    let mut remaining_chunks = estimate.num_chunks;
+
+    let mut input_index = 0;
+    let mut output_index = 0;
+
+    {
+        let length_of_fast_decode_chunks = input.len().saturating_sub(trailing_bytes_to_skip);
+
+        // Fast loop, stage 1
+        // manual unroll to CHUNKS_PER_FAST_LOOP_BLOCK of u64s to amortize slice bounds checks
+        if let Some(max_start_index) = length_of_fast_decode_chunks.checked_sub(INPUT_BLOCK_LEN) {
+            while input_index <= max_start_index {
+                let input_slice = &input[input_index..(input_index + INPUT_BLOCK_LEN)];
+                let output_slice = &mut output[output_index..(output_index + DECODED_BLOCK_LEN)];
+
+                decode_chunk(
+                    &input_slice[0..],
+                    input_index,
+                    decode_table,
+                    &mut output_slice[0..],
+                )?;
+                decode_chunk(
+                    &input_slice[8..],
+                    input_index + 8,
+                    decode_table,
+                    &mut output_slice[6..],
+                )?;
+                decode_chunk(
+                    &input_slice[16..],
+                    input_index + 16,
+                    decode_table,
+                    &mut output_slice[12..],
+                )?;
+                decode_chunk(
+                    &input_slice[24..],
+                    input_index + 24,
+                    decode_table,
+                    &mut output_slice[18..],
+                )?;
+
+                input_index += INPUT_BLOCK_LEN;
+                output_index += DECODED_BLOCK_LEN - DECODED_CHUNK_SUFFIX;
+                remaining_chunks -= CHUNKS_PER_FAST_LOOP_BLOCK;
+            }
+        }
+
+        // Fast loop, stage 2 (aka still pretty fast loop)
+        // 8 bytes at a time for whatever we didn't do in stage 1.
+        if let Some(max_start_index) = length_of_fast_decode_chunks.checked_sub(INPUT_CHUNK_LEN) {
+            while input_index < max_start_index {
+                decode_chunk(
+                    &input[input_index..(input_index + INPUT_CHUNK_LEN)],
+                    input_index,
+                    decode_table,
+                    &mut output
+                        [output_index..(output_index + DECODED_CHUNK_LEN + DECODED_CHUNK_SUFFIX)],
+                )?;
+
+                output_index += DECODED_CHUNK_LEN;
+                input_index += INPUT_CHUNK_LEN;
+                remaining_chunks -= 1;
+            }
+        }
+    }
+
+    // Stage 3
+    // If input length was such that a chunk had to be deferred until after the fast loop
+    // because decoding it would have produced 2 trailing bytes that wouldn't then be
+    // overwritten, we decode that chunk here. This way is slower but doesn't write the 2
+    // trailing bytes.
+    // However, we still need to avoid the last chunk (partial or complete) because it could
+    // have padding, so we always do 1 fewer to avoid the last chunk.
+    for _ in 1..remaining_chunks {
+        decode_chunk_precise(
+            &input[input_index..],
+            input_index,
+            decode_table,
+            &mut output[output_index..(output_index + DECODED_CHUNK_LEN)],
+        )?;
+
+        input_index += INPUT_CHUNK_LEN;
+        output_index += DECODED_CHUNK_LEN;
+    }
+
+    // always have one more (possibly partial) block of 8 input
+    debug_assert!(input.len() - input_index > 1 || input.is_empty());
+    debug_assert!(input.len() - input_index <= 8);
+
+    super::decode_suffix::decode_suffix(
+        input,
+        input_index,
+        output,
+        output_index,
+        decode_table,
+        decode_allow_trailing_bits,
+        padding_mode,
+    )
+}
+
+/// Decode 8 bytes of input into 6 bytes of output. 8 bytes of output will be written, but only the
+/// first 6 of those contain meaningful data.
+///
+/// `input` is the bytes to decode, of which the first 8 bytes will be processed.
+/// `index_at_start_of_input` is the offset in the overall input (used for reporting errors
+/// accurately)
+/// `decode_table` is the lookup table for the particular base64 alphabet.
+/// `output` will have its first 8 bytes overwritten, of which only the first 6 are valid decoded
+/// data.
+// yes, really inline (worth 30-50% speedup)
+#[inline(always)]
+fn decode_chunk(
+    input: &[u8],
+    index_at_start_of_input: usize,
+    decode_table: &[u8; 256],
+    output: &mut [u8],
+) -> Result<(), DecodeError> {
+    let morsel = decode_table[input[0] as usize];
+    if morsel == INVALID_VALUE {
+        return Err(DecodeError::InvalidByte(index_at_start_of_input, input[0]));
+    }
+    let mut accum = (morsel as u64) << 58;
+
+    let morsel = decode_table[input[1] as usize];
+    if morsel == INVALID_VALUE {
+        return Err(DecodeError::InvalidByte(
+            index_at_start_of_input + 1,
+            input[1],
+        ));
+    }
+    accum |= (morsel as u64) << 52;
+
+    let morsel = decode_table[input[2] as usize];
+    if morsel == INVALID_VALUE {
+        return Err(DecodeError::InvalidByte(
+            index_at_start_of_input + 2,
+            input[2],
+        ));
+    }
+    accum |= (morsel as u64) << 46;
+
+    let morsel = decode_table[input[3] as usize];
+    if morsel == INVALID_VALUE {
+        return Err(DecodeError::InvalidByte(
+            index_at_start_of_input + 3,
+            input[3],
+        ));
+    }
+    accum |= (morsel as u64) << 40;
+
+    let morsel = decode_table[input[4] as usize];
+    if morsel == INVALID_VALUE {
+        return Err(DecodeError::InvalidByte(
+            index_at_start_of_input + 4,
+            input[4],
+        ));
+    }
+    accum |= (morsel as u64) << 34;
+
+    let morsel = decode_table[input[5] as usize];
+    if morsel == INVALID_VALUE {
+        return Err(DecodeError::InvalidByte(
+            index_at_start_of_input + 5,
+            input[5],
+        ));
+    }
+    accum |= (morsel as u64) << 28;
+
+    let morsel = decode_table[input[6] as usize];
+    if morsel == INVALID_VALUE {
+        return Err(DecodeError::InvalidByte(
+            index_at_start_of_input + 6,
+            input[6],
+        ));
+    }
+    accum |= (morsel as u64) << 22;
+
+    let morsel = decode_table[input[7] as usize];
+    if morsel == INVALID_VALUE {
+        return Err(DecodeError::InvalidByte(
+            index_at_start_of_input + 7,
+            input[7],
+        ));
+    }
+    accum |= (morsel as u64) << 16;
+
+    write_u64(output, accum);
+
+    Ok(())
+}
+
+/// Decode an 8-byte chunk, but only write the 6 bytes actually decoded instead of including 2
+/// trailing garbage bytes.
+#[inline]
+fn decode_chunk_precise(
+    input: &[u8],
+    index_at_start_of_input: usize,
+    decode_table: &[u8; 256],
+    output: &mut [u8],
+) -> Result<(), DecodeError> {
+    let mut tmp_buf = [0_u8; 8];
+
+    decode_chunk(
+        input,
+        index_at_start_of_input,
+        decode_table,
+        &mut tmp_buf[..],
+    )?;
+
+    output[0..6].copy_from_slice(&tmp_buf[0..6]);
+
+    Ok(())
+}
+
+#[inline]
+fn write_u64(output: &mut [u8], value: u64) {
+    output[..8].copy_from_slice(&value.to_be_bytes());
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    use crate::engine::general_purpose::STANDARD;
+
+    #[test]
+    fn decode_chunk_precise_writes_only_6_bytes() {
+        let input = b"Zm9vYmFy"; // "foobar"
+        let mut output = [0_u8, 1, 2, 3, 4, 5, 6, 7];
+
+        decode_chunk_precise(&input[..], 0, &STANDARD.decode_table, &mut output).unwrap();
+        assert_eq!(&vec![b'f', b'o', b'o', b'b', b'a', b'r', 6, 7], &output);
+    }
+
+    #[test]
+    fn decode_chunk_writes_8_bytes() {
+        let input = b"Zm9vYmFy"; // "foobar"
+        let mut output = [0_u8, 1, 2, 3, 4, 5, 6, 7];
+
+        decode_chunk(&input[..], 0, &STANDARD.decode_table, &mut output).unwrap();
+        assert_eq!(&vec![b'f', b'o', b'o', b'b', b'a', b'r', 0, 0], &output);
+    }
+}
diff --git a/third_party/rust/base64/src/engine/general_purpose/decode_suffix.rs b/third_party/rust/base64/src/engine/general_purpose/decode_suffix.rs
new file mode 100644
index 0000000000..5652035d0e
--- /dev/null
+++ b/third_party/rust/base64/src/engine/general_purpose/decode_suffix.rs
@@ -0,0 +1,161 @@
+use crate::{
+    engine::{general_purpose::INVALID_VALUE, DecodePaddingMode},
+    DecodeError, PAD_BYTE,
+};
+
+/// Decode the last 1-8 bytes, checking for trailing set bits and padding per the provided
+/// parameters.
+///
+/// Returns the total number of bytes decoded, including the ones indicated as already written by
+/// `output_index`.
+pub(crate) fn decode_suffix(
+    input: &[u8],
+    input_index: usize,
+    output: &mut [u8],
+    mut output_index: usize,
+    decode_table: &[u8; 256],
+    decode_allow_trailing_bits: bool,
+    padding_mode: DecodePaddingMode,
+) -> Result<usize, DecodeError> {
+    // Decode any leftovers that aren't a complete input block of 8 bytes.
+    // Use a u64 as a stack-resident 8 byte buffer.
+    let mut leftover_bits: u64 = 0;
+    let mut morsels_in_leftover = 0;
+    let mut padding_bytes = 0;
+    let mut first_padding_index: usize = 0;
+    let mut last_symbol = 0_u8;
+    let start_of_leftovers = input_index;
+
+    for (i, &b) in input[start_of_leftovers..].iter().enumerate() {
+        // '=' padding
+        if b == PAD_BYTE {
+            // There can be bad padding bytes in a few ways:
+            // 1 - Padding with non-padding characters after it
+            // 2 - Padding after zero or one characters in the current quad (should only
+            //     be after 2 or 3 chars)
+            // 3 - More than two characters of padding. If 3 or 4 padding chars
+            //     are in the same quad, that implies it will be caught by #2.
+            //     If it spreads from one quad to another, it will be an invalid byte
+            //     in the first quad.
+            // 4 - Non-canonical padding -- 1 byte when it should be 2, etc.
+            //     Per config, non-canonical but still functional non- or partially-padded base64
+            //     may be treated as an error condition.
+
+            if i % 4 < 2 {
+                // Check for case #2.
+                let bad_padding_index = start_of_leftovers
+                    + if padding_bytes > 0 {
+                        // If we've already seen padding, report the first padding index.
+                        // This is to be consistent with the normal decode logic: it will report an
+                        // error on the first padding character (since it doesn't expect to see
+                        // anything but actual encoded data).
+                        // This could only happen if the padding started in the previous quad since
+                        // otherwise this case would have been hit at i % 4 == 0 if it was the same
+                        // quad.
+                        first_padding_index
+                    } else {
+                        // haven't seen padding before, just use where we are now
+                        i
+                    };
+                return Err(DecodeError::InvalidByte(bad_padding_index, b));
+            }
+
+            if padding_bytes == 0 {
+                first_padding_index = i;
+            }
+
+            padding_bytes += 1;
+            continue;
+        }
+
+        // Check for case #1.
+        // To make '=' handling consistent with the main loop, don't allow
+        // non-suffix '=' in trailing chunk either. Report error as first
+        // erroneous padding.
+        if padding_bytes > 0 {
+            return Err(DecodeError::InvalidByte(
+                start_of_leftovers + first_padding_index,
+                PAD_BYTE,
+            ));
+        }
+
+        last_symbol = b;
+
+        // can use up to 8 * 6 = 48 bits of the u64, if last chunk has no padding.
+        // Pack the leftovers from left to right.
+        let shift = 64 - (morsels_in_leftover + 1) * 6;
+        let morsel = decode_table[b as usize];
+        if morsel == INVALID_VALUE {
+            return Err(DecodeError::InvalidByte(start_of_leftovers + i, b));
+        }
+
+        leftover_bits |= (morsel as u64) << shift;
+        morsels_in_leftover += 1;
+    }
+
+    match padding_mode {
+        DecodePaddingMode::Indifferent => { /* everything we care about was already checked */ }
+        DecodePaddingMode::RequireCanonical => {
+            if (padding_bytes + morsels_in_leftover) % 4 != 0 {
+                return Err(DecodeError::InvalidPadding);
+            }
+        }
+        DecodePaddingMode::RequireNone => {
+            if padding_bytes > 0 {
+                // check at the end to make sure we let the cases of padding that should be InvalidByte
+                // get hit
+                return Err(DecodeError::InvalidPadding);
+            }
+        }
+    }
+
+    // When encoding 1 trailing byte (e.g. 0xFF), 2 base64 bytes ("/w") are needed.
+    // / is the symbol for 63 (0x3F, bottom 6 bits all set) and w is 48 (0x30, top 2 bits
+    // of bottom 6 bits set).
+    // When decoding two symbols back to one trailing byte, any final symbol higher than
+    // w would still decode to the original byte because we only care about the top two
+    // bits in the bottom 6, but would be a non-canonical encoding. So, we calculate a
+    // mask based on how many bits are used for just the canonical encoding, and optionally
+    // error if any other bits are set. In the example of one encoded byte -> 2 symbols,
+    // 2 symbols can technically encode 12 bits, but the last 4 are non canonical, and
+    // useless since there are no more symbols to provide the necessary 4 additional bits
+    // to finish the second original byte.
