util: Don't allow base58-decoding of std::string:s containing non-base58 characters

src/util/strencodings.cpp

@@ -4,6 +4,7 @@
 // file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
 #include <util/strencodings.h>
+#include <util/string.h>
 
 #include <tinyformat.h>
 
@@ -269,7 +270,7 @@ NODISCARD static bool ParsePrechecks(const std::string& str)
         return false;
     if (str.size() >= 1 && (IsSpace(str[0]) || IsSpace(str[str.size()-1]))) // No padding allowed
         return false;
-    if (str.size() != strlen(str.c_str())) // No embedded NUL characters allowed
+    if (!ValidAsCString(str)) // No embedded NUL characters allowed
         return false;
     return true;
 }

src/util/string.h

@@ -5,6 +5,9 @@
 #ifndef BITCOIN_UTIL_STRING_H
 #define BITCOIN_UTIL_STRING_H
 
+#include <attributes.h>
+
+#include <cstring>
 #include <string>
 #include <vector>
 
@@ -31,4 +34,12 @@ inline std::string Join(const std::vector<std::string>& list, const std::string&
     return Join(list, separator, [](const std::string& i) { return i; });
 }
 
+/**
+ * Check if a string does not contain any embedded NUL (\0) characters
+ */
+NODISCARD inline bool ValidAsCString(const std::string& str) noexcept
+{
+    return str.size() == strlen(str.c_str());
+}
+
 #endif // BITCOIN_UTIL_STRENCODINGS_H