key.cpp

// Copyright (c) 2009-2022 The Bitcoin Core developers
// Copyright (c) 2017 The Zcash developers
// Distributed under the MIT software license, see the accompanying
// file COPYING or http://www.opensource.org/licenses/mit-license.php.

#include <key.h>

#include <crypto/common.h>
#include <crypto/hmac_sha512.h>
#include <hash.h>
#include <random.h>

#include <secp256k1.h>
#include <secp256k1_ellswift.h>
#include <secp256k1_extrakeys.h>
#include <secp256k1_recovery.h>
#include <secp256k1_schnorrsig.h>

static secp256k1_context* secp256k1_context_sign = nullptr;

/** These functions are taken from the libsecp256k1 distribution and are very ugly. */

/**
 * This parses a format loosely based on a DER encoding of the ECPrivateKey type from
 * section C.4 of SEC 1 <https://www.secg.org/sec1-v2.pdf>, with the following caveats:
 *
 * * The octet-length of the SEQUENCE must be encoded as 1 or 2 octets. It is not
 *   required to be encoded as one octet if it is less than 256, as DER would require.
 * * The octet-length of the SEQUENCE must not be greater than the remaining
 *   length of the key encoding, but need not match it (i.e. the encoding may contain
 *   junk after the encoded SEQUENCE).
 * * The privateKey OCTET STRING is zero-filled on the left to 32 octets.
 * * Anything after the encoding of the privateKey OCTET STRING is ignored, whether
 *   or not it is validly encoded DER.
 *
 * out32 must point to an output buffer of length at least 32 bytes.
 */
int ec_seckey_import_der(const secp256k1_context* ctx, unsigned char *out32, const unsigned char *seckey, size_t seckeylen) {
    const unsigned char *end = seckey + seckeylen;
    memset(out32, 0, 32);
    /* sequence header */
    if (end - seckey < 1 || *seckey != 0x30u) {
        return 0;
    }
    seckey++;
    /* sequence length constructor */
    if (end - seckey < 1 || !(*seckey & 0x80u)) {
        return 0;
    }
    ptrdiff_t lenb = *seckey & ~0x80u; seckey++;
    if (lenb < 1 || lenb > 2) {
        return 0;
    }
    if (end - seckey < lenb) {
        return 0;
    }
    /* sequence length */
    ptrdiff_t len = seckey[lenb-1] | (lenb > 1 ? seckey[lenb-2] << 8 : 0u);
    seckey += lenb;
    if (end - seckey < len) {
        return 0;
    }
    /* sequence element 0: version number (=1) */
    if (end - seckey < 3 || seckey[0] != 0x02u || seckey[1] != 0x01u || seckey[2] != 0x01u) {
        return 0;
    }
    seckey += 3;
    /* sequence element 1: octet string, up to 32 bytes */
    if (end - seckey < 2 || seckey[0] != 0x04u) {
        return 0;
    }
    ptrdiff_t oslen = seckey[1];
    seckey += 2;
    if (oslen > 32 || end - seckey < oslen) {
        return 0;
    }
    memcpy(out32 + (32 - oslen), seckey, oslen);
    if (!secp256k1_ec_seckey_verify(ctx, out32)) {
        memset(out32, 0, 32);
        return 0;
    }
    return 1;
}

