2012-11-09 03:52:11

by Jorgen Lundman

[permalink] [raw]
Subject: CCM-AES Compatibility

Hello list,

I hope it is ok to ask a question on how to use the crypto API in the Linux
kernel, even though the list appears to be mostly PATCH mails.

I am currently porting some code from Solaris kernel to Linux, which uses
"SUN_CKM_AES_CCM" mech, with ivsize of 12, and hmac computed of len 16.

Looking in tcrypt.c tester, which appears to be the only example that I can
find, I should be using AEAD. But I must admit I could not get my AEAD
example to work. My test code calls crypto_aead_encrypt() and I get -22
back (EINVAL).


But, since ivsize is 16, and there are no API calls to change it to 12,
perhaps I will not be able to use the supplied ccm-aes? Is it a lost cause?
Will I be pasting in the entire AES sources that I need into my module?

Going over the Solaris sources, they appear to call regular AES, then call
the ccm_encrypt_final() function:

* For CCM mode, aes_ccm_encrypt_final() will take care of any
* left-over unprocessed data, and compute the MAC
if (aes_ctx->ac_flags & CCM_MODE) {
ret = ccm_encrypt_final((ccm_ctx_t *)aes_ctx, ciphertext,
AES_BLOCK_LEN, aes_encrypt_block, aes_xor_block);

Which makes me think that perhaps I can do the same on Linux. Ignore AEAD
(which I can not get to work) and use blkcipher "cbc(aes)" instead. Then
port in the ccm_encrypt_final() sources over, which calls final, and
computes the hmac. Could that work? Any examples?

I would very much appreciate some hints here,


ps. Some peculiarities that I came across are:

1) aead_request_set_assoc() takes a scatterlist *, and a length. And
scatterlist is a pointer, and length. Seems redundant, do I set both, or is
one ignored?

2) crypto_blkcipher_set_iv() takes a pointer and length, which implies you
can set the "ivsize", but length is only used in the iv copy. "->ivsize" is
not modified, and remains the original value. Is it useful to be able to
only partially set the iv?

Jorgen Lundman | <[email protected]>
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