+
+    let leftover_bits_ready_to_append = match morsels_in_leftover {
+        0 => 0,
+        2 => 8,
+        3 => 16,
+        4 => 24,
+        6 => 32,
+        7 => 40,
+        8 => 48,
+        // can also be detected as case #2 bad padding above
+        _ => unreachable!(
+            "Impossible: must only have 0 to 8 input bytes in last chunk, with no invalid lengths"
+        ),
+    };
+
+    // if there are bits set outside the bits we care about, last symbol encodes trailing bits that
+    // will not be included in the output
+    let mask = !0 >> leftover_bits_ready_to_append;
+    if !decode_allow_trailing_bits && (leftover_bits & mask) != 0 {
+        // last morsel is at `morsels_in_leftover` - 1
+        return Err(DecodeError::InvalidLastSymbol(
+            start_of_leftovers + morsels_in_leftover - 1,
+            last_symbol,
+        ));
+    }
+
+    // TODO benchmark simply converting to big endian bytes
+    let mut leftover_bits_appended_to_buf = 0;
+    while leftover_bits_appended_to_buf < leftover_bits_ready_to_append {
+        // `as` simply truncates the higher bits, which is what we want here
+        let selected_bits = (leftover_bits >> (56 - leftover_bits_appended_to_buf)) as u8;
+        output[output_index] = selected_bits;
+        output_index += 1;
+
+        leftover_bits_appended_to_buf += 8;
+    }
+
+    Ok(output_index)
+}
diff --git a/third_party/rust/base64/src/engine/general_purpose/mod.rs b/third_party/rust/base64/src/engine/general_purpose/mod.rs
new file mode 100644
index 0000000000..af8897bc2b
--- /dev/null
+++ b/third_party/rust/base64/src/engine/general_purpose/mod.rs
@@ -0,0 +1,349 @@
+//! Provides the [GeneralPurpose] engine and associated config types.
+use crate::{
+    alphabet,
+    alphabet::Alphabet,
+    engine::{Config, DecodePaddingMode},
+    DecodeError,
+};
+use core::convert::TryInto;
+
+mod decode;
+pub(crate) mod decode_suffix;
+pub use decode::GeneralPurposeEstimate;
+
+pub(crate) const INVALID_VALUE: u8 = 255;
+
+/// A general-purpose base64 engine.
+///
+/// - It uses no vector CPU instructions, so it will work on any system.
+/// - It is reasonably fast (~2-3GiB/s).
+/// - It is not constant-time, though, so it is vulnerable to timing side-channel attacks. For loading cryptographic keys, etc, it is suggested to use the forthcoming constant-time implementation.
+pub struct GeneralPurpose {
+    encode_table: [u8; 64],
+    decode_table: [u8; 256],
+    config: GeneralPurposeConfig,
+}
+
+impl GeneralPurpose {
+    /// Create a `GeneralPurpose` engine from an [Alphabet].
+    ///
+    /// While not very expensive to initialize, ideally these should be cached
+    /// if the engine will be used repeatedly.
+    pub const fn new(alphabet: &Alphabet, config: GeneralPurposeConfig) -> Self {
+        Self {
+            encode_table: encode_table(alphabet),
+            decode_table: decode_table(alphabet),
+            config,
+        }
+    }
+}
+
+impl super::Engine for GeneralPurpose {
+    type Config = GeneralPurposeConfig;
+    type DecodeEstimate = GeneralPurposeEstimate;
+
+    fn internal_encode(&self, input: &[u8], output: &mut [u8]) -> usize {
+        let mut input_index: usize = 0;
+
+        const BLOCKS_PER_FAST_LOOP: usize = 4;
+        const LOW_SIX_BITS: u64 = 0x3F;
+
+        // we read 8 bytes at a time (u64) but only actually consume 6 of those bytes. Thus, we need
+        // 2 trailing bytes to be available to read..
+        let last_fast_index = input.len().saturating_sub(BLOCKS_PER_FAST_LOOP * 6 + 2);
+        let mut output_index = 0;
+
+        if last_fast_index > 0 {
+            while input_index <= last_fast_index {
+                // Major performance wins from letting the optimizer do the bounds check once, mostly
+                // on the output side
+                let input_chunk =
+                    &input[input_index..(input_index + (BLOCKS_PER_FAST_LOOP * 6 + 2))];
+                let output_chunk =
+                    &mut output[output_index..(output_index + BLOCKS_PER_FAST_LOOP * 8)];
+
+                // Hand-unrolling for 32 vs 16 or 8 bytes produces yields performance about equivalent
+                // to unsafe pointer code on a Xeon E5-1650v3. 64 byte unrolling was slightly better for
+                // large inputs but significantly worse for 50-byte input, unsurprisingly. I suspect
+                // that it's a not uncommon use case to encode smallish chunks of data (e.g. a 64-byte
+                // SHA-512 digest), so it would be nice if that fit in the unrolled loop at least once.
+                // Plus, single-digit percentage performance differences might well be quite different
+                // on different hardware.
+
+                let input_u64 = read_u64(&input_chunk[0..]);
+
+                output_chunk[0] = self.encode_table[((input_u64 >> 58) & LOW_SIX_BITS) as usize];
+                output_chunk[1] = self.encode_table[((input_u64 >> 52) & LOW_SIX_BITS) as usize];
+                output_chunk[2] = self.encode_table[((input_u64 >> 46) & LOW_SIX_BITS) as usize];
+                output_chunk[3] = self.encode_table[((input_u64 >> 40) & LOW_SIX_BITS) as usize];
+                output_chunk[4] = self.encode_table[((input_u64 >> 34) & LOW_SIX_BITS) as usize];
+                output_chunk[5] = self.encode_table[((input_u64 >> 28) & LOW_SIX_BITS) as usize];
+                output_chunk[6] = self.encode_table[((input_u64 >> 22) & LOW_SIX_BITS) as usize];
+                output_chunk[7] = self.encode_table[((input_u64 >> 16) & LOW_SIX_BITS) as usize];
+
+                let input_u64 = read_u64(&input_chunk[6..]);
+
+                output_chunk[8] = self.encode_table[((input_u64 >> 58) & LOW_SIX_BITS) as usize];
+                output_chunk[9] = self.encode_table[((input_u64 >> 52) & LOW_SIX_BITS) as usize];
+                output_chunk[10] = self.encode_table[((input_u64 >> 46) & LOW_SIX_BITS) as usize];
+                output_chunk[11] = self.encode_table[((input_u64 >> 40) & LOW_SIX_BITS) as usize];
+                output_chunk[12] = self.encode_table[((input_u64 >> 34) & LOW_SIX_BITS) as usize];
+                output_chunk[13] = self.encode_table[((input_u64 >> 28) & LOW_SIX_BITS) as usize];
+                output_chunk[14] = self.encode_table[((input_u64 >> 22) & LOW_SIX_BITS) as usize];
+                output_chunk[15] = self.encode_table[((input_u64 >> 16) & LOW_SIX_BITS) as usize];
+
+                let input_u64 = read_u64(&input_chunk[12..]);
+
+                output_chunk[16] = self.encode_table[((input_u64 >> 58) & LOW_SIX_BITS) as usize];
+                output_chunk[17] = self.encode_table[((input_u64 >> 52) & LOW_SIX_BITS) as usize];
+                output_chunk[18] = self.encode_table[((input_u64 >> 46) & LOW_SIX_BITS) as usize];
+                output_chunk[19] = self.encode_table[((input_u64 >> 40) & LOW_SIX_BITS) as usize];
+                output_chunk[20] = self.encode_table[((input_u64 >> 34) & LOW_SIX_BITS) as usize];
+                output_chunk[21] = self.encode_table[((input_u64 >> 28) & LOW_SIX_BITS) as usize];
+                output_chunk[22] = self.encode_table[((input_u64 >> 22) & LOW_SIX_BITS) as usize];
+                output_chunk[23] = self.encode_table[((input_u64 >> 16) & LOW_SIX_BITS) as usize];
+
+                let input_u64 = read_u64(&input_chunk[18..]);
+
+                output_chunk[24] = self.encode_table[((input_u64 >> 58) & LOW_SIX_BITS) as usize];
+                output_chunk[25] = self.encode_table[((input_u64 >> 52) & LOW_SIX_BITS) as usize];
+                output_chunk[26] = self.encode_table[((input_u64 >> 46) & LOW_SIX_BITS) as usize];
+                output_chunk[27] = self.encode_table[((input_u64 >> 40) & LOW_SIX_BITS) as usize];
+                output_chunk[28] = self.encode_table[((input_u64 >> 34) & LOW_SIX_BITS) as usize];
+                output_chunk[29] = self.encode_table[((input_u64 >> 28) & LOW_SIX_BITS) as usize];
+                output_chunk[30] = self.encode_table[((input_u64 >> 22) & LOW_SIX_BITS) as usize];
+                output_chunk[31] = self.encode_table[((input_u64 >> 16) & LOW_SIX_BITS) as usize];
+
+                output_index += BLOCKS_PER_FAST_LOOP * 8;
+                input_index += BLOCKS_PER_FAST_LOOP * 6;
+            }
+        }
+
+        // Encode what's left after the fast loop.
+
+        const LOW_SIX_BITS_U8: u8 = 0x3F;
+
+        let rem = input.len() % 3;
+        let start_of_rem = input.len() - rem;
+
+        // start at the first index not handled by fast loop, which may be 0.
+
+        while input_index < start_of_rem {
+            let input_chunk = &input[input_index..(input_index + 3)];
+            let output_chunk = &mut output[output_index..(output_index + 4)];
+
+            output_chunk[0] = self.encode_table[(input_chunk[0] >> 2) as usize];
+            output_chunk[1] = self.encode_table
+                [((input_chunk[0] << 4 | input_chunk[1] >> 4) & LOW_SIX_BITS_U8) as usize];
+            output_chunk[2] = self.encode_table
+                [((input_chunk[1] << 2 | input_chunk[2] >> 6) & LOW_SIX_BITS_U8) as usize];
+            output_chunk[3] = self.encode_table[(input_chunk[2] & LOW_SIX_BITS_U8) as usize];
+
+            input_index += 3;
+            output_index += 4;
+        }
+
+        if rem == 2 {
+            output[output_index] = self.encode_table[(input[start_of_rem] >> 2) as usize];
+            output[output_index + 1] =
+                self.encode_table[((input[start_of_rem] << 4 | input[start_of_rem + 1] >> 4)
+                    & LOW_SIX_BITS_U8) as usize];
+            output[output_index + 2] =
+                self.encode_table[((input[start_of_rem + 1] << 2) & LOW_SIX_BITS_U8) as usize];
+            output_index += 3;
+        } else if rem == 1 {
+            output[output_index] = self.encode_table[(input[start_of_rem] >> 2) as usize];
+            output[output_index + 1] =
+                self.encode_table[((input[start_of_rem] << 4) & LOW_SIX_BITS_U8) as usize];
+            output_index += 2;
+        }
+
+        output_index
+    }
+
+    fn internal_decoded_len_estimate(&self, input_len: usize) -> Self::DecodeEstimate {
+        GeneralPurposeEstimate::new(input_len)
+    }
+
+    fn internal_decode(
+        &self,
+        input: &[u8],
+        output: &mut [u8],
+        estimate: Self::DecodeEstimate,
+    ) -> Result<usize, DecodeError> {
+        decode::decode_helper(
+            input,
+            estimate,
+            output,
+            &self.decode_table,
+            self.config.decode_allow_trailing_bits,
+            self.config.decode_padding_mode,
+        )
+    }
+
+    fn config(&self) -> &Self::Config {
+        &self.config
+    }
+}
+
+/// Returns a table mapping a 6-bit index to the ASCII byte encoding of the index
+pub(crate) const fn encode_table(alphabet: &Alphabet) -> [u8; 64] {
+    // the encode table is just the alphabet:
+    // 6-bit index lookup -> printable byte
+    let mut encode_table = [0_u8; 64];
+    {
+        let mut index = 0;
+        while index < 64 {
+            encode_table[index] = alphabet.symbols[index];
+            index += 1;
+        }
+    }
+
+    encode_table
+}
+
+/// Returns a table mapping base64 bytes as the lookup index to either:
+/// - [INVALID_VALUE] for bytes that aren't members of the alphabet
+/// - a byte whose lower 6 bits are the value that was encoded into the index byte
+pub(crate) const fn decode_table(alphabet: &Alphabet) -> [u8; 256] {
+    let mut decode_table = [INVALID_VALUE; 256];
+
+    // Since the table is full of `INVALID_VALUE` already, we only need to overwrite
+    // the parts that are valid.
+    let mut index = 0;
+    while index < 64 {
+        // The index in the alphabet is the 6-bit value we care about.
+        // Since the index is in 0-63, it is safe to cast to u8.
+        decode_table[alphabet.symbols[index] as usize] = index as u8;
+        index += 1;
+    }
+
+    decode_table
+}
+
+#[inline]
+fn read_u64(s: &[u8]) -> u64 {
+    u64::from_be_bytes(s[..8].try_into().unwrap())
+}
+
+/// Contains configuration parameters for base64 encoding and decoding.
+///
+/// ```
+/// # use base64::engine::GeneralPurposeConfig;
+/// let config = GeneralPurposeConfig::new()
+///     .with_encode_padding(false);
+///     // further customize using `.with_*` methods as needed
+/// ```
+///
+/// The constants [PAD] and [NO_PAD] cover most use cases.
+///
+/// To specify the characters used, see [Alphabet].
+#[derive(Clone, Copy, Debug)]
+pub struct GeneralPurposeConfig {
+    encode_padding: bool,
+    decode_allow_trailing_bits: bool,
+    decode_padding_mode: DecodePaddingMode,
+}
+
+impl GeneralPurposeConfig {
+    /// Create a new config with `padding` = `true`, `decode_allow_trailing_bits` = `false`, and
+    /// `decode_padding_mode = DecodePaddingMode::RequireCanonicalPadding`.
+    ///
+    /// This probably matches most people's expectations, but consider disabling padding to save
+    /// a few bytes unless you specifically need it for compatibility with some legacy system.