/**
 * This serializes to a DER encoding of the ECPrivateKey type from section C.4 of SEC 1
 * <https://www.secg.org/sec1-v2.pdf>. The optional parameters and publicKey fields are
 * included.
 *
 * seckey must point to an output buffer of length at least CKey::SIZE bytes.
 * seckeylen must initially be set to the size of the seckey buffer. Upon return it
 * will be set to the number of bytes used in the buffer.
 * key32 must point to a 32-byte raw private key.
 */
int ec_seckey_export_der(const secp256k1_context *ctx, unsigned char *seckey, size_t *seckeylen, const unsigned char *key32, bool compressed) {
    assert(*seckeylen >= CKey::SIZE);
    secp256k1_pubkey pubkey;
    size_t pubkeylen = 0;
    if (!secp256k1_ec_pubkey_create(ctx, &pubkey, key32)) {
        *seckeylen = 0;
        return 0;
    }
    if (compressed) {
        static const unsigned char begin[] = {
            0x30,0x81,0xD3,0x02,0x01,0x01,0x04,0x20
        };
        static const unsigned char middle[] = {
            0xA0,0x81,0x85,0x30,0x81,0x82,0x02,0x01,0x01,0x30,0x2C,0x06,0x07,0x2A,0x86,0x48,
            0xCE,0x3D,0x01,0x01,0x02,0x21,0x00,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,
            0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,
            0xFF,0xFF,0xFE,0xFF,0xFF,0xFC,0x2F,0x30,0x06,0x04,0x01,0x00,0x04,0x01,0x07,0x04,
            0x21,0x02,0x79,0xBE,0x66,0x7E,0xF9,0xDC,0xBB,0xAC,0x55,0xA0,0x62,0x95,0xCE,0x87,
            0x0B,0x07,0x02,0x9B,0xFC,0xDB,0x2D,0xCE,0x28,0xD9,0x59,0xF2,0x81,0x5B,0x16,0xF8,
            0x17,0x98,0x02,0x21,0x00,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,
            0xFF,0xFF,0xFF,0xFF,0xFE,0xBA,0xAE,0xDC,0xE6,0xAF,0x48,0xA0,0x3B,0xBF,0xD2,0x5E,
            0x8C,0xD0,0x36,0x41,0x41,0x02,0x01,0x01,0xA1,0x24,0x03,0x22,0x00
        };
        unsigned char *ptr = seckey;
        memcpy(ptr, begin, sizeof(begin)); ptr += sizeof(begin);
        memcpy(ptr, key32, 32); ptr += 32;
        memcpy(ptr, middle, sizeof(middle)); ptr += sizeof(middle);
        pubkeylen = CPubKey::COMPRESSED_SIZE;
        secp256k1_ec_pubkey_serialize(ctx, ptr, &pubkeylen, &pubkey, SECP256K1_EC_COMPRESSED);
        ptr += pubkeylen;
        *seckeylen = ptr - seckey;
        assert(*seckeylen == CKey::COMPRESSED_SIZE);
    } else {
        static const unsigned char begin[] = {
            0x30,0x82,0x01,0x13,0x02,0x01,0x01,0x04,0x20
        };
        static const unsigned char middle[] = {
            0xA0,0x81,0xA5,0x30,0x81,0xA2,0x02,0x01,0x01,0x30,0x2C,0x06,0x07,0x2A,0x86,0x48,
            0xCE,0x3D,0x01,0x01,0x02,0x21,0x00,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,
            0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,
            0xFF,0xFF,0xFE,0xFF,0xFF,0xFC,0x2F,0x30,0x06,0x04,0x01,0x00,0x04,0x01,0x07,0x04,
            0x41,0x04,0x79,0xBE,0x66,0x7E,0xF9,0xDC,0xBB,0xAC,0x55,0xA0,0x62,0x95,0xCE,0x87,
            0x0B,0x07,0x02,0x9B,0xFC,0xDB,0x2D,0xCE,0x28,0xD9,0x59,0xF2,0x81,0x5B,0x16,0xF8,
            0x17,0x98,0x48,0x3A,0xDA,0x77,0x26,0xA3,0xC4,0x65,0x5D,0xA4,0xFB,0xFC,0x0E,0x11,
            0x08,0xA8,0xFD,0x17,0xB4,0x48,0xA6,0x85,0x54,0x19,0x9C,0x47,0xD0,0x8F,0xFB,0x10,
            0xD4,0xB8,0x02,0x21,0x00,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,
            0xFF,0xFF,0xFF,0xFF,0xFE,0xBA,0xAE,0xDC,0xE6,0xAF,0x48,0xA0,0x3B,0xBF,0xD2,0x5E,
            0x8C,0xD0,0x36,0x41,0x41,0x02,0x01,0x01,0xA1,0x44,0x03,0x42,0x00
        };
        unsigned char *ptr = seckey;
        memcpy(ptr, begin, sizeof(begin)); ptr += sizeof(begin);
        memcpy(ptr, key32, 32); ptr += 32;
        memcpy(ptr, middle, sizeof(middle)); ptr += sizeof(middle);
        pubkeylen = CPubKey::SIZE;
        secp256k1_ec_pubkey_serialize(ctx, ptr, &pubkeylen, &pubkey, SECP256K1_EC_UNCOMPRESSED);
        ptr += pubkeylen;
        *seckeylen = ptr - seckey;
        assert(*seckeylen == CKey::SIZE);
    }
    return 1;
}

bool CKey::Check(const unsigned char *vch) {
    return secp256k1_ec_seckey_verify(secp256k1_context_sign, vch);
}

void CKey::MakeNewKey(bool fCompressedIn) {
    MakeKeyData();
    do {
        GetStrongRandBytes(*keydata);
    } while (!Check(keydata->data()));
    fCompressed = fCompressedIn;
}

bool CKey::Negate()
{
    assert(keydata);
    return secp256k1_ec_seckey_negate(secp256k1_context_sign, keydata->data());
}

