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diff --git a/security/manager/ssl/DER.sys.mjs b/security/manager/ssl/DER.sys.mjs
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+++ b/security/manager/ssl/DER.sys.mjs
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+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+// A minimal ASN.1 DER decoder. Supports input lengths up to 65539 (one byte for
+// the outer tag, one byte for the 0x82 length-length indicator, two bytes
+// indicating a contents length of 65535, and then the 65535 bytes of contents).
+// Intended to be used like so:
+//
+// let bytes = <an array of bytes describing a SEQUENCE OF INTEGER>;
+// let der = new DER.DERDecoder(bytes);
+// let contents = new DER.DERDecoder(der.readTagAndGetContents(DER.SEQUENCE));
+// while (!contents.atEnd()) {
+//   let integerBytes = contents.readTagAndGetContents(DER.INTEGER);
+//   <... do something with integerBytes ...>
+// }
+// der.assertAtEnd();
+//
+// For CHOICE, use readTLVChoice and pass an array of acceptable tags.
+// The convenience function readBIT_STRING is provided to handle the unused bits
+// aspect of BIT STRING. It returns an object that has the properties contents
+// (an array of bytes consisting of the bytes making up the BIT STRING) and
+// unusedBits (indicating the number of unused bits at the end).
+// All other functions generally return an array of bytes or a single byte as
+// appropriate.
+// peekTag can be used to see if the next tag is an expected given tag.
+// readTLV reads and returns an entire (tag, length, value) tuple (again
+// returned as an array of bytes).
+//
+// NB: While DERDecoder must be given an array, it does not validate that each
+// element in the array is an integer in the range [0, 255]. If the input to be
+// decoded could conceivably violate this property, callers should perform this
+// check before using DERDecoder.
+
+const UNIVERSAL = 0 << 6;
+const CONSTRUCTED = 1 << 5;
+const CONTEXT_SPECIFIC = 2 << 6;
+
+const INTEGER = UNIVERSAL | 0x02; // 0x02
+const BIT_STRING = UNIVERSAL | 0x03; // 0x03
+const NULL = UNIVERSAL | 0x05; // 0x05
+const OBJECT_IDENTIFIER = UNIVERSAL | 0x06; // 0x06
+const PrintableString = UNIVERSAL | 0x13; // 0x13
+const TeletexString = UNIVERSAL | 0x14; // 0x14
+const IA5String = UNIVERSAL | 0x16; // 0x16
+const UTCTime = UNIVERSAL | 0x17; // 0x17
+const GeneralizedTime = UNIVERSAL | 0x18; // 0x18
+const UTF8String = UNIVERSAL | 0x0c; // 0x0c
+const SEQUENCE = UNIVERSAL | CONSTRUCTED | 0x10; // 0x30
+const SET = UNIVERSAL | CONSTRUCTED | 0x11; // 0x31
+
+const ERROR_INVALID_INPUT = "invalid input";
+const ERROR_DATA_TRUNCATED = "data truncated";
+const ERROR_EXTRA_DATA = "extra data";
+const ERROR_INVALID_LENGTH = "invalid length";
+const ERROR_UNSUPPORTED_ASN1 = "unsupported asn.1";
+const ERROR_UNSUPPORTED_LENGTH = "unsupported length";
+const ERROR_INVALID_BIT_STRING = "invalid BIT STRING encoding";
+
+/** Class representing a decoded BIT STRING. */
+class BitString {
+  /**
+   * @param {number} unusedBits the number of unused bits
+   * @param {number[]} contents an array of bytes comprising the BIT STRING
+   */
+  constructor(unusedBits, contents) {
+    this._unusedBits = unusedBits;
+    this._contents = contents;
+  }
+
+  /**
+   * Get the number of unused bits in the BIT STRING
+   *
+   * @returns {number} the number of unused bits
+   */
+  get unusedBits() {
+    return this._unusedBits;
+  }
+
+  /**
+   * Get the contents of the BIT STRING
+   *
+   * @returns {number[]} an array of bytes representing the contents
+   */
+  get contents() {
+    return this._contents;
+  }
+}
+
+/** Class representing DER-encoded data. Provides methods for decoding it. */
+class DERDecoder {
+  /**
+   * @param {number[]} bytes an array of bytes representing the encoded data
+   */
+  constructor(bytes) {
+    // Reject non-array inputs.
