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![Nathan Huckleberry](/assets/img/avatar_default.png)
Add a generic implementation of XCTR mode as a template. XCTR is a blockcipher mode similar to CTR mode. XCTR uses XORs and little-endian addition rather than big-endian arithmetic which has two advantages: It is slightly faster on little-endian CPUs and it is less likely to be implemented incorrect since integer overflows are not possible on practical input sizes. XCTR is used as a component to implement HCTR2. More information on XCTR mode can be found in the HCTR2 paper: https://eprint.iacr.org/2021/1441.pdf Signed-off-by: Nathan Huckleberry <nhuck@google.com> Reviewed-by: Eric Biggers <ebiggers@google.com> Reviewed-by: Ard Biesheuvel <ardb@kernel.org> Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
191 lines
5.3 KiB
C
191 lines
5.3 KiB
C
// SPDX-License-Identifier: GPL-2.0-or-later
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/*
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* XCTR: XOR Counter mode - Adapted from ctr.c
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*
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* (C) Copyright IBM Corp. 2007 - Joy Latten <latten@us.ibm.com>
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* Copyright 2021 Google LLC
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*/
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/*
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* XCTR mode is a blockcipher mode of operation used to implement HCTR2. XCTR is
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* closely related to the CTR mode of operation; the main difference is that CTR
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* generates the keystream using E(CTR + IV) whereas XCTR generates the
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* keystream using E(CTR ^ IV). This allows implementations to avoid dealing
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* with multi-limb integers (as is required in CTR mode). XCTR is also specified
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* using little-endian arithmetic which makes it slightly faster on LE machines.
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*
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* See the HCTR2 paper for more details:
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* Length-preserving encryption with HCTR2
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* (https://eprint.iacr.org/2021/1441.pdf)
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*/
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#include <crypto/algapi.h>
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#include <crypto/internal/cipher.h>
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#include <crypto/internal/skcipher.h>
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#include <linux/err.h>
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#include <linux/init.h>
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#include <linux/kernel.h>
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#include <linux/module.h>
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#include <linux/slab.h>
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/* For now this implementation is limited to 16-byte blocks for simplicity */
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#define XCTR_BLOCKSIZE 16
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static void crypto_xctr_crypt_final(struct skcipher_walk *walk,
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struct crypto_cipher *tfm, u32 byte_ctr)
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{
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u8 keystream[XCTR_BLOCKSIZE];
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const u8 *src = walk->src.virt.addr;
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u8 *dst = walk->dst.virt.addr;
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unsigned int nbytes = walk->nbytes;
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__le32 ctr32 = cpu_to_le32(byte_ctr / XCTR_BLOCKSIZE + 1);
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crypto_xor(walk->iv, (u8 *)&ctr32, sizeof(ctr32));
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crypto_cipher_encrypt_one(tfm, keystream, walk->iv);
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crypto_xor_cpy(dst, keystream, src, nbytes);
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crypto_xor(walk->iv, (u8 *)&ctr32, sizeof(ctr32));
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}
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static int crypto_xctr_crypt_segment(struct skcipher_walk *walk,
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struct crypto_cipher *tfm, u32 byte_ctr)
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{
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void (*fn)(struct crypto_tfm *, u8 *, const u8 *) =
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crypto_cipher_alg(tfm)->cia_encrypt;
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const u8 *src = walk->src.virt.addr;
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u8 *dst = walk->dst.virt.addr;
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unsigned int nbytes = walk->nbytes;
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__le32 ctr32 = cpu_to_le32(byte_ctr / XCTR_BLOCKSIZE + 1);
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do {
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crypto_xor(walk->iv, (u8 *)&ctr32, sizeof(ctr32));
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fn(crypto_cipher_tfm(tfm), dst, walk->iv);
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crypto_xor(dst, src, XCTR_BLOCKSIZE);
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crypto_xor(walk->iv, (u8 *)&ctr32, sizeof(ctr32));
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le32_add_cpu(&ctr32, 1);
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src += XCTR_BLOCKSIZE;
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dst += XCTR_BLOCKSIZE;
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} while ((nbytes -= XCTR_BLOCKSIZE) >= XCTR_BLOCKSIZE);
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return nbytes;
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}
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static int crypto_xctr_crypt_inplace(struct skcipher_walk *walk,
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struct crypto_cipher *tfm, u32 byte_ctr)
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{
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void (*fn)(struct crypto_tfm *, u8 *, const u8 *) =
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crypto_cipher_alg(tfm)->cia_encrypt;
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unsigned long alignmask = crypto_cipher_alignmask(tfm);
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unsigned int nbytes = walk->nbytes;
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u8 *data = walk->src.virt.addr;
