Merge #13666: Always create signatures with Low R values

e306be7429 Use 72 byte dummy signatures when watching only inputs may be used (Andrew Chow)
48b1473c89 Use 71 byte signature for DUMMY_SIGNATURE_CREATOR (Andrew Chow)
18dfea0dd0 Always create 70 byte signatures with low R values (Andrew Chow)

Pull request description:

  When creating signatures for transactions, always make one which has a 32 byte or smaller R and 32 byte or smaller S value. This results in signatures that are always less than 71 bytes (32 byte R + 32 byte S + 6 bytes DER + 1 byte sighash) with low R values. In most cases, the signature will be 71 bytes.

  Because R is not mutable in the same way that S is, a low R value can only be found by trying different nonces. RFC 6979 for deterministic nonce generation has the option to specify additional entropy, so we simply use that and add a uin32_t counter which we increment in order to try different nonces. Nonces are sill deterministically generated as the nonce used will the be the first one where the counter results in a nonce that results in a low R value. Because different nonces need to be tried, time to produce a signature does increase. On average, it takes twice as long to make a signature as two signatures need to be created, on average, to find one with a low R.

  Having a fixed size signature makes size calculations easier and also saves half a byte of transaction size, on average.

  DUMMY_SIGNATURE_CREATOR has been modified to produce 71 byte dummy signatures instead of 72 byte signatures.

Tree-SHA512: 3cd791505126ce92da7c631856a97ba0b59e87d9c132feff6e0eef1dc47768e81fbb38bfbe970371bedf9714b7f61a13a5fe9f30f962c81734092a4d19a4ef33

src/key.cpp

@@ -189,7 +189,20 @@ CPubKey CKey::GetPubKey() const {
     return result;
 }
 
-bool CKey::Sign(const uint256 &hash, std::vector<unsigned char>& vchSig, uint32_t test_case) const {
+// 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 (!fValid)
         return false;
     vchSig.resize(CPubKey::SIGNATURE_SIZE);
@@ -197,7 +210,14 @@ bool CKey::Sign(const uint256 &hash, std::vector<unsigned char>& vchSig, uint32_
     unsigned char extra_entropy[32] = {0};
     WriteLE32(extra_entropy, test_case);
     secp256k1_ecdsa_signature sig;
-    int ret = secp256k1_ecdsa_sign(secp256k1_context_sign, &sig, hash.begin(), begin(), secp256k1_nonce_function_rfc6979, test_case ? extra_entropy : nullptr);
+    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);