Add BIP324Cipher, encapsulating key agreement, derivation, and stream/AEAD ciphers

Co-authored-by: dhruv <856960+dhruv@users.noreply.github.com>

Author: sipa

src/Makefile.am

@@ -124,6 +124,7 @@ BITCOIN_CORE_H = \
   banman.h \
   base58.h \
   bech32.h \
+  bip324.h \
   blockencodings.h \
   blockfilter.h \
   chain.h \
@@ -376,6 +377,7 @@ libbitcoin_node_a_SOURCES = \
   addrdb.cpp \
   addrman.cpp \
   banman.cpp \
+  bip324.cpp \
   blockencodings.cpp \
   blockfilter.cpp \
   chain.cpp \

src/Makefile.test.include

@@ -74,6 +74,7 @@ BITCOIN_TESTS =\
   test/base64_tests.cpp \
   test/bech32_tests.cpp \
   test/bip32_tests.cpp \
+  test/bip324_tests.cpp \
   test/blockchain_tests.cpp \
   test/blockencodings_tests.cpp \
   test/blockfilter_index_tests.cpp \
@@ -246,6 +247,7 @@ test_fuzz_fuzz_SOURCES = \
  test/fuzz/banman.cpp \
  test/fuzz/base_encode_decode.cpp \
  test/fuzz/bech32.cpp \
+ test/fuzz/bip324.cpp \
  test/fuzz/bitdeque.cpp \
  test/fuzz/block.cpp \
  test/fuzz/block_header.cpp \

