Re: [OAUTH-WG] JWT BCP on Compression in JWE

Yaron Sheffer <yaronf.ietf@gmail.com> Sat, 29 July 2017 20:22 UTC

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To: Jim Manico <jim@manicode.com>
Cc: oauth@ietf.org
References: <mailman.4424.1501277231.4234.oauth@ietf.org> <08197d4f-7512-d877-f99c-fe0ca03d3e19@gmail.com> <A07A5549-3554-42D6-B0ED-3CC62306D1DA@manicode.com>
From: Yaron Sheffer <yaronf.ietf@gmail.com>
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Date: Sat, 29 Jul 2017 23:22:18 +0300
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Subject: Re: [OAUTH-WG] JWT BCP on Compression in JWE
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Hi Jim,

The problem is not the encryption of attacker-controlled data. The 
problem is the interaction between this encryption and compression.

If you don't need compression, you're good. You're mostly OK if you can 
compress only the non-attacker controlled data, however this could 
potentially leak information about ciphertext.

This is all very use-case specific and fragile, so I think a reasonable 
recommendation is:

- Avoid transparent compression in generic JWS/JWE libraries.
- Only compress data at the application layer, but bear in mind that the 
length of compressed+encrypted data leaks information about cleartext.

Thanks,
	Yaron

On 29/07/17 21:32, Jim Manico wrote:
> Yaron,
> 
> As a developer, I can think of many scenarios where the attacker controls some of the plaintext yet I still need encryption services of some kind. What are the proper crypto controls that allow developers to do this safely? I think that's the better question right now.
> 
> Aloha,
> --
> Jim Manico
> @Manicode
> 
>> On Jul 28, 2017, at 7:57 PM, Yaron Sheffer <yaronf.ietf@gmail.com>; wrote:
>>
>> Hi Brian,
>>
>> These two attacks on TLS are only examples of the breakage that can occur when the adversary can control the plaintext to some degree (even a small piece of the plaintext, e.g. a malleable HTTP cookie can result in decryption of the whole message). Similar attacks were demonstrated in IPsec. Can you please add details on why typical use of JWT would not be susceptible to these attacks?
>>
>> Thanks,
>>     Yaron
>>
>>> On critique of JWT I've seen a few times can be paraphrased as "JWT
>>> supports compressed plaintext so, because of CRIME and BREACH, it is
>>> dangerous and stupid."  It's very possible that I am stupid (many on this
>>> list will likely attest to it) but I don't see the applicability of those
>>> kinds of chosen plaintext attacks aimed at recovering sensitive data to how
>>> JWT/JWE are typically used.
>>>
>>> I think it would be useful, if during the development of the JWT BCP, the
>>> authors or chairs or WG could somehow engage some experts (CFRG?) to
>>> understand if there's any real practical advice that can be given about
>>> using compression with JWE and the risks involved.
>>>
>>
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