[CRYPTO] ctr: Add CTR (Counter) block cipher mode

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>

1 files changed, 9 insertions, 0 deletions

diff --git a/crypto/Kconfig b/crypto/Kconfig
index 083d2e1..1f32071 100644
--- a/crypto/Kconfig
+++ b/crypto/Kconfig
@@ -195,6 +195,15 @@ config CRYPTO_XTS
 	  key size 256, 384 or 512 bits. This implementation currently
 	  can't handle a sectorsize which is not a multiple of 16 bytes.
 
+config CRYPTO_CTR
+	tristate "CTR support"
+	select CRYPTO_BLKCIPHER
+	select CRYPTO_MANAGER
+	default m
+	help
+	  CTR: Counter mode
+	  This block cipher algorithm is required for IPSec.
+
 config CRYPTO_CRYPTD
 	tristate "Software async crypto daemon"
 	select CRYPTO_ABLKCIPHER