Implement public key checking logic

SignalServiceKit/src/Loki/ECKeyPair.swift

@@ -9,4 +9,14 @@ public extension ECKeyPair {
         // Prefixing with "05" is necessary for what seems to be a sort of Signal public key versioning system
         return "05" + publicKey.map { String(format: "%02hhx", $0) }.joined()
     }
+    
+    static func isValidHexEncodedPublicKey(candidate: String) -> Bool {
+        // Check that it's a valid hexadecimal encoding
+        let allowedCharacters = CharacterSet(charactersIn: "0123456789ABCDEF")
+        guard candidate.uppercased().unicodeScalars.allSatisfy({ allowedCharacters.contains($0) }) else { return false }
+        // Check that it has either length 33 and a leading "05" or length 32
+        guard (candidate.count == 33 && candidate.hasPrefix("05")) || candidate.count == 32 else { return false }
+        // It appears to be a valid public key
+        return true
+    }
 }