// SPDX-License-Identifier: GPL-2.0-or-later
/*
 * XCTR: XOR Counter mode - Adapted from ctr.c
 *
 * (C) Copyright IBM Corp. 2007 - Joy Latten <latten@us.ibm.com>
 * Copyright 2021 Google LLC
 */

/*
 * XCTR mode is a blockcipher mode of operation used to implement HCTR2. XCTR is
 * closely related to the CTR mode of operation; the main difference is that CTR
 * generates the keystream using E(CTR + IV) whereas XCTR generates the
 * keystream using E(CTR ^ IV). This allows implementations to avoid dealing
 * with multi-limb integers (as is required in CTR mode). XCTR is also specified
 * using little-endian arithmetic which makes it slightly faster on LE machines.
 *
 * See the HCTR2 paper for more details:
 *      Length-preserving encryption with HCTR2
 *      (https://eprint.iacr.org/2021/1441.pdf)
 */

#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/slab.h>

/* For now this implementation is limited to 16-byte blocks for simplicity */
#define XCTR_BLOCKSIZE 16

static void crypto_xctr_crypt_final(struct skcipher_walk *walk,
                                   struct crypto_cipher *tfm, u32 byte_ctr)
{
        u8 keystream[XCTR_BLOCKSIZE];
        const u8 *src = walk->src.virt.addr;
        u8 *dst = walk->dst.virt.addr;
        unsigned int nbytes = walk->nbytes;
        __le32 ctr32 = cpu_to_le32(byte_ctr / XCTR_BLOCKSIZE + 1);

        crypto_xor(walk->iv, (u8 *)&ctr32, sizeof(ctr32));
        crypto_cipher_encrypt_one(tfm, keystream, walk->iv);
        crypto_xor_cpy(dst, keystream, src, nbytes);
        crypto_xor(walk->iv, (u8 *)&ctr32, sizeof(ctr32));
}

static int crypto_xctr_crypt_segment(struct skcipher_walk *walk,
                                    struct crypto_cipher *tfm, u32 byte_ctr)
{
        void (*fn)(struct crypto_tfm *, u8 *, const u8 *) =
                   crypto_cipher_alg(tfm)->cia_encrypt;
        const u8 *src = walk->src.virt.addr;
        u8 *dst = walk->dst.virt.addr;
        unsigned int nbytes = walk->nbytes;
        __le32 ctr32 = cpu_to_le32(byte_ctr / XCTR_BLOCKSIZE + 1);

        do {
                crypto_xor(walk->iv, (u8 *)&ctr32, sizeof(ctr32));
                fn(crypto_cipher_tfm(tfm), dst, walk->iv);
                crypto_xor(dst, src, XCTR_BLOCKSIZE);
                crypto_xor(walk->iv, (u8 *)&ctr32, sizeof(ctr32));

                le32_add_cpu(&ctr32, 1);

                src += XCTR_BLOCKSIZE;
                dst += XCTR_BLOCKSIZE;
        } while ((nbytes -= XCTR_BLOCKSIZE) >= XCTR_BLOCKSIZE);

        return nbytes;
}

static int crypto_xctr_crypt_inplace(struct skcipher_walk *walk,
                                    struct crypto_cipher *tfm, u32 byte_ctr)
{
        void (*fn)(struct crypto_tfm *, u8 *, const u8 *) =
                   crypto_cipher_alg(tfm)->cia_encrypt;
        unsigned long alignmask = crypto_cipher_alignmask(tfm);
        unsigned int nbytes = walk->nbytes;
        u8 *data = walk->dst.virt.addr;
        u8 tmp[XCTR_BLOCKSIZE + MAX_CIPHER_ALIGNMASK];
        u8 *keystream = PTR_ALIGN(tmp + 0, alignmask + 1);
        __le32 ctr32 = cpu_to_le32(byte_ctr / XCTR_BLOCKSIZE + 1);

        do {
                crypto_xor(walk->iv, (u8 *)&ctr32, sizeof(ctr32));
                fn(crypto_cipher_tfm(tfm), keystream, walk->iv);
                crypto_xor(data, keystream, XCTR_BLOCKSIZE);
                crypto_xor(walk->iv, (u8 *)&ctr32, sizeof(ctr32));

                le32_add_cpu(&ctr32, 1);

                data += XCTR_BLOCKSIZE;
        } while ((nbytes -= XCTR_BLOCKSIZE) >= XCTR_BLOCKSIZE);

        return nbytes;
}

static int crypto_xctr_crypt(struct skcipher_request *req)
{
        struct crypto_skcipher *tfm = crypto_skcipher_reqtfm(req);
        struct crypto_cipher *cipher = skcipher_cipher_simple(tfm);
        struct skcipher_walk walk;
        unsigned int nbytes;
        int err;
        u32 byte_ctr = 0;

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

        while (walk.nbytes >= XCTR_BLOCKSIZE) {
                if (walk.src.virt.addr == walk.dst.virt.addr)
                        nbytes = crypto_xctr_crypt_inplace(&walk, cipher,
                                                           byte_ctr);
                else
                        nbytes = crypto_xctr_crypt_segment(&walk, cipher,
                                                           byte_ctr);

                byte_ctr += walk.nbytes - nbytes;
                err = skcipher_walk_done(&walk, nbytes);
        }

        if (walk.nbytes) {
                crypto_xctr_crypt_final(&walk, cipher, byte_ctr);
                err = skcipher_walk_done(&walk, 0);
        }

        return err;
}

static int crypto_xctr_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);

        /* Block size must be 16 bytes. */
        err = -EINVAL;
        if (alg->cra_blocksize != XCTR_BLOCKSIZE)
                goto out_free_inst;

        /* XCTR 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_xctr_crypt;
        inst->alg.decrypt = crypto_xctr_crypt;

        err = skcipher_register_instance(tmpl, inst);
        if (err) {
out_free_inst:
                inst->free(inst);
        }

        return err;
}

static struct crypto_template crypto_xctr_tmpl = {
        .name = "xctr",
        .create = crypto_xctr_create,
        .module = THIS_MODULE,
};

static int __init crypto_xctr_module_init(void)
{
        return crypto_register_template(&crypto_xctr_tmpl);
}

static void __exit crypto_xctr_module_exit(void)
{
        crypto_unregister_template(&crypto_xctr_tmpl);
}

module_init(crypto_xctr_module_init);
module_exit(crypto_xctr_module_exit);

MODULE_LICENSE("GPL");
MODULE_DESCRIPTION("XCTR block cipher mode of operation");
MODULE_ALIAS_CRYPTO("xctr");
MODULE_IMPORT_NS("CRYPTO_INTERNAL");