+    pub const fn new() -> Self {
+        Self {
+            // RFC states that padding must be applied by default
+            encode_padding: true,
+            decode_allow_trailing_bits: false,
+            decode_padding_mode: DecodePaddingMode::RequireCanonical,
+        }
+    }
+
+    /// Create a new config based on `self` with an updated `padding` setting.
+    ///
+    /// If `padding` is `true`, encoding will append either 1 or 2 `=` padding characters as needed
+    /// to produce an output whose length is a multiple of 4.
+    ///
+    /// Padding is not needed for correct decoding and only serves to waste bytes, but it's in the
+    /// [spec](https://datatracker.ietf.org/doc/html/rfc4648#section-3.2).
+    ///
+    /// For new applications, consider not using padding if the decoders you're using don't require
+    /// padding to be present.
+    pub const fn with_encode_padding(self, padding: bool) -> Self {
+        Self {
+            encode_padding: padding,
+            ..self
+        }
+    }
+
+    /// Create a new config based on `self` with an updated `decode_allow_trailing_bits` setting.
+    ///
+    /// Most users will not need to configure this. It's useful if you need to decode base64
+    /// produced by a buggy encoder that has bits set in the unused space on the last base64
+    /// character as per [forgiving-base64 decode](https://infra.spec.whatwg.org/#forgiving-base64-decode).
+    /// If invalid trailing bits are present and this is `true`, those bits will
+    /// be silently ignored, else `DecodeError::InvalidLastSymbol` will be emitted.
+    pub const fn with_decode_allow_trailing_bits(self, allow: bool) -> Self {
+        Self {
+            decode_allow_trailing_bits: allow,
+            ..self
+        }
+    }
+
+    /// Create a new config based on `self` with an updated `decode_padding_mode` setting.
+    ///
+    /// Padding is not useful in terms of representing encoded data -- it makes no difference to
+    /// the decoder if padding is present or not, so if you have some un-padded input to decode, it
+    /// is perfectly fine to use `DecodePaddingMode::Indifferent` to prevent errors from being
+    /// emitted.
+    ///
+    /// However, since in practice
+    /// [people who learned nothing from BER vs DER seem to expect base64 to have one canonical encoding](https://eprint.iacr.org/2022/361),
+    /// the default setting is the stricter `DecodePaddingMode::RequireCanonicalPadding`.
+    ///
+    /// Or, if "canonical" in your circumstance means _no_ padding rather than padding to the
+    /// next multiple of four, there's `DecodePaddingMode::RequireNoPadding`.
+    pub const fn with_decode_padding_mode(self, mode: DecodePaddingMode) -> Self {
+        Self {
+            decode_padding_mode: mode,
+            ..self
+        }
+    }
+}
+
+impl Default for GeneralPurposeConfig {
+    /// Delegates to [GeneralPurposeConfig::new].
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
+impl Config for GeneralPurposeConfig {
+    fn encode_padding(&self) -> bool {
+        self.encode_padding
+    }
+}
+
+/// A [GeneralPurpose] engine using the [alphabet::STANDARD] base64 alphabet and [PAD] config.
+pub const STANDARD: GeneralPurpose = GeneralPurpose::new(&alphabet::STANDARD, PAD);
+
+/// A [GeneralPurpose] engine using the [alphabet::STANDARD] base64 alphabet and [NO_PAD] config.
+pub const STANDARD_NO_PAD: GeneralPurpose = GeneralPurpose::new(&alphabet::STANDARD, NO_PAD);
+
+/// A [GeneralPurpose] engine using the [alphabet::URL_SAFE] base64 alphabet and [PAD] config.
+pub const URL_SAFE: GeneralPurpose = GeneralPurpose::new(&alphabet::URL_SAFE, PAD);
+
+/// A [GeneralPurpose] engine using the [alphabet::URL_SAFE] base64 alphabet and [NO_PAD] config.
+pub const URL_SAFE_NO_PAD: GeneralPurpose = GeneralPurpose::new(&alphabet::URL_SAFE, NO_PAD);
+
+/// Include padding bytes when encoding, and require that they be present when decoding.
+///
+/// This is the standard per the base64 RFC, but consider using [NO_PAD] instead as padding serves
+/// little purpose in practice.
+pub const PAD: GeneralPurposeConfig = GeneralPurposeConfig::new();
+
+/// Don't add padding when encoding, and require no padding when decoding.
+pub const NO_PAD: GeneralPurposeConfig = GeneralPurposeConfig::new()
+    .with_encode_padding(false)
+    .with_decode_padding_mode(DecodePaddingMode::RequireNone);
diff --git a/third_party/rust/base64/src/engine/mod.rs b/third_party/rust/base64/src/engine/mod.rs
new file mode 100644
index 0000000000..12dfaa8845
--- /dev/null
+++ b/third_party/rust/base64/src/engine/mod.rs
@@ -0,0 +1,410 @@
+//! Provides the [Engine] abstraction and out of the box implementations.
+#[cfg(any(feature = "alloc", feature = "std", test))]
+use crate::chunked_encoder;
+use crate::{
+    encode::{encode_with_padding, EncodeSliceError},
+    encoded_len, DecodeError, DecodeSliceError,
+};
+#[cfg(any(feature = "alloc", feature = "std", test))]
+use alloc::vec::Vec;
+
+#[cfg(any(feature = "alloc", feature = "std", test))]
+use alloc::{string::String, vec};
+
+pub mod general_purpose;
+
+#[cfg(test)]
+mod naive;
+
+#[cfg(test)]
+mod tests;
+
+pub use general_purpose::{GeneralPurpose, GeneralPurposeConfig};
+
+/// An `Engine` provides low-level encoding and decoding operations that all other higher-level parts of the API use. Users of the library will generally not need to implement this.
+///
+/// Different implementations offer different characteristics. The library currently ships with
+/// [GeneralPurpose] that offers good speed and works on any CPU, with more choices
+/// coming later, like a constant-time one when side channel resistance is called for, and vendor-specific vectorized ones for more speed.
+///
+/// See [general_purpose::STANDARD_NO_PAD] if you just want standard base64. Otherwise, when possible, it's
+/// recommended to store the engine in a `const` so that references to it won't pose any lifetime
+/// issues, and to avoid repeating the cost of engine setup.
+///
+/// Since almost nobody will need to implement `Engine`, docs for internal methods are hidden.
+// When adding an implementation of Engine, include them in the engine test suite:
+// - add an implementation of [engine::tests::EngineWrapper]
+// - add the implementation to the `all_engines` macro
+// All tests run on all engines listed in the macro.
+pub trait Engine: Send + Sync {
+    /// The config type used by this engine
+    type Config: Config;
+    /// The decode estimate used by this engine
+    type DecodeEstimate: DecodeEstimate;
+
+    /// This is not meant to be called directly; it is only for `Engine` implementors.
+    /// See the other `encode*` functions on this trait.
+    ///
+    /// Encode the `input` bytes into the `output` buffer based on the mapping in `encode_table`.
+    ///
+    /// `output` will be long enough to hold the encoded data.
+    ///
+    /// Returns the number of bytes written.
+    ///
+    /// No padding should be written; that is handled separately.
+    ///
+    /// Must not write any bytes into the output slice other than the encoded data.
+    #[doc(hidden)]
+    fn internal_encode(&self, input: &[u8], output: &mut [u8]) -> usize;
+
+    /// This is not meant to be called directly; it is only for `Engine` implementors.
+    ///
+    /// As an optimization to prevent the decoded length from being calculated twice, it is
+    /// sometimes helpful to have a conservative estimate of the decoded size before doing the
+    /// decoding, so this calculation is done separately and passed to [Engine::decode()] as needed.
+    ///
+    /// # Panics
+    ///
+    /// Panics if decoded length estimation overflows.
+    #[doc(hidden)]
+    fn internal_decoded_len_estimate(&self, input_len: usize) -> Self::DecodeEstimate;
+
+    /// This is not meant to be called directly; it is only for `Engine` implementors.
+    /// See the other `decode*` functions on this trait.
+    ///
+    /// Decode `input` base64 bytes into the `output` buffer.
+    ///
+    /// `decode_estimate` is the result of [Engine::internal_decoded_len_estimate()], which is passed in to avoid
+    /// calculating it again (expensive on short inputs).`
+    ///
+    /// Returns the number of bytes written to `output`.
+    ///
+    /// Each complete 4-byte chunk of encoded data decodes to 3 bytes of decoded data, but this
+    /// function must also handle the final possibly partial chunk.
+    /// If the input length is not a multiple of 4, or uses padding bytes to reach a multiple of 4,
+    /// the trailing 2 or 3 bytes must decode to 1 or 2 bytes, respectively, as per the
+    /// [RFC](https://tools.ietf.org/html/rfc4648#section-3.5).
+    ///
+    /// Decoding must not write any bytes into the output slice other than the decoded data.
+    ///
+    /// Non-canonical trailing bits in the final tokens or non-canonical padding must be reported as
+    /// errors unless the engine is configured otherwise.
+    ///
+    /// # Panics
+    ///
+    /// Panics if `output` is too small.
+    #[doc(hidden)]
+    fn internal_decode(
+        &self,
+        input: &[u8],
+        output: &mut [u8],
+        decode_estimate: Self::DecodeEstimate,
+    ) -> Result<usize, DecodeError>;
+
+    /// Returns the config for this engine.
+    fn config(&self) -> &Self::Config;
+
+    /// Encode arbitrary octets as base64 using the provided `Engine`.
+    /// Returns a `String`.
+    ///
+    /// # Example
+    ///
+    /// ```rust
+    /// use base64::{Engine as _, engine::{self, general_purpose}, alphabet};
+    ///
+    /// let b64 = general_purpose::STANDARD.encode(b"hello world~");
+    /// println!("{}", b64);
+    ///
+    /// const CUSTOM_ENGINE: engine::GeneralPurpose =
+    ///     engine::GeneralPurpose::new(&alphabet::URL_SAFE, general_purpose::NO_PAD);
+    ///
+    /// let b64_url = CUSTOM_ENGINE.encode(b"hello internet~");
+    #[cfg(any(feature = "alloc", feature = "std", test))]
+    fn encode<T: AsRef<[u8]>>(&self, input: T) -> String {
+        let encoded_size = encoded_len(input.as_ref().len(), self.config().encode_padding())
+            .expect("integer overflow when calculating buffer size");
+        let mut buf = vec![0; encoded_size];
+
+        encode_with_padding(input.as_ref(), &mut buf[..], self, encoded_size);
+
+        String::from_utf8(buf).expect("Invalid UTF8")
+    }
+
+    /// Encode arbitrary octets as base64 into a supplied `String`.
+    /// Writes into the supplied `String`, which may allocate if its internal buffer isn't big enough.
+    ///
+    /// # Example
+    ///
+    /// ```rust
+    /// use base64::{Engine as _, engine::{self, general_purpose}, alphabet};
+    /// const CUSTOM_ENGINE: engine::GeneralPurpose =
+    ///     engine::GeneralPurpose::new(&alphabet::URL_SAFE, general_purpose::NO_PAD);
+    ///
+    /// fn main() {
+    ///     let mut buf = String::new();
+    ///     general_purpose::STANDARD.encode_string(b"hello world~", &mut buf);
+    ///     println!("{}", buf);
+    ///
+    ///     buf.clear();
+    ///     CUSTOM_ENGINE.encode_string(b"hello internet~", &mut buf);
+    ///     println!("{}", buf);
+    /// }
+    /// ```
+    #[cfg(any(feature = "alloc", feature = "std", test))]
+    fn encode_string<T: AsRef<[u8]>>(&self, input: T, output_buf: &mut String) {
+        let input_bytes = input.as_ref();
+
+        {
+            let mut sink = chunked_encoder::StringSink::new(output_buf);
+
+            chunked_encoder::ChunkedEncoder::new(self)
+                .encode(input_bytes, &mut sink)
+                .expect("Writing to a String shouldn't fail");
+        }
+    }
+
+    /// Encode arbitrary octets as base64 into a supplied slice.
+    /// Writes into the supplied output buffer.
+    ///
+    /// This is useful if you wish to avoid allocation entirely (e.g. encoding into a stack-resident
+    /// or statically-allocated buffer).
+    ///
+    /// # Example
+    ///
+    /// ```rust
+    /// use base64::{Engine as _, engine::general_purpose};
+    /// let s = b"hello internet!";
+    /// let mut buf = Vec::new();
+    /// // make sure we'll have a slice big enough for base64 + padding
+    /// buf.resize(s.len() * 4 / 3 + 4, 0);
+    ///
+    /// let bytes_written = general_purpose::STANDARD.encode_slice(s, &mut buf).unwrap();
+    ///
+    /// // shorten our vec down to just what was written
+    /// buf.truncate(bytes_written);
+    ///
+    /// assert_eq!(s, general_purpose::STANDARD.decode(&buf).unwrap().as_slice());
+    /// ```
+    fn encode_slice<T: AsRef<[u8]>>(
+        &self,
+        input: T,
+        output_buf: &mut [u8],
+    ) -> Result<usize, EncodeSliceError> {
+        let input_bytes = input.as_ref();
+
+        let encoded_size = encoded_len(input_bytes.len(), self.config().encode_padding())
+            .expect("usize overflow when calculating buffer size");
+
+        if output_buf.len() < encoded_size {
+            return Err(EncodeSliceError::OutputSliceTooSmall);
+        }
+
+        let b64_output = &mut output_buf[0..encoded_size];
+
+        encode_with_padding(input_bytes, b64_output, self, encoded_size);
+
+        Ok(encoded_size)
+    }
+
+    /// Decode from string reference as octets using the specified [Engine].
+    /// Returns a `Result` containing a `Vec<u8>`.
