Method GetLength

src/crypto/chacha_poly_aead.cpp:107–126

}

bool ChaCha20Poly1305AEAD::GetLength(uint32_t* len24_out, uint64_t seqnr_aad, int aad_pos, const uint8_t* ciphertext)
{
    // enforce valid aad position to avoid accessing outside of the 64byte keystream cache
    // (there is space for 21 times 3 bytes)
    assert(aad_pos >= 0 && aad_pos < CHACHA20_ROUND_OUTPUT - CHACHA20_POLY1305_AEAD_AAD_LEN);
    if (m_cached_aad_seqnr != seqnr_aad) {
        // we need to calculate the 64 keystream bytes since we reached a new aad sequence number
        m_cached_aad_seqnr = seqnr_aad;
        m_chacha_header.SetIV(seqnr_aad);                                         // use LE for the nonce
        m_chacha_header.Seek(0);                                                  // block counter 0
        m_chacha_header.Keystream(m_aad_keystream_buffer, CHACHA20_ROUND_OUTPUT); // write keystream to the cache
    }

    // decrypt the ciphertext length by XORing the right position of the 64byte keystream cache with the ciphertext
    *len24_out = (ciphertext[0] ^ m_aad_keystream_buffer[aad_pos + 0]) |
                 (ciphertext[1] ^ m_aad_keystream_buffer[aad_pos + 1]) << 8 |
                 (ciphertext[2] ^ m_aad_keystream_buffer[aad_pos + 2]) << 16;

    return true;
}