Add SCRIPT_VERIFY_MINIMALDATA (BIP62 rules 3 and 4)

Also use the new flag as a standard rule, and replace the IsCanonicalPush
standardness check with it (as it is more complete).

5 files changed, 60 insertions, 46 deletions

diff --git a/src/script/interpreter.cpp b/src/script/interpreter.cpp
index e463de8cc2..11c909472a 100644
--- a/src/script/interpreter.cpp
+++ b/src/script/interpreter.cpp
@@ -157,6 +157,29 @@ bool static CheckPubKeyEncoding(const valtype &vchSig, unsigned int flags) {
     return true;
 }
 
+bool static CheckMinimalPush(const valtype& data, opcodetype opcode) {
+    if (data.size() == 0) {
+        // Could have used OP_0.
+        return opcode == OP_0;
+    } else if (data.size() == 1 && data[0] >= 1 && data[0] <= 16) {
+        // Could have used OP_1 .. OP_16.
+        return opcode == OP_1 + (data[0] - 1);
+    } else if (data.size() == 1 && data[0] == 0x81) {
+        // Could have used OP_1NEGATE.
+        return opcode == OP_1NEGATE;
+    } else if (data.size() <= 75) {
+        // Could have used a direct push (opcode indicating number of bytes pushed + those bytes).
+        return opcode == data.size();
+    } else if (data.size() <= 255) {
+        // Could have used OP_PUSHDATA.
+        return opcode == OP_PUSHDATA1;
+    } else if (data.size() <= 65535) {
+        // Could have used OP_PUSHDATA2.
+        return opcode == OP_PUSHDATA2;
+    }
+    return true;
+}
+
 bool EvalScript(vector<vector<unsigned char> >& stack, const CScript& script, unsigned int flags, const BaseSignatureChecker& checker)
 {
     CScript::const_iterator pc = script.begin();
@@ -169,6 +192,7 @@ bool EvalScript(vector<vector<unsigned char> >& stack, const CScript& script, un
     if (script.size() > 10000)
         return false;
     int nOpCount = 0;
+    bool fRequireMinimal = (flags & SCRIPT_VERIFY_MINIMALDATA) != 0;
 
     try
     {
@@ -205,9 +229,12 @@ bool EvalScript(vector<vector<unsigned char> >& stack, const CScript& script, un
                 opcode == OP_RSHIFT)
                 return false; // Disabled opcodes.
 
-            if (fExec && 0 <= opcode && opcode <= OP_PUSHDATA4)
+            if (fExec && 0 <= opcode && opcode <= OP_PUSHDATA4) {
+                if (fRequireMinimal && !CheckMinimalPush(vchPushValue, opcode)) {
+                    return false;
+                }
                 stack.push_back(vchPushValue);
-            else if (fExec || (OP_IF <= opcode && opcode <= OP_ENDIF))
+            } else if (fExec || (OP_IF <= opcode && opcode <= OP_ENDIF))
             switch (opcode)
             {
                 //
@@ -234,6 +261,8 @@ bool EvalScript(vector<vector<unsigned char> >& stack, const CScript& script, un
                     // ( -- value)
                     CScriptNum bn((int)opcode - (int)(OP_1 - 1));
                     stack.push_back(bn.getvch());
+                    // The result of these opcodes should always be the minimal way to push the data
+                    // they push, so no need for a CheckMinimalPush here.
                 }
                 break;
 