+    ///
+    /// # Example
+    ///
+    /// ```rust
+    /// use base64::{Engine as _, alphabet, engine::{self, general_purpose}};
+    ///
+    /// let bytes = general_purpose::STANDARD
+    ///     .decode("aGVsbG8gd29ybGR+Cg==").unwrap();
+    /// println!("{:?}", bytes);
+    ///
+    /// // custom engine setup
+    /// let bytes_url = engine::GeneralPurpose::new(
+    ///              &alphabet::URL_SAFE,
+    ///              general_purpose::NO_PAD)
+    ///     .decode("aGVsbG8gaW50ZXJuZXR-Cg").unwrap();
+    /// println!("{:?}", bytes_url);
+    /// ```
+    ///
+    /// # Panics
+    ///
+    /// Panics if decoded length estimation overflows.
+    /// This would happen for sizes within a few bytes of the maximum value of `usize`.
+    #[cfg(any(feature = "alloc", feature = "std", test))]
+    fn decode<T: AsRef<[u8]>>(&self, input: T) -> Result<Vec<u8>, DecodeError> {
+        let input_bytes = input.as_ref();
+
+        let estimate = self.internal_decoded_len_estimate(input_bytes.len());
+        let mut buffer = vec![0; estimate.decoded_len_estimate()];
+
+        let bytes_written = self.internal_decode(input_bytes, &mut buffer, estimate)?;
+        buffer.truncate(bytes_written);
+
+        Ok(buffer)
+    }
+
+    /// Decode from string reference as octets.
+    /// Writes into the supplied `Vec`, which may allocate if its internal buffer isn't big enough.
+    /// Returns a `Result` containing an empty tuple, aka `()`.
+    ///
+    /// # Example
+    ///
+    /// ```rust
+    /// use base64::{Engine as _, alphabet, engine::{self, general_purpose}};
+    /// const CUSTOM_ENGINE: engine::GeneralPurpose =
+    ///     engine::GeneralPurpose::new(&alphabet::URL_SAFE, general_purpose::PAD);
+    ///
+    /// fn main() {
+    ///     use base64::Engine;
+    ///     let mut buffer = Vec::<u8>::new();
+    ///     // with the default engine
+    ///     general_purpose::STANDARD
+    ///         .decode_vec("aGVsbG8gd29ybGR+Cg==", &mut buffer,).unwrap();
+    ///     println!("{:?}", buffer);
+    ///
+    ///     buffer.clear();
+    ///
+    ///     // with a custom engine
+    ///     CUSTOM_ENGINE.decode_vec(
+    ///         "aGVsbG8gaW50ZXJuZXR-Cg==",
+    ///         &mut buffer,
+    ///     ).unwrap();
+    ///     println!("{:?}", buffer);
+    /// }
+    /// ```
+    ///
+    /// # Panics
+    ///
+    /// Panics if decoded length estimation overflows.
+    /// This would happen for sizes within a few bytes of the maximum value of `usize`.
+    #[cfg(any(feature = "alloc", feature = "std", test))]
+    fn decode_vec<T: AsRef<[u8]>>(
+        &self,
+        input: T,
+        buffer: &mut Vec<u8>,
+    ) -> Result<(), DecodeError> {
+        let input_bytes = input.as_ref();
+
+        let starting_output_len = buffer.len();
+
+        let estimate = self.internal_decoded_len_estimate(input_bytes.len());
+        let total_len_estimate = estimate
+            .decoded_len_estimate()
+            .checked_add(starting_output_len)
+            .expect("Overflow when calculating output buffer length");
+        buffer.resize(total_len_estimate, 0);
+
+        let buffer_slice = &mut buffer.as_mut_slice()[starting_output_len..];
+        let bytes_written = self.internal_decode(input_bytes, buffer_slice, estimate)?;
+
+        buffer.truncate(starting_output_len + bytes_written);
+
+        Ok(())
+    }
+
+    /// Decode the input into the provided output slice.
+    ///
+    /// Returns an error if `output` is smaller than the estimated decoded length.
+    ///
+    /// This will not write any bytes past exactly what is decoded (no stray garbage bytes at the end).
+    ///
+    /// See [crate::decoded_len_estimate] for calculating buffer sizes.
+    ///
+    /// See [Engine::decode_slice_unchecked] for a version that panics instead of returning an error
+    /// if the output buffer is too small.
+    ///
+    /// # Panics
+    ///
+    /// Panics if decoded length estimation overflows.
+    /// This would happen for sizes within a few bytes of the maximum value of `usize`.
+    fn decode_slice<T: AsRef<[u8]>>(
+        &self,
+        input: T,
+        output: &mut [u8],
+    ) -> Result<usize, DecodeSliceError> {
+        let input_bytes = input.as_ref();
+
+        let estimate = self.internal_decoded_len_estimate(input_bytes.len());
+        if output.len() < estimate.decoded_len_estimate() {
+            return Err(DecodeSliceError::OutputSliceTooSmall);
+        }
+
+        self.internal_decode(input_bytes, output, estimate)
+            .map_err(|e| e.into())
+    }
+
+    /// Decode the input into the provided output slice.
+    ///
+    /// This will not write any bytes past exactly what is decoded (no stray garbage bytes at the end).
+    ///
+    /// See [crate::decoded_len_estimate] for calculating buffer sizes.
+    ///
+    /// See [Engine::decode_slice] for a version that returns an error instead of panicking if the output
+    /// buffer is too small.
+    ///
+    /// # Panics
+    ///
+    /// Panics if decoded length estimation overflows.
+    /// This would happen for sizes within a few bytes of the maximum value of `usize`.
+    ///
+    /// Panics if the provided output buffer is too small for the decoded data.
+    fn decode_slice_unchecked<T: AsRef<[u8]>>(
+        &self,
+        input: T,
+        output: &mut [u8],
+    ) -> Result<usize, DecodeError> {
+        let input_bytes = input.as_ref();
+
+        self.internal_decode(
+            input_bytes,
+            output,
+            self.internal_decoded_len_estimate(input_bytes.len()),
+        )
+    }
+}
+
+/// The minimal level of configuration that engines must support.
+pub trait Config {
+    /// Returns `true` if padding should be added after the encoded output.
+    ///
+    /// Padding is added outside the engine's encode() since the engine may be used
+    /// to encode only a chunk of the overall output, so it can't always know when
+    /// the output is "done" and would therefore need padding (if configured).
+    // It could be provided as a separate parameter when encoding, but that feels like
+    // leaking an implementation detail to the user, and it's hopefully more convenient
+    // to have to only pass one thing (the engine) to any part of the API.
+    fn encode_padding(&self) -> bool;
+}
+
+/// The decode estimate used by an engine implementation. Users do not need to interact with this;
+/// it is only for engine implementors.
+///
+/// Implementors may store relevant data here when constructing this to avoid having to calculate
+/// them again during actual decoding.
+pub trait DecodeEstimate {
+    /// Returns a conservative (err on the side of too big) estimate of the decoded length to use
+    /// for pre-allocating buffers, etc.
+    ///
+    /// The estimate must be no larger than the next largest complete triple of decoded bytes.
+    /// That is, the final quad of tokens to decode may be assumed to be complete with no padding.
+    ///
+    /// # Panics
+    ///
+    /// Panics if decoded length estimation overflows.
+    /// This would happen for sizes within a few bytes of the maximum value of `usize`.
+    fn decoded_len_estimate(&self) -> usize;
+}
+
+/// Controls how pad bytes are handled when decoding.
+///
+/// Each [Engine] must support at least the behavior indicated by
+/// [DecodePaddingMode::RequireCanonical], and may support other modes.
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+pub enum DecodePaddingMode {
+    /// Canonical padding is allowed, but any fewer padding bytes than that is also allowed.
+    Indifferent,
+    /// Padding must be canonical (0, 1, or 2 `=` as needed to produce a 4 byte suffix).
+    RequireCanonical,
+    /// Padding must be absent -- for when you want predictable padding, without any wasted bytes.
+    RequireNone,
+}
diff --git a/third_party/rust/base64/src/engine/naive.rs b/third_party/rust/base64/src/engine/naive.rs
new file mode 100644
index 0000000000..6665c5eb41
--- /dev/null
+++ b/third_party/rust/base64/src/engine/naive.rs
@@ -0,0 +1,219 @@
+use crate::{
+    alphabet::Alphabet,
+    engine::{
+        general_purpose::{self, decode_table, encode_table},
+        Config, DecodeEstimate, DecodePaddingMode, Engine,
+    },
+    DecodeError, PAD_BYTE,
+};
+use alloc::ops::BitOr;
+use std::ops::{BitAnd, Shl, Shr};
+
+/// Comparatively simple implementation that can be used as something to compare against in tests
+pub struct Naive {
+    encode_table: [u8; 64],
+    decode_table: [u8; 256],
+    config: NaiveConfig,
+}
+
+impl Naive {
+    const ENCODE_INPUT_CHUNK_SIZE: usize = 3;
+    const DECODE_INPUT_CHUNK_SIZE: usize = 4;
+
+    pub const fn new(alphabet: &Alphabet, config: NaiveConfig) -> Self {
+        Self {
+            encode_table: encode_table(alphabet),
+            decode_table: decode_table(alphabet),
+            config,
+        }
+    }
+
+    fn decode_byte_into_u32(&self, offset: usize, byte: u8) -> Result<u32, DecodeError> {
+        let decoded = self.decode_table[byte as usize];
+
+        if decoded == general_purpose::INVALID_VALUE {
+            return Err(DecodeError::InvalidByte(offset, byte));
+        }
+
+        Ok(decoded as u32)
+    }
+}
+
+impl Engine for Naive {
+    type Config = NaiveConfig;
+    type DecodeEstimate = NaiveEstimate;
+
+    fn internal_encode(&self, input: &[u8], output: &mut [u8]) -> usize {
+        // complete chunks first
+
+        const LOW_SIX_BITS: u32 = 0x3F;
+
+        let rem = input.len() % Self::ENCODE_INPUT_CHUNK_SIZE;
+        // will never underflow
+        let complete_chunk_len = input.len() - rem;
+
+        let mut input_index = 0_usize;
+        let mut output_index = 0_usize;
+        if let Some(last_complete_chunk_index) =
+            complete_chunk_len.checked_sub(Self::ENCODE_INPUT_CHUNK_SIZE)
+        {
+            while input_index <= last_complete_chunk_index {
+                let chunk = &input[input_index..input_index + Self::ENCODE_INPUT_CHUNK_SIZE];
+
+                // populate low 24 bits from 3 bytes
+                let chunk_int: u32 =
+                    (chunk[0] as u32).shl(16) | (chunk[1] as u32).shl(8) | (chunk[2] as u32);
+                // encode 4x 6-bit output bytes
+                output[output_index] = self.encode_table[chunk_int.shr(18) as usize];
+                output[output_index + 1] =
+                    self.encode_table[chunk_int.shr(12_u8).bitand(LOW_SIX_BITS) as usize];
+                output[output_index + 2] =
+                    self.encode_table[chunk_int.shr(6_u8).bitand(LOW_SIX_BITS) as usize];
+                output[output_index + 3] =
+                    self.encode_table[chunk_int.bitand(LOW_SIX_BITS) as usize];
+
+                input_index += Self::ENCODE_INPUT_CHUNK_SIZE;
+                output_index += 4;
+            }
+        }
+
+        // then leftovers
+        if rem == 2 {
+            let chunk = &input[input_index..input_index + 2];
+
+            // high six bits of chunk[0]
+            output[output_index] = self.encode_table[chunk[0].shr(2) as usize];
+            // bottom 2 bits of [0], high 4 bits of [1]
+            output[output_index + 1] =
+                self.encode_table[(chunk[0].shl(4_u8).bitor(chunk[1].shr(4_u8)) as u32)
+                    .bitand(LOW_SIX_BITS) as usize];
+            // bottom 4 bits of [1], with the 2 bottom bits as zero
+            output[output_index + 2] =
+                self.encode_table[(chunk[1].shl(2_u8) as u32).bitand(LOW_SIX_BITS) as usize];
+
+            output_index += 3;
+        } else if rem == 1 {
+            let byte = input[input_index];
+            output[output_index] = self.encode_table[byte.shr(2) as usize];
+            output[output_index + 1] =
+                self.encode_table[(byte.shl(4_u8) as u32).bitand(LOW_SIX_BITS) as usize];
+            output_index += 2;
+        }
+
+        output_index
+    }
+
+    fn internal_decoded_len_estimate(&self, input_len: usize) -> Self::DecodeEstimate {
+        NaiveEstimate::new(input_len)
+    }
+
+    fn internal_decode(
+        &self,
+        input: &[u8],
+        output: &mut [u8],
+        estimate: Self::DecodeEstimate,
+    ) -> Result<usize, DecodeError> {
+        if estimate.rem == 1 {
+            // trailing whitespace is so common that it's worth it to check the last byte to
+            // possibly return a better error message
+            if let Some(b) = input.last() {
+                if *b != PAD_BYTE
+                    && self.decode_table[*b as usize] == general_purpose::INVALID_VALUE
+                {
+                    return Err(DecodeError::InvalidByte(input.len() - 1, *b));
+                }
+            }
+
+            return Err(DecodeError::InvalidLength);
+        }
+
+        let mut input_index = 0_usize;
+        let mut output_index = 0_usize;
+        const BOTTOM_BYTE: u32 = 0xFF;
+
+        // can only use the main loop on non-trailing chunks
+        if input.len() > Self::DECODE_INPUT_CHUNK_SIZE {
+            // skip the last chunk, whether it's partial or full, since it might
+            // have padding, and start at the beginning of the chunk before that
+            let last_complete_chunk_start_index = estimate.complete_chunk_len
+                - if estimate.rem == 0 {
+                    // Trailing chunk is also full chunk, so there must be at least 2 chunks, and
+                    // this won't underflow
+                    Self::DECODE_INPUT_CHUNK_SIZE * 2
+                } else {
+                    // Trailing chunk is partial, so it's already excluded in
+                    // complete_chunk_len
+                    Self::DECODE_INPUT_CHUNK_SIZE
+                };
+
+            while input_index <= last_complete_chunk_start_index {
+                let chunk = &input[input_index..input_index + Self::DECODE_INPUT_CHUNK_SIZE];
+                let decoded_int: u32 = self.decode_byte_into_u32(input_index, chunk[0])?.shl(18)
+                    | self
+                        .decode_byte_into_u32(input_index + 1, chunk[1])?
