Refactor message hashing into a utility function

And add unit test for it.

The purpose of using a preamble or "magic" text as part of signing and
verifying a message was not given when the code was repeated in a few
locations. Make a test showing how it is used to prevent inadvertently
signing a transaction.

2 files changed, 23 insertions, 12 deletions

diff --git a/src/util/message.cpp b/src/util/message.cpp
index 22ace2dd75..17603a43d2 100644
--- a/src/util/message.cpp
+++ b/src/util/message.cpp
@@ -15,7 +15,11 @@
 #include <string>
 #include <vector>
 
-const std::string strMessageMagic = "Bitcoin Signed Message:\n";
+/**
+ * Text used to signify that a signed message follows and to prevent
+ * inadvertently signing a transaction.
+ */
+const std::string MESSAGE_MAGIC = "Bitcoin Signed Message:\n";
 
 MessageVerificationResult MessageVerify(
     const std::string& address,
@@ -37,12 +41,8 @@ MessageVerificationResult MessageVerify(
         return MessageVerificationResult::ERR_MALFORMED_SIGNATURE;
     }
 
-    CHashWriter ss(SER_GETHASH, 0);
-    ss << strMessageMagic;
-    ss << message;
-
     CPubKey pubkey;
-    if (!pubkey.RecoverCompact(ss.GetHash(), signature_bytes)) {
+    if (!pubkey.RecoverCompact(MessageHash(message), signature_bytes)) {
         return MessageVerificationResult::ERR_PUBKEY_NOT_RECOVERED;
     }
 
@@ -58,13 +58,9 @@ bool MessageSign(
     const std::string& message,
     std::string& signature)
 {
-    CHashWriter ss(SER_GETHASH, 0);
-    ss << strMessageMagic;
-    ss << message;
-
     std::vector<unsigned char> signature_bytes;
 
-    if (!privkey.SignCompact(ss.GetHash(), signature_bytes)) {
+    if (!privkey.SignCompact(MessageHash(message), signature_bytes)) {
         return false;
     }
 
@@ -72,3 +68,11 @@ bool MessageSign(
 
     return true;
 }
+
+uint256 MessageHash(const std::string& message)
+{
+    CHashWriter hasher(SER_GETHASH, 0);
+    hasher << MESSAGE_MAGIC << message;
+
+    return hasher.GetHash();
+}
diff --git a/src/util/message.h b/src/util/message.h
index 1a1ba88daf..01fd14ce2d 100644
--- a/src/util/message.h
+++ b/src/util/message.h
@@ -7,10 +7,11 @@
 #define BITCOIN_UTIL_MESSAGE_H
 
 #include <key.h> // For CKey
+#include <uint256.h>
 
 #include <string>
 
-extern const std::string strMessageMagic;
+extern const std::string MESSAGE_MAGIC;
 
 /** The result of a signed message verification.
  * Message verification takes as an input:
@@ -58,4 +59,10 @@ bool MessageSign(
     const std::string& message,
     std::string& signature);
 
+/**
+ * Hashes a message for signing and verification in a manner that prevents
+ * inadvertently signing a transaction.
+ */
+uint256 MessageHash(const std::string& message);
+
 #endif // BITCOIN_UTIL_MESSAGE_H