[cryptography] Compositing Ciphers?

[Eugen Leitl]

----- Forwarded message from Natanael <natanael.l at gmail.com> -----

Date: Sat, 7 Sep 2013 02:53:22 +0200
From: Natanael <natanael.l at gmail.com>
To: noloader at gmail.com
Cc: Cryptography List <cryptography at randombit.net>
Subject: Re: [cryptography] Compositing Ciphers?

http://blog.cryptographyengineering.com/2012/02/multiple-encryption.html
Apparently it's called "cascade encryption" or "cascade encipherment",
and the implementations are apparently called "robust combiners". And
by the way, Truecrypt already lets you pick your chosen combo of AES
and two other ciphers.

I think you should worry about your PRNG and it's seed before you
focus on AES. Your key should both have enough entropy and be secret.
Is your PRNG backdoored already? And I'm guessing the cipher mode
probably matters a bit more than the exact choice of algorithm.

On Sat, Sep 7, 2013 at 2:27 AM, Jeffrey Walton <noloader at gmail.com> wrote:
> Hi All,
>
> With all the talk of the NSA poisoning NIST, would it be wise to
> composite ciphers? (NY Times, Guardian, Dr. Green's blog, et seq).
>
> I've been thinking about running a fast inner stream cipher (Salsa20
> without a MAC) and wrapping it in AES with an authenticated encryption
> mode (or CBC mode with {HMAC|CMAC}).
>
> I'm aware of, for example, NSA's Fishbowl running IPSec at the network
> layer (the "outer" encryption") and then SRTP and the application
> level (the "inner" encryption). But I'd like to focus on hardening one
> cipherstream at one level, and not cross OSI boundaries.
>
> I'm also aware of the NSA's lightweight block ciphers
> (http://eprint.iacr.org/2013/404). I may have been born at night, but
> it was not last night....
>
> Has anyone studied the configuration and security properties of a
> inner stream cipher with an outer block cipher?
>
> Jeff
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