+                        .shl(12)
+                    | self.decode_byte_into_u32(input_index + 2, chunk[2])?.shl(6)
+                    | self.decode_byte_into_u32(input_index + 3, chunk[3])?;
+
+                output[output_index] = decoded_int.shr(16_u8).bitand(BOTTOM_BYTE) as u8;
+                output[output_index + 1] = decoded_int.shr(8_u8).bitand(BOTTOM_BYTE) as u8;
+                output[output_index + 2] = decoded_int.bitand(BOTTOM_BYTE) as u8;
+
+                input_index += Self::DECODE_INPUT_CHUNK_SIZE;
+                output_index += 3;
+            }
+        }
+
+        general_purpose::decode_suffix::decode_suffix(
+            input,
+            input_index,
+            output,
+            output_index,
+            &self.decode_table,
+            self.config.decode_allow_trailing_bits,
+            self.config.decode_padding_mode,
+        )
+    }
+
+    fn config(&self) -> &Self::Config {
+        &self.config
+    }
+}
+
+pub struct NaiveEstimate {
+    /// remainder from dividing input by `Naive::DECODE_CHUNK_SIZE`
+    rem: usize,
+    /// Length of input that is in complete `Naive::DECODE_CHUNK_SIZE`-length chunks
+    complete_chunk_len: usize,
+}
+
+impl NaiveEstimate {
+    fn new(input_len: usize) -> Self {
+        let rem = input_len % Naive::DECODE_INPUT_CHUNK_SIZE;
+        let complete_chunk_len = input_len - rem;
+
+        Self {
+            rem,
+            complete_chunk_len,
+        }
+    }
+}
+
+impl DecodeEstimate for NaiveEstimate {
+    fn decoded_len_estimate(&self) -> usize {
+        ((self.complete_chunk_len / 4) + ((self.rem > 0) as usize)) * 3
+    }
+}
+
+#[derive(Clone, Copy, Debug)]
+pub struct NaiveConfig {
+    pub encode_padding: bool,
+    pub decode_allow_trailing_bits: bool,
+    pub decode_padding_mode: DecodePaddingMode,
+}
+
+impl Config for NaiveConfig {
+    fn encode_padding(&self) -> bool {
+        self.encode_padding
+    }
+}
diff --git a/third_party/rust/base64/src/engine/tests.rs b/third_party/rust/base64/src/engine/tests.rs
new file mode 100644
index 0000000000..906bba04d8
--- /dev/null
+++ b/third_party/rust/base64/src/engine/tests.rs
@@ -0,0 +1,1430 @@
+// rstest_reuse template functions have unused variables
+#![allow(unused_variables)]
+
+use rand::{
+    self,
+    distributions::{self, Distribution as _},
+    rngs, Rng as _, SeedableRng as _,
+};
+use rstest::rstest;
+use rstest_reuse::{apply, template};
+use std::{collections, fmt};
+
+use crate::{
+    alphabet::{Alphabet, STANDARD},
+    encode::add_padding,
+    encoded_len,
+    engine::{general_purpose, naive, Config, DecodeEstimate, DecodePaddingMode, Engine},
+    tests::{assert_encode_sanity, random_alphabet, random_config},
+    DecodeError, PAD_BYTE,
+};
+
+// the case::foo syntax includes the "foo" in the generated test method names
+#[template]
+#[rstest(engine_wrapper,
+case::general_purpose(GeneralPurposeWrapper {}),
+case::naive(NaiveWrapper {}),
+)]
+fn all_engines<E: EngineWrapper>(engine_wrapper: E) {}
+
+#[apply(all_engines)]
+fn rfc_test_vectors_std_alphabet<E: EngineWrapper>(engine_wrapper: E) {
+    let data = vec![
+        ("", ""),
+        ("f", "Zg=="),
+        ("fo", "Zm8="),
+        ("foo", "Zm9v"),
+        ("foob", "Zm9vYg=="),
+        ("fooba", "Zm9vYmE="),
+        ("foobar", "Zm9vYmFy"),
+    ];
+
+    let engine = E::standard();
+    let engine_no_padding = E::standard_unpadded();
+
+    for (orig, encoded) in &data {
+        let encoded_without_padding = encoded.trim_end_matches('=');
+
+        // unpadded
+        {
+            let mut encode_buf = [0_u8; 8];
+            let mut decode_buf = [0_u8; 6];
+
+            let encode_len =
+                engine_no_padding.internal_encode(orig.as_bytes(), &mut encode_buf[..]);
+            assert_eq!(
+                &encoded_without_padding,
+                &std::str::from_utf8(&encode_buf[0..encode_len]).unwrap()
+            );
+            let decode_len = engine_no_padding
+                .decode_slice_unchecked(encoded_without_padding.as_bytes(), &mut decode_buf[..])
+                .unwrap();
+            assert_eq!(orig.len(), decode_len);
+
+            assert_eq!(
+                orig,
+                &std::str::from_utf8(&decode_buf[0..decode_len]).unwrap()
+            );
+
+            // if there was any padding originally, the no padding engine won't decode it
+            if encoded.as_bytes().contains(&PAD_BYTE) {
+                assert_eq!(
+                    Err(DecodeError::InvalidPadding),
+                    engine_no_padding.decode(encoded)
+                )
+            }
+        }
+
+        // padded
+        {
+            let mut encode_buf = [0_u8; 8];
+            let mut decode_buf = [0_u8; 6];
+
+            let encode_len = engine.internal_encode(orig.as_bytes(), &mut encode_buf[..]);
+            assert_eq!(
+                // doesn't have padding added yet
+                &encoded_without_padding,
+                &std::str::from_utf8(&encode_buf[0..encode_len]).unwrap()
+            );
+            let pad_len = add_padding(orig.len(), &mut encode_buf[encode_len..]);
+            assert_eq!(encoded.as_bytes(), &encode_buf[..encode_len + pad_len]);
+
+            let decode_len = engine
+                .decode_slice_unchecked(encoded.as_bytes(), &mut decode_buf[..])
+                .unwrap();
+            assert_eq!(orig.len(), decode_len);
+
+            assert_eq!(
+                orig,
+                &std::str::from_utf8(&decode_buf[0..decode_len]).unwrap()
+            );
+
+            // if there was (canonical) padding, and we remove it, the standard engine won't decode
+            if encoded.as_bytes().contains(&PAD_BYTE) {
+                assert_eq!(
+                    Err(DecodeError::InvalidPadding),
+                    engine.decode(encoded_without_padding)
+                )
+            }
+        }
+    }
+}
+
+#[apply(all_engines)]
+fn roundtrip_random<E: EngineWrapper>(engine_wrapper: E) {
+    let mut rng = seeded_rng();
+
+    let mut orig_data = Vec::<u8>::new();
+    let mut encode_buf = Vec::<u8>::new();
+    let mut decode_buf = Vec::<u8>::new();
+
+    let len_range = distributions::Uniform::new(1, 1_000);
+
+    for _ in 0..10_000 {
+        let engine = E::random(&mut rng);
+
+        orig_data.clear();
+        encode_buf.clear();
+        decode_buf.clear();
+
+        let (orig_len, _, encoded_len) = generate_random_encoded_data(
+            &engine,
+            &mut orig_data,
+            &mut encode_buf,
+            &mut rng,
+            &len_range,
+        );
+
+        // exactly the right size
+        decode_buf.resize(orig_len, 0);
+
+        let dec_len = engine
+            .decode_slice_unchecked(&encode_buf[0..encoded_len], &mut decode_buf[..])
+            .unwrap();
+
+        assert_eq!(orig_len, dec_len);
+        assert_eq!(&orig_data[..], &decode_buf[..dec_len]);
+    }
+}
+
+#[apply(all_engines)]
+fn encode_doesnt_write_extra_bytes<E: EngineWrapper>(engine_wrapper: E) {
+    let mut rng = seeded_rng();
+
+    let mut orig_data = Vec::<u8>::new();
+    let mut encode_buf = Vec::<u8>::new();
+    let mut encode_buf_backup = Vec::<u8>::new();
+
+    let input_len_range = distributions::Uniform::new(0, 1000);
+
+    for _ in 0..10_000 {
+        let engine = E::random(&mut rng);
+        let padded = engine.config().encode_padding();
+
+        orig_data.clear();
+        encode_buf.clear();
+        encode_buf_backup.clear();
+
+        let orig_len = fill_rand(&mut orig_data, &mut rng, &input_len_range);
+
+        let prefix_len = 1024;
+        // plenty of prefix and suffix
+        fill_rand_len(&mut encode_buf, &mut rng, prefix_len * 2 + orig_len * 2);
+        encode_buf_backup.extend_from_slice(&encode_buf[..]);
+
+        let expected_encode_len_no_pad = encoded_len(orig_len, false).unwrap();
+
+        let encoded_len_no_pad =
+            engine.internal_encode(&orig_data[..], &mut encode_buf[prefix_len..]);
+        assert_eq!(expected_encode_len_no_pad, encoded_len_no_pad);
+
+        // no writes past what it claimed to write
+        assert_eq!(&encode_buf_backup[..prefix_len], &encode_buf[..prefix_len]);
+        assert_eq!(
+            &encode_buf_backup[(prefix_len + encoded_len_no_pad)..],
+            &encode_buf[(prefix_len + encoded_len_no_pad)..]
+        );
+
+        let encoded_data = &encode_buf[prefix_len..(prefix_len + encoded_len_no_pad)];
+        assert_encode_sanity(
+            std::str::from_utf8(encoded_data).unwrap(),
+            // engines don't pad
+            false,
+            orig_len,
+        );
+
+        // pad so we can decode it in case our random engine requires padding
+        let pad_len = if padded {
+            add_padding(orig_len, &mut encode_buf[prefix_len + encoded_len_no_pad..])
+        } else {
+            0
+        };
+
+        assert_eq!(
+            orig_data,
+            engine
+                .decode(&encode_buf[prefix_len..(prefix_len + encoded_len_no_pad + pad_len)],)
+                .unwrap()
+        );
+    }
+}
+
+#[apply(all_engines)]
+fn encode_engine_slice_fits_into_precisely_sized_slice<E: EngineWrapper>(engine_wrapper: E) {
+    let mut orig_data = Vec::new();
+    let mut encoded_data = Vec::new();
+    let mut decoded = Vec::new();
+
+    let input_len_range = distributions::Uniform::new(0, 1000);
+
+    let mut rng = rngs::SmallRng::from_entropy();
+
+    for _ in 0..10_000 {
+        orig_data.clear();
+        encoded_data.clear();
+        decoded.clear();
+
+        let input_len = input_len_range.sample(&mut rng);
+
+        for _ in 0..input_len {
+            orig_data.push(rng.gen());
+        }
+
+        let engine = E::random(&mut rng);
+
+        let encoded_size = encoded_len(input_len, engine.config().encode_padding()).unwrap();
+
+        encoded_data.resize(encoded_size, 0);
+
+        assert_eq!(
+            encoded_size,
+            engine.encode_slice(&orig_data, &mut encoded_data).unwrap()
+        );
+
+        assert_encode_sanity(
+            std::str::from_utf8(&encoded_data[0..encoded_size]).unwrap(),
+            engine.config().encode_padding(),
+            input_len,
+        );
+
+        engine
+            .decode_vec(&encoded_data[0..encoded_size], &mut decoded)
+            .unwrap();
+        assert_eq!(orig_data, decoded);
+    }
+}
+
+#[apply(all_engines)]
+fn decode_doesnt_write_extra_bytes<E>(engine_wrapper: E)
+where
+    E: EngineWrapper,
+    <<E as EngineWrapper>::Engine as Engine>::Config: fmt::Debug,
+{
+    let mut rng = seeded_rng();
+
+    let mut orig_data = Vec::<u8>::new();
+    let mut encode_buf = Vec::<u8>::new();
+    let mut decode_buf = Vec::<u8>::new();
+    let mut decode_buf_backup = Vec::<u8>::new();
+
+    let len_range = distributions::Uniform::new(1, 1_000);
+
+    for _ in 0..10_000 {
+        let engine = E::random(&mut rng);
+
+        orig_data.clear();
+        encode_buf.clear();
+        decode_buf.clear();
+        decode_buf_backup.clear();
+
+        let orig_len = fill_rand(&mut orig_data, &mut rng, &len_range);
+        encode_buf.resize(orig_len * 2 + 100, 0);
+
+        let encoded_len = engine
+            .encode_slice(&orig_data[..], &mut encode_buf[..])
+            .unwrap();
+        encode_buf.truncate(encoded_len);
+
+        // oversize decode buffer so we can easily tell if it writes anything more than
+        // just the decoded data
+        let prefix_len = 1024;
+        // plenty of prefix and suffix
+        fill_rand_len(&mut decode_buf, &mut rng, prefix_len * 2 + orig_len * 2);
+        decode_buf_backup.extend_from_slice(&decode_buf[..]);
+
+        let dec_len = engine
+            .decode_slice_unchecked(&encode_buf, &mut decode_buf[prefix_len..])
+            .unwrap();
+
+        assert_eq!(orig_len, dec_len);
+        assert_eq!(
+            &orig_data[..],
+            &decode_buf[prefix_len..prefix_len + dec_len]
+        );
+        assert_eq!(&decode_buf_backup[..prefix_len], &decode_buf[..prefix_len]);
+        assert_eq!(
+            &decode_buf_backup[prefix_len + dec_len..],
+            &decode_buf[prefix_len + dec_len..]
