# Identifying a PKI cert uniquely

A Web Application wants the user to digitally sign a transaction using a PKI certificate. All users are pre-registered with the web-application. Pre-registeration includes uploading the Public part of the certificate to be used by the user for signing.

In such a case, at the time of registration what details should the application store about the user's certificate. This will be used to confirm that the same certificate is used later on for signing.

I was thinking for each user, the following 2 details would be enough - the serial no - the issuer string.

Is this enough for identifying the certificate uniquely?

Or is it easier/better to just store the whole certificate (public part) & compare it. If this is better what's the best format to store the certificate etc.

Note that this question is not about verifying the signature or making sure the cert used for signing corresponds to the public key attached to the signature etc. All that is already taken care of.

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"the Public part of the certificate" $\:$ = $\:$ "the entire certificate" $\;\;\;$ –  Ricky Demer Jul 8 '13 at 20:17