CPrivKey CKey::GetPrivKey() const {
    assert(keydata);
    CPrivKey seckey;
    int ret;
    size_t seckeylen;
    seckey.resize(SIZE);
    seckeylen = SIZE;
    ret = ec_seckey_export_der(secp256k1_context_sign, seckey.data(), &seckeylen, begin(), fCompressed);
    assert(ret);
    seckey.resize(seckeylen);
    return seckey;
}

CPubKey CKey::GetPubKey() const {
    assert(keydata);
    secp256k1_pubkey pubkey;
    size_t clen = CPubKey::SIZE;
    CPubKey result;
    int ret = secp256k1_ec_pubkey_create(secp256k1_context_sign, &pubkey, begin());
    assert(ret);
    secp256k1_ec_pubkey_serialize(secp256k1_context_sign, (unsigned char*)result.begin(), &clen, &pubkey, fCompressed ? SECP256K1_EC_COMPRESSED : SECP256K1_EC_UNCOMPRESSED);
    assert(result.size() == clen);
    assert(result.IsValid());
    return result;
}

// Check that the sig has a low R value and will be less than 71 bytes
bool SigHasLowR(const secp256k1_ecdsa_signature* sig)
{
    unsigned char compact_sig[64];
    secp256k1_ecdsa_signature_serialize_compact(secp256k1_context_sign, compact_sig, sig);

    // In DER serialization, all values are interpreted as big-endian, signed integers. The highest bit in the integer indicates
    // its signed-ness; 0 is positive, 1 is negative. When the value is interpreted as a negative integer, it must be converted
    // to a positive value by prepending a 0x00 byte so that the highest bit is 0. We can avoid this prepending by ensuring that
    // our highest bit is always 0, and thus we must check that the first byte is less than 0x80.
    return compact_sig[0] < 0x80;
}

bool CKey::Sign(const uint256 &hash, std::vector<unsigned char>& vchSig, bool grind, uint32_t test_case) const {
    if (!keydata)
        return false;
    vchSig.resize(CPubKey::SIGNATURE_SIZE);
    size_t nSigLen = CPubKey::SIGNATURE_SIZE;
    unsigned char extra_entropy[32] = {0};
    WriteLE32(extra_entropy, test_case);
    secp256k1_ecdsa_signature sig;
    uint32_t counter = 0;
    int ret = secp256k1_ecdsa_sign(secp256k1_context_sign, &sig, hash.begin(), begin(), secp256k1_nonce_function_rfc6979, (!grind && test_case) ? extra_entropy : nullptr);

    // Grind for low R
    while (ret && !SigHasLowR(&sig) && grind) {
        WriteLE32(extra_entropy, ++counter);
        ret = secp256k1_ecdsa_sign(secp256k1_context_sign, &sig, hash.begin(), begin(), secp256k1_nonce_function_rfc6979, extra_entropy);
    }
    assert(ret);
    secp256k1_ecdsa_signature_serialize_der(secp256k1_context_sign, vchSig.data(), &nSigLen, &sig);
    vchSig.resize(nSigLen);
    // Additional verification step to prevent using a potentially corrupted signature
    secp256k1_pubkey pk;
    ret = secp256k1_ec_pubkey_create(secp256k1_context_sign, &pk, begin());
    assert(ret);
    ret = secp256k1_ecdsa_verify(secp256k1_context_static, &sig, hash.begin(), &pk);
    assert(ret);
    return true;
}

bool CKey::VerifyPubKey(const CPubKey& pubkey) const {
    if (pubkey.IsCompressed() != fCompressed) {
        return false;
    }
    unsigned char rnd[8];
    std::string str = "Bitcoin key verification\n";
    GetRandBytes(rnd);
    uint256 hash{Hash(str, rnd)};
    std::vector<unsigned char> vchSig;
    Sign(hash, vchSig);
    return pubkey.Verify(hash, vchSig);
}

