1// Copyright (c) 2023-present The Bitcoin Core developers
2// Distributed under the MIT software license, see the accompanying
3// file COPYING or http://www.opensource.org/licenses/mit-license.php.
4
5#include <crypto/chacha20poly1305.h>
6
7#include <crypto/chacha20.h>
8#include <crypto/common.h>
9#include <crypto/poly1305.h>
10#include <span.h>
11#include <support/cleanse.h>
12
13#include <cassert>
14#include <cstddef>
15
16AEADChaCha20Poly1305::AEADChaCha20Poly1305(std::span<const std::byte> key) noexcept : m_chacha20(key)
17{
18    assert(key.size() == KEYLEN);
19}
20
21void AEADChaCha20Poly1305::SetKey(std::span<const std::byte> key) noexcept
22{
23    assert(key.size() == KEYLEN);
24    m_chacha20.SetKey(key);
25}
26
27namespace {
28
29int timingsafe_bcmp_internal(const unsigned char* b1, const unsigned char* b2, size_t n) noexcept
30{
31    const unsigned char *p1 = b1, *p2 = b2;
32    int ret = 0;
33    for (; n > 0; n--)
34        ret |= *p1++ ^ *p2++;
35    return (ret != 0);
36}
37
38/** Compute poly1305 tag. chacha20 must be set to the right nonce, block 0. Will be at block 1 after. */
39void ComputeTag(ChaCha20& chacha20, std::span<const std::byte> aad, std::span<const std::byte> cipher, std::span<std::byte> tag) noexcept
40{
41    static const std::byte PADDING[16] = {{}};
42
43    // Get block of keystream (use a full 64 byte buffer to avoid the need for chacha20's own buffering).
44    std::byte first_block[ChaCha20Aligned::BLOCKLEN];
45    chacha20.Keystream(first_block);
46
47    // Use the first 32 bytes of the first keystream block as poly1305 key.
48    Poly1305 poly1305{std::span{first_block}.first(Poly1305::KEYLEN)};
49
50    // Compute tag:
51    // - Process the padded AAD with Poly1305.
52    const unsigned aad_padding_length = (16 - (aad.size() % 16)) % 16;
53    poly1305.Update(aad).Update(std::span{PADDING}.first(aad_padding_length));
54    // - Process the padded ciphertext with Poly1305.
55    const unsigned cipher_padding_length = (16 - (cipher.size() % 16)) % 16;
56    poly1305.Update(cipher).Update(std::span{PADDING}.first(cipher_padding_length));
57    // - Process the AAD and plaintext length with Poly1305.
58    std::byte length_desc[Poly1305::TAGLEN];
59    WriteLE64(length_desc, aad.size());
60    WriteLE64(length_desc + 8, cipher.size());
61    poly1305.Update(length_desc);
62
63    // Output tag.
64    poly1305.Finalize(tag);
65}
66
67} // namespace
68
69void AEADChaCha20Poly1305::Encrypt(std::span<const std::byte> plain1, std::span<const std::byte> plain2, std::span<const std::byte> aad, Nonce96 nonce, std::span<std::byte> cipher) noexcept
70{
71    assert(cipher.size() == plain1.size() + plain2.size() + EXPANSION);
72
73    // Encrypt using ChaCha20 (starting at block 1).
74    m_chacha20.Seek(nonce, 1);
75    m_chacha20.Crypt(plain1, cipher.first(plain1.size()));
76    m_chacha20.Crypt(plain2, cipher.subspan(plain1.size()).first(plain2.size()));
77
78    // Seek to block 0, and compute tag using key drawn from there.
79    m_chacha20.Seek(nonce, 0);
80    ComputeTag(m_chacha20, aad, cipher.first(cipher.size() - EXPANSION), cipher.last(EXPANSION));
81}
82
83bool AEADChaCha20Poly1305::Decrypt(std::span<const std::byte> cipher, std::span<const std::byte> aad, Nonce96 nonce, std::span<std::byte> plain1, std::span<std::byte> plain2) noexcept
84{
85    assert(cipher.size() == plain1.size() + plain2.size() + EXPANSION);
86
87    // Verify tag (using key drawn from block 0).
88    m_chacha20.Seek(nonce, 0);
89    std::byte expected_tag[EXPANSION];
90    ComputeTag(m_chacha20, aad, cipher.first(cipher.size() - EXPANSION), expected_tag);
91    if (timingsafe_bcmp_internal(UCharCast(expected_tag), UCharCast(cipher.last(EXPANSION).data()), EXPANSION)) return false;
92
93    // Decrypt (starting at block 1).
94    m_chacha20.Crypt(cipher.first(plain1.size()), plain1);
95    m_chacha20.Crypt(cipher.subspan(plain1.size()).first(plain2.size()), plain2);
96    return true;
97}
98
99void AEADChaCha20Poly1305::Keystream(Nonce96 nonce, std::span<std::byte> keystream) noexcept
100{
101    // Skip the first output block, as it's used for generating the poly1305 key.
102    m_chacha20.Seek(nonce, 1);
103    m_chacha20.Keystream(keystream);
104}
105
106void FSChaCha20Poly1305::NextPacket() noexcept
107{
108    if (++m_packet_counter == m_rekey_interval) {
109        // Generate a full block of keystream, to avoid needing the ChaCha20 buffer, even though
110        // we only need KEYLEN (32) bytes.
111        std::byte one_block[ChaCha20Aligned::BLOCKLEN];
112        m_aead.Keystream({0xFFFFFFFF, m_rekey_counter}, one_block);
113        // Switch keys.
114        m_aead.SetKey(std::span{one_block}.first(KEYLEN));
115        // Wipe the generated keystream (a copy remains inside m_aead, which will be cleaned up
116        // once it cycles again, or is destroyed).
117        memory_cleanse(one_block, sizeof(one_block));
118        // Update counters.
119        m_packet_counter = 0;
120        ++m_rekey_counter;
121    }
122}
123
124void FSChaCha20Poly1305::Encrypt(std::span<const std::byte> plain1, std::span<const std::byte> plain2, std::span<const std::byte> aad, std::span<std::byte> cipher) noexcept
125{
126    m_aead.Encrypt(plain1, plain2, aad, {m_packet_counter, m_rekey_counter}, cipher);
127    NextPacket();
128}
129
130bool FSChaCha20Poly1305::Decrypt(std::span<const std::byte> cipher, std::span<const std::byte> aad, std::span<std::byte> plain1, std::span<std::byte> plain2) noexcept
131{
132    bool ret = m_aead.Decrypt(cipher, aad, {m_packet_counter, m_rekey_counter}, plain1, plain2);
133    NextPacket();
134    return ret;
135}
136