+        );
+    }
+}
+
+#[apply(all_engines)]
+fn decode_detect_invalid_last_symbol<E: EngineWrapper>(engine_wrapper: E) {
+    // 0xFF -> "/w==", so all letters > w, 0-9, and '+', '/' should get InvalidLastSymbol
+    let engine = E::standard();
+
+    assert_eq!(Ok(vec![0x89, 0x85]), engine.decode("iYU="));
+    assert_eq!(Ok(vec![0xFF]), engine.decode("/w=="));
+
+    for (suffix, offset) in vec![
+        // suffix, offset of bad byte from start of suffix
+        ("/x==", 1_usize),
+        ("/z==", 1_usize),
+        ("/0==", 1_usize),
+        ("/9==", 1_usize),
+        ("/+==", 1_usize),
+        ("//==", 1_usize),
+        // trailing 01
+        ("iYV=", 2_usize),
+        // trailing 10
+        ("iYW=", 2_usize),
+        // trailing 11
+        ("iYX=", 2_usize),
+    ] {
+        for prefix_quads in 0..256 {
+            let mut encoded = "AAAA".repeat(prefix_quads);
+            encoded.push_str(suffix);
+
+            assert_eq!(
+                Err(DecodeError::InvalidLastSymbol(
+                    encoded.len() - 4 + offset,
+                    suffix.as_bytes()[offset],
+                )),
+                engine.decode(encoded.as_str())
+            );
+        }
+    }
+}
+
+#[apply(all_engines)]
+fn decode_detect_invalid_last_symbol_when_length_is_also_invalid<E: EngineWrapper>(
+    engine_wrapper: E,
+) {
+    let mut rng = seeded_rng();
+
+    // check across enough lengths that it would likely cover any implementation's various internal
+    // small/large input division
+    for len in (0_usize..256).map(|len| len * 4 + 1) {
+        let engine = E::random_alphabet(&mut rng, &STANDARD);
+
+        let mut input = vec![b'A'; len];
+
+        // with a valid last char, it's InvalidLength
+        assert_eq!(Err(DecodeError::InvalidLength), engine.decode(&input));
+        // after mangling the last char, it's InvalidByte
+        input[len - 1] = b'"';
+        assert_eq!(
+            Err(DecodeError::InvalidByte(len - 1, b'"')),
+            engine.decode(&input)
+        );
+    }
+}
+
+#[apply(all_engines)]
+fn decode_detect_invalid_last_symbol_every_possible_two_symbols<E: EngineWrapper>(
+    engine_wrapper: E,
+) {
+    let engine = E::standard();
+
+    let mut base64_to_bytes = collections::HashMap::new();
+
+    for b in 0_u8..=255 {
+        let mut b64 = vec![0_u8; 4];
+        assert_eq!(2, engine.internal_encode(&[b], &mut b64[..]));
+        let _ = add_padding(1, &mut b64[2..]);
+
+        assert!(base64_to_bytes.insert(b64, vec![b]).is_none());
+    }
+
+    // every possible combination of trailing symbols must either decode to 1 byte or get InvalidLastSymbol, with or without any leading chunks
+
+    let mut prefix = Vec::new();
+    for _ in 0..256 {
+        let mut clone = prefix.clone();
+
+        let mut symbols = [0_u8; 4];
+        for &s1 in STANDARD.symbols.iter() {
+            symbols[0] = s1;
+            for &s2 in STANDARD.symbols.iter() {
+                symbols[1] = s2;
+                symbols[2] = PAD_BYTE;
+                symbols[3] = PAD_BYTE;
+
+                // chop off previous symbols
+                clone.truncate(prefix.len());
+                clone.extend_from_slice(&symbols[..]);
+                let decoded_prefix_len = prefix.len() / 4 * 3;
+
+                match base64_to_bytes.get(&symbols[..]) {
+                    Some(bytes) => {
+                        let res = engine
+                            .decode(&clone)
+                            // remove prefix
+                            .map(|decoded| decoded[decoded_prefix_len..].to_vec());
+
+                        assert_eq!(Ok(bytes.clone()), res);
+                    }
+                    None => assert_eq!(
+                        Err(DecodeError::InvalidLastSymbol(1, s2)),
+                        engine.decode(&symbols[..])
+                    ),
+                }
+            }
+        }
+
+        prefix.extend_from_slice(b"AAAA");
+    }
+}
+
+#[apply(all_engines)]
+fn decode_detect_invalid_last_symbol_every_possible_three_symbols<E: EngineWrapper>(
+    engine_wrapper: E,
+) {
+    let engine = E::standard();
+
+    let mut base64_to_bytes = collections::HashMap::new();
+
+    let mut bytes = [0_u8; 2];
+    for b1 in 0_u8..=255 {
+        bytes[0] = b1;
+        for b2 in 0_u8..=255 {
+            bytes[1] = b2;
+            let mut b64 = vec![0_u8; 4];
+            assert_eq!(3, engine.internal_encode(&bytes, &mut b64[..]));
+            let _ = add_padding(2, &mut b64[3..]);
+
+            let mut v = Vec::with_capacity(2);
+            v.extend_from_slice(&bytes[..]);
+
+            assert!(base64_to_bytes.insert(b64, v).is_none());
+        }
+    }
+
+    // every possible combination of symbols must either decode to 2 bytes or get InvalidLastSymbol, with or without any leading chunks
+
+    let mut prefix = Vec::new();
+    for _ in 0..256 {
+        let mut input = prefix.clone();
+
+        let mut symbols = [0_u8; 4];
+        for &s1 in STANDARD.symbols.iter() {
+            symbols[0] = s1;
+            for &s2 in STANDARD.symbols.iter() {
+                symbols[1] = s2;
+                for &s3 in STANDARD.symbols.iter() {
+                    symbols[2] = s3;
+                    symbols[3] = PAD_BYTE;
+
+                    // chop off previous symbols
+                    input.truncate(prefix.len());
+                    input.extend_from_slice(&symbols[..]);
+                    let decoded_prefix_len = prefix.len() / 4 * 3;
+
+                    match base64_to_bytes.get(&symbols[..]) {
+                        Some(bytes) => {
+                            let res = engine
+                                .decode(&input)
+                                // remove prefix
+                                .map(|decoded| decoded[decoded_prefix_len..].to_vec());
+
+                            assert_eq!(Ok(bytes.clone()), res);
+                        }
+                        None => assert_eq!(
+                            Err(DecodeError::InvalidLastSymbol(2, s3)),
+                            engine.decode(&symbols[..])
+                        ),
+                    }
+                }
+            }
+        }
+        prefix.extend_from_slice(b"AAAA");
+    }
+}
+
+#[apply(all_engines)]
+fn decode_invalid_trailing_bits_ignored_when_configured<E: EngineWrapper>(engine_wrapper: E) {
+    let strict = E::standard();
+    let forgiving = E::standard_allow_trailing_bits();
+
+    fn assert_tolerant_decode<E: Engine>(
+        engine: &E,
+        input: &mut String,
+        b64_prefix_len: usize,
+        expected_decode_bytes: Vec<u8>,
+        data: &str,
+    ) {
+        let prefixed = prefixed_data(input, b64_prefix_len, data);
+        let decoded = engine.decode(prefixed);
+        // prefix is always complete chunks
+        let decoded_prefix_len = b64_prefix_len / 4 * 3;
+        assert_eq!(
+            Ok(expected_decode_bytes),
+            decoded.map(|v| v[decoded_prefix_len..].to_vec())
+        );
+    }
+
+    let mut prefix = String::new();
+    for _ in 0..256 {
+        let mut input = prefix.clone();
+
+        // example from https://github.com/marshallpierce/rust-base64/issues/75
+        assert!(strict
+            .decode(prefixed_data(&mut input, prefix.len(), "/w=="))
+            .is_ok());
+        assert!(strict
+            .decode(prefixed_data(&mut input, prefix.len(), "iYU="))
+            .is_ok());
+        // trailing 01
+        assert_tolerant_decode(&forgiving, &mut input, prefix.len(), vec![255], "/x==");
+        assert_tolerant_decode(&forgiving, &mut input, prefix.len(), vec![137, 133], "iYV=");
+        // trailing 10
+        assert_tolerant_decode(&forgiving, &mut input, prefix.len(), vec![255], "/y==");
+        assert_tolerant_decode(&forgiving, &mut input, prefix.len(), vec![137, 133], "iYW=");
+        // trailing 11
+        assert_tolerant_decode(&forgiving, &mut input, prefix.len(), vec![255], "/z==");
+        assert_tolerant_decode(&forgiving, &mut input, prefix.len(), vec![137, 133], "iYX=");
+
+        prefix.push_str("AAAA");
+    }
+}
+
+#[apply(all_engines)]
+fn decode_invalid_byte_error<E: EngineWrapper>(engine_wrapper: E) {
+    let mut rng = seeded_rng();
+
+    let mut orig_data = Vec::<u8>::new();
+    let mut encode_buf = Vec::<u8>::new();
+    let mut decode_buf = Vec::<u8>::new();
+
+    let len_range = distributions::Uniform::new(1, 1_000);
+
+    for _ in 0..10_000 {
+        let alphabet = random_alphabet(&mut rng);
+        let engine = E::random_alphabet(&mut rng, alphabet);
+
+        orig_data.clear();
+        encode_buf.clear();
+        decode_buf.clear();
+
+        let (orig_len, encoded_len_just_data, encoded_len_with_padding) =
+            generate_random_encoded_data(
+                &engine,
+                &mut orig_data,
+                &mut encode_buf,
+                &mut rng,
+                &len_range,
+            );
+
+        // exactly the right size
+        decode_buf.resize(orig_len, 0);
+
+        // replace one encoded byte with an invalid byte
+        let invalid_byte: u8 = loop {
+            let byte: u8 = rng.gen();
+
+            if alphabet.symbols.contains(&byte) {
+                continue;
+            } else {
+                break byte;
+            }
+        };
+
+        let invalid_range = distributions::Uniform::new(0, orig_len);
+        let invalid_index = invalid_range.sample(&mut rng);
+        encode_buf[invalid_index] = invalid_byte;
+
+        assert_eq!(
+            Err(DecodeError::InvalidByte(invalid_index, invalid_byte)),
+            engine.decode_slice_unchecked(
+                &encode_buf[0..encoded_len_with_padding],
+                &mut decode_buf[..],
+            )
+        );
+    }
+}
+
+/// Any amount of padding anywhere before the final non padding character = invalid byte at first
+/// pad byte.
+/// From this, we know padding must extend to the end of the input.
+#[apply(all_engines)]
+fn decode_padding_before_final_non_padding_char_error_invalid_byte<E: EngineWrapper>(
+    engine_wrapper: E,
+) {
+    let mut rng = seeded_rng();
+
+    // the different amounts of proper padding, w/ offset from end for the last non-padding char
+    let suffixes = vec![("/w==", 2), ("iYu=", 1), ("zzzz", 0)];
+
+    let prefix_quads_range = distributions::Uniform::from(0..=256);
+
+    for mode in all_pad_modes() {
+        // we don't encode so we don't care about encode padding
+        let engine = E::standard_with_pad_mode(true, mode);
+
+        for _ in 0..100_000 {
+            for (suffix, offset) in suffixes.iter() {
+                let mut s = "ABCD".repeat(prefix_quads_range.sample(&mut rng));
+                s.push_str(suffix);
+                let mut encoded = s.into_bytes();
+
+                // calculate a range to write padding into that leaves at least one non padding char
+                let last_non_padding_offset = encoded.len() - 1 - offset;
+
+                // don't include last non padding char as it must stay not padding
+                let padding_end = rng.gen_range(0..last_non_padding_offset);
+
+                // don't use more than 100 bytes of padding, but also use shorter lengths when
+                // padding_end is near the start of the encoded data to avoid biasing to padding
+                // the entire prefix on short lengths
+                let padding_len = rng.gen_range(1..=usize::min(100, padding_end + 1));
+                let padding_start = padding_end.saturating_sub(padding_len);
+
+                encoded[padding_start..=padding_end].fill(PAD_BYTE);
+
+                assert_eq!(
+                    Err(DecodeError::InvalidByte(padding_start, PAD_BYTE)),
+                    engine.decode(&encoded),
+                );
+            }
+        }
+    }
+}
+
+/// Any amount of padding before final chunk that crosses over into final chunk with 1-4 bytes =
+/// invalid byte at first pad byte (except for 1 byte suffix = invalid length).
+/// From this we know the padding must start in the final chunk.
+#[apply(all_engines)]
+fn decode_padding_starts_before_final_chunk_error_invalid_byte<E: EngineWrapper>(
+    engine_wrapper: E,
+) {
+    let mut rng = seeded_rng();
+
+    // must have at least one prefix quad
+    let prefix_quads_range = distributions::Uniform::from(1..256);
+    // including 1 just to make sure that it really does produce invalid length
+    let suffix_pad_len_range = distributions::Uniform::from(1..=4);
+    for mode in all_pad_modes() {
+        // we don't encode so we don't care about encode padding
+        let engine = E::standard_with_pad_mode(true, mode);
+        for _ in 0..100_000 {
+            let suffix_len = suffix_pad_len_range.sample(&mut rng);
+            let mut encoded = "ABCD"
+                .repeat(prefix_quads_range.sample(&mut rng))
+                .into_bytes();
+            encoded.resize(encoded.len() + suffix_len, PAD_BYTE);
+
+            // amount of padding must be long enough to extend back from suffix into previous
+            // quads
+            let padding_len = rng.gen_range(suffix_len + 1..encoded.len());
+            // no non-padding after padding in this test, so padding goes to the end
+            let padding_start = encoded.len() - padding_len;
+            encoded[padding_start..].fill(PAD_BYTE);
+
+            if suffix_len == 1 {
+                assert_eq!(Err(DecodeError::InvalidLength), engine.decode(&encoded),);
+            } else {
+                assert_eq!(
+                    Err(DecodeError::InvalidByte(padding_start, PAD_BYTE)),
+                    engine.decode(&encoded),
+                );
+            }
+        }
+    }
+}
+
+/// 0-1 bytes of data before any amount of padding in final chunk = invalid byte, since padding
+/// is not valid data (consistent with error for pad bytes in earlier chunks).
