Re: [Cfrg] Analysis of ipcrypt?

Jean-Philippe Aumasson <jeanphilippe.aumasson@gmail.com> Thu, 22 February 2018 06:55 UTC

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From: Jean-Philippe Aumasson <jeanphilippe.aumasson@gmail.com>
Date: Thu, 22 Feb 2018 06:55:17 +0000
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To: Paul Hoffman <paul.hoffman@icann.org>
Cc: "Jason A. Donenfeld" <Jason@zx2c4.com>, "cfrg@irtf.org" <cfrg@irtf.org>
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Subject: Re: [Cfrg] Analysis of ipcrypt?
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Hi!

I designed ipcrypt as a low-security toy cipher to encrypt IPv4 addresses
for some log analysis application. It may be good enough for this purpose,
however it has very low security:

* because of 32-bit blocks, a chosen-plaintext codebook attack will work in
time 2^32 (or much less for specific IP ranges)

* known-plaintext codebook attacks will work similarly but in O(n log n),
or 2^37 (coupon collector problem)

* there is a generic ~2^16 distinguisher that works by looking for a
collision in a sequence of blocks

* worse, Jason just found a high-probability differential that seems
detectable with fewer than 2^24 chosen-plaintext pair, and which may speed
up key recovery



On Thu, 22 Feb 2018 at 03:03, Paul Hoffman <paul.hoffman@icann.org>; wrote:

> Greetings. ipcrypt is a format-preserving cipher for IPv4 addresses. It
> has a 32-bit blocksize for input and output, and 128-bit blocksize for the
> key. It was developed by Jean-Philippe Aumasson and is described at:
>    https://github.com/veorq/ipcrypt
> There doesn't appear to be any formal paper describing the algorithm, but
> the Python and Go code is trivial to follow.
>
> This algorithm is now being considered by a few different projects that
> want to obfuscate IPv4 addresses. Has anyone analyzed the algorithm? I
> could not find analyses, but certainly could have missed them.
>
> 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?
>
> (Note that there are other options for this use case, which have different
> positive and negative features. What we'd like to know is how good is
> ipcrypt if we chose it.)
>
> --Paul Hoffman_______________________________________________
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