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diff --git a/third_party/rust/digest/src/core_api.rs b/third_party/rust/digest/src/core_api.rs
new file mode 100644
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+++ b/third_party/rust/digest/src/core_api.rs
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+//! Low-level traits operating on blocks and wrappers around them.
+//!
+//! Usage of traits in this module in user code is discouraged. Instead use
+//! core algorithm wrapped by the wrapper types, which implement the
+//! higher-level traits.
+use crate::InvalidOutputSize;
+
+pub use crypto_common::{AlgorithmName, Block, BlockSizeUser, OutputSizeUser, Reset};
+
+use block_buffer::{BlockBuffer, BufferKind};
+use crypto_common::{
+    typenum::{IsLess, Le, NonZero, U256},
+    Output,
+};
+
+mod ct_variable;
+mod rt_variable;
+mod wrapper;
+mod xof_reader;
+
+pub use ct_variable::CtVariableCoreWrapper;
+pub use rt_variable::RtVariableCoreWrapper;
+pub use wrapper::{CoreProxy, CoreWrapper};
+pub use xof_reader::XofReaderCoreWrapper;
+
+/// Buffer type used by type which implements [`BufferKindUser`].
+pub type Buffer<S> =
+    BlockBuffer<<S as BlockSizeUser>::BlockSize, <S as BufferKindUser>::BufferKind>;
+
+/// Types which consume data in blocks.
+pub trait UpdateCore: BlockSizeUser {
+    /// Update state using the provided data blocks.
+    fn update_blocks(&mut self, blocks: &[Block<Self>]);
+}
+
+/// Types which use [`BlockBuffer`] functionality.
+pub trait BufferKindUser: BlockSizeUser {
+    /// Block buffer kind over which type operates.
+    type BufferKind: BufferKind;
+}
+
+/// Core trait for hash functions with fixed output size.
+pub trait FixedOutputCore: UpdateCore + BufferKindUser + OutputSizeUser
+where
+    Self::BlockSize: IsLess<U256>,
+    Le<Self::BlockSize, U256>: NonZero,
+{
+    /// Finalize state using remaining data stored in the provided block buffer,
+    /// write result into provided array and leave `self` in a dirty state.
+    fn finalize_fixed_core(&mut self, buffer: &mut Buffer<Self>, out: &mut Output<Self>);
+}
+
+/// Core trait for hash functions with extendable (XOF) output size.
+pub trait ExtendableOutputCore: UpdateCore + BufferKindUser
+where
+    Self::BlockSize: IsLess<U256>,
+    Le<Self::BlockSize, U256>: NonZero,
+{
+    /// XOF reader core state.
+    type ReaderCore: XofReaderCore;
+
+    /// Retrieve XOF reader using remaining data stored in the block buffer
+    /// and leave hasher in a dirty state.
+    fn finalize_xof_core(&mut self, buffer: &mut Buffer<Self>) -> Self::ReaderCore;
+}
+
+/// Core reader trait for extendable-output function (XOF) result.
+pub trait XofReaderCore: BlockSizeUser {
+    /// Read next XOF block.
+    fn read_block(&mut self) -> Block<Self>;
+}
+
+/// Core trait for hash functions with variable output size.
+///
+/// Maximum output size is equal to [`OutputSizeUser::OutputSize`].
+/// Users are expected to truncate result returned by the
+/// [`finalize_variable_core`] to `output_size` passed to the [`new`] method
+/// during construction. Truncation side is defined by the [`TRUNC_SIDE`]
+/// associated constant.
+///
+/// [`finalize_variable_core`]: VariableOutputCore::finalize_variable_core
+/// [`new`]: VariableOutputCore::new
+/// [`TRUNC_SIDE`]: VariableOutputCore::TRUNC_SIDE
+pub trait VariableOutputCore: UpdateCore + OutputSizeUser + BufferKindUser + Sized
+where
+    Self::BlockSize: IsLess<U256>,
+    Le<Self::BlockSize, U256>: NonZero,
+{
+    /// Side which should be used in a truncated result.
+    const TRUNC_SIDE: TruncSide;
+
+    /// Initialize hasher state for given output size.
+    ///
+    /// Returns [`InvalidOutputSize`] if `output_size` is not valid for
+    /// the algorithm, e.g. if it's bigger than the [`OutputSize`]
+    /// associated type.
+    ///
+    /// [`OutputSize`]: OutputSizeUser::OutputSize
+    fn new(output_size: usize) -> Result<Self, InvalidOutputSize>;
+
+    /// Finalize hasher and write full hashing result into the `out` buffer.
+    ///
+    /// The result must be truncated to `output_size` used during hasher
+    /// construction. Truncation side is defined by the [`TRUNC_SIDE`]
+    /// associated constant.
+    ///
+    /// [`TRUNC_SIDE`]: VariableOutputCore::TRUNC_SIDE
+    fn finalize_variable_core(&mut self, buffer: &mut Buffer<Self>, out: &mut Output<Self>);
+}
+
+/// Type which used for defining truncation side in the [`VariableOutputCore`]
+/// trait.
+#[derive(Copy, Clone, Debug)]
+pub enum TruncSide {
+    /// Truncate left side, i.e. `&out[..n]`.
+    Left,
+    /// Truncate right side, i.e. `&out[m..]`.
+    Right,
+}