// SPDX-License-Identifier: GPL-2.0
  
  /*
   * OFB: Output FeedBack mode
   *
   * Copyright (C) 2018 ARM Limited or its affiliates.
   * All rights reserved.

   */
  
  #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>

  static int crypto_ofb_crypt(struct skcipher_request *req)

  {

  	struct crypto_skcipher *tfm = crypto_skcipher_reqtfm(req);

  	struct crypto_cipher *cipher = skcipher_cipher_simple(tfm);

  	const unsigned int bsize = crypto_cipher_blocksize(cipher);
  	struct skcipher_walk walk;
  	int err;

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

  	while (walk.nbytes >= bsize) {
  		const u8 *src = walk.src.virt.addr;
  		u8 *dst = walk.dst.virt.addr;
  		u8 * const iv = walk.iv;
  		unsigned int nbytes = walk.nbytes;

  		do {
  			crypto_cipher_encrypt_one(cipher, iv, iv);
  			crypto_xor_cpy(dst, src, iv, bsize);
  			dst += bsize;
  			src += bsize;
  		} while ((nbytes -= bsize) >= bsize);

  		err = skcipher_walk_done(&walk, nbytes);
  	}

  	if (walk.nbytes) {
  		crypto_cipher_encrypt_one(cipher, walk.iv, walk.iv);
  		crypto_xor_cpy(walk.dst.virt.addr, walk.src.virt.addr, walk.iv,
  			       walk.nbytes);
  		err = skcipher_walk_done(&walk, 0);
  	}
  	return err;

  }

  static int crypto_ofb_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);

  	/* OFB 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_ofb_crypt;
  	inst->alg.decrypt = crypto_ofb_crypt;

  	err = skcipher_register_instance(tmpl, inst);
  	if (err)

  		inst->free(inst);

  	return err;

  }
  
  static struct crypto_template crypto_ofb_tmpl = {
  	.name = "ofb",
  	.create = crypto_ofb_create,
  	.module = THIS_MODULE,
  };
  
  static int __init crypto_ofb_module_init(void)
  {
  	return crypto_register_template(&crypto_ofb_tmpl);
  }
  
  static void __exit crypto_ofb_module_exit(void)
  {
  	crypto_unregister_template(&crypto_ofb_tmpl);
  }

  subsys_initcall(crypto_ofb_module_init);

  module_exit(crypto_ofb_module_exit);
  
  MODULE_LICENSE("GPL");

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

  MODULE_ALIAS_CRYPTO("ofb");