cts.c

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/*
 * CTS: Cipher Text Stealing mode
 *
 * COPYRIGHT (c) 2008
 * The Regents of the University of Michigan
 * ALL RIGHTS RESERVED
 *
 * Permission is granted to use, copy, create derivative works
 * and redistribute this software and such derivative works
 * for any purpose, so long as the name of The University of
 * Michigan is not used in any advertising or publicity
 * pertaining to the use of distribution of this software
 * without specific, written prior authorization.  If the
 * above copyright notice or any other identification of the
 * University of Michigan is included in any copy of any
 * portion of this software, then the disclaimer below must
 * also be included.
 *
 * THIS SOFTWARE IS PROVIDED AS IS, WITHOUT REPRESENTATION
 * FROM THE UNIVERSITY OF MICHIGAN AS TO ITS FITNESS FOR ANY
 * PURPOSE, AND WITHOUT WARRANTY BY THE UNIVERSITY OF
 * MICHIGAN OF ANY KIND, EITHER EXPRESS OR IMPLIED, INCLUDING
 * WITHOUT LIMITATION THE IMPLIED WARRANTIES OF
 * MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. THE
 * REGENTS OF THE UNIVERSITY OF MICHIGAN SHALL NOT BE LIABLE
 * FOR ANY DAMAGES, INCLUDING SPECIAL, INDIRECT, INCIDENTAL, OR
 * CONSEQUENTIAL DAMAGES, WITH RESPECT TO ANY CLAIM ARISING
 * OUT OF OR IN CONNECTION WITH THE USE OF THE SOFTWARE, EVEN
 * IF IT HAS BEEN OR IS HEREAFTER ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGES.
 */

/* Derived from various:
 *  Copyright (c) 2006 Herbert Xu <[email protected]>
 */

/*
 * This is the Cipher Text Stealing mode as described by
 * Section 8 of rfc2040 and referenced by rfc3962.
 * rfc3962 includes errata information in its Appendix A.
 */

#include <crypto/algapi.h>
#include <linux/err.h>
#include <linux/init.h>
#include <linux/kernel.h>
#include <linux/log2.h>
#include <linux/module.h>
#include <linux/scatterlist.h>
#include <crypto/scatterwalk.h>
#include <linux/slab.h>

struct crypto_cts_ctx {
    struct crypto_blkcipher *child;
};

static int crypto_cts_setkey(struct crypto_tfm *parent, const u8 *key,
                 unsigned int keylen)
{
    struct crypto_cts_ctx *ctx = crypto_tfm_ctx(parent);
    struct crypto_blkcipher *child = ctx->child;
    int err;

    crypto_blkcipher_clear_flags(child, CRYPTO_TFM_REQ_MASK);
    crypto_blkcipher_set_flags(child, crypto_tfm_get_flags(parent) &
                       CRYPTO_TFM_REQ_MASK);
    err = crypto_blkcipher_setkey(child, key, keylen);
    crypto_tfm_set_flags(parent, crypto_blkcipher_get_flags(child) &
                     CRYPTO_TFM_RES_MASK);
    return err;
}

static int cts_cbc_encrypt(struct crypto_cts_ctx *ctx,
               struct blkcipher_desc *desc,
               struct scatterlist *dst,
               struct scatterlist *src,
               unsigned int offset,
               unsigned int nbytes)
{
    int bsize = crypto_blkcipher_blocksize(desc->tfm);
    u8 tmp[bsize], tmp2[bsize];
    struct blkcipher_desc lcldesc;
    struct scatterlist sgsrc[1], sgdst[1];
    int lastn = nbytes - bsize;
    u8 iv[bsize];
    u8 s[bsize * 2], d[bsize * 2];
    int err;

    if (lastn < 0)
        return -EINVAL;

    sg_init_table(sgsrc, 1);
    sg_init_table(sgdst, 1);

    memset(s, 0, sizeof(s));
    scatterwalk_map_and_copy(s, src, offset, nbytes, 0);

    memcpy(iv, desc->info, bsize);

    lcldesc.tfm = ctx->child;
    lcldesc.info = iv;
    lcldesc.flags = desc->flags;

    sg_set_buf(&sgsrc[0], s, bsize);
    sg_set_buf(&sgdst[0], tmp, bsize);
    err = crypto_blkcipher_encrypt_iv(&lcldesc, sgdst, sgsrc, bsize);

    memcpy(d + bsize, tmp, lastn);

    lcldesc.info = tmp;

    sg_set_buf(&sgsrc[0], s + bsize, bsize);
    sg_set_buf(&sgdst[0], tmp2, bsize);
    err = crypto_blkcipher_encrypt_iv(&lcldesc, sgdst, sgsrc, bsize);

