Date: Mon, 6 May 2002 11:06:14 -0400 From: David Dagon <dagon@cc.gatech.edu> To: jack xiao <jack_xiao99@hotmail.com> Cc: freebsd-security@FreeBSD.ORG Subject: Re: AES(rijndael) Message-ID: <20020506110614.B90233@fritz.cc.gt.atl.ga.us> In-Reply-To: <OE64AAkjtjsX3Ra5cNt000073cd@hotmail.com>; from jack_xiao99@hotmail.com on Mon, May 06, 2002 at 10:26:47AM -0400 References: <OE64AAkjtjsX3Ra5cNt000073cd@hotmail.com>
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On Mon, May 06, 2002 at 10:26:47AM -0400, jack xiao wrote: > I have some questions about AES(rijndael) algorithm. AS far as I know, The > AES algorithm is capable of using cryptographic keys of 128, 192, and 256 > bits to encypt and decrypt data in blocks of 128 bits. This is correct. With 128 bit keys, there are ~3.4 x 10^38 keys, with 192 bits, there are ~6.2 x 10^57 keys, and 256 bits yields ~1.1 x 10^77. You can examine a reference implementation from the authors at: http://csrc.nist.gov/encryption/aes/rijndael/rijndael-unix-refc.tar and also: /usr/src/sys/crypto/rijndael > Is that to say, AES is capable using more kinds of keys than 128, > 192, 256 bits long? Could you please give me your thoughts? FYI, DES uses a 64 bit input as a key but only 56 bits are used for the actual key itself. (The other bits are 'parity', or were discarded in the standard to weaken DES, depending on your perspective.) Most modern systems also add salt to increase the strength of DES. Freebsd has des_cipher(3) that accepts up to 24 bits of salt for ~16M variations on simple DES. The FIPS 197 standard for AES lists only 128, 192, 256 bit key sizes. While it may be possible to write a program that takes larger keys, one would have to first investigate whether the larger keys would yield expansions/shifts that create congruences, or would then be vulnerable to weak keys, etc. -- David Dagon /"\ "When cryptography dagon@cc.gatech.edu \ / ASCII RIBBON CAMPAIGN is outlawed, bayl Georgia Inst. of Tech. X AGAINST HTML MAIL bhgynjf jvyy unir / \ cevinpl." To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-security" in the body of the message
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