@@ -458,7 +487,7 @@ bool EvalScript(vector<vector<unsigned char> >& stack, const CScript& script, un
                     // (xn ... x2 x1 x0 n - ... x2 x1 x0 xn)
                     if (stack.size() < 2)
                         return false;
-                    int n = CScriptNum(stacktop(-1)).getint();
+                    int n = CScriptNum(stacktop(-1), fRequireMinimal).getint();
                     popstack(stack);
                     if (n < 0 || n >= (int)stack.size())
                         return false;
@@ -557,7 +586,7 @@ bool EvalScript(vector<vector<unsigned char> >& stack, const CScript& script, un
                     // (in -- out)
                     if (stack.size() < 1)
                         return false;
-                    CScriptNum bn(stacktop(-1));
+                    CScriptNum bn(stacktop(-1), fRequireMinimal);
                     switch (opcode)
                     {
                     case OP_1ADD:       bn += bnOne; break;
@@ -590,8 +619,8 @@ bool EvalScript(vector<vector<unsigned char> >& stack, const CScript& script, un
                     // (x1 x2 -- out)
                     if (stack.size() < 2)
                         return false;
-                    CScriptNum bn1(stacktop(-2));
-                    CScriptNum bn2(stacktop(-1));
+                    CScriptNum bn1(stacktop(-2), fRequireMinimal);
+                    CScriptNum bn2(stacktop(-1), fRequireMinimal);
                     CScriptNum bn(0);
                     switch (opcode)
                     {
@@ -635,9 +664,9 @@ bool EvalScript(vector<vector<unsigned char> >& stack, const CScript& script, un
                     // (x min max -- out)
                     if (stack.size() < 3)
                         return false;
-                    CScriptNum bn1(stacktop(-3));
-                    CScriptNum bn2(stacktop(-2));
-                    CScriptNum bn3(stacktop(-1));
+                    CScriptNum bn1(stacktop(-3), fRequireMinimal);
+                    CScriptNum bn2(stacktop(-2), fRequireMinimal);
+                    CScriptNum bn3(stacktop(-1), fRequireMinimal);
                     bool fValue = (bn2 <= bn1 && bn1 < bn3);
                     popstack(stack);
                     popstack(stack);
@@ -727,7 +756,7 @@ bool EvalScript(vector<vector<unsigned char> >& stack, const CScript& script, un
                     if ((int)stack.size() < i)
                         return false;
 
-                    int nKeysCount = CScriptNum(stacktop(-i)).getint();
+                    int nKeysCount = CScriptNum(stacktop(-i), fRequireMinimal).getint();
                     if (nKeysCount < 0 || nKeysCount > 20)
                         return false;
                     nOpCount += nKeysCount;
@@ -738,7 +767,7 @@ bool EvalScript(vector<vector<unsigned char> >& stack, const CScript& script, un
                     if ((int)stack.size() < i)
                         return false;
 
-                    int nSigsCount = CScriptNum(stacktop(-i)).getint();
+                    int nSigsCount = CScriptNum(stacktop(-i), fRequireMinimal).getint();
                     if (nSigsCount < 0 || nSigsCount > nKeysCount)
                         return false;
                     int isig = ++i;
diff --git a/src/script/interpreter.h b/src/script/interpreter.h
index 085cd867da..5133c80aab 100644
--- a/src/script/interpreter.h
+++ b/src/script/interpreter.h
@@ -49,6 +49,13 @@ enum
 
     // Using a non-push operator in the scriptSig causes script failure (softfork safe, BIP62 rule 2).
     SCRIPT_VERIFY_SIGPUSHONLY = (1U << 5),
+
+    // Require minimal encodings for all push operations (OP_0... OP_16, OP_1NEGATE where possible, direct
+    // pushes up to 75 bytes, OP_PUSHDATA up to 255 bytes, OP_PUSHDATA2 for anything larger). Evaluating
+    // any other push causes the script to fail (BIP62 rule 3).
+    // In addition, whenever a stack element is interpreted as a number, it must be of minimal length (BIP62 rule 4).
+    // (softfork safe)
+    SCRIPT_VERIFY_MINIMALDATA = (1U << 6)
 };
 
 uint256 SignatureHash(const CScript &scriptCode, const CTransaction& txTo, unsigned int nIn, int nHashType);
diff --git a/src/script/script.cpp b/src/script/script.cpp
index bbcfe9dfdd..b879d72d6b 100644
--- a/src/script/script.cpp
+++ b/src/script/script.cpp
@@ -12,7 +12,7 @@ namespace {
 inline std::string ValueString(const std::vector<unsigned char>& vch)
 {
     if (vch.size() <= 4)
-        return strprintf("%d", CScriptNum(vch).getint());
+        return strprintf("%d", CScriptNum(vch, false).getint());
     else
         return HexStr(vch);
 }
@@ -238,33 +238,6 @@ bool CScript::IsPushOnly() const
     return true;
 }
 
