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Use constant-time comparison for MAC verification #1656

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Use constant-time comparison for MAC verification #1656

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5 changes: 4 additions & 1 deletion src/Utils.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -81,7 +81,10 @@ 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 comparison to prevent timing side-channel attacks
uint8_t diff = 0;
for (int i = 0; i < CIPHER_MAC_SIZE; i++) diff |= hmac[i] ^ src[i];
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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