Add Protocol::SignedMessage::valid + tests

Protocol/src/include/protocol/protocol.hxx

@@ -19,7 +19,7 @@ struct SignedMessage {
                     std::string plaintext);
 
       std::vector<uint8_t> serialize();
-      // bool valid();
+      bool valid(unsigned char pub_key[crypto_sign_PUBLICKEYBYTES]);
 };
 
 } // namespace Protocol

Protocol/src/protocol.cxx

@@ -1,5 +1,5 @@
-#include "sodium/crypto_sign_ed25519.h"
 #include <chrono>
+#include <cstring>
 #include <protocol/protocol.hxx>
 #include <sodium.h>
 
@@ -39,4 +39,22 @@ std::vector<uint8_t> SignedMessage::serialize() {
       return bytes;
 }
 
+bool SignedMessage::valid(unsigned char pub_key[crypto_sign_PUBLICKEYBYTES]) {
+      unsigned char pub_key_fingerprint[crypto_hash_sha256_BYTES];
+      crypto_hash_sha256(pub_key_fingerprint, pub_key,
+                         crypto_sign_PUBLICKEYBYTES);
+
+      if (std::memcmp(pub_key_fingerprint, sender_key_fingerprint,
+                      crypto_sign_PUBLICKEYBYTES) != 0) {
+            return false;
+      }
+
+      auto serialized_data = serialize();
+      if (crypto_sign_verify_detached(signature, serialized_data.data(),
+                                      serialized_data.size(), pub_key) != 0) {
+            return false;
+      }
+      return true;
+}
+
 } // namespace Protocol