//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/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, &alg);
  	if (IS_ERR(inst))
  		return PTR_ERR(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);

  	crypto_mod_put(alg);

  	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");