Merge #18422: [consensus] MOVEONLY: Move single-sig checking EvalScript code to EvalChecksig

14e8cf974a7a317796ef8e97e5cf9c355ceff0ee [consensus] MOVEONLY: Move single-sig checking EvalScript code to EvalChecksig (Pieter Wuille)

Pull request description:

  This is another small refactor pulled out of the Schnorr/Taproot PR #17977.

  This is in preparation for adding different signature verification rules,
  specifically tapscript (BIP 342), which interprets opcode 0xac and 0xad
  as Schnorr signature verifications.

ACKs for top commit:
  sipa:
    ACK 14e8cf974a7a317796ef8e97e5cf9c355ceff0ee, verified move-only.
  MarcoFalke:
    ACK 14e8cf974a7a317796ef8e97e5cf9c355ceff0ee, reviewed with "git show 14e8cf974a7a317796ef8e97e5cf9c355ceff0ee --color-moved=dimmed-zebra --color-moved-ws=ignore-all-space -W" 👆
  fjahr:
    Code-review ACK 14e8cf974a7a317796ef8e97e5cf9c355ceff0ee, verified that it's move-only.
  instagibbs:
    code review ACK https://github.com/bitcoin/bitcoin/pull/18422/commits/14e8cf974a7a317796ef8e97e5cf9c355ceff0ee, verified move-only
  theStack:
    Code-Review ACK https://github.com/bitcoin/bitcoin/commit/14e8cf974a7a317796ef8e97e5cf9c355ceff0ee
  jonatack:
    ACK 14e8cf974a7a317796ef8e97e5cf9c355ceff0ee

Tree-SHA512: af2efce9ae39d5ec01db5b9ef0ff383fe252ef5f33b3483927308ae17d91a619266cb45951f32ea1ce54807a4c0f052bcdefb47e244465d3a726393221c227b1

1 files changed, 31 insertions, 19 deletions

diff --git a/src/script/interpreter.cpp b/src/script/interpreter.cpp
index 083022fbdd..50f7806ba5 100644
--- a/src/script/interpreter.cpp
+++ b/src/script/interpreter.cpp
@@ -342,6 +342,35 @@ public:
 };
 }
 
+/** Helper for OP_CHECKSIG and OP_CHECKSIGVERIFY
+ *
+ * A return value of false means the script fails entirely. When true is returned, the
+ * fSuccess variable indicates whether the signature check itself succeeded.
+ */
+static bool EvalChecksig(const valtype& vchSig, const valtype& vchPubKey, CScript::const_iterator pbegincodehash, CScript::const_iterator pend, unsigned int flags, const BaseSignatureChecker& checker, SigVersion sigversion, ScriptError* serror, bool& fSuccess)
+{
+    // Subset of script starting at the most recent codeseparator
+    CScript scriptCode(pbegincodehash, pend);
+
+    // Drop the signature in pre-segwit scripts but not segwit scripts
+    if (sigversion == SigVersion::BASE) {
+        int found = FindAndDelete(scriptCode, CScript() << vchSig);
+        if (found > 0 && (flags & SCRIPT_VERIFY_CONST_SCRIPTCODE))
+            return set_error(serror, SCRIPT_ERR_SIG_FINDANDDELETE);
+    }
+
+    if (!CheckSignatureEncoding(vchSig, flags, serror) || !CheckPubKeyEncoding(vchPubKey, flags, sigversion, serror)) {
+        //serror is set
+        return false;
+    }
+    fSuccess = checker.CheckSig(vchSig, vchPubKey, scriptCode, sigversion);
+
+    if (!fSuccess && (flags & SCRIPT_VERIFY_NULLFAIL) && vchSig.size())
+        return set_error(serror, SCRIPT_ERR_SIG_NULLFAIL);
+
+    return true;
+}
+
 bool EvalScript(std::vector<std::vector<unsigned char> >& stack, const CScript& script, unsigned int flags, const BaseSignatureChecker& checker, SigVersion sigversion, ScriptError* serror)
 {
     static const CScriptNum bnZero(0);
@@ -985,25 +1014,8 @@ bool EvalScript(std::vector<std::vector<unsigned char> >& stack, const CScript&
                     valtype& vchSig    = stacktop(-2);
                     valtype& vchPubKey = stacktop(-1);
 
-                    // Subset of script starting at the most recent codeseparator
-                    CScript scriptCode(pbegincodehash, pend);
-
-                    // Drop the signature in pre-segwit scripts but not segwit scripts
-                    if (sigversion == SigVersion::BASE) {
-                        int found = FindAndDelete(scriptCode, CScript() << vchSig);
-                        if (found > 0 && (flags & SCRIPT_VERIFY_CONST_SCRIPTCODE))
-                            return set_error(serror, SCRIPT_ERR_SIG_FINDANDDELETE);
-                    }
-
-                    if (!CheckSignatureEncoding(vchSig, flags, serror) || !CheckPubKeyEncoding(vchPubKey, flags, sigversion, serror)) {
-                        //serror is set
-                        return false;
-                    }
-                    bool fSuccess = checker.CheckSig(vchSig, vchPubKey, scriptCode, sigversion);
-
-                    if (!fSuccess && (flags & SCRIPT_VERIFY_NULLFAIL) && vchSig.size())
-                        return set_error(serror, SCRIPT_ERR_SIG_NULLFAIL);
-
+                    bool fSuccess = true;
+                    if (!EvalChecksig(vchSig, vchPubKey, pbegincodehash, pend, flags, checker, sigversion, serror, fSuccess)) return false;
                     popstack(stack);
                     popstack(stack);
                     stack.push_back(fSuccess ? vchTrue : vchFalse);