How can I determine what encryption algorithm someone used if I have their PGP Key ID and Fingerprint?

I just learned how to lookup a user's info on pgp.mit.edu. Is there some way to determine what algorithm the user is using?

trusktr

Posted 2014-01-24T23:40:41.967

Reputation: 1 521

2This question appears to be off-topic because it does not handle cryptography itself, but rather asks about a "how to" related to the MIT PGP Public Key Server and software usage. (Hint: read the related documentation.) – e-sushi – 2014-01-25T04:38:43.540

@e-sushi Would superuser be a better place to ask? – trusktr – 2014-01-25T10:39:11.583

1I think so, but to be honest - I'm not really sure. Since moderators also have the ability to move/migrate questions if it makes sense, I'll flag it for a moderator to look at it and decide if it makes sense to move it there or not. Could take a bit though, so you might want to be prepared for a little waiting time (yet... think in hours, not weeks). – e-sushi – 2014-01-25T13:50:27.267

Import the key into your GnuPG keyring and examine it. See the documentation of GnuPG. – None – 2014-01-25T20:52:24.823

1This question appears to be off-topic, because it is about usage of specific cryptographic software (not its cryptographic internals). Such question may be more appropriate for [su]. – Ilmari Karonen – 2014-01-26T16:35:49.533

Answers

That's basically right. The details of what algorithm his key-pair is for is actually encoded as part of the public-key itself! That is the public-key is not really a raw key but instead it is packaged a bit to specify some details of the key (minimally the algorithm the key is for) as well as the key bit-string itself.

You can find more by reading about RSA PKCS#1 [published by IETF as RFC3447]: http://tools.ietf.org/html/rfc3447#section-3.1

Ram

Posted 2014-01-24T23:40:41.967

Reputation: 977

I'm still don't fully understand. Is the following correct? When the recipient creates his key pair, he has chosen an algorithm for it. When the data on my side gets encrypted with the data encryption key, that will be using my algorithm of choice, and then the data encryption key gets encrypted with the recipient's public key and therefore with his chosen algorithm. He decrypts the data encryption key with the algorithm that matches with his private key, then he decrypts my final message with the algorithm I've used. Is that right? – trusktr – 2014-02-01T19:52:52.090