+/// From this we know there must be 2-3 bytes of data before padding
+#[apply(all_engines)]
+fn decode_too_little_data_before_padding_error_invalid_byte<E: EngineWrapper>(engine_wrapper: E) {
+    let mut rng = seeded_rng();
+
+    // want to test no prefix quad case, so start at 0
+    let prefix_quads_range = distributions::Uniform::from(0_usize..256);
+    let suffix_data_len_range = distributions::Uniform::from(0_usize..=1);
+    for mode in all_pad_modes() {
+        // we don't encode so we don't care about encode padding
+        let engine = E::standard_with_pad_mode(true, mode);
+        for _ in 0..100_000 {
+            let suffix_data_len = suffix_data_len_range.sample(&mut rng);
+            let prefix_quad_len = prefix_quads_range.sample(&mut rng);
+
+            // ensure there is a suffix quad
+            let min_padding = usize::from(suffix_data_len == 0);
+
+            // for all possible padding lengths
+            for padding_len in min_padding..=(4 - suffix_data_len) {
+                let mut encoded = "ABCD".repeat(prefix_quad_len).into_bytes();
+                encoded.resize(encoded.len() + suffix_data_len, b'A');
+                encoded.resize(encoded.len() + padding_len, PAD_BYTE);
+
+                if suffix_data_len + padding_len == 1 {
+                    assert_eq!(Err(DecodeError::InvalidLength), engine.decode(&encoded),);
+                } else {
+                    assert_eq!(
+                        Err(DecodeError::InvalidByte(
+                            prefix_quad_len * 4 + suffix_data_len,
+                            PAD_BYTE,
+                        )),
+                        engine.decode(&encoded),
+                        "suffix data len {} pad len {}",
+                        suffix_data_len,
+                        padding_len
+                    );
+                }
+            }
+        }
+    }
+}
+
+// https://eprint.iacr.org/2022/361.pdf table 2, test 1
+#[apply(all_engines)]
+fn decode_malleability_test_case_3_byte_suffix_valid<E: EngineWrapper>(engine_wrapper: E) {
+    assert_eq!(
+        b"Hello".as_slice(),
+        &E::standard().decode("SGVsbG8=").unwrap()
+    );
+}
+
+// https://eprint.iacr.org/2022/361.pdf table 2, test 2
+#[apply(all_engines)]
+fn decode_malleability_test_case_3_byte_suffix_invalid_trailing_symbol<E: EngineWrapper>(
+    engine_wrapper: E,
+) {
+    assert_eq!(
+        DecodeError::InvalidLastSymbol(6, 0x39),
+        E::standard().decode("SGVsbG9=").unwrap_err()
+    );
+}
+
+// https://eprint.iacr.org/2022/361.pdf table 2, test 3
+#[apply(all_engines)]
+fn decode_malleability_test_case_3_byte_suffix_no_padding<E: EngineWrapper>(engine_wrapper: E) {
+    assert_eq!(
+        DecodeError::InvalidPadding,
+        E::standard().decode("SGVsbG9").unwrap_err()
+    );
+}
+
+// https://eprint.iacr.org/2022/361.pdf table 2, test 4
+#[apply(all_engines)]
+fn decode_malleability_test_case_2_byte_suffix_valid_two_padding_symbols<E: EngineWrapper>(
+    engine_wrapper: E,
+) {
+    assert_eq!(
+        b"Hell".as_slice(),
+        &E::standard().decode("SGVsbA==").unwrap()
+    );
+}
+
+// https://eprint.iacr.org/2022/361.pdf table 2, test 5
+#[apply(all_engines)]
+fn decode_malleability_test_case_2_byte_suffix_short_padding<E: EngineWrapper>(engine_wrapper: E) {
+    assert_eq!(
+        DecodeError::InvalidPadding,
+        E::standard().decode("SGVsbA=").unwrap_err()
+    );
+}
+
+// https://eprint.iacr.org/2022/361.pdf table 2, test 6
+#[apply(all_engines)]
+fn decode_malleability_test_case_2_byte_suffix_no_padding<E: EngineWrapper>(engine_wrapper: E) {
+    assert_eq!(
+        DecodeError::InvalidPadding,
+        E::standard().decode("SGVsbA").unwrap_err()
+    );
+}
+
+// https://eprint.iacr.org/2022/361.pdf table 2, test 7
+#[apply(all_engines)]
+fn decode_malleability_test_case_2_byte_suffix_too_much_padding<E: EngineWrapper>(
+    engine_wrapper: E,
+) {
+    assert_eq!(
+        DecodeError::InvalidByte(6, PAD_BYTE),
+        E::standard().decode("SGVsbA====").unwrap_err()
+    );
+}
+
+/// Requires canonical padding -> accepts 2 + 2, 3 + 1, 4 + 0 final quad configurations
+#[apply(all_engines)]
+fn decode_pad_mode_requires_canonical_accepts_canonical<E: EngineWrapper>(engine_wrapper: E) {
+    assert_all_suffixes_ok(
+        E::standard_with_pad_mode(true, DecodePaddingMode::RequireCanonical),
+        vec!["/w==", "iYU=", "AAAA"],
+    );
+}
+
+/// Requires canonical padding -> rejects 2 + 0-1, 3 + 0 final chunk configurations
+#[apply(all_engines)]
+fn decode_pad_mode_requires_canonical_rejects_non_canonical<E: EngineWrapper>(engine_wrapper: E) {
+    let engine = E::standard_with_pad_mode(true, DecodePaddingMode::RequireCanonical);
+
+    let suffixes = vec!["/w", "/w=", "iYU"];
+    for num_prefix_quads in 0..256 {
+        for &suffix in suffixes.iter() {
+            let mut encoded = "AAAA".repeat(num_prefix_quads);
+            encoded.push_str(suffix);
+
+            let res = engine.decode(&encoded);
+
+            assert_eq!(Err(DecodeError::InvalidPadding), res);
+        }
+    }
+}
+
+/// Requires no padding -> accepts 2 + 0, 3 + 0, 4 + 0 final chunk configuration
+#[apply(all_engines)]
+fn decode_pad_mode_requires_no_padding_accepts_no_padding<E: EngineWrapper>(engine_wrapper: E) {
+    assert_all_suffixes_ok(
+        E::standard_with_pad_mode(true, DecodePaddingMode::RequireNone),
+        vec!["/w", "iYU", "AAAA"],
+    );
+}
+
+/// Requires no padding -> rejects 2 + 1-2, 3 + 1 final chunk configuration
+#[apply(all_engines)]
+fn decode_pad_mode_requires_no_padding_rejects_any_padding<E: EngineWrapper>(engine_wrapper: E) {
+    let engine = E::standard_with_pad_mode(true, DecodePaddingMode::RequireNone);
+
+    let suffixes = vec!["/w=", "/w==", "iYU="];
+    for num_prefix_quads in 0..256 {
+        for &suffix in suffixes.iter() {
+            let mut encoded = "AAAA".repeat(num_prefix_quads);
+            encoded.push_str(suffix);
+
+            let res = engine.decode(&encoded);
+
+            assert_eq!(Err(DecodeError::InvalidPadding), res);
+        }
+    }
+}
+
+/// Indifferent padding accepts 2 + 0-2, 3 + 0-1, 4 + 0 final chunk configuration
+#[apply(all_engines)]
+fn decode_pad_mode_indifferent_padding_accepts_anything<E: EngineWrapper>(engine_wrapper: E) {
+    assert_all_suffixes_ok(
+        E::standard_with_pad_mode(true, DecodePaddingMode::Indifferent),
+        vec!["/w", "/w=", "/w==", "iYU", "iYU=", "AAAA"],
+    );
+}
+
+//this is a MAY in the rfc: https://tools.ietf.org/html/rfc4648#section-3.3
+#[apply(all_engines)]
+fn decode_pad_byte_in_penultimate_quad_error<E: EngineWrapper>(engine_wrapper: E) {
+    for mode in all_pad_modes() {
+        // we don't encode so we don't care about encode padding
+        let engine = E::standard_with_pad_mode(true, mode);
+
+        for num_prefix_quads in 0..256 {
+            // leave room for at least one pad byte in penultimate quad
+            for num_valid_bytes_penultimate_quad in 0..4 {
+                // can't have 1 or it would be invalid length
+                for num_pad_bytes_in_final_quad in 2..=4 {
+                    let mut s: String = "ABCD".repeat(num_prefix_quads);
+
+                    // varying amounts of padding in the penultimate quad
+                    for _ in 0..num_valid_bytes_penultimate_quad {
+                        s.push('A');
+                    }
+                    // finish penultimate quad with padding
+                    for _ in num_valid_bytes_penultimate_quad..4 {
+                        s.push('=');
+                    }
+                    // and more padding in the final quad
+                    for _ in 0..num_pad_bytes_in_final_quad {
+                        s.push('=');
+                    }
+
+                    // padding should be an invalid byte before the final quad.
+                    // Could argue that the *next* padding byte (in the next quad) is technically the first
+                    // erroneous one, but reporting that accurately is more complex and probably nobody cares
+                    assert_eq!(
+                        DecodeError::InvalidByte(
+                            num_prefix_quads * 4 + num_valid_bytes_penultimate_quad,
+                            b'=',
+                        ),
+                        engine.decode(&s).unwrap_err()
+                    );
+                }
+            }
+        }
+    }
+}
+
+#[apply(all_engines)]
+fn decode_bytes_after_padding_in_final_quad_error<E: EngineWrapper>(engine_wrapper: E) {
+    for mode in all_pad_modes() {
+        // we don't encode so we don't care about encode padding
+        let engine = E::standard_with_pad_mode(true, mode);
+
+        for num_prefix_quads in 0..256 {
+            // leave at least one byte in the quad for padding
+            for bytes_after_padding in 1..4 {
+                let mut s: String = "ABCD".repeat(num_prefix_quads);
+
+                // every invalid padding position with a 3-byte final quad: 1 to 3 bytes after padding
+                for _ in 0..(3 - bytes_after_padding) {
+                    s.push('A');
+                }
+                s.push('=');
+                for _ in 0..bytes_after_padding {
+                    s.push('A');
+                }
+
+                // First (and only) padding byte is invalid.
+                assert_eq!(
+                    DecodeError::InvalidByte(
+                        num_prefix_quads * 4 + (3 - bytes_after_padding),
+                        b'='
+                    ),
+                    engine.decode(&s).unwrap_err()
+                );
+            }
+        }
+    }
+}
+
+#[apply(all_engines)]
+fn decode_absurd_pad_error<E: EngineWrapper>(engine_wrapper: E) {
+    for mode in all_pad_modes() {
+        // we don't encode so we don't care about encode padding
+        let engine = E::standard_with_pad_mode(true, mode);
+
+        for num_prefix_quads in 0..256 {
+            let mut s: String = "ABCD".repeat(num_prefix_quads);
+            s.push_str("==Y=Wx===pY=2U=====");
+
+            // first padding byte
+            assert_eq!(
+                DecodeError::InvalidByte(num_prefix_quads * 4, b'='),
+                engine.decode(&s).unwrap_err()
+            );
+        }
+    }
+}
+
+#[apply(all_engines)]
+fn decode_too_much_padding_returns_error<E: EngineWrapper>(engine_wrapper: E) {
+    for mode in all_pad_modes() {
+        // we don't encode so we don't care about encode padding
+        let engine = E::standard_with_pad_mode(true, mode);
+
+        for num_prefix_quads in 0..256 {
+            // add enough padding to ensure that we'll hit all decode stages at the different lengths
+            for pad_bytes in 1..=64 {
+                let mut s: String = "ABCD".repeat(num_prefix_quads);
+                let padding: String = "=".repeat(pad_bytes);
+                s.push_str(&padding);
+
+                if pad_bytes % 4 == 1 {
+                    assert_eq!(DecodeError::InvalidLength, engine.decode(&s).unwrap_err());
+                } else {
+                    assert_eq!(
+                        DecodeError::InvalidByte(num_prefix_quads * 4, b'='),
+                        engine.decode(&s).unwrap_err()
+                    );
+                }
+            }
+        }
+    }
+}
+
+#[apply(all_engines)]
+fn decode_padding_followed_by_non_padding_returns_error<E: EngineWrapper>(engine_wrapper: E) {
+    for mode in all_pad_modes() {
+        // we don't encode so we don't care about encode padding
+        let engine = E::standard_with_pad_mode(true, mode);
+
+        for num_prefix_quads in 0..256 {
+            for pad_bytes in 0..=32 {
+                let mut s: String = "ABCD".repeat(num_prefix_quads);
+                let padding: String = "=".repeat(pad_bytes);
+                s.push_str(&padding);
+                s.push('E');
+
+                if pad_bytes % 4 == 0 {
+                    assert_eq!(DecodeError::InvalidLength, engine.decode(&s).unwrap_err());
+                } else {
+                    assert_eq!(
+                        DecodeError::InvalidByte(num_prefix_quads * 4, b'='),
+                        engine.decode(&s).unwrap_err()
+                    );
+                }
+            }
+        }
+    }
+}
+
+#[apply(all_engines)]
+fn decode_one_char_in_final_quad_with_padding_error<E: EngineWrapper>(engine_wrapper: E) {
+    for mode in all_pad_modes() {
+        // we don't encode so we don't care about encode padding
+        let engine = E::standard_with_pad_mode(true, mode);
+
+        for num_prefix_quads in 0..256 {
+            let mut s: String = "ABCD".repeat(num_prefix_quads);
+            s.push_str("E=");
+
+            assert_eq!(
+                DecodeError::InvalidByte(num_prefix_quads * 4 + 1, b'='),
+                engine.decode(&s).unwrap_err()
+            );
+
+            // more padding doesn't change the error
+            s.push('=');
+            assert_eq!(
+                DecodeError::InvalidByte(num_prefix_quads * 4 + 1, b'='),
+                engine.decode(&s).unwrap_err()
+            );
+
+            s.push('=');
+            assert_eq!(
+                DecodeError::InvalidByte(num_prefix_quads * 4 + 1, b'='),
+                engine.decode(&s).unwrap_err()
+            );
+        }
+    }
+}
+
+#[apply(all_engines)]
+fn decode_too_few_symbols_in_final_quad_error<E: EngineWrapper>(engine_wrapper: E) {
+    for mode in all_pad_modes() {
+        // we don't encode so we don't care about encode padding
+        let engine = E::standard_with_pad_mode(true, mode);
+
+        for num_prefix_quads in 0..256 {
+            // <2 is invalid
+            for final_quad_symbols in 0..2 {
+                for padding_symbols in 0..=(4 - final_quad_symbols) {
+                    let mut s: String = "ABCD".repeat(num_prefix_quads);
+
+                    for _ in 0..final_quad_symbols {
+                        s.push('A');
+                    }
+                    for _ in 0..padding_symbols {
+                        s.push('=');
+                    }
+
+                    match final_quad_symbols + padding_symbols {
+                        0 => continue,
+                        1 => {
+                            assert_eq!(DecodeError::InvalidLength, engine.decode(&s).unwrap_err());
+                        }
+                        _ => {
+                            // error reported at first padding byte
+                            assert_eq!(
+                                DecodeError::InvalidByte(
+                                    num_prefix_quads * 4 + final_quad_symbols,
+                                    b'=',
+                                ),
+                                engine.decode(&s).unwrap_err()
+                            );
+                        }
+                    }
+                }
+            }
+        }
+    }
+}
+
+#[apply(all_engines)]
+fn decode_invalid_trailing_bytes<E: EngineWrapper>(engine_wrapper: E) {
+    for mode in all_pad_modes() {
+        // we don't encode so we don't care about encode padding
+        let engine = E::standard_with_pad_mode(true, mode);
+
+        for num_prefix_quads in 0..256 {
+            let mut s: String = "ABCD".repeat(num_prefix_quads);
+            s.push_str("Cg==\n");
+
+            // The case of trailing newlines is common enough to warrant a test for a good error
+            // message.
