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/test/key_tests.cpp

@@ -152,4 +152,40 @@ BOOST_AUTO_TEST_CASE(key_test1)
     BOOST_CHECK(detsigc == ParseHex("2052d8a32079c11e79db95af63bb9600c5b04f21a9ca33dc129c2bfa8ac9dc1cd561d8ae5e0f6c1a16bde3719c64c2fd70e404b6428ab9a69566962e8771b5944d"));
 }
 
+BOOST_AUTO_TEST_CASE(key_signature_tests)
+{
+    // When entropy is specified, we should see at least one high R signature within 20 signatures
+    CKey key = DecodeSecret(strSecret1);
+    std::string msg = "A message to be signed";
+    uint256 msg_hash = Hash(msg.begin(), msg.end());
+    std::vector<unsigned char> sig;
+    bool found = false;
+
+    for (int i = 1; i <=20; ++i) {
+        sig.clear();
+        key.Sign(msg_hash, sig, false, i);
+        found = sig[3] == 0x21 && sig[4] == 0x00;
+        if (found) {
+            break;
+        }
+    }
+    BOOST_CHECK(found);
+
+    // When entropy is not specified, we should always see low R signatures that are less than 70 bytes in 256 tries
+    // We should see at least one signature that is less than 70 bytes.
+    found = true;
+    bool found_small = false;
+    for (int i = 0; i < 256; ++i) {
+        sig.clear();
+        std::string msg = "A message to be signed" + std::to_string(i);
+        msg_hash = Hash(msg.begin(), msg.end());
+        key.Sign(msg_hash, sig);
+        found = sig[3] == 0x20;
+        BOOST_CHECK(sig.size() <= 70);
+        found_small |= sig.size() < 70;
+    }
+    BOOST_CHECK(found);
+    BOOST_CHECK(found_small);
+}
+
 BOOST_AUTO_TEST_SUITE_END()