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u8 tmp[XCTR_BLOCKSIZE + MAX_CIPHER_ALIGNMASK];
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u8 *keystream = PTR_ALIGN(tmp + 0, alignmask + 1);
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__le32 ctr32 = cpu_to_le32(byte_ctr / XCTR_BLOCKSIZE + 1);
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do {
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crypto_xor(walk->iv, (u8 *)&ctr32, sizeof(ctr32));
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fn(crypto_cipher_tfm(tfm), keystream, walk->iv);
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crypto_xor(data, keystream, XCTR_BLOCKSIZE);
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crypto_xor(walk->iv, (u8 *)&ctr32, sizeof(ctr32));
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le32_add_cpu(&ctr32, 1);
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data += XCTR_BLOCKSIZE;
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} while ((nbytes -= XCTR_BLOCKSIZE) >= XCTR_BLOCKSIZE);
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return nbytes;
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}
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static int crypto_xctr_crypt(struct skcipher_request *req)
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{
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struct crypto_skcipher *tfm = crypto_skcipher_reqtfm(req);
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struct crypto_cipher *cipher = skcipher_cipher_simple(tfm);
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struct skcipher_walk walk;
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unsigned int nbytes;
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int err;
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u32 byte_ctr = 0;
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err = skcipher_walk_virt(&walk, req, false);
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while (walk.nbytes >= XCTR_BLOCKSIZE) {
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if (walk.src.virt.addr == walk.dst.virt.addr)
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nbytes = crypto_xctr_crypt_inplace(&walk, cipher,
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byte_ctr);
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else
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nbytes = crypto_xctr_crypt_segment(&walk, cipher,
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byte_ctr);
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byte_ctr += walk.nbytes - nbytes;
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err = skcipher_walk_done(&walk, nbytes);
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}
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if (walk.nbytes) {
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crypto_xctr_crypt_final(&walk, cipher, byte_ctr);
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err = skcipher_walk_done(&walk, 0);
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}
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return err;
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}
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static int crypto_xctr_create(struct crypto_template *tmpl, struct rtattr **tb)
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{
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struct skcipher_instance *inst;
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struct crypto_alg *alg;
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int err;
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inst = skcipher_alloc_instance_simple(tmpl, tb);
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if (IS_ERR(inst))
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return PTR_ERR(inst);
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alg = skcipher_ialg_simple(inst);
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/* Block size must be 16 bytes. */
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err = -EINVAL;
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if (alg->cra_blocksize != XCTR_BLOCKSIZE)
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goto out_free_inst;
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/* XCTR mode is a stream cipher. */
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inst->alg.base.cra_blocksize = 1;
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/*
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* To simplify the implementation, configure the skcipher walk to only
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* give a partial block at the very end, never earlier.
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*/
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inst->alg.chunksize = alg->cra_blocksize;
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inst->alg.encrypt = crypto_xctr_crypt;
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inst->alg.decrypt = crypto_xctr_crypt;
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err = skcipher_register_instance(tmpl, inst);
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if (err) {
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out_free_inst:
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inst->free(inst);
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}
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return err;
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}
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static struct crypto_template crypto_xctr_tmpl = {
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.name = "xctr",
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.create = crypto_xctr_create,
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.module = THIS_MODULE,
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};
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static int __init crypto_xctr_module_init(void)
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{
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return crypto_register_template(&crypto_xctr_tmpl);
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}
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static void __exit crypto_xctr_module_exit(void)
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{
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crypto_unregister_template(&crypto_xctr_tmpl);
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}
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subsys_initcall(crypto_xctr_module_init);
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module_exit(crypto_xctr_module_exit);
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MODULE_LICENSE("GPL");
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MODULE_DESCRIPTION("XCTR block cipher mode of operation");
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MODULE_ALIAS_CRYPTO("xctr");
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MODULE_IMPORT_NS(CRYPTO_INTERNAL);
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