    memcpy(d, tmp2, bsize);

    scatterwalk_map_and_copy(d, dst, offset, nbytes, 1);

    memcpy(desc->info, tmp2, bsize);

    return err;
}

static int crypto_cts_encrypt(struct blkcipher_desc *desc,
                  struct scatterlist *dst, struct scatterlist *src,
                  unsigned int nbytes)
{
    struct crypto_cts_ctx *ctx = crypto_blkcipher_ctx(desc->tfm);
    int bsize = crypto_blkcipher_blocksize(desc->tfm);
    int tot_blocks = (nbytes + bsize - 1) / bsize;
    int cbc_blocks = tot_blocks > 2 ? tot_blocks - 2 : 0;
    struct blkcipher_desc lcldesc;
    int err;

    lcldesc.tfm = ctx->child;
    lcldesc.info = desc->info;
    lcldesc.flags = desc->flags;

    if (tot_blocks == 1) {
        err = crypto_blkcipher_encrypt_iv(&lcldesc, dst, src, bsize);
    } else if (nbytes <= bsize * 2) {
        err = cts_cbc_encrypt(ctx, desc, dst, src, 0, nbytes);
    } else {
        /* do normal function for tot_blocks - 2 */
        err = crypto_blkcipher_encrypt_iv(&lcldesc, dst, src,
                            cbc_blocks * bsize);
        if (err == 0) {
            /* do cts for final two blocks */
            err = cts_cbc_encrypt(ctx, desc, dst, src,
                        cbc_blocks * bsize,
                        nbytes - (cbc_blocks * bsize));
        }
    }

    return err;
}

static int cts_cbc_decrypt(struct crypto_cts_ctx *ctx,
               struct blkcipher_desc *desc,
               struct scatterlist *dst,
               struct scatterlist *src,
               unsigned int offset,
               unsigned int nbytes)
{
    int bsize = crypto_blkcipher_blocksize(desc->tfm);
    u8 tmp[bsize];
    struct blkcipher_desc lcldesc;
    struct scatterlist sgsrc[1], sgdst[1];
    int lastn = nbytes - bsize;
    u8 iv[bsize];
    u8 s[bsize * 2], d[bsize * 2];
    int err;

    if (lastn < 0)
        return -EINVAL;

    sg_init_table(sgsrc, 1);
    sg_init_table(sgdst, 1);

    scatterwalk_map_and_copy(s, src, offset, nbytes, 0);

    lcldesc.tfm = ctx->child;
    lcldesc.info = iv;
    lcldesc.flags = desc->flags;

    /* 1. Decrypt Cn-1 (s) to create Dn (tmp)*/
    memset(iv, 0, sizeof(iv));
    sg_set_buf(&sgsrc[0], s, bsize);
    sg_set_buf(&sgdst[0], tmp, bsize);
    err = crypto_blkcipher_decrypt_iv(&lcldesc, sgdst, sgsrc, bsize);
    if (err)
        return err;
    /* 2. Pad Cn with zeros at the end to create C of length BB */
    memset(iv, 0, sizeof(iv));
    memcpy(iv, s + bsize, lastn);
    /* 3. Exclusive-or Dn (tmp) with C (iv) to create Xn (tmp) */
    crypto_xor(tmp, iv, bsize);
    /* 4. Select the first Ln bytes of Xn (tmp) to create Pn */
    memcpy(d + bsize, tmp, lastn);

    /* 5. Append the tail (BB - Ln) bytes of Xn (tmp) to Cn to create En */
    memcpy(s + bsize + lastn, tmp + lastn, bsize - lastn);
    /* 6. Decrypt En to create Pn-1 */
    memset(iv, 0, sizeof(iv));
    sg_set_buf(&sgsrc[0], s + bsize, bsize);
    sg_set_buf(&sgdst[0], d, bsize);
    err = crypto_blkcipher_decrypt_iv(&lcldesc, sgdst, sgsrc, bsize);

    /* XOR with previous block */
    crypto_xor(d, desc->info, bsize);

    scatterwalk_map_and_copy(d, dst, offset, nbytes, 1);

    memcpy(desc->info, s, bsize);
    return err;
}

static int crypto_cts_decrypt(struct blkcipher_desc *desc,
                  struct scatterlist *dst, struct scatterlist *src,
                  unsigned int nbytes)
{
    struct crypto_cts_ctx *ctx = crypto_blkcipher_ctx(desc->tfm);
    int bsize = crypto_blkcipher_blocksize(desc->tfm);
    int tot_blocks = (nbytes + bsize - 1) / bsize;
    int cbc_blocks = tot_blocks > 2 ? tot_blocks - 2 : 0;
    struct blkcipher_desc lcldesc;
    int err;

