This patch adds countersize to CTR mode.
The template is now ctr(algo,noncesize,ivsize,countersize).
For example, ctr(aes,4,8,4) indicates the counterblock
will be composed of a salt/nonce that is 4 bytes, an iv
that is 8 bytes and the counter is 4 bytes.
When noncesize + ivsize < blocksize, CTR initializes the
last block - ivsize - noncesize portion of the block to
zero. Otherwise the counter block is composed of the IV
(and nonce if necessary).
If noncesize + ivsize == blocksize, then this indicates that
user is passing in entire counterblock. Thus countersize
indicates the amount of bytes in counterblock to use as
the counter for incrementing. CTR will increment counter
portion by 1, and begin encryption with that value.
Note that CTR assumes the counter portion of the block that
will be incremented is stored in big endian.
Signed-off-by: Joy Latten <latten@austin.ibm.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
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.
Signed-off-by: Joy Latten <latten@austin.ibm.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Joy Latten