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Advance replay window only after CCM tag verifies #5

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Advance replay window only after CCM tag verifies #5

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21 changes: 19 additions & 2 deletions encryption.c
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Original file line number Diff line number Diff line change
Expand Up @@ -250,13 +250,27 @@ bool verifyNonceReplay(uint8_t* nonce)
}
}

/* Do NOT advance the replay window here: a forged frame with a fresh
* counter passes this check but must not desync the window before its
* CCM tag is verified. advanceNonceReplay() is called only after the
* tag verifies successfully. */
return true;
}

void advanceNonceReplay(uint8_t* nonce)
{
if (!encryptionSession.authenticated) return;

uint64_t nonce_counter = 0;
for (int i = 0; i < 8; i++) {
nonce_counter = (nonce_counter << 8) | nonce[8 + i];
}

if (nonce_counter > encryptionSession.last_seen_counter)
encryptionSession.last_seen_counter = nonce_counter;

encryptionSession.replay_window[encryptionSession.replay_idx] = nonce_counter;
encryptionSession.replay_idx = (encryptionSession.replay_idx + 1) % 64;

return true;
}

bool handleAuthenticate(uint8_t* data, uint16_t len,
Expand Down Expand Up @@ -482,6 +496,9 @@ bool decryptCommand(uint8_t* ciphertext, uint16_t ciphertext_len,
memcpy(plaintext, decrypted_with_length + 1, payload_length);
}
*plaintext_len = payload_length;
/* Tag verified: only now advance the replay window so a forged
* frame (bad tag) cannot desync it. */
advanceNonceReplay(nonce_full);
encryptionSession.integrity_failures = 0;
updateEncryptionSessionActivity();
memset(decrypted_with_length, 0, sizeof(decrypted_with_length));
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2 changes: 2 additions & 0 deletions encryption.h
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Original file line number Diff line number Diff line change
Expand Up @@ -31,6 +31,8 @@ void incrementNonceCounter(void);

bool verifyNonceReplay(uint8_t* nonce);

void advanceNonceReplay(uint8_t* nonce);

bool decryptCommand(uint8_t* ciphertext, uint16_t ciphertext_len,
uint8_t* plaintext, uint16_t* plaintext_len,
uint8_t* nonce_full, uint8_t* auth_tag,
Expand Down