rfc6979: add K-163 test vector; fix nonaligned use#781
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The first HMAC-DRBG output appears correct (albeit truncated to the byte size of the modulus): However, RFC6979 suggests the correct candidate value is this: The problem is that our candidate value is 168-bits, and our modulus is 163-bits. So, we need to add a right shift with the difference: ...so we need our implementation to count the bit length of the modulus first, then perform a right shift if necessary. |
RFC6979 Appendix A.1. provides a "Detailed Example" which exercises several edge cases in the protocol: - `bits2int` for an input which is not byte-aligned - Rejecting inputs which exceed the modulus This commit adds what was missing from the previous implementation which assumed inputs were always aligned to the size of the digest output: a constant-time right shift by the number of bits by which the modulus is smaller than a byte-aligned value.
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This was referenced Jan 17, 2024
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RFC6979 Appendix A.1. provides a "Detailed Example" which exercises several edge cases in the protocol:
bits2intfor an input which is not byte-alignedThis PR adds what was missing from the previous implementation which assumed inputs were always aligned to the size of the digest output: a constant-time right shift by the number of bits by which the modulus is smaller than a byte-aligned value.