Why should we set a timestamp when we do a codesigning?

Question

If I set a timestamp with signing, what happens?
What if I don't set?

Is it essential? Why is it recommended?

Answer 1

Timestamping is used to specify time when the digital signature is made. This is needed to properly validate the signature.

If signature timestamp is present, the application which validates (verifies) the signature, will check whether the certificates involved into signature validation were valid at the moment of signing. If there's no timestamp for the signature, certificate validity is checked for the moment of signature validation, which is not always acceptable.

Example:
Certificate is valid from: 1st of January, 2008
Certificate is valid to: 31st of December, 2010
Signature is made on: 4th of July, 2009
Signature is verified on: 30th of April, 2012

With timestamp: signature is ok (signature was made during certificate validity period) Without timestamp: signature is not valid (certificate has expired by the moment of signature verification).

Timestamping should be used if the signature is supposed to be used (to proof authenticity of the document author or data originator) in long term, i.e. longer than one or several days.

Timestamping is not necessary when you, for example, send a short signed note to the colleague and this note is expected to be read and disposed of the same day as it has been written. Of course, timestamping can not be used when it's not supported by the signing technologies or when timestamping authority is not available.

On the other hand, timestamping is a must when you create signed documents for wide distribution or for long-term storage and archiving purposes. Timestamping is also used when signing the executable modules of software applications.

Update: the timestamp is also signed with a certificate. This signature is also validate using regular rules, which means that the certificate used to sign the timestamp must be valid at the moment of signature validation. In the above example if the timestamping certificate expired on the 1st of April, 2012, then the timestamp will be reported as not valid and won't be counted during validation of the signature.