//SPDX-License-Identifier: GPL-2.0

/*

* CFB: Cipher FeedBack mode

*

* Copyright (c) 2018 James.Bottomley@HansenPartnership.com

*

* CFB is a stream cipher mode which is layered on to a block

* encryption scheme. It works very much like a one time pad where

* the pad is generated initially from the encrypted IV and then

* subsequently from the encrypted previous block of ciphertext. The

* pad is XOR'd into the plain text to get the final ciphertext.

*

* The scheme of CFB is best described by wikipedia:

*

* https://en.wikipedia.org/wiki/Block_cipher_mode_of_operation#CFB

*

* Note that since the pad for both encryption and decryption is

* generated by an encryption operation, CFB never uses the block

* decryption function.

*/

#include <crypto/algapi.h>

#include <crypto/internal/cipher.h>

#include <crypto/internal/skcipher.h>

#include <linux/err.h>

#include <linux/init.h>

#include <linux/kernel.h>

#include <linux/module.h>

#include <linux/string.h>

static unsigned int crypto_cfb_bsize(struct crypto_skcipher *tfm)

{

return crypto_cipher_blocksize(skcipher_cipher_simple(tfm));

}

static void crypto_cfb_encrypt_one(struct crypto_skcipher *tfm,

const u8 *src, u8 *dst)

{

crypto_cipher_encrypt_one(skcipher_cipher_simple(tfm), dst, src);

}

/* final encrypt and decrypt is the same */

static void crypto_cfb_final(struct skcipher_walk *walk,

struct crypto_skcipher *tfm)

{

const unsigned long alignmask = crypto_skcipher_alignmask(tfm);

u8 tmp[MAX_CIPHER_BLOCKSIZE + MAX_CIPHER_ALIGNMASK];

u8 *stream = PTR_ALIGN(tmp + 0, alignmask + 1);

u8 *src = walk->src.virt.addr;

u8 *dst = walk->dst.virt.addr;

u8 *iv = walk->iv;

unsigned int nbytes = walk->nbytes;

crypto_cfb_encrypt_one(tfm, iv, stream);

crypto_xor_cpy(dst, stream, src, nbytes);

}

static int crypto_cfb_encrypt_segment(struct skcipher_walk *walk,

struct crypto_skcipher *tfm)

{

const unsigned int bsize = crypto_cfb_bsize(tfm);

unsigned int nbytes = walk->nbytes;

u8 *src = walk->src.virt.addr;

u8 *dst = walk->dst.virt.addr;

u8 *iv = walk->iv;

do {

crypto_cfb_encrypt_one(tfm, iv, dst);

crypto_xor(dst, src, bsize);

iv = dst;

src += bsize;

dst += bsize;

} while ((nbytes -= bsize) >= bsize);

memcpy(walk->iv, iv, bsize);

return nbytes;

}

static int crypto_cfb_encrypt_inplace(struct skcipher_walk *walk,

struct crypto_skcipher *tfm)

{

const unsigned int bsize = crypto_cfb_bsize(tfm);

unsigned int nbytes = walk->nbytes;

u8 *src = walk->src.virt.addr;

u8 *iv = walk->iv;

u8 tmp[MAX_CIPHER_BLOCKSIZE];

do {

crypto_cfb_encrypt_one(tfm, iv, tmp);

crypto_xor(src, tmp, bsize);

iv = src;

src += bsize;

} while ((nbytes -= bsize) >= bsize);

memcpy(walk->iv, iv, bsize);

return nbytes;

}

static int crypto_cfb_encrypt(struct skcipher_request *req)

{

struct crypto_skcipher *tfm = crypto_skcipher_reqtfm(req);

struct skcipher_walk walk;

unsigned int bsize = crypto_cfb_bsize(tfm);

int err;

err = skcipher_walk_virt(&walk, req, false);

while (walk.nbytes >= bsize) {

if (walk.src.virt.addr == walk.dst.virt.addr)

err = crypto_cfb_encrypt_inplace(&walk, tfm);

else

err = crypto_cfb_encrypt_segment(&walk, tfm);

err = skcipher_walk_done(&walk, err);