src/bip324.cpp

@@ -0,0 +1,110 @@
+// 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 <bip324.h>
+
+#include <chainparams.h>
+#include <crypto/chacha20.h>
+#include <crypto/chacha20poly1305.h>
+#include <crypto/hkdf_sha256_32.h>
+#include <random.h>
+#include <span.h>
+#include <support/cleanse.h>
+
+#include <algorithm>
+#include <assert.h>
+#include <cstdint>
+#include <cstddef>
+
+BIP324Cipher::BIP324Cipher() noexcept
+{
+    m_key.MakeNewKey(true);
+    uint256 entropy = GetRandHash();
+    m_our_pubkey = m_key.EllSwiftCreate(MakeByteSpan(entropy));
+}
+
+BIP324Cipher::BIP324Cipher(const CKey& key, Span<const std::byte> ent32) noexcept :
+    m_key(key)
+{
+    m_our_pubkey = m_key.EllSwiftCreate(ent32);
+}
+
+BIP324Cipher::BIP324Cipher(const CKey& key, const EllSwiftPubKey& pubkey) noexcept :
+    m_key(key), m_our_pubkey(pubkey) {}
+
+void BIP324Cipher::Initialize(const EllSwiftPubKey& their_pubkey, bool initiator) noexcept
+{
+    // Determine salt (fixed string + network magic bytes)
+    const auto& message_header = Params().MessageStart();
+    std::string salt = std::string{"bitcoin_v2_shared_secret"} + std::string(std::begin(message_header), std::end(message_header));
+
+    // Perform ECDH to derive shared secret.
+    ECDHSecret ecdh_secret = m_key.ComputeBIP324ECDHSecret(their_pubkey, m_our_pubkey, initiator);
+
+    // Derive encryption keys from shared secret, and initialize stream ciphers and AEADs.
+    CHKDF_HMAC_SHA256_L32 hkdf(UCharCast(ecdh_secret.data()), ecdh_secret.size(), salt);
+    std::array<std::byte, 32> hkdf_32_okm;
+    hkdf.Expand32("initiator_L", UCharCast(hkdf_32_okm.data()));
+    (initiator ? m_send_l_cipher : m_recv_l_cipher).emplace(hkdf_32_okm, REKEY_INTERVAL);
+    hkdf.Expand32("initiator_P", UCharCast(hkdf_32_okm.data()));
+    (initiator ? m_send_p_cipher : m_recv_p_cipher).emplace(hkdf_32_okm, REKEY_INTERVAL);
+    hkdf.Expand32("responder_L", UCharCast(hkdf_32_okm.data()));
+    (initiator ? m_recv_l_cipher : m_send_l_cipher).emplace(hkdf_32_okm, REKEY_INTERVAL);
+    hkdf.Expand32("responder_P", UCharCast(hkdf_32_okm.data()));
+    (initiator ? m_recv_p_cipher : m_send_p_cipher).emplace(hkdf_32_okm, REKEY_INTERVAL);
+
+    // Derive garbage terminators from shared secret.
+    hkdf.Expand32("garbage_terminators", UCharCast(hkdf_32_okm.data()));
+    std::copy(std::begin(hkdf_32_okm), std::begin(hkdf_32_okm) + GARBAGE_TERMINATOR_LEN,
+        (initiator ? m_send_garbage_terminator : m_recv_garbage_terminator).begin());
+    std::copy(std::end(hkdf_32_okm) - GARBAGE_TERMINATOR_LEN, std::end(hkdf_32_okm),
+        (initiator ? m_recv_garbage_terminator : m_send_garbage_terminator).begin());
+
+    // Derive session id from shared secret.
+    hkdf.Expand32("session_id", UCharCast(m_session_id.data()));
+
+    // Wipe all variables that contain information which could be used to re-derive encryption keys.
+    memory_cleanse(ecdh_secret.data(), ecdh_secret.size());
+    memory_cleanse(hkdf_32_okm.data(), sizeof(hkdf_32_okm));
+    memory_cleanse(&hkdf, sizeof(hkdf));
+    m_key = CKey();
+}
+
+void BIP324Cipher::Encrypt(Span<const std::byte> contents, Span<const std::byte> aad, bool ignore, Span<std::byte> output) noexcept
+{
+    assert(output.size() == contents.size() + EXPANSION);
+
+    // Encrypt length.
+    std::byte len[LENGTH_LEN];
+    len[0] = std::byte{(uint8_t)(contents.size() & 0xFF)};
+    len[1] = std::byte{(uint8_t)((contents.size() >> 8) & 0xFF)};
+    len[2] = std::byte{(uint8_t)((contents.size() >> 16) & 0xFF)};
+    m_send_l_cipher->Crypt(len, output.first(LENGTH_LEN));
+
+    // Encrypt plaintext.
+    std::byte header[HEADER_LEN] = {ignore ? IGNORE_BIT : std::byte{0}};
+    m_send_p_cipher->Encrypt(header, contents, aad, output.subspan(LENGTH_LEN));
+}
+
+uint32_t BIP324Cipher::DecryptLength(Span<const std::byte> input) noexcept
+{
+    assert(input.size() == LENGTH_LEN);
+
+    std::byte buf[LENGTH_LEN];
+    // Decrypt length
+    m_recv_l_cipher->Crypt(input, buf);
+    // Convert to number.
+    return uint32_t(buf[0]) + (uint32_t(buf[1]) << 8) + (uint32_t(buf[2]) << 16);
+}
+
+bool BIP324Cipher::Decrypt(Span<const std::byte> input, Span<const std::byte> aad, bool& ignore, Span<std::byte> contents) noexcept
+{
+    assert(input.size() + LENGTH_LEN == contents.size() + EXPANSION);
+
+    std::byte header[HEADER_LEN];
+    if (!m_recv_p_cipher->Decrypt(input, aad, header, contents)) return false;
+
+    ignore = (header[0] & IGNORE_BIT) == IGNORE_BIT;
+    return true;
+}

