Hash the pad byte of K without a branch - #1283
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The MSB of the shared secret K decides whether its mpint encoding takes a 0 pad byte. CreateMpint now derives the pad with masks, and HashMpintHeader hashes the length and the optional pad as one update of LENGTH_SZ + kPad bytes: no branch on the pad in wolfSSH, and no zero-length update, which the SE050 and TI hash ports reject. - apply to DoKexDhReply, SendKexDhReply, and the in-tree GenerateKey - add wolfSSH_TestCreateMpint and a unit test of the pad, leading-zero, empty-input and NULL-argument cases Issue: F-8808
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🟡 Changes recommended
The security-sensitive padded hashing path lacks known-answer test coverage.
Review effort: Balanced
Findings: 1
Open (1)
What changed in this PR
Refactors shared-secret mpint hashing to avoid branching on the pad byte.
Changes:
- Adds branchless mpint header hashing.
- Updates key exchange and key derivation paths.
- Adds
CreateMpint()edge-case tests.
| File | Description |
|---|---|
src/internal.c |
Implements branchless pad handling and hashing. |
wolfssh/internal.h |
Exposes the internal test wrapper. |
tests/unit.c |
Tests mpint normalization and validation. |
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| pad &= 1; | ||
| c32toa(sz + pad, hdr); | ||
| hdr[LENGTH_SZ] = 0; | ||
| return HashUpdate(hash, type, hdr, LENGTH_SZ + pad); |
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The MSB of the shared secret K decides whether its mpint encoding takes a 0 pad byte. The pad is now derived with masks and hashed with the length in a single update, with no branch on it in wolfSSH (F-8808).