Summer of Code: Small steps

Yesterday I got my first results encrypting and decrypting OpenPGP messages using PGPainless, my fork of bouncy-gpg. There were some interesting hurdles that I want to discuss though.

GnuPG

As a first step towards working encryption and decryption, I obviously needed to create some PGP keys for testing purposes. As a regular user of OpenPGP I knew how to create keys using the command line tool GnuPG, so I started up the key creation by typing “gpg –generate-key”. I chose the key type to be RSA with a length of 2048 bits, as those settings are also the defaults recommended by GnuPG itself. When it came to entering user id information though, things got a little more complicated. GnuPG asks for the name of the user, their email address and a comment. XEP-0373 states, that the user id packet of a PGP key MUST be of the format “xmpp:juliet@capulet.lit”. My first thing to figure out was, if I should enter that String as the name, email or as a comment. I first tried with the name, upon which GnuPG complained, that neither name, nor comment is allowed to contain an email address. Logically my next step was to enter the String as the users email address. Again, GnuPG complained, this time it stated, that “xmpp:juliet@capulet.lit” was not a valid Email address. So I got stuck.

Luckily I knew that Philipp Hรถrist was working on an experimental OX plugin for Gajim. He could hint me to a process of “Unattended Key Generation“, which reads input values from a script file. This input would not be validated by GnuPG as strictly as when using the wizard, so now I was able to successfully create my testing keys. Big thanks for the help ๐Ÿ™‚

Update: Apparently GnuPG provides an additional flag “–allow-freeform-uid”, which does exactly that. Allowing uids of any form. Using that flag allows easy generation and editing of keys with freely chosen uids. Thanks to Wiktor from the Conversations.im chat room ๐Ÿ™‚

Bouncy-gpg

As a next step, I wrote a little JUnit test which signs and encrypts a little piece of text, followed by decryption and signature validation. Here I came across my next problem.

Bouncy-gpg provides a class called Rfc4880KeySelectionStrategy, which is used to select keys from the users keyring following a certain strategy. In my testing code, I created two keyrings for Romeo and Juliet and added their respective public and private keys like you would do in a real life scenario. The issue I then encountered was, that when I tried to encrypt my message from Juliet for Romeos public key, I got the error, that “no suitable key was found”. How could that be? I did some more debugging and was able to verify that the keys were in fact added to the keyrings just as I intended.

To explain the cause of this issue, I have to explain in a little more depth, how the user id field is formatted in OpenPGP.
RFC4880 states, the following:

A User ID packet consists of UTF-8 text that is intended to represent
the name and email address of the key holder.  By convention, it
includes an RFC 2822 [RFC2822] mail name-addr, but there are no
restrictions on its content.

A “mail name-addr” follows this format: “Juliet Capulet (The Juliet from Shakespear’s play) <juliet@capulet.lit>”.
First there is the name of the key owner, followed by a comment in parentheses, followed by the email address in angle brackets. The usage of brackets makes it unambiguous, which value is which, so all values are optional.
“<juliet@capulet.lit>” would still be a valid user id for example.

So what was the problem?
The user id of my testing key looked like this: “xmpp:juliet@capulet.lit”. Note that there are no angle brackets or parentheses around the text, so the string would be interpreted as name.
Bouncy-gpg’s Rfc4880KeySelectionStrategy however contained some code, which would check, whether the query the user entered to search for a key would be enclosed in angle brackets, to follow the email address format. In case it doesn’t, the code would add the angle brackets prior to executing the search. Instead of searching for “xmpp:juliet@capulet.lit”, the selection strategy would look out for keys with the user id “<xmpp:juliet@capulet.lit>”.
My solution to the problem was to create my own KeySelectionStrategy, which would leave the query as is, in order for it to match my keys user id. Figuring that out took me quite a while ๐Ÿ˜€

Conclusions

So what conclusions can I draw from my experiences?
First of all, I’m not sure, if it is a good idea to give the user the ability to import their own PGP keys. GnuPGs behaviour of forbidding the user to add user ids which don’t follow the mail name-addr format will make it very hard for a user to create a key with a valid user id. (Update: Users can use the flag “–allow-freeform-uid” to generate new keys and edit existing ones with unconventional uids.) Philipp Hรถrist suggested that implementations of XEP-0373 should instead create a key for the user on the first use and I think I aggree with him. As a logical next step I have to figure out, how to create PGP keys using Bouncycastle ๐Ÿ˜€

I hope you liked my little update post, which grew longer than I expected ๐Ÿ˜€

Happy Hacking!

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