Re: [Cfrg] [Ext] Re: Analysis of ipcrypt?

Greg Rose <ggr@seer-grog.net> Fri, 23 February 2018 01:53 UTC

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From: Greg Rose <ggr@seer-grog.net>
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Date: Thu, 22 Feb 2018 17:52:59 -0800
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Cc: Greg Rose <ggr@seer-grog.net>, "cfrg@irtf.org" <cfrg@irtf.org>
To: Paul Hoffman <paul.hoffman@icann.org>
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Subject: Re: [Cfrg] [Ext] Re: Analysis of ipcrypt?
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> On Feb 22, 2018, at 16:19 , Paul Hoffman <paul.hoffman@icann.org>; wrote:
> 
> On Feb 22, 2018, at 4:14 PM, Greg Rose <ggr@seer-grog.net>; wrote:
>> Anyone who wants to do 32-bit encryption with a key longer than 80 bits already needs to have their threat model reviewed ;-).
> 
> OK, so please review what I said at the top of the thread:
> 
> For a project I'm on, ipcrypt is attractive if an attacker cannot derive the 128-bit random key without a lot (maybe 2^80ish) effort. For cases in common use, assume that the attacker has 2^24 known plaintext/ciphertext pairs under a single 128-bit random key. For additional ciphertexts, how much effort must the attacker expend to get the key in order to decrypt additional unknown ciphertexts?
> 
> The threat model then is that an attacker with 2^24 known plaintext/ciphertext pairs wants to determine the 128-bit random key that was used so that the attacker can de-anonymize addresses that are not in their current set.
> 
> Why is that threat model worth a smiley?

I think J-P's earlier posting already described that he considered ipcrypt to be "a low-security toy cipher". He also very well spelled out the various generic attacks. You yourself said "2^80ish" work to recover the key. So my smiley was about the fact that recovering an 80-bit key in 2^80ish work is just as hard as recovering a 128- or 256-bit key in 2^80ish work. And it happened that I had an 80-bit key 32-bit block cipher lying around, so I mentioned it. I did not mean to give offense.

Greg.