Extract DKIM signature from gmail email

Question

Gmail signs emails between .gmail.com addresses with DKIM. How can I extract the DKIM signature from a gmail email so I can prove to others it was sent at a specific date by a specific person? Cryptographically speaking this isn't a problem, since the signatures are signed using Google's public key, but I don't know how to actually find and reformat the signatures.

Answer 1

Ghm... let me tell you how DKIM is working.

Google signing outgoing messages with a private DKIM key that lives in a secure place on their servers(not a public key), you don't have access to it. Integrity of sent messages verified on receiving servers by taking publicly available public key from Coogle's DNS record (you can get it too by running: host -t txt 20120113._domainkey.gmail.com for example)

You can extract process of verification on receiving email server by opening your mail as a source(most of email clients has such option in menu.Check it how to see full message with headers ) with all headers included that show all route how particular email came to your email box. There should be (if receiving server DKIM aware) a header that show status of verification of DKIM, it either pass or failed. If you will reformat your message DKIM signature will not match original message, that is the whole point to sign it cryptographically to protect integrity of message.