+            assert_eq!(
+                Err(DecodeError::InvalidByte(num_prefix_quads * 4 + 4, b'\n')),
+                engine.decode(&s)
+            );
+
+            // extra padding, however, is still InvalidLength
+            let s = s.replace('\n', "=");
+            assert_eq!(Err(DecodeError::InvalidLength), engine.decode(s));
+        }
+    }
+}
+
+#[apply(all_engines)]
+fn decode_wrong_length_error<E: EngineWrapper>(engine_wrapper: E) {
+    let engine = E::standard_with_pad_mode(true, DecodePaddingMode::Indifferent);
+
+    for num_prefix_quads in 0..256 {
+        // at least one token, otherwise it wouldn't be a final quad
+        for num_tokens_final_quad in 1..=4 {
+            for num_padding in 0..=(4 - num_tokens_final_quad) {
+                let mut s: String = "IIII".repeat(num_prefix_quads);
+                for _ in 0..num_tokens_final_quad {
+                    s.push('g');
+                }
+                for _ in 0..num_padding {
+                    s.push('=');
+                }
+
+                let res = engine.decode(&s);
+                if num_tokens_final_quad >= 2 {
+                    assert!(res.is_ok());
+                } else if num_tokens_final_quad == 1 && num_padding > 0 {
+                    // = is invalid if it's too early
+                    assert_eq!(
+                        Err(DecodeError::InvalidByte(
+                            num_prefix_quads * 4 + num_tokens_final_quad,
+                            61
+                        )),
+                        res
+                    );
+                } else if num_padding > 2 {
+                    assert_eq!(Err(DecodeError::InvalidPadding), res);
+                } else {
+                    assert_eq!(Err(DecodeError::InvalidLength), res);
+                }
+            }
+        }
+    }
+}
+
+#[apply(all_engines)]
+fn decode_into_slice_fits_in_precisely_sized_slice<E: EngineWrapper>(engine_wrapper: E) {
+    let mut orig_data = Vec::new();
+    let mut encoded_data = String::new();
+    let mut decode_buf = Vec::new();
+
+    let input_len_range = distributions::Uniform::new(0, 1000);
+    let mut rng = rngs::SmallRng::from_entropy();
+
+    for _ in 0..10_000 {
+        orig_data.clear();
+        encoded_data.clear();
+        decode_buf.clear();
+
+        let input_len = input_len_range.sample(&mut rng);
+
+        for _ in 0..input_len {
+            orig_data.push(rng.gen());
+        }
+
+        let engine = E::random(&mut rng);
+        engine.encode_string(&orig_data, &mut encoded_data);
+        assert_encode_sanity(&encoded_data, engine.config().encode_padding(), input_len);
+
+        decode_buf.resize(input_len, 0);
+
+        // decode into the non-empty buf
+        let decode_bytes_written = engine
+            .decode_slice_unchecked(encoded_data.as_bytes(), &mut decode_buf[..])
+            .unwrap();
+
+        assert_eq!(orig_data.len(), decode_bytes_written);
+        assert_eq!(orig_data, decode_buf);
+    }
+}
+
+#[apply(all_engines)]
+fn decode_length_estimate_delta<E: EngineWrapper>(engine_wrapper: E) {
+    for engine in [E::standard(), E::standard_unpadded()] {
+        for &padding in &[true, false] {
+            for orig_len in 0..1000 {
+                let encoded_len = encoded_len(orig_len, padding).unwrap();
+
+                let decoded_estimate = engine
+                    .internal_decoded_len_estimate(encoded_len)
+                    .decoded_len_estimate();
+                assert!(decoded_estimate >= orig_len);
+                assert!(
+                    decoded_estimate - orig_len < 3,
+                    "estimate: {}, encoded: {}, orig: {}",
+                    decoded_estimate,
+                    encoded_len,
+                    orig_len
+                );
+            }
+        }
+    }
+}
+
+/// Returns a tuple of the original data length, the encoded data length (just data), and the length including padding.
+///
+/// Vecs provided should be empty.
+fn generate_random_encoded_data<E: Engine, R: rand::Rng, D: distributions::Distribution<usize>>(
+    engine: &E,
+    orig_data: &mut Vec<u8>,
+    encode_buf: &mut Vec<u8>,
+    rng: &mut R,
+    length_distribution: &D,
+) -> (usize, usize, usize) {
+    let padding: bool = engine.config().encode_padding();
+
+    let orig_len = fill_rand(orig_data, rng, length_distribution);
+    let expected_encoded_len = encoded_len(orig_len, padding).unwrap();
+    encode_buf.resize(expected_encoded_len, 0);
+
+    let base_encoded_len = engine.internal_encode(&orig_data[..], &mut encode_buf[..]);
+
+    let enc_len_with_padding = if padding {
+        base_encoded_len + add_padding(orig_len, &mut encode_buf[base_encoded_len..])
+    } else {
+        base_encoded_len
+    };
+
+    assert_eq!(expected_encoded_len, enc_len_with_padding);
+
+    (orig_len, base_encoded_len, enc_len_with_padding)
+}
+
+// fill to a random length
+fn fill_rand<R: rand::Rng, D: distributions::Distribution<usize>>(
+    vec: &mut Vec<u8>,
+    rng: &mut R,
+    length_distribution: &D,
+) -> usize {
+    let len = length_distribution.sample(rng);
+    for _ in 0..len {
+        vec.push(rng.gen());
+    }
+
+    len
+}
+
+fn fill_rand_len<R: rand::Rng>(vec: &mut Vec<u8>, rng: &mut R, len: usize) {
+    for _ in 0..len {
+        vec.push(rng.gen());
+    }
+}
+
+fn prefixed_data<'i, 'd>(
+    input_with_prefix: &'i mut String,
+    prefix_len: usize,
+    data: &'d str,
+) -> &'i str {
+    input_with_prefix.truncate(prefix_len);
+    input_with_prefix.push_str(data);
+    input_with_prefix.as_str()
+}
+
+/// A wrapper to make using engines in rstest fixtures easier.
+/// The functions don't need to be instance methods, but rstest does seem
+/// to want an instance, so instances are passed to test functions and then ignored.
+trait EngineWrapper {
+    type Engine: Engine;
+
+    /// Return an engine configured for RFC standard base64
+    fn standard() -> Self::Engine;
+
+    /// Return an engine configured for RFC standard base64, except with no padding appended on
+    /// encode, and required no padding on decode.
+    fn standard_unpadded() -> Self::Engine;
+
+    /// Return an engine configured for RFC standard alphabet with the provided encode and decode
+    /// pad settings
+    fn standard_with_pad_mode(encode_pad: bool, decode_pad_mode: DecodePaddingMode)
+        -> Self::Engine;
+
+    /// Return an engine configured for RFC standard base64 that allows invalid trailing bits
+    fn standard_allow_trailing_bits() -> Self::Engine;
+
+    /// Return an engine configured with a randomized alphabet and config
+    fn random<R: rand::Rng>(rng: &mut R) -> Self::Engine;
+
+    /// Return an engine configured with the specified alphabet and randomized config
+    fn random_alphabet<R: rand::Rng>(rng: &mut R, alphabet: &Alphabet) -> Self::Engine;
+}
+
+struct GeneralPurposeWrapper {}
+
+impl EngineWrapper for GeneralPurposeWrapper {
+    type Engine = general_purpose::GeneralPurpose;
+
+    fn standard() -> Self::Engine {
+        general_purpose::GeneralPurpose::new(&STANDARD, general_purpose::PAD)
+    }
+
+    fn standard_unpadded() -> Self::Engine {
+        general_purpose::GeneralPurpose::new(&STANDARD, general_purpose::NO_PAD)
+    }
+
+    fn standard_with_pad_mode(
+        encode_pad: bool,
+        decode_pad_mode: DecodePaddingMode,
+    ) -> Self::Engine {
+        general_purpose::GeneralPurpose::new(
+            &STANDARD,
+            general_purpose::GeneralPurposeConfig::new()
+                .with_encode_padding(encode_pad)
+                .with_decode_padding_mode(decode_pad_mode),
+        )
+    }
+
+    fn standard_allow_trailing_bits() -> Self::Engine {
+        general_purpose::GeneralPurpose::new(
+            &STANDARD,
+            general_purpose::GeneralPurposeConfig::new().with_decode_allow_trailing_bits(true),
+        )
+    }
+
+    fn random<R: rand::Rng>(rng: &mut R) -> Self::Engine {
+        let alphabet = random_alphabet(rng);
+
+        Self::random_alphabet(rng, alphabet)
+    }
+
+    fn random_alphabet<R: rand::Rng>(rng: &mut R, alphabet: &Alphabet) -> Self::Engine {
+        general_purpose::GeneralPurpose::new(alphabet, random_config(rng))
+    }
+}
+
+struct NaiveWrapper {}
+
+impl EngineWrapper for NaiveWrapper {
+    type Engine = naive::Naive;
+
+    fn standard() -> Self::Engine {
+        naive::Naive::new(
+            &STANDARD,
+            naive::NaiveConfig {
+                encode_padding: true,
+                decode_allow_trailing_bits: false,
+                decode_padding_mode: DecodePaddingMode::RequireCanonical,
+            },
+        )
+    }
+
+    fn standard_unpadded() -> Self::Engine {
+        naive::Naive::new(
+            &STANDARD,
+            naive::NaiveConfig {
+                encode_padding: false,
+                decode_allow_trailing_bits: false,
+                decode_padding_mode: DecodePaddingMode::RequireNone,
+            },
+        )
+    }
+
+    fn standard_with_pad_mode(
+        encode_pad: bool,
+        decode_pad_mode: DecodePaddingMode,
+    ) -> Self::Engine {
+        naive::Naive::new(
+            &STANDARD,
+            naive::NaiveConfig {
+                encode_padding: false,
+                decode_allow_trailing_bits: false,
+                decode_padding_mode: decode_pad_mode,
+            },
+        )
+    }
+
+    fn standard_allow_trailing_bits() -> Self::Engine {
+        naive::Naive::new(
+            &STANDARD,
+            naive::NaiveConfig {
+                encode_padding: true,
+                decode_allow_trailing_bits: true,
+                decode_padding_mode: DecodePaddingMode::RequireCanonical,
+            },
+        )
+    }
+
+    fn random<R: rand::Rng>(rng: &mut R) -> Self::Engine {
+        let alphabet = random_alphabet(rng);
+
+        Self::random_alphabet(rng, alphabet)
+    }
+
+    fn random_alphabet<R: rand::Rng>(rng: &mut R, alphabet: &Alphabet) -> Self::Engine {
+        let mode = rng.gen();
+
+        let config = naive::NaiveConfig {
+            encode_padding: match mode {
+                DecodePaddingMode::Indifferent => rng.gen(),
+                DecodePaddingMode::RequireCanonical => true,
+                DecodePaddingMode::RequireNone => false,
+            },
+            decode_allow_trailing_bits: rng.gen(),
+            decode_padding_mode: mode,
+        };
+
+        naive::Naive::new(alphabet, config)
+    }
+}
+
+fn seeded_rng() -> impl rand::Rng {
+    rngs::SmallRng::from_entropy()
+}
+
+fn all_pad_modes() -> Vec<DecodePaddingMode> {
+    vec![
+        DecodePaddingMode::Indifferent,
+        DecodePaddingMode::RequireCanonical,
+        DecodePaddingMode::RequireNone,
+    ]
+}
+
+fn assert_all_suffixes_ok<E: Engine>(engine: E, suffixes: Vec<&str>) {
+    for num_prefix_quads in 0..256 {
+        for &suffix in suffixes.iter() {
+            let mut encoded = "AAAA".repeat(num_prefix_quads);
+            encoded.push_str(suffix);
+
+            let res = &engine.decode(&encoded);
+            assert!(res.is_ok());
+        }
+    }
+}