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fix(crypto): constant-time MAC compare #2530

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fix(crypto): constant-time MAC compare #2530

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6 changes: 5 additions & 1 deletion src/Utils.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -81,7 +81,11 @@ int Utils::MACThenDecrypt(const uint8_t* shared_secret, uint8_t* dest, const uin
sha.update(src + CIPHER_MAC_SIZE, src_len - CIPHER_MAC_SIZE);
sha.finalizeHMAC(shared_secret, PUB_KEY_SIZE, hmac, CIPHER_MAC_SIZE);
}
if (memcmp(hmac, src, CIPHER_MAC_SIZE) == 0) {
// Constant-time MAC compare: memcmp() returns at the first non-matching byte,
// leaking byte-of-first-mismatch via timing.
uint8_t diff = 0;
for (int k = 0; k < CIPHER_MAC_SIZE; k++) diff |= hmac[k] ^ src[k];
if (diff == 0) {
return decrypt(shared_secret, dest, src + CIPHER_MAC_SIZE, src_len - CIPHER_MAC_SIZE);
}
return 0; // invalid HMAC
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