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// Copyright (c) 2023 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/chacha20poly1305.h>
#include <crypto/common.h>
#include <crypto/chacha20.h>
#include <crypto/poly1305.h>
#include <span.h>
#include <support/cleanse.h>
#include <assert.h>
#include <cstddef>
AEADChaCha20Poly1305::AEADChaCha20Poly1305(Span<const std::byte> key) noexcept : m_chacha20(key)
{
assert(key.size() == KEYLEN);
}
void AEADChaCha20Poly1305::SetKey(Span<const std::byte> key) noexcept
{
assert(key.size() == KEYLEN);
m_chacha20.SetKey(key);
}
namespace {
int timingsafe_bcmp_internal(const unsigned char* b1, const unsigned char* b2, size_t n) noexcept
{
const unsigned char *p1 = b1, *p2 = b2;
int ret = 0;
for (; n > 0; n--)
ret |= *p1++ ^ *p2++;
return (ret != 0);
}
/** Compute poly1305 tag. chacha20 must be set to the right nonce, block 0. Will be at block 1 after. */
void ComputeTag(ChaCha20& chacha20, Span<const std::byte> aad, Span<const std::byte> cipher, Span<std::byte> tag) noexcept
{
static const std::byte PADDING[16] = {{}};
// Get block of keystream (use a full 64 byte buffer to avoid the need for chacha20's own buffering).
std::byte first_block[ChaCha20Aligned::BLOCKLEN];
chacha20.Keystream(first_block);
// Use the first 32 bytes of the first keystream block as poly1305 key.
Poly1305 poly1305{Span{first_block}.first(Poly1305::KEYLEN)};
// Compute tag:
// - Process the padded AAD with Poly1305.
const unsigned aad_padding_length = (16 - (aad.size() % 16)) % 16;
poly1305.Update(aad).Update(Span{PADDING}.first(aad_padding_length));
// - Process the padded ciphertext with Poly1305.
const unsigned cipher_padding_length = (16 - (cipher.size() % 16)) % 16;
poly1305.Update(cipher).Update(Span{PADDING}.first(cipher_padding_length));
// - Process the AAD and plaintext length with Poly1305.
std::byte length_desc[Poly1305::TAGLEN];
WriteLE64(UCharCast(length_desc), aad.size());
WriteLE64(UCharCast(length_desc + 8), cipher.size());
poly1305.Update(length_desc);
// Output tag.
poly1305.Finalize(tag);
}
} // namespace
void AEADChaCha20Poly1305::Encrypt(Span<const std::byte> plain1, Span<const std::byte> plain2, Span<const std::byte> aad, Nonce96 nonce, Span<std::byte> cipher) noexcept
{
assert(cipher.size() == plain1.size() + plain2.size() + EXPANSION);
// Encrypt using ChaCha20 (starting at block 1).
m_chacha20.Seek(nonce, 1);
m_chacha20.Crypt(plain1, cipher.first(plain1.size()));
m_chacha20.Crypt(plain2, cipher.subspan(plain1.size()).first(plain2.size()));
// Seek to block 0, and compute tag using key drawn from there.
m_chacha20.Seek(nonce, 0);
ComputeTag(m_chacha20, aad, cipher.first(cipher.size() - EXPANSION), cipher.last(EXPANSION));
}
bool AEADChaCha20Poly1305::Decrypt(Span<const std::byte> cipher, Span<const std::byte> aad, Nonce96 nonce, Span<std::byte> plain1, Span<std::byte> plain2) noexcept
{
assert(cipher.size() == plain1.size() + plain2.size() + EXPANSION);
// Verify tag (using key drawn from block 0).
m_chacha20.Seek(nonce, 0);
std::byte expected_tag[EXPANSION];
ComputeTag(m_chacha20, aad, cipher.first(cipher.size() - EXPANSION), expected_tag);
if (timingsafe_bcmp_internal(UCharCast(expected_tag), UCharCast(cipher.last(EXPANSION).data()), EXPANSION)) return false;
// Decrypt (starting at block 1).
m_chacha20.Crypt(cipher.first(plain1.size()), plain1);
m_chacha20.Crypt(cipher.subspan(plain1.size()).first(plain2.size()), plain2);
return true;
}
void AEADChaCha20Poly1305::Keystream(Nonce96 nonce, Span<std::byte> keystream) noexcept
{
// Skip the first output block, as it's used for generating the poly1305 key.
m_chacha20.Seek(nonce, 1);
m_chacha20.Keystream(keystream);
}
void FSChaCha20Poly1305::NextPacket() noexcept
{
if (++m_packet_counter == m_rekey_interval) {
// Generate a full block of keystream, to avoid needing the ChaCha20 buffer, even though
// we only need KEYLEN (32) bytes.
std::byte one_block[ChaCha20Aligned::BLOCKLEN];
m_aead.Keystream({0xFFFFFFFF, m_rekey_counter}, one_block);
// Switch keys.
m_aead.SetKey(Span{one_block}.first(KEYLEN));
// Wipe the generated keystream (a copy remains inside m_aead, which will be cleaned up
// once it cycles again, or is destroyed).
memory_cleanse(one_block, sizeof(one_block));
// Update counters.
m_packet_counter = 0;
++m_rekey_counter;
}
}
void FSChaCha20Poly1305::Encrypt(Span<const std::byte> plain1, Span<const std::byte> plain2, Span<const std::byte> aad, Span<std::byte> cipher) noexcept
{
m_aead.Encrypt(plain1, plain2, aad, {m_packet_counter, m_rekey_counter}, cipher);
NextPacket();
}
bool FSChaCha20Poly1305::Decrypt(Span<const std::byte> cipher, Span<const std::byte> aad, Span<std::byte> plain1, Span<std::byte> plain2) noexcept
{
bool ret = m_aead.Decrypt(cipher, aad, {m_packet_counter, m_rekey_counter}, plain1, plain2);
NextPacket();
return ret;
}
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