src/bip324.h

@@ -0,0 +1,90 @@
+// 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.
+
+#ifndef BITCOIN_BIP324_H
+#define BITCOIN_BIP324_H
+
+#include <cstddef>
+#include <optional>
+
+#include <crypto/chacha20.h>
+#include <crypto/chacha20poly1305.h>
+#include <key.h>
+#include <pubkey.h>
+#include <span.h>
+
+/** The BIP324 packet cipher, encapsulating its key derivation, stream cipher, and AEAD. */
+class BIP324Cipher
+{
+public:
+    static constexpr unsigned SESSION_ID_LEN{32};
+    static constexpr unsigned GARBAGE_TERMINATOR_LEN{16};
+    static constexpr unsigned REKEY_INTERVAL{224};
+    static constexpr unsigned LENGTH_LEN{3};
+    static constexpr unsigned HEADER_LEN{1};
+    static constexpr unsigned EXPANSION = LENGTH_LEN + HEADER_LEN + FSChaCha20Poly1305::EXPANSION;
+    static constexpr std::byte IGNORE_BIT{0x80};
+
+private:
+    std::optional<FSChaCha20> m_send_l_cipher;
+    std::optional<FSChaCha20> m_recv_l_cipher;
+    std::optional<FSChaCha20Poly1305> m_send_p_cipher;
+    std::optional<FSChaCha20Poly1305> m_recv_p_cipher;
+
+    CKey m_key;
+    EllSwiftPubKey m_our_pubkey;
+
+    std::array<std::byte, SESSION_ID_LEN> m_session_id;
+    std::array<std::byte, GARBAGE_TERMINATOR_LEN> m_send_garbage_terminator;
+    std::array<std::byte, GARBAGE_TERMINATOR_LEN> m_recv_garbage_terminator;
+
+public:
+    /** Initialize a BIP324 cipher with securely generated random keys. */
+    BIP324Cipher() noexcept;
+
+    /** Initialize a BIP324 cipher with specified key and encoding entropy (testing only). */
+    BIP324Cipher(const CKey& key, Span<const std::byte> ent32) noexcept;
+
+    /** Initialize a BIP324 cipher with specified key (testing only). */
+    BIP324Cipher(const CKey& key, const EllSwiftPubKey& pubkey) noexcept;
+
+    /** Retrieve our public key. */
+    const EllSwiftPubKey& GetOurPubKey() const noexcept { return m_our_pubkey; }
+
+    /** Initialize when the other side's public key is received. Can only be called once. */
+    void Initialize(const EllSwiftPubKey& their_pubkey, bool initiator) noexcept;
+
+    /** Determine whether this cipher is fully initialized. */
+    explicit operator bool() const noexcept { return m_send_l_cipher.has_value(); }
+
+    /** Encrypt a packet. Only after Initialize().
+     *
+     * It must hold that output.size() == contents.size() + EXPANSION.
+     */
+    void Encrypt(Span<const std::byte> contents, Span<const std::byte> aad, bool ignore, Span<std::byte> output) noexcept;
+
+    /** Decrypt the length of a packet. Only after Initialize().
+     *
+     * It must hold that input.size() == LENGTH_LEN.
+     */
+    unsigned DecryptLength(Span<const std::byte> input) noexcept;
+
+    /** Decrypt a packet. Only after Initialize().
+     *
+     * It must hold that input.size() + LENGTH_LEN == contents.size() + EXPANSION.
+     * Contents.size() must equal the length returned by DecryptLength.
+     */
+    bool Decrypt(Span<const std::byte> input, Span<const std::byte> aad, bool& ignore, Span<std::byte> contents) noexcept;
+
+    /** Get the Session ID. Only after Initialize(). */
+    Span<const std::byte> GetSessionID() const noexcept { return m_session_id; }
+
+    /** Get the Garbage Terminator to send. Only after Initialize(). */
+    Span<const std::byte> GetSendGarbageTerminator() const noexcept { return m_send_garbage_terminator; }
+
+    /** Get the expected Garbage Terminator to receive. Only after Initialize(). */
+    Span<const std::byte> GetReceiveGarbageTerminator() const noexcept { return m_recv_garbage_terminator; }
+};
+
+#endif // BITCOIN_BIP324_H

src/pubkey.h

@@ -299,6 +299,9 @@ struct EllSwiftPubKey
     std::array<std::byte, SIZE> m_pubkey;
 
 public:
+    /** Default constructor creates all-zero pubkey (which is valid). */
+    EllSwiftPubKey() noexcept = default;
+
     /** Construct a new ellswift public key from a given serialization. */
     EllSwiftPubKey(const std::array<std::byte, SIZE>& ellswift) :
         m_pubkey(ellswift) {}