+    if (!Array.isArray(bytes)) {
+      throw new Error(ERROR_INVALID_INPUT);
+    }
+    if (bytes.length > 65539) {
+      throw new Error(ERROR_UNSUPPORTED_LENGTH);
+    }
+    this._bytes = bytes;
+    this._cursor = 0;
+  }
+
+  /**
+   * Asserts that the decoder is at the end of the given data. Throws an error
+   * if this is not the case.
+   */
+  assertAtEnd() {
+    if (!this.atEnd()) {
+      throw new Error(ERROR_EXTRA_DATA);
+    }
+  }
+
+  /**
+   * Determines whether or not the decoder is at the end of the given data.
+   *
+   * @returns {boolean} true if the decoder is at the end and false otherwise
+   */
+  atEnd() {
+    return this._cursor == this._bytes.length;
+  }
+
+  /**
+   * Reads the next byte of data. Throws if no more data is available.
+   *
+   * @returns {number} the next byte of data
+   */
+  readByte() {
+    if (this._cursor >= this._bytes.length) {
+      throw new Error(ERROR_DATA_TRUNCATED);
+    }
+    let val = this._bytes[this._cursor];
+    this._cursor++;
+    return val;
+  }
+
+  /**
+   * Given the next expected tag, reads and asserts that the next tag is in fact
+   * the given tag.
+   *
+   * @param {number} expectedTag the expected next tag
+   */
+  _readExpectedTag(expectedTag) {
+    let tag = this.readByte();
+    if (tag != expectedTag) {
+      throw new Error(`unexpected tag: found ${tag} instead of ${expectedTag}`);
+    }
+  }
+
+  /**
+   * Decodes and returns the length portion of an ASN.1 TLV tuple. Throws if the
+   * length is incorrectly encoded or if it describes a length greater than
+   * 65535 bytes. Indefinite-length encoding is not supported.
+   *
+   * @returns {number} the length of the value of the TLV tuple
+   */
+  _readLength() {
+    let nextByte = this.readByte();
+    if (nextByte < 0x80) {
+      return nextByte;
+    }
+    if (nextByte == 0x80) {
+      throw new Error(ERROR_UNSUPPORTED_ASN1);
+    }
+    if (nextByte == 0x81) {
+      let length = this.readByte();
+      if (length < 0x80) {
+        throw new Error(ERROR_INVALID_LENGTH);
+      }
+      return length;
+    }
+    if (nextByte == 0x82) {
+      let length1 = this.readByte();
+      let length2 = this.readByte();
+      let length = (length1 << 8) | length2;
+      if (length < 256) {
+        throw new Error(ERROR_INVALID_LENGTH);
+      }
+      return length;
+    }
+    throw new Error(ERROR_UNSUPPORTED_LENGTH);
+  }
+
+  /**
+   * Reads <length> bytes of data if available. Throws if less than <length>
+   * bytes are available.
+   *
+   * @param {number} length the number of bytes to read. Must be non-negative.
+   * @returns {number[]} the next <length> bytes of data
+   */
+  readBytes(length) {
+    if (length < 0) {
+      throw new Error(ERROR_INVALID_LENGTH);
+    }
+    if (this._cursor + length > this._bytes.length) {
+      throw new Error(ERROR_DATA_TRUNCATED);
+    }
+    let bytes = this._bytes.slice(this._cursor, this._cursor + length);
+    this._cursor += length;
+    return bytes;
+  }
+
+  /**
+   * Given an expected next ASN.1 tag, ensures that that tag is next and returns
+   * the contents of that tag. Throws if a different tag is encountered or if
+   * the data is otherwise incorrectly encoded.
