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 <bip324.h>
6
7#include <chainparams.h>
8#include <crypto/chacha20.h>
9#include <crypto/chacha20poly1305.h>
10#include <crypto/hkdf_sha256_32.h>
11#include <key.h>
12#include <pubkey.h>
13#include <random.h>
14#include <span.h>
15#include <support/cleanse.h>
16#include <uint256.h>
17
18#include <algorithm>
19#include <cassert>
20#include <cstddef>
21#include <cstdint>
22#include <iterator>
23#include <string>
24
25BIP324Cipher::BIP324Cipher(const CKey& key, std::span<const std::byte> ent32) noexcept
26    : m_key(key)
27{
28    m_our_pubkey = m_key.EllSwiftCreate(ent32);
29}
30
31BIP324Cipher::BIP324Cipher(const CKey& key, const EllSwiftPubKey& pubkey) noexcept :
32    m_key(key), m_our_pubkey(pubkey) {}
33
34void BIP324Cipher::Initialize(const EllSwiftPubKey& their_pubkey, bool initiator, bool self_decrypt) noexcept
35{
36    // Determine salt (fixed string + network magic bytes)
37    const auto& message_header = Params().MessageStart();
38    std::string salt = std::string{"bitcoin_v2_shared_secret"} + std::string(std::begin(message_header), std::end(message_header));
39
40    // Perform ECDH to derive shared secret.
41    ECDHSecret ecdh_secret = m_key.ComputeBIP324ECDHSecret(their_pubkey, m_our_pubkey, initiator);
42
43    // Derive encryption keys from shared secret, and initialize stream ciphers and AEADs.
44    bool side = (initiator != self_decrypt);
45    CHKDF_HMAC_SHA256_L32 hkdf(UCharCast(ecdh_secret.data()), ecdh_secret.size(), salt);
46    std::array<std::byte, 32> hkdf_32_okm;
47    hkdf.Expand32("initiator_L", UCharCast(hkdf_32_okm.data()));
48    (side ? m_send_l_cipher : m_recv_l_cipher).emplace(hkdf_32_okm, REKEY_INTERVAL);
49    hkdf.Expand32("initiator_P", UCharCast(hkdf_32_okm.data()));
50    (side ? m_send_p_cipher : m_recv_p_cipher).emplace(hkdf_32_okm, REKEY_INTERVAL);
51    hkdf.Expand32("responder_L", UCharCast(hkdf_32_okm.data()));
52    (side ? m_recv_l_cipher : m_send_l_cipher).emplace(hkdf_32_okm, REKEY_INTERVAL);
53    hkdf.Expand32("responder_P", UCharCast(hkdf_32_okm.data()));
54    (side ? m_recv_p_cipher : m_send_p_cipher).emplace(hkdf_32_okm, REKEY_INTERVAL);
55
56    // Derive garbage terminators from shared secret.
57    hkdf.Expand32("garbage_terminators", UCharCast(hkdf_32_okm.data()));
58    std::copy(std::begin(hkdf_32_okm), std::begin(hkdf_32_okm) + GARBAGE_TERMINATOR_LEN,
59        (initiator ? m_send_garbage_terminator : m_recv_garbage_terminator).begin());
60    std::copy(std::end(hkdf_32_okm) - GARBAGE_TERMINATOR_LEN, std::end(hkdf_32_okm),
61        (initiator ? m_recv_garbage_terminator : m_send_garbage_terminator).begin());
62
63    // Derive session id from shared secret.
64    hkdf.Expand32("session_id", UCharCast(m_session_id.data()));
65
66    // Wipe all variables that contain information which could be used to re-derive encryption keys.
67    memory_cleanse(ecdh_secret.data(), ecdh_secret.size());
68    memory_cleanse(hkdf_32_okm.data(), sizeof(hkdf_32_okm));
69    memory_cleanse(&hkdf, sizeof(hkdf));
70    m_key = CKey();
71}
72
73void BIP324Cipher::Encrypt(std::span<const std::byte> contents, std::span<const std::byte> aad, bool ignore, std::span<std::byte> output) noexcept
74{
75    assert(output.size() == contents.size() + EXPANSION);
76
77    // Encrypt length.
78    std::byte len[LENGTH_LEN];
79    len[0] = std::byte{(uint8_t)(contents.size() & 0xFF)};
80    len[1] = std::byte{(uint8_t)((contents.size() >> 8) & 0xFF)};
81    len[2] = std::byte{(uint8_t)((contents.size() >> 16) & 0xFF)};
82    m_send_l_cipher->Crypt(len, output.first(LENGTH_LEN));
83
84    // Encrypt plaintext.
85    std::byte header[HEADER_LEN] = {ignore ? IGNORE_BIT : std::byte{0}};
86    m_send_p_cipher->Encrypt(header, contents, aad, output.subspan(LENGTH_LEN));
87}
88
89uint32_t BIP324Cipher::DecryptLength(std::span<const std::byte> input) noexcept
90{
91    assert(input.size() == LENGTH_LEN);
92
93    std::byte buf[LENGTH_LEN];
94    // Decrypt length
95    m_recv_l_cipher->Crypt(input, buf);
96    // Convert to number.
97    return uint32_t(buf[0]) + (uint32_t(buf[1]) << 8) + (uint32_t(buf[2]) << 16);
98}
99
100bool BIP324Cipher::Decrypt(std::span<const std::byte> input, std::span<const std::byte> aad, bool& ignore, std::span<std::byte> contents) noexcept
101{
102    assert(input.size() + LENGTH_LEN == contents.size() + EXPANSION);
103
104    std::byte header[HEADER_LEN];
105    if (!m_recv_p_cipher->Decrypt(input, aad, header, contents)) return false;
106
107    ignore = (header[0] & IGNORE_BIT) == IGNORE_BIT;
108    return true;
109}
110