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Summer of Code: Bug found!

BouncyCastle

The mystery has been solved! I finally found out, why the OpenPGP keys I generated for my project had a broken format. Turns out, there was a bug in BouncyCastle.
Big thanks to Heiko Stamer, who quickly identified the issue in the bug report I created for pgpdump, as well as Kazu Yamamoto and David Hook, who helped identify and confirm the issue.

The bug was, that BouncyCastle, when exporting a secret key without a password, was appending 20 bytes of the SHA1 hash after the secret key material. That is only supposed to happen, when the key in fact is password protected. In case of unprotected keys, BouncyCastle is supposed to add a two byte checksum instead. BouncyCastles wrong behaviour cause pgpdump to interpret random bytes as packet tags, which resulted in a wrong key id being printed out.

The relevant part of RFC-4880 is found in section 5.5.3:

      -If the string-to-key usage octet is zero or 255, then a two-octet
       checksum of the plaintext of the algorithm-specific portion (sum
       of all octets, mod 65536).  If the string-to-key usage octet was
       254, then a 20-octet SHA-1 hash of the plaintext of the
       algorithm-specific portion.

Shortly after I filed a bug report for BouncyCastle, Vincent Breitmoser, one of the Authors of XEP-0373 and XEP-0374 submitted a fix for the bug. This is a nice little example of how free software projects can work together to improve each other. Big thanks for that :)

Working OX Test Client!

I spent the last night to create a command line chat client that can “speak” OX. Everything is a little bit rough around the edges, but the core functionality works.
The user has to do actions like publishing and fetching keys by hand, but encrypted, signed messages can be exchanged. Having working code, I can now start to formulate a general API which will enable multiple OpenPGP back-ends. I will spend some more time to polish that client up and eventually publish it in a separate git repository.

EFAIL

I totally forgot to talk about EFAIL in my last blog posts. It was a little shock when I woke up on Monday, the first day of the coding phase, only to read sentences like “Are you okay?” or “Is the GSoC project in danger?” :D
I’m sure you all have read about the EFAIL attack somewhere in the media, so I’m not going into too much detail here (the EFF already did a great job *cough cough*). The E-Fail website describes the attack as follows:

“In a nutshell, EFAIL abuses active content of HTML emails, for example externally loaded images or styles, to exfiltrate plaintext through requested URLs.”

Is EFAIL applicable to XMPP?
Probably not to the XEPs I’m implementing. In case of E-Mail, it is relatively easy to prepend the image tag to the message. XEP-0373 however specifies, that the transported extension elements (eg. the body of the message) is wrapped inside of an additional extension element, which is then encrypted. Additionally this element (eg. <signcrypt/>) carries a random length, random content padding element, so it is very hard to nearly impossible for an attacker to guess, where the actual body starts, and in turn where they’d have to insert an “extraction channel” (eg. image tag) to the message.

In legacy OpenPGP for XMPP (XEP-0027) it is theoretically possible to at least execute the first part of the attack made in EFAIL. An attacker could insert an image tag to make a link out of the message. However, external images are usually shared by using XEP-0066 (Out of Band Data) by adding an x-element with the oob namespace to the message, which contains the URL to the image. Note, that this element is added outside the body though, so we should be fine, as so the attack would only work if the user tried to open the linkified message in a browser :)

Another option for the attacker would be to attack XHTML-IM (XEP-0071) messages, but I think those do not support legacy OpenPGP in the first place. Also XHTML-IM has been deprecated recently *phew*.

In the end, I’m by no means a security expert, so please do not quote me on my wild thoughts here :)
However it is important to learn from that example to not make the same mistakes some Email clients did.

Happy Hacking!

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