bitcoin/src/script/sigcache.cpp

// Copyright (c) 2009-2010 Satoshi Nakamoto
// Copyright (c) 2009-2020 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 <script/sigcache.h>

#include <pubkey.h>
#include <random.h>
#include <uint256.h>
#include <util/system.h>

#include <cuckoocache.h>

#include <algorithm>
#include <mutex>
#include <shared_mutex>
#include <vector>

namespace {
/**
 * Valid signature cache, to avoid doing expensive ECDSA signature checking
 * twice for every transaction (once when accepted into memory pool, and
 * again when accepted into the block chain)
 */
class CSignatureCache
{
private:
     //! Entries are SHA256(nonce || 'E' or 'S' || 31 zero bytes || signature hash || public key || signature):
    CSHA256 m_salted_hasher_ecdsa;
    CSHA256 m_salted_hasher_schnorr;
    typedef CuckooCache::cache<uint256, SignatureCacheHasher> map_type;
    map_type setValid;
    std::shared_mutex cs_sigcache;

public:
    CSignatureCache()
    {
        uint256 nonce = GetRandHash();
        // We want the nonce to be 64 bytes long to force the hasher to process
        // this chunk, which makes later hash computations more efficient. We
        // just write our 32-byte entropy, and then pad with 'E' for ECDSA and
        // 'S' for Schnorr (followed by 0 bytes).
        static constexpr unsigned char PADDING_ECDSA[32] = {'E'};
        static constexpr unsigned char PADDING_SCHNORR[32] = {'S'};
        m_salted_hasher_ecdsa.Write(nonce.begin(), 32);
        m_salted_hasher_ecdsa.Write(PADDING_ECDSA, 32);
        m_salted_hasher_schnorr.Write(nonce.begin(), 32);
        m_salted_hasher_schnorr.Write(PADDING_SCHNORR, 32);
    }

    void
    ComputeEntryECDSA(uint256& entry, const uint256 &hash, const std::vector<unsigned char>& vchSig, const CPubKey& pubkey) const
    {
        CSHA256 hasher = m_salted_hasher_ecdsa;
        hasher.Write(hash.begin(), 32).Write(pubkey.data(), pubkey.size()).Write(vchSig.data(), vchSig.size()).Finalize(entry.begin());
    }

    void
    ComputeEntrySchnorr(uint256& entry, const uint256 &hash, Span<const unsigned char> sig, const XOnlyPubKey& pubkey) const
    {
        CSHA256 hasher = m_salted_hasher_schnorr;
        hasher.Write(hash.begin(), 32).Write(pubkey.data(), pubkey.size()).Write(sig.data(), sig.size()).Finalize(entry.begin());
    }

    bool
    Get(const uint256& entry, const bool erase)
    {
        std::shared_lock<std::shared_mutex> lock(cs_sigcache);
        return setValid.contains(entry, erase);
    }

    void Set(const uint256& entry)
    {
        std::unique_lock<std::shared_mutex> lock(cs_sigcache);
        setValid.insert(entry);
    }
    uint32_t setup_bytes(size_t n)
    {
        return setValid.setup_bytes(n);
    }
};

/* In previous versions of this code, signatureCache was a local static variable
 * in CachingTransactionSignatureChecker::VerifySignature.  We initialize
 * signatureCache outside of VerifySignature to avoid the atomic operation per
 * call overhead associated with local static variables even though
 * signatureCache could be made local to VerifySignature.
*/
static CSignatureCache signatureCache;
} // namespace

// To be called once in AppInitMain/BasicTestingSetup to initialize the
// signatureCache.
void InitSignatureCache()
{
    // nMaxCacheSize is unsigned. If -maxsigcachesize is set to zero,
    // setup_bytes creates the minimum possible cache (2 elements).
    size_t nMaxCacheSize = std::min(std::max((int64_t)0, gArgs.GetIntArg("-maxsigcachesize", DEFAULT_MAX_SIG_CACHE_SIZE) / 2), MAX_MAX_SIG_CACHE_SIZE) * ((size_t) 1 << 20);
    size_t nElems = signatureCache.setup_bytes(nMaxCacheSize);
    LogPrintf("Using %zu MiB out of %zu/2 requested for signature cache, able to store %zu elements\n",
            (nElems*sizeof(uint256)) >>20, (nMaxCacheSize*2)>>20, nElems);
}

bool CachingTransactionSignatureChecker::VerifyECDSASignature(const std::vector<unsigned char>& vchSig, const CPubKey& pubkey, const uint256& sighash) const
{
    uint256 entry;
    signatureCache.ComputeEntryECDSA(entry, sighash, vchSig, pubkey);
    if (signatureCache.Get(entry, !store))
        return true;
    if (!TransactionSignatureChecker::VerifyECDSASignature(vchSig, pubkey, sighash))
        return false;
    if (store)
        signatureCache.Set(entry);
    return true;
}

bool CachingTransactionSignatureChecker::VerifySchnorrSignature(Span<const unsigned char> sig, const XOnlyPubKey& pubkey, const uint256& sighash) const
{
    uint256 entry;
    signatureCache.ComputeEntrySchnorr(entry, sighash, sig, pubkey);
    if (signatureCache.Get(entry, !store)) return true;
    if (!TransactionSignatureChecker::VerifySchnorrSignature(sig, pubkey, sighash)) return false;
    if (store) signatureCache.Set(entry);
    return true;
}