xref: /openbmc/linux/crypto/cfb.c (revision f5c27da4)
1  // SPDX-License-Identifier: GPL-2.0
2  /*
3   * CFB: Cipher FeedBack mode
4   *
5   * Copyright (c) 2018 James.Bottomley@HansenPartnership.com
6   *
7   * CFB is a stream cipher mode which is layered on to a block
8   * encryption scheme.  It works very much like a one time pad where
9   * the pad is generated initially from the encrypted IV and then
10   * subsequently from the encrypted previous block of ciphertext.  The
11   * pad is XOR'd into the plain text to get the final ciphertext.
12   *
13   * The scheme of CFB is best described by wikipedia:
14   *
15   * https://en.wikipedia.org/wiki/Block_cipher_mode_of_operation#CFB
16   *
17   * Note that since the pad for both encryption and decryption is
18   * generated by an encryption operation, CFB never uses the block
19   * decryption function.
20   */
21  
22  #include <crypto/algapi.h>
23  #include <crypto/internal/cipher.h>
24  #include <crypto/internal/skcipher.h>
25  #include <linux/err.h>
26  #include <linux/init.h>
27  #include <linux/kernel.h>
28  #include <linux/module.h>
29  #include <linux/string.h>
30  
31  static unsigned int crypto_cfb_bsize(struct crypto_skcipher *tfm)
32  {
33  	return crypto_cipher_blocksize(skcipher_cipher_simple(tfm));
34  }
35  
36  static void crypto_cfb_encrypt_one(struct crypto_skcipher *tfm,
37  					  const u8 *src, u8 *dst)
38  {
39  	crypto_cipher_encrypt_one(skcipher_cipher_simple(tfm), dst, src);
40  }
41  
42  /* final encrypt and decrypt is the same */
43  static void crypto_cfb_final(struct skcipher_walk *walk,
44  			     struct crypto_skcipher *tfm)
45  {
46  	const unsigned long alignmask = crypto_skcipher_alignmask(tfm);
47  	u8 tmp[MAX_CIPHER_BLOCKSIZE + MAX_CIPHER_ALIGNMASK];
48  	u8 *stream = PTR_ALIGN(tmp + 0, alignmask + 1);
49  	u8 *src = walk->src.virt.addr;
50  	u8 *dst = walk->dst.virt.addr;
51  	u8 *iv = walk->iv;
52  	unsigned int nbytes = walk->nbytes;
53  
54  	crypto_cfb_encrypt_one(tfm, iv, stream);
55  	crypto_xor_cpy(dst, stream, src, nbytes);
56  }
57  
58  static int crypto_cfb_encrypt_segment(struct skcipher_walk *walk,
59  				      struct crypto_skcipher *tfm)
60  {
61  	const unsigned int bsize = crypto_cfb_bsize(tfm);
62  	unsigned int nbytes = walk->nbytes;
63  	u8 *src = walk->src.virt.addr;
64  	u8 *dst = walk->dst.virt.addr;
65  	u8 *iv = walk->iv;
66  
67  	do {
68  		crypto_cfb_encrypt_one(tfm, iv, dst);
69  		crypto_xor(dst, src, bsize);
70  		iv = dst;
71  
72  		src += bsize;
73  		dst += bsize;
74  	} while ((nbytes -= bsize) >= bsize);
75  
76  	memcpy(walk->iv, iv, bsize);
77  
78  	return nbytes;
79  }
80  
81  static int crypto_cfb_encrypt_inplace(struct skcipher_walk *walk,
82  				      struct crypto_skcipher *tfm)
83  {
84  	const unsigned int bsize = crypto_cfb_bsize(tfm);
85  	unsigned int nbytes = walk->nbytes;
86  	u8 *src = walk->src.virt.addr;
87  	u8 *iv = walk->iv;
88  	u8 tmp[MAX_CIPHER_BLOCKSIZE];
89  
90  	do {
91  		crypto_cfb_encrypt_one(tfm, iv, tmp);
92  		crypto_xor(src, tmp, bsize);
93  		iv = src;
94  
95  		src += bsize;
96  	} while ((nbytes -= bsize) >= bsize);
97  
98  	memcpy(walk->iv, iv, bsize);
99  
100  	return nbytes;
101  }
102  
103  static int crypto_cfb_encrypt(struct skcipher_request *req)
104  {
105  	struct crypto_skcipher *tfm = crypto_skcipher_reqtfm(req);
106  	struct skcipher_walk walk;
107  	unsigned int bsize = crypto_cfb_bsize(tfm);
108  	int err;
109  
110  	err = skcipher_walk_virt(&walk, req, false);
111  
112  	while (walk.nbytes >= bsize) {
113  		if (walk.src.virt.addr == walk.dst.virt.addr)
114  			err = crypto_cfb_encrypt_inplace(&walk, tfm);
115  		else
116  			err = crypto_cfb_encrypt_segment(&walk, tfm);
117  		err = skcipher_walk_done(&walk, err);
