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