Only create signatures with even S, and verification mode to check.

To fix a minor malleability found by Sergio Lerner (reported here:
https://bitcointalk.org/index.php?topic=8392.msg1245898#msg1245898)

The problem is that if (R,S) is a valid ECDSA signature for a given
message and public key, (R,-S) is also valid. Modulo N (the order
of the secp256k1 curve), this means that both (R,S) and (R,N-S) are
valid. Given that N is odd, S and N-S have a different lowest bit.
We solve the problem by forcing signatures to have an even S value,
excluding one of the alternatives.

This commit just changes the signing code to always produce even S
values, and adds a verification mode to check it. This code is not
enabled anywhere yet. Existing tests in key_tests.cpp verify that
the produced signatures are still valid.

1 files changed, 17 insertions, 9 deletions

diff --git a/src/script.cpp b/src/script.cpp
index 5699fbfb6a..2df2e9f0d5 100644
--- a/src/script.cpp
+++ b/src/script.cpp
@@ -227,7 +227,10 @@ const char* GetOpName(opcodetype opcode)
     }
 }
 
-bool IsCanonicalPubKey(const valtype &vchPubKey) {
+bool IsCanonicalPubKey(const valtype &vchPubKey, unsigned int flags) {
+    if (!(flags & SCRIPT_VERIFY_STRICTENC))
+        return true;
+
     if (vchPubKey.size() < 33)
         return error("Non-canonical public key: too short");
     if (vchPubKey[0] == 0x04) {
@@ -242,7 +245,10 @@ bool IsCanonicalPubKey(const valtype &vchPubKey) {
     return true;
 }
 
-bool IsCanonicalSignature(const valtype &vchSig) {
+bool IsCanonicalSignature(const valtype &vchSig, unsigned int flags) {
+    if (!(flags & SCRIPT_VERIFY_STRICTENC))
+        return true;
+
     // See https://bitcointalk.org/index.php?topic=8392.msg127623#msg127623
     // A canonical signature exists of: <30> <total len> <02> <len R> <R> <02> <len S> <S> <hashtype>
     // Where R and S are not negative (their first byte has its highest bit not set), and not
@@ -286,6 +292,11 @@ bool IsCanonicalSignature(const valtype &vchSig) {
     if (nLenS > 1 && (S[0] == 0x00) && !(S[1] & 0x80))
         return error("Non-canonical signature: S value excessively padded");
 
+    if (flags & SCRIPT_VERIFY_EVEN_S) {
+        if (S[nLenS-1] & 1)
+            return error("Non-canonical signature: S value odd");
+    }
+
     return true;
 }
 
@@ -302,7 +313,6 @@ bool EvalScript(vector<vector<unsigned char> >& stack, const CScript& script, co
     if (script.size() > 10000)
         return false;
     int nOpCount = 0;
-    bool fStrictEncodings = flags & SCRIPT_VERIFY_STRICTENC;
 
     try
     {
@@ -841,9 +851,8 @@ bool EvalScript(vector<vector<unsigned char> >& stack, const CScript& script, co
                     // Drop the signature, since there's no way for a signature to sign itself
                     scriptCode.FindAndDelete(CScript(vchSig));
 
-                    bool fSuccess = (!fStrictEncodings || (IsCanonicalSignature(vchSig) && IsCanonicalPubKey(vchPubKey)));
-                    if (fSuccess)
-                        fSuccess = CheckSig(vchSig, vchPubKey, scriptCode, txTo, nIn, nHashType, flags);
+                    bool fSuccess = IsCanonicalSignature(vchSig, flags) && IsCanonicalPubKey(vchPubKey, flags) &&
+                        CheckSig(vchSig, vchPubKey, scriptCode, txTo, nIn, nHashType, flags);
 
                     popstack(stack);
                     popstack(stack);
@@ -903,9 +912,8 @@ bool EvalScript(vector<vector<unsigned char> >& stack, const CScript& script, co
                         valtype& vchPubKey = stacktop(-ikey);
 
                         // Check signature
-                        bool fOk = (!fStrictEncodings || (IsCanonicalSignature(vchSig) && IsCanonicalPubKey(vchPubKey)));
-                        if (fOk)
-                            fOk = CheckSig(vchSig, vchPubKey, scriptCode, txTo, nIn, nHashType, flags);
+                        bool fOk = IsCanonicalSignature(vchSig, flags) && IsCanonicalPubKey(vchPubKey, flags) &&
+                            CheckSig(vchSig, vchPubKey, scriptCode, txTo, nIn, nHashType, flags);
 
                         if (fOk) {
                             isig++;