A valid digital signature, by itself, proves that a message is new and has never been reused.

### Why this is right A signature can help verify the signer’s key and whether the signed content has changed, but the described signing and verification steps do not establish when the message was created or whether it was sent before. A previously signed message could be presented again unless the surrounding protocol checks freshness. ### The concept **Freshness** means that a message is new for the exchange at hand rather than a replay of an earlier message. A signature binds a cryptographic signature to content, but the same signed content and signature can still be copied and presented again. The course material identifies newness as a concern, but its OpenSSL example only signs and verifies a file. It does not show a timestamp, challenge, or other freshness check. Do not confuse **integrity**, meaning the content has not changed relative to the signature, with **freshness**, meaning the message is not a reused one. ### Watch out The word “valid” can tempt you to assume that a signature proves every security property. It only supports the checks performed by the signature process. ### Terms - **Freshness**: evidence that a message is new for the current exchange, not simply an old message resent. - **Replay**: resending or reusing a previously valid message. - **Integrity**: assurance that data has not been altered relative to the signature. - **Digital signature**: cryptographic information checked against data with a public key. ### Remember A signature can detect alteration, but freshness needs a separate check.