Fundamentals of InfoSecadvanced
A colleague says, “Because I received a valid signature, the document is secret and the message is definitely new.” Evaluate both claims using the properties described in the material.
Answer
Neither claim follows from signature verification alone. A digital signature supports authenticity and integrity checks but does not hide the document's contents. It also does not establish freshness, because an old signed message and its signature could be reused. Secrecy would require a confidentiality measure, and freshness would require a separate check not shown in the signing workflow.
Explanation
### The concept
A digital signature addresses two main questions in the supplied material: whether the signature is associated with the expected sender’s key, and whether the document has changed since it was signed. Those properties are called **authenticity** and **integrity**. They are different from **confidentiality**, which means preventing unauthorized people from reading the contents. The signature process shown does not encrypt the file, so a recipient who can read the file is not prevented from doing so by the signature. **Freshness** is also separate: a copy of a previously signed message can be resent with its original signature. The material raises the need to ensure a message is new, but the demonstrated signing and verification commands do not perform that check.
### What a good answer covers
- Reject the claim that signature verification makes a document secret.
- Explain that the described process signs but does not encrypt the file.
- Reject the claim that a valid signature proves the message is new.
- Distinguish authenticity and integrity from confidentiality and freshness.
### Watch out
“Valid signature” does not mean “all security goals have been met.” State exactly what the verification checks and do not infer unshown protections.
### Terms
- **Authenticity**: assurance that a signature is associated with the expected sender’s key.
- **Integrity**: assurance that the document has not changed relative to its signature.
- **Confidentiality**: protection against unauthorized reading of the document.
- **Freshness**: evidence that the message is new for the current exchange.
- **Encryption**: a process that conceals data from people without the needed key.
- **Replay**: reuse of a previously valid message.
### Remember
A signature checks who-key association and alteration; it does not automatically provide secrecy or newness.
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