To Re-validate a Signature

From the Digital Signatures dialog, select a signature that is invalid.
Invalid signatures have an X in the verified column.
Click the re-validate a Signature tool.
The signature is re-validated with the current state of the file. The signature now displays verified (green check mark) and the red X is removed.

CAUTION: Re-validating a signature does not re-validate the signatures that depend on it. Conversely, a dependent signature cannot be re-validated until all of its prerequisite signatures have been re-validated. In general, the signatures in a hierarchy must be re-validated in the reverse of the order in which they were created.

Note: If a model signature element is copied, the copy will be a signature but will be invalid. The signer (and only the signer) could then recreate it, thus updating its signing time.

When a signed design file is modified, the signature is invalidated and marked with a red cross. The MS_SIGNATURE_DISPLAY_UNVERIFIED configuration variable can be used to change how unverified digital signatures are crossed out. Additionally, the MS_SIGNATURE_SHOW_FULL_NAME variable can be set to display the full subject and issuer names from the signer's certificate in the Properties balloon.
The table of uncategorized configuration variables has complete information on descriptions and values.