+   *
+   * @param {number} tag the next expected ASN.1 tag
+   * @returns {number[]} the contents of the tag
+   */
+  readTagAndGetContents(tag) {
+    this._readExpectedTag(tag);
+    let length = this._readLength();
+    return this.readBytes(length);
+  }
+
+  /**
+   * Returns the next byte without advancing the decoder. Throws if no more data
+   * is available.
+   *
+   * @returns {number} the next available byte
+   */
+  _peekByte() {
+    if (this._cursor >= this._bytes.length) {
+      throw new Error(ERROR_DATA_TRUNCATED);
+    }
+    return this._bytes[this._cursor];
+  }
+
+  /**
+   * Given an expected tag, reads the next entire ASN.1 TLV tuple, asserting
+   * that the tag matches.
+   *
+   * @param {number} tag the expected tag
+   * @returns {number[]} an array of bytes representing the TLV tuple
+   */
+  _readExpectedTLV(tag) {
+    let mark = this._cursor;
+    this._readExpectedTag(tag);
+    let length = this._readLength();
+    // read the bytes so we know they're there (also to advance the cursor)
+    this.readBytes(length);
+    return this._bytes.slice(mark, this._cursor);
+  }
+
+  /**
+   * Reads the next ASN.1 tag, length, and value and returns them as an array of
+   * bytes.
+   *
+   * @returns {number[]} an array of bytes representing the next ASN.1 TLV
+   */
+  readTLV() {
+    let nextTag = this._peekByte();
+    return this._readExpectedTLV(nextTag);
+  }
+
+  /**
+   * Convenience function for decoding a BIT STRING. Reads and returns the
+   * contents of the expected next BIT STRING. Throws if the next TLV isn't a
+   * BIT STRING or if the BIT STRING is incorrectly encoded.
+   *
+   * @returns {BitString} the next BIT STRING
+   */
+  readBIT_STRING() {
+    let contents = this.readTagAndGetContents(BIT_STRING);
+    if (contents.length < 1) {
+      throw new Error(ERROR_INVALID_BIT_STRING);
+    }
+    let unusedBits = contents[0];
+    if (unusedBits > 7) {
+      throw new Error(ERROR_INVALID_BIT_STRING);
+    }
+    // Zero bytes of content but some amount of padding is invalid.
+    if (contents.length == 1 && unusedBits != 0) {
+      throw new Error(ERROR_INVALID_BIT_STRING);
+    }
+    return new BitString(unusedBits, contents.slice(1, contents.length));
+  }
+
+  /**
+   * Looks to see if the next ASN.1 tag is the expected given tag.
+   *
+   * @param {number} tag the expected next ASN.1 tag
+   * @returns {boolean} true if the next tag is the given one and false otherwise
+   */
+  peekTag(tag) {
+    if (this._cursor >= this._bytes.length) {
+      return false;
+    }
+    return this._bytes[this._cursor] == tag;
+  }
+
+  /**
+   * Given a list of possible next ASN.1 tags, returns the next TLV if the next
+   * tag is in the list. Throws if the next tag is not in the list or if the
+   * data is incorrectly encoded.
+   *
+   * @param {number[]} tagList the list of potential next tags
+   * @returns {number[]} the contents of the next TLV if the next tag is in
+   *                    <tagList>
+   */
+  readTLVChoice(tagList) {
+    let tag = this._peekByte();
+    if (!tagList.includes(tag)) {
+      throw new Error(
+        `unexpected tag: found ${tag} instead of one of ${tagList}`
+      );
+    }
+    return this._readExpectedTLV(tag);
+  }
+}
+
+export const DER = {
+  UNIVERSAL,
+  CONSTRUCTED,
+  CONTEXT_SPECIFIC,
+  INTEGER,
+  BIT_STRING,
+  NULL,
+  OBJECT_IDENTIFIER,
+  PrintableString,
+  TeletexString,
+  IA5String,
+  UTCTime,
+  GeneralizedTime,
+  UTF8String,
+  SEQUENCE,
+  SET,
+  DERDecoder,
+};