118  	}
119  
120  	if (walk.nbytes) {
121  		crypto_cfb_final(&walk, tfm);
122  		err = skcipher_walk_done(&walk, 0);
123  	}
124  
125  	return err;
126  }
127  
128  static int crypto_cfb_decrypt_segment(struct skcipher_walk *walk,
129  				      struct crypto_skcipher *tfm)
130  {
131  	const unsigned int bsize = crypto_cfb_bsize(tfm);
132  	unsigned int nbytes = walk->nbytes;
133  	u8 *src = walk->src.virt.addr;
134  	u8 *dst = walk->dst.virt.addr;
135  	u8 *iv = walk->iv;
136  
137  	do {
138  		crypto_cfb_encrypt_one(tfm, iv, dst);
139  		crypto_xor(dst, src, bsize);
140  		iv = src;
141  
142  		src += bsize;
143  		dst += bsize;
144  	} while ((nbytes -= bsize) >= bsize);
145  
146  	memcpy(walk->iv, iv, bsize);
147  
148  	return nbytes;
149  }
150  
151  static int crypto_cfb_decrypt_inplace(struct skcipher_walk *walk,
152  				      struct crypto_skcipher *tfm)
153  {
154  	const unsigned int bsize = crypto_cfb_bsize(tfm);
155  	unsigned int nbytes = walk->nbytes;
156  	u8 *src = walk->src.virt.addr;
157  	u8 * const iv = walk->iv;
158  	u8 tmp[MAX_CIPHER_BLOCKSIZE];
159  
160  	do {
161  		crypto_cfb_encrypt_one(tfm, iv, tmp);
162  		memcpy(iv, src, bsize);
163  		crypto_xor(src, tmp, bsize);
164  		src += bsize;
165  	} while ((nbytes -= bsize) >= bsize);
166  
167  	return nbytes;
168  }
169  
170  static int crypto_cfb_decrypt_blocks(struct skcipher_walk *walk,
171  				     struct crypto_skcipher *tfm)
172  {
173  	if (walk->src.virt.addr == walk->dst.virt.addr)
174  		return crypto_cfb_decrypt_inplace(walk, tfm);
175  	else
176  		return crypto_cfb_decrypt_segment(walk, tfm);
177  }
178  
179  static int crypto_cfb_decrypt(struct skcipher_request *req)
180  {
181  	struct crypto_skcipher *tfm = crypto_skcipher_reqtfm(req);
182  	struct skcipher_walk walk;
183  	const unsigned int bsize = crypto_cfb_bsize(tfm);
184  	int err;
185  
186  	err = skcipher_walk_virt(&walk, req, false);
187  
188  	while (walk.nbytes >= bsize) {
189  		err = crypto_cfb_decrypt_blocks(&walk, tfm);
190  		err = skcipher_walk_done(&walk, err);
191  	}
192  
193  	if (walk.nbytes) {
194  		crypto_cfb_final(&walk, tfm);
195  		err = skcipher_walk_done(&walk, 0);
196  	}
197  
198  	return err;
199  }
200  
201  static int crypto_cfb_create(struct crypto_template *tmpl, struct rtattr **tb)
202  {
203  	struct skcipher_instance *inst;
204  	struct crypto_alg *alg;
205  	int err;
206  
207  	inst = skcipher_alloc_instance_simple(tmpl, tb);
208  	if (IS_ERR(inst))
209  		return PTR_ERR(inst);
210  
211  	alg = skcipher_ialg_simple(inst);
212  
213  	/* CFB mode is a stream cipher. */
214  	inst->alg.base.cra_blocksize = 1;
215  
216  	/*
217  	 * To simplify the implementation, configure the skcipher walk to only
218  	 * give a partial block at the very end, never earlier.
219  	 */
220  	inst->alg.chunksize = alg->cra_blocksize;
221  
222  	inst->alg.encrypt = crypto_cfb_encrypt;
223  	inst->alg.decrypt = crypto_cfb_decrypt;
224  
225  	err = skcipher_register_instance(tmpl, inst);
226  	if (err)
227  		inst->free(inst);
228  
229  	return err;
230  }
231  
232  static struct crypto_template crypto_cfb_tmpl = {
233  	.name = "cfb",
234  	.create = crypto_cfb_create,
235  	.module = THIS_MODULE,
236  };
237  
238  static int __init crypto_cfb_module_init(void)
239  {
240  	return crypto_register_template(&crypto_cfb_tmpl);
241  }
242  
243  static void __exit crypto_cfb_module_exit(void)
244  {
245  	crypto_unregister_template(&crypto_cfb_tmpl);
246  }
247  
248  subsys_initcall(crypto_cfb_module_init);
249  module_exit(crypto_cfb_module_exit);
250  
251  MODULE_LICENSE("GPL");
252  MODULE_DESCRIPTION("CFB block cipher mode of operation");
253  MODULE_ALIAS_CRYPTO("cfb");
254  MODULE_IMPORT_NS(CRYPTO_INTERNAL);
255