    lcldesc.tfm = ctx->child;
    lcldesc.info = desc->info;
    lcldesc.flags = desc->flags;

    if (tot_blocks == 1) {
        err = crypto_blkcipher_decrypt_iv(&lcldesc, dst, src, bsize);
    } else if (nbytes <= bsize * 2) {
        err = cts_cbc_decrypt(ctx, desc, dst, src, 0, nbytes);
    } else {
        /* do normal function for tot_blocks - 2 */
        err = crypto_blkcipher_decrypt_iv(&lcldesc, dst, src,
                            cbc_blocks * bsize);
        if (err == 0) {
            /* do cts for final two blocks */
            err = cts_cbc_decrypt(ctx, desc, dst, src,
                        cbc_blocks * bsize,
                        nbytes - (cbc_blocks * bsize));
        }
    }
    return err;
}

static int crypto_cts_init_tfm(struct crypto_tfm *tfm)
{
    struct crypto_instance *inst = (void *)tfm->__crt_alg;
    struct crypto_spawn *spawn = crypto_instance_ctx(inst);
    struct crypto_cts_ctx *ctx = crypto_tfm_ctx(tfm);
    struct crypto_blkcipher *cipher;

    cipher = crypto_spawn_blkcipher(spawn);
    if (IS_ERR(cipher))
        return PTR_ERR(cipher);

    ctx->child = cipher;
    return 0;
}

static void crypto_cts_exit_tfm(struct crypto_tfm *tfm)
{
    struct crypto_cts_ctx *ctx = crypto_tfm_ctx(tfm);
    crypto_free_blkcipher(ctx->child);
}

static struct crypto_instance *crypto_cts_alloc(struct rtattr **tb)
{
    struct crypto_instance *inst;
    struct crypto_alg *alg;
    int err;

    err = crypto_check_attr_type(tb, CRYPTO_ALG_TYPE_BLKCIPHER);
    if (err)
        return ERR_PTR(err);

    alg = crypto_attr_alg(tb[1], CRYPTO_ALG_TYPE_BLKCIPHER,
                  CRYPTO_ALG_TYPE_MASK);
    err = PTR_ERR(alg);
    if (IS_ERR(alg))
        return ERR_PTR(err);

    inst = ERR_PTR(-EINVAL);
    if (!is_power_of_2(alg->cra_blocksize))
        goto out_put_alg;

    inst = crypto_alloc_instance("cts", alg);
    if (IS_ERR(inst))
        goto out_put_alg;

    inst->alg.cra_flags = CRYPTO_ALG_TYPE_BLKCIPHER;
    inst->alg.cra_priority = alg->cra_priority;
    inst->alg.cra_blocksize = alg->cra_blocksize;
    inst->alg.cra_alignmask = alg->cra_alignmask;
    inst->alg.cra_type = &crypto_blkcipher_type;

    /* We access the data as u32s when xoring. */
    inst->alg.cra_alignmask |= __alignof__(u32) - 1;

    inst->alg.cra_blkcipher.ivsize = alg->cra_blocksize;
    inst->alg.cra_blkcipher.min_keysize = alg->cra_blkcipher.min_keysize;
    inst->alg.cra_blkcipher.max_keysize = alg->cra_blkcipher.max_keysize;

    inst->alg.cra_blkcipher.geniv = "seqiv";

    inst->alg.cra_ctxsize = sizeof(struct crypto_cts_ctx);

    inst->alg.cra_init = crypto_cts_init_tfm;
    inst->alg.cra_exit = crypto_cts_exit_tfm;

    inst->alg.cra_blkcipher.setkey = crypto_cts_setkey;
    inst->alg.cra_blkcipher.encrypt = crypto_cts_encrypt;
    inst->alg.cra_blkcipher.decrypt = crypto_cts_decrypt;

out_put_alg:
    crypto_mod_put(alg);
    return inst;
}

static void crypto_cts_free(struct crypto_instance *inst)
{
    crypto_drop_spawn(crypto_instance_ctx(inst));
    kfree(inst);
}

static struct crypto_template crypto_cts_tmpl = {
    .name = "cts",
    .alloc = crypto_cts_alloc,
    .free = crypto_cts_free,
    .module = THIS_MODULE,
};

static int __init crypto_cts_module_init(void)
{
    return crypto_register_template(&crypto_cts_tmpl);
}

static void __exit crypto_cts_module_exit(void)
{
    crypto_unregister_template(&crypto_cts_tmpl);
}

module_init(crypto_cts_module_init);
module_exit(crypto_cts_module_exit);

MODULE_LICENSE("Dual BSD/GPL");
MODULE_DESCRIPTION("CTS-CBC CipherText Stealing for CBC");