-bool CScript::HasCanonicalPushes() const
-{
-    const_iterator pc = begin();
-    while (pc < end())
-    {
-        opcodetype opcode;
-        std::vector<unsigned char> data;
-        if (!GetOp(pc, opcode, data))
-            return false;
-        if (opcode > OP_16)
-            continue;
-        if (opcode < OP_PUSHDATA1 && opcode > OP_0 && (data.size() == 1 && data[0] <= 16))
-            // Could have used an OP_n code, rather than a 1-byte push.
-            return false;
-        if (opcode == OP_PUSHDATA1 && data.size() < OP_PUSHDATA1)
-            // Could have used a normal n-byte push, rather than OP_PUSHDATA1.
-            return false;
-        if (opcode == OP_PUSHDATA2 && data.size() <= 0xFF)
-            // Could have used an OP_PUSHDATA1.
-            return false;
-        if (opcode == OP_PUSHDATA4 && data.size() <= 0xFFFF)
-            // Could have used an OP_PUSHDATA2.
-            return false;
-    }
-    return true;
-}
-
 std::string CScript::ToString() const
 {
     std::string str;
diff --git a/src/script/script.h b/src/script/script.h
index 706a85a29b..e97967dcea 100644
--- a/src/script/script.h
+++ b/src/script/script.h
@@ -192,10 +192,14 @@ public:
         m_value = n;
     }
 
-    explicit CScriptNum(const std::vector<unsigned char>& vch)
+    explicit CScriptNum(const std::vector<unsigned char>& vch, bool fRequireMinimal)
     {
-        if (vch.size() > nMaxNumSize)
-            throw scriptnum_error("CScriptNum(const std::vector<unsigned char>&) : overflow");
+        if (vch.size() > nMaxNumSize) {
+            throw scriptnum_error("script number overflow");
+        }
+        if (fRequireMinimal && vch.size() > 0 && (vch.back() & 0x7f) == 0 && (vch.size() <= 1 || (vch[vch.size() - 2] & 0x80) == 0)) {
+            throw scriptnum_error("non-minimally encoded script number");
+        }
         m_value = set_vch(vch);
     }
 
@@ -319,7 +323,6 @@ private:
     int64_t m_value;
 };
 
-
 /** Serialized script, used inside transaction inputs and outputs */
 class CScript : public std::vector<unsigned char>
 {
@@ -330,6 +333,10 @@ protected:
         {
             push_back(n + (OP_1 - 1));
         }
+        else if (n == 0)
+        {
+            push_back(OP_0);
+        }
         else
         {
             *this << CScriptNum::serialize(n);
@@ -554,9 +561,6 @@ public:
     // Called by IsStandardTx and P2SH/BIP62 VerifyScript (which makes it consensus-critical).
     bool IsPushOnly() const;
 
-    // Called by IsStandardTx.
-    bool HasCanonicalPushes() const;
-
     // Returns whether the script is guaranteed to fail at execution,
     // regardless of the initial stack. This allows outputs to be pruned
     // instantly when entering the UTXO set.
diff --git a/src/script/standard.h b/src/script/standard.h
index 961b214c89..248b941a64 100644
--- a/src/script/standard.h
+++ b/src/script/standard.h
@@ -41,6 +41,7 @@ static const unsigned int MANDATORY_SCRIPT_VERIFY_FLAGS = SCRIPT_VERIFY_P2SH;
 // blocks and we must accept those blocks.
 static const unsigned int STANDARD_SCRIPT_VERIFY_FLAGS = MANDATORY_SCRIPT_VERIFY_FLAGS |
                                                          SCRIPT_VERIFY_STRICTENC |
+                                                         SCRIPT_VERIFY_MINIMALDATA |
                                                          SCRIPT_VERIFY_NULLDUMMY;
 
 // For convenience, standard but not mandatory verify flags.