bool CKey::SignCompact(const uint256 &hash, std::vector<unsigned char>& vchSig) const {
    if (!keydata)
        return false;
    vchSig.resize(CPubKey::COMPACT_SIGNATURE_SIZE);
    int rec = -1;
    secp256k1_ecdsa_recoverable_signature rsig;
    int ret = secp256k1_ecdsa_sign_recoverable(secp256k1_context_sign, &rsig, hash.begin(), begin(), secp256k1_nonce_function_rfc6979, nullptr);
    assert(ret);
    ret = secp256k1_ecdsa_recoverable_signature_serialize_compact(secp256k1_context_sign, &vchSig[1], &rec, &rsig);
    assert(ret);
    assert(rec != -1);
    vchSig[0] = 27 + rec + (fCompressed ? 4 : 0);
    // Additional verification step to prevent using a potentially corrupted signature
    secp256k1_pubkey epk, rpk;
    ret = secp256k1_ec_pubkey_create(secp256k1_context_sign, &epk, begin());
    assert(ret);
    ret = secp256k1_ecdsa_recover(secp256k1_context_static, &rpk, &rsig, hash.begin());
    assert(ret);
    ret = secp256k1_ec_pubkey_cmp(secp256k1_context_static, &epk, &rpk);
    assert(ret == 0);
    return true;
}

bool CKey::SignSchnorr(const uint256& hash, Span<unsigned char> sig, const uint256* merkle_root, const uint256& aux) const
{
    assert(sig.size() == 64);
    secp256k1_keypair keypair;
    if (!secp256k1_keypair_create(secp256k1_context_sign, &keypair, begin())) return false;
    if (merkle_root) {
        secp256k1_xonly_pubkey pubkey;
        if (!secp256k1_keypair_xonly_pub(secp256k1_context_sign, &pubkey, nullptr, &keypair)) return false;
        unsigned char pubkey_bytes[32];
        if (!secp256k1_xonly_pubkey_serialize(secp256k1_context_sign, pubkey_bytes, &pubkey)) return false;
        uint256 tweak = XOnlyPubKey(pubkey_bytes).ComputeTapTweakHash(merkle_root->IsNull() ? nullptr : merkle_root);
        if (!secp256k1_keypair_xonly_tweak_add(secp256k1_context_static, &keypair, tweak.data())) return false;
    }
    bool ret = secp256k1_schnorrsig_sign32(secp256k1_context_sign, sig.data(), hash.data(), &keypair, aux.data());
    if (ret) {
        // Additional verification step to prevent using a potentially corrupted signature
        secp256k1_xonly_pubkey pubkey_verify;
        ret = secp256k1_keypair_xonly_pub(secp256k1_context_static, &pubkey_verify, nullptr, &keypair);
        ret &= secp256k1_schnorrsig_verify(secp256k1_context_static, sig.data(), hash.begin(), 32, &pubkey_verify);
    }
    if (!ret) memory_cleanse(sig.data(), sig.size());
    memory_cleanse(&keypair, sizeof(keypair));
    return ret;
}

bool CKey::Load(const CPrivKey &seckey, const CPubKey &vchPubKey, bool fSkipCheck=false) {
    MakeKeyData();
    if (!ec_seckey_import_der(secp256k1_context_sign, (unsigned char*)begin(), seckey.data(), seckey.size())) {
        ClearKeyData();
        return false;
    }
    fCompressed = vchPubKey.IsCompressed();

    if (fSkipCheck)
        return true;

    return VerifyPubKey(vchPubKey);
}

bool CKey::Derive(CKey& keyChild, ChainCode &ccChild, unsigned int nChild, const ChainCode& cc) const {
    assert(IsValid());
    assert(IsCompressed());
    std::vector<unsigned char, secure_allocator<unsigned char>> vout(64);
    if ((nChild >> 31) == 0) {
        CPubKey pubkey = GetPubKey();
        assert(pubkey.size() == CPubKey::COMPRESSED_SIZE);
        BIP32Hash(cc, nChild, *pubkey.begin(), pubkey.begin()+1, vout.data());
    } else {
        assert(size() == 32);
        BIP32Hash(cc, nChild, 0, begin(), vout.data());
    }
    memcpy(ccChild.begin(), vout.data()+32, 32);
    keyChild.Set(begin(), begin() + 32, true);
    bool ret = secp256k1_ec_seckey_tweak_add(secp256k1_context_sign, (unsigned char*)keyChild.begin(), vout.data());
    if (!ret) keyChild.ClearKeyData();
    return ret;
}

EllSwiftPubKey CKey::EllSwiftCreate(Span<const std::byte> ent32) const
{
    assert(keydata);
    assert(ent32.size() == 32);
    std::array<std::byte, EllSwiftPubKey::size()> encoded_pubkey;

    auto success = secp256k1_ellswift_create(secp256k1_context_sign,
                                             UCharCast(encoded_pubkey.data()),
                                             keydata->data(),
                                             UCharCast(ent32.data()));

