crypto(3) ?

peter <>
Thu Mar 4 21:30:36 CET 2004


Hi,  

It is covered, but you have to use a special case of the des_cbc_encrypt/3
function as follows (referring to the example in your letter below):

  E(Ki,Dj) = crypto:des_cbc_encrypt(Ki, 0, Dj), i=0,Kmax, j=0,..Dmax,

where Ki and Dj are 8 byte quantities (for each Ki only 7 bits are 
considered).

The des functions in `crypto' are for the Cipher Block Chaining (CBC)
mode, where results of the previous encryption/decryption step is fed into
the next step by the initializing vector, IV, but can be used for the 
basic DES mode as well:

If IV is set to zero (it is xored with the first 8 byte segment of input),
and 8 byte is provided as input, you get the basic DES
encryption/decryption of an 8 byte quantity.

This is not at all obvious, but true. FIPS 81 states the details.

/Peter


On Thu, 4 Mar 2004, Torbjörn Törnkvist wrote:

> Hi,
> 
> I've been comparing the crypto(3) man page with the CIFS  description
> on how the (SMB) session key is computed, trying to understand if they
> are the same. Perhaps someone (Peter ?) could comment on this?
> 
> The CIFS explanation:
>  ------------------------------------
> E(K, D) denote the DES block mode encryption function [FIPS 81] , which
> accepts a seven byte key (K) and an eight byte data block (D) and produces
> an eight byte encrypted data block as its value.
> 
> Ex(K,D) denote the extension of DES to longer keys and data blocks. If the
> data to be encrypted is longer than eight bytes, the encryption function is
> applied to each block of eight bytes in sequence and the results are 
> concatenated
> together. If the key is longer than seven bytes, each 8 byte block of 
> data is first
> completely encrypted using the first seven bytes of the key, then the 
> second seven
> bytes, etc., appending the results each time. For example, to encrypt 
> the 16 byte
> quantity D0D1 with the 14 byte key K0K1,
> Ex(K0K1,D0D1) = concat(E(K0,D0),E(K0,D1),E(K1,D0),E(K1,D1))
> 
> So is this algorithm covered by the crypto module?
> This just gives me headache...
> 
> Cheers, Tobbe
> 
> 
> 
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