From: Joy Latten <latten@austin.ibm.com>
Subject: Re: [PATCH 1/1]: Revised CTR mode implementation
Date: Wed, 10 Oct 2007 11:08:26 -0500
Message-ID: <1192032506.2477.285.camel@faith.austin.ibm.com>
References: <200710091944.l99JiedH009971@faith.austin.ibm.com>
	 <20071010151743.GA8337@gondor.apana.org.au>
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Cc: linux-crypto@vger.kernel.org
To: Herbert Xu <herbert@gondor.apana.org.au>
In-Reply-To: <20071010151743.GA8337@gondor.apana.org.au>
Sender: linux-crypto-owner@vger.kernel.org

On Wed, 2007-10-10 at 23:17 +0800, Herbert Xu wrote:
> On Tue, Oct 09, 2007 at 02:44:40PM -0500, Joy Latten wrote:
> > This should contain the geniv as well as all the
> > improvements discussed. All the testcases pass.
> 
> This looks pretty good!
> 
> I'm going to apply this once I fix up the geniv problems found
> by Sebastian.
> 
> BTW, could you please send me a final changeset description
> and Signed-off-by?
> 

Description:

This patch implements CTR mode for IPsec.
It is based off of RFC 3686.

Please note:
1. CTR turns a block cipher into a stream cipher.
Encryption is done in blocks, however the last block
may be a partial block.

A "counter block" is encrypted, creating a keystream
that is xor'ed with the plaintext. The counter portion
of the counter block is incremented after each block
of plaintext is encrypted.
Decryption is performed in same manner.

2. The CTR counterblock is composed of,
        nonce + IV + counter

The size of the counterblock is equivalent to the
blocksize of the cipher.
        sizeof(nonce) + sizeof(IV) + sizeof(counter) = blocksize

The CTR template requires the name of the cipher
algorithm, the sizeof the nonce, and the sizeof the iv.
        ctr(cipher,sizeof_nonce,sizeof_iv)

So for example,
        ctr(aes,4,8)
specifies the counterblock will be composed of 4 bytes
from a nonce, 8 bytes from the iv, and 4 bytes for counter
since aes has a blocksize of 16 bytes.

3. The counter portion of the counter block is stored
in big endian for conformance to rfc 3686.

Regards,
Joy

Signed-off-by: Joy Latten <latten@austin.ibm.com>