    // Should always succeed for valid keys (asserted above).
    assert(success);
    return {encoded_pubkey};
}

ECDHSecret CKey::ComputeBIP324ECDHSecret(const EllSwiftPubKey& their_ellswift, const EllSwiftPubKey& our_ellswift, bool initiating) const
{
    assert(keydata);

    ECDHSecret output;
    // BIP324 uses the initiator as party A, and the responder as party B. Remap the inputs
    // accordingly:
    bool success = secp256k1_ellswift_xdh(secp256k1_context_sign,
                                          UCharCast(output.data()),
                                          UCharCast(initiating ? our_ellswift.data() : their_ellswift.data()),
                                          UCharCast(initiating ? their_ellswift.data() : our_ellswift.data()),
                                          keydata->data(),
                                          initiating ? 0 : 1,
                                          secp256k1_ellswift_xdh_hash_function_bip324,
                                          nullptr);
    // Should always succeed for valid keys (assert above).
    assert(success);
    return output;
}

bool CExtKey::Derive(CExtKey &out, unsigned int _nChild) const {
    if (nDepth == std::numeric_limits<unsigned char>::max()) return false;
    out.nDepth = nDepth + 1;
    CKeyID id = key.GetPubKey().GetID();
    memcpy(out.vchFingerprint, &id, 4);
    out.nChild = _nChild;
    return key.Derive(out.key, out.chaincode, _nChild, chaincode);
}

void CExtKey::SetSeed(Span<const std::byte> seed)
{
    static const unsigned char hashkey[] = {'B','i','t','c','o','i','n',' ','s','e','e','d'};
    std::vector<unsigned char, secure_allocator<unsigned char>> vout(64);
    CHMAC_SHA512{hashkey, sizeof(hashkey)}.Write(UCharCast(seed.data()), seed.size()).Finalize(vout.data());
    key.Set(vout.data(), vout.data() + 32, true);
    memcpy(chaincode.begin(), vout.data() + 32, 32);
    nDepth = 0;
    nChild = 0;
    memset(vchFingerprint, 0, sizeof(vchFingerprint));
}

CExtPubKey CExtKey::Neuter() const {
    CExtPubKey ret;
    ret.nDepth = nDepth;
    memcpy(ret.vchFingerprint, vchFingerprint, 4);
    ret.nChild = nChild;
    ret.pubkey = key.GetPubKey();
    ret.chaincode = chaincode;
    return ret;
}

void CExtKey::Encode(unsigned char code[BIP32_EXTKEY_SIZE]) const {
    code[0] = nDepth;
    memcpy(code+1, vchFingerprint, 4);
    WriteBE32(code+5, nChild);
    memcpy(code+9, chaincode.begin(), 32);
    code[41] = 0;
    assert(key.size() == 32);
    memcpy(code+42, key.begin(), 32);
}

void CExtKey::Decode(const unsigned char code[BIP32_EXTKEY_SIZE]) {
    nDepth = code[0];
    memcpy(vchFingerprint, code+1, 4);
    nChild = ReadBE32(code+5);
    memcpy(chaincode.begin(), code+9, 32);
    key.Set(code+42, code+BIP32_EXTKEY_SIZE, true);
    if ((nDepth == 0 && (nChild != 0 || ReadLE32(vchFingerprint) != 0)) || code[41] != 0) key = CKey();
}

bool ECC_InitSanityCheck() {
    CKey key;
    key.MakeNewKey(true);
    CPubKey pubkey = key.GetPubKey();
    return key.VerifyPubKey(pubkey);
}

void ECC_Start() {
    assert(secp256k1_context_sign == nullptr);

    secp256k1_context *ctx = secp256k1_context_create(SECP256K1_CONTEXT_NONE);
    assert(ctx != nullptr);

    {
        // Pass in a random blinding seed to the secp256k1 context.
        std::vector<unsigned char, secure_allocator<unsigned char>> vseed(32);
        GetRandBytes(vseed);
        bool ret = secp256k1_context_randomize(ctx, vseed.data());
        assert(ret);
    }

    secp256k1_context_sign = ctx;
}

void ECC_Stop() {
    secp256k1_context *ctx = secp256k1_context_sign;
    secp256k1_context_sign = nullptr;

    if (ctx) {
        secp256k1_context_destroy(ctx);
    }
}