}

if (walk.nbytes) {

crypto_cfb_final(&walk, tfm);

err = skcipher_walk_done(&walk, 0);

}

return err;

}

static int crypto_cfb_decrypt_segment(struct skcipher_walk *walk,

struct crypto_skcipher *tfm)

{

const unsigned int bsize = crypto_cfb_bsize(tfm);

unsigned int nbytes = walk->nbytes;

u8 *src = walk->src.virt.addr;

u8 *dst = walk->dst.virt.addr;

u8 *iv = walk->iv;

do {

crypto_cfb_encrypt_one(tfm, iv, dst);

crypto_xor(dst, src, bsize);

iv = src;

src += bsize;

dst += bsize;

} while ((nbytes -= bsize) >= bsize);

memcpy(walk->iv, iv, bsize);

return nbytes;

}

static int crypto_cfb_decrypt_inplace(struct skcipher_walk *walk,

struct crypto_skcipher *tfm)

{

const unsigned int bsize = crypto_cfb_bsize(tfm);

unsigned int nbytes = walk->nbytes;

u8 *src = walk->src.virt.addr;

u8 * const iv = walk->iv;

u8 tmp[MAX_CIPHER_BLOCKSIZE];

do {

crypto_cfb_encrypt_one(tfm, iv, tmp);

memcpy(iv, src, bsize);

crypto_xor(src, tmp, bsize);

src += bsize;

} while ((nbytes -= bsize) >= bsize);

return nbytes;

}

static int crypto_cfb_decrypt_blocks(struct skcipher_walk *walk,

struct crypto_skcipher *tfm)

{

if (walk->src.virt.addr == walk->dst.virt.addr)

return crypto_cfb_decrypt_inplace(walk, tfm);

else

return crypto_cfb_decrypt_segment(walk, tfm);

}

static int crypto_cfb_decrypt(struct skcipher_request *req)

{

struct crypto_skcipher *tfm = crypto_skcipher_reqtfm(req);

struct skcipher_walk walk;

const unsigned int bsize = crypto_cfb_bsize(tfm);

int err;

err = skcipher_walk_virt(&walk, req, false);

while (walk.nbytes >= bsize) {

err = crypto_cfb_decrypt_blocks(&walk, tfm);

err = skcipher_walk_done(&walk, err);

}

if (walk.nbytes) {

crypto_cfb_final(&walk, tfm);

err = skcipher_walk_done(&walk, 0);

}

return err;

}

static int crypto_cfb_create(struct crypto_template *tmpl, struct rtattr **tb)

{

struct skcipher_instance *inst;

struct crypto_alg *alg;

int err;

inst = skcipher_alloc_instance_simple(tmpl, tb);

if (IS_ERR(inst))

return PTR_ERR(inst);

alg = skcipher_ialg_simple(inst);

/* CFB mode is a stream cipher. */

inst->alg.base.cra_blocksize = 1;

/*

* To simplify the implementation, configure the skcipher walk to only

* give a partial block at the very end, never earlier.

*/

inst->alg.chunksize = alg->cra_blocksize;

inst->alg.encrypt = crypto_cfb_encrypt;

inst->alg.decrypt = crypto_cfb_decrypt;

err = skcipher_register_instance(tmpl, inst);

if (err)

inst->free(inst);

return err;

}

static struct crypto_template crypto_cfb_tmpl = {

.name = "cfb",

.create = crypto_cfb_create,

.module = THIS_MODULE,

};

static int __init crypto_cfb_module_init(void)

{

return crypto_register_template(&crypto_cfb_tmpl);

}

static void __exit crypto_cfb_module_exit(void)

{

crypto_unregister_template(&crypto_cfb_tmpl);

}

subsys_initcall(crypto_cfb_module_init);

module_exit(crypto_cfb_module_exit);

MODULE_LICENSE("GPL");

MODULE_DESCRIPTION("CFB block cipher mode of operation");

MODULE_ALIAS_CRYPTO("cfb");

MODULE_IMPORT_NS(CRYPTO_INTERNAL);