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// Copyright (c) 2019 The Bitcoin Core developers
// Distributed under the MIT software license, see the accompanying
// file COPYING or http://www.opensource.org/licenses/mit-license.php.
#include <crypto/chacha_poly_aead.h>
#include <crypto/poly1305.h>
#include <support/cleanse.h>
#include <assert.h>
#include <string.h>
#include <cstdio>
#include <limits>
#ifndef HAVE_TIMINGSAFE_BCMP
int timingsafe_bcmp(const unsigned char* b1, const unsigned char* b2, size_t n)
{
const unsigned char *p1 = b1, *p2 = b2;
int ret = 0;
for (; n > 0; n--)
ret |= *p1++ ^ *p2++;
return (ret != 0);
}
#endif // TIMINGSAFE_BCMP
ChaCha20Poly1305AEAD::ChaCha20Poly1305AEAD(const unsigned char* K_1, size_t K_1_len, const unsigned char* K_2, size_t K_2_len)
{
assert(K_1_len == CHACHA20_POLY1305_AEAD_KEY_LEN);
assert(K_2_len == CHACHA20_POLY1305_AEAD_KEY_LEN);
m_chacha_main.SetKey(K_1, CHACHA20_POLY1305_AEAD_KEY_LEN);
m_chacha_header.SetKey(K_2, CHACHA20_POLY1305_AEAD_KEY_LEN);
// set the cached sequence number to uint64 max which hints for an unset cache.
// we can't hit uint64 max since the rekey rule (which resets the sequence number) is 1GB
m_cached_aad_seqnr = std::numeric_limits<uint64_t>::max();
}
bool ChaCha20Poly1305AEAD::Crypt(uint64_t seqnr_payload, uint64_t seqnr_aad, int aad_pos, unsigned char* dest, size_t dest_len /* length of the output buffer for sanity checks */, const unsigned char* src, size_t src_len, bool is_encrypt)
{
// check buffer boundaries
if (
// if we encrypt, make sure the source contains at least the expected AAD and the destination has at least space for the source + MAC
(is_encrypt && (src_len < CHACHA20_POLY1305_AEAD_AAD_LEN || dest_len < src_len + POLY1305_TAGLEN)) ||
// if we decrypt, make sure the source contains at least the expected AAD+MAC and the destination has at least space for the source - MAC
(!is_encrypt && (src_len < CHACHA20_POLY1305_AEAD_AAD_LEN + POLY1305_TAGLEN || dest_len < src_len - POLY1305_TAGLEN))) {
return false;
}
unsigned char expected_tag[POLY1305_TAGLEN], poly_key[POLY1305_KEYLEN];
memset(poly_key, 0, sizeof(poly_key));
m_chacha_main.SetIV(seqnr_payload);
// block counter 0 for the poly1305 key
// use lower 32bytes for the poly1305 key
// (throws away 32 unused bytes (upper 32) from this ChaCha20 round)
m_chacha_main.Seek(0);
m_chacha_main.Crypt(poly_key, poly_key, sizeof(poly_key));
// if decrypting, verify the tag prior to decryption
if (!is_encrypt) {
const unsigned char* tag = src + src_len - POLY1305_TAGLEN;
poly1305_auth(expected_tag, src, src_len - POLY1305_TAGLEN, poly_key);
// constant time compare the calculated MAC with the provided MAC
if (timingsafe_bcmp(expected_tag, tag, POLY1305_TAGLEN) != 0) {
memory_cleanse(expected_tag, sizeof(expected_tag));
memory_cleanse(poly_key, sizeof(poly_key));
return false;
}
memory_cleanse(expected_tag, sizeof(expected_tag));
// MAC has been successfully verified, make sure we don't covert it in decryption
src_len -= POLY1305_TAGLEN;
}
// calculate and cache the next 64byte keystream block if requested sequence number is not yet the cache
if (m_cached_aad_seqnr != seqnr_aad) {
m_cached_aad_seqnr = seqnr_aad;
m_chacha_header.SetIV(seqnr_aad);
m_chacha_header.Seek(0);
m_chacha_header.Keystream(m_aad_keystream_buffer, CHACHA20_ROUND_OUTPUT);
}
// crypt the AAD (3 bytes message length) with given position in AAD cipher instance keystream
dest[0] = src[0] ^ m_aad_keystream_buffer[aad_pos];
dest[1] = src[1] ^ m_aad_keystream_buffer[aad_pos + 1];
dest[2] = src[2] ^ m_aad_keystream_buffer[aad_pos + 2];
// Set the playload ChaCha instance block counter to 1 and crypt the payload
m_chacha_main.Seek(1);
m_chacha_main.Crypt(src + CHACHA20_POLY1305_AEAD_AAD_LEN, dest + CHACHA20_POLY1305_AEAD_AAD_LEN, src_len - CHACHA20_POLY1305_AEAD_AAD_LEN);
// If encrypting, calculate and append tag
if (is_encrypt) {
// the poly1305 tag expands over the AAD (3 bytes length) & encrypted payload
poly1305_auth(dest + src_len, dest, src_len, poly_key);
}
// cleanse no longer required MAC and polykey
memory_cleanse(poly_key, sizeof(poly_key));
return true;
}
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;
}
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