2 * Copyright 2020 The OpenSSL Project Authors. All Rights Reserved.
4 * Licensed under the Apache License 2.0 (the "License"). You may not use
5 * this file except in compliance with the License. You can obtain a copy
6 * in the file LICENSE in the source distribution or at
7 * https://www.openssl.org/source/license.html
11 * Helper functions for AES CBC CTS ciphers.
13 * The function dispatch tables are embedded into cipher_aes.c
14 * using cipher_aes_cts.inc
18 * Refer to SP800-38A-Addendum
20 * Ciphertext stealing encrypts plaintext using a block cipher, without padding
21 * the message to a multiple of the block size, so the ciphertext is the same
22 * size as the plaintext.
23 * It does this by altering processing of the last two blocks of the message.
24 * The processing of all but the last two blocks is unchanged, but a portion of
25 * the second-last block's ciphertext is "stolen" to pad the last plaintext
26 * block. The padded final block is then encrypted as usual.
27 * The final ciphertext for the last two blocks, consists of the partial block
28 * (with the "stolen" portion omitted) plus the full final block,
29 * which are the same size as the original plaintext.
30 * Decryption requires decrypting the final block first, then restoring the
31 * stolen ciphertext to the partial block, which can then be decrypted as usual.
33 * AES_CBC_CTS has 3 variants:
34 * (1) CS1 The NIST variant.
35 * If the length is a multiple of the blocksize it is the same as CBC mode.
36 * otherwise it produces C1||C2||(C(n-1))*||Cn.
37 * Where C(n-1)* is a partial block.
39 * If the length is a multiple of the blocksize it is the same as CBC mode.
40 * otherwise it produces C1||C2||Cn||(C(n-1))*.
41 * Where C(n-1)* is a partial block.
42 * (3) CS3 The Kerberos5 variant.
43 * Produces C1||C2||Cn||(C(n-1))* regardless of the length.
44 * If the length is a multiple of the blocksize it looks similar to CBC mode
45 * with the last 2 blocks swapped.
46 * Otherwise it is the same as CS2.
49 #include "e_os.h" /* strcasecmp */
50 #include <openssl/core_names.h>
51 #include <openssl/aes.h>
52 #include "prov/ciphercommon.h"
53 #include "internal/nelem.h"
54 #include "cipher_aes_cts.h"
56 /* The value assigned to 0 is the default */
63 unsigned char c[AES_BLOCK_SIZE];
66 typedef struct cts_mode_name2id_st {
71 static CTS_MODE_NAME2ID cts_modes[] =
73 { CTS_CS1, OSSL_CIPHER_CTS_MODE_CS1 },
74 { CTS_CS2, OSSL_CIPHER_CTS_MODE_CS2 },
75 { CTS_CS3, OSSL_CIPHER_CTS_MODE_CS3 },
78 const char *ossl_aes_cbc_cts_mode_id2name(unsigned int id)
82 for (i = 0; i < OSSL_NELEM(cts_modes); ++i) {
83 if (cts_modes[i].id == id)
84 return cts_modes[i].name;
89 int ossl_aes_cbc_cts_mode_name2id(const char *name)
93 for (i = 0; i < OSSL_NELEM(cts_modes); ++i) {
94 if (strcasecmp(name, cts_modes[i].name) == 0)
95 return (int)cts_modes[i].id;
100 static size_t cts128_cs1_encrypt(PROV_CIPHER_CTX *ctx, const unsigned char *in,
101 unsigned char *out, size_t len)
103 aligned_16bytes tmp_in;
106 residue = len % AES_BLOCK_SIZE;
108 if (!ctx->hw->cipher(ctx, out, in, len))
117 memset(tmp_in.c, 0, sizeof(tmp_in));
118 memcpy(tmp_in.c, in, residue);
119 if (!ctx->hw->cipher(ctx, out - AES_BLOCK_SIZE + residue, tmp_in.c,
122 return len + residue;
125 static void do_xor(const unsigned char *in1, const unsigned char *in2,
126 size_t len, unsigned char *out)
130 for (i = 0; i < len; ++i)
131 out[i] = in1[i] ^ in2[i];
134 static size_t cts128_cs1_decrypt(PROV_CIPHER_CTX *ctx, const unsigned char *in,
135 unsigned char *out, size_t len)
137 aligned_16bytes mid_iv, ct_mid, pt_last;
140 residue = len % AES_BLOCK_SIZE;
142 /* If there are no partial blocks then it is the same as CBC mode */
143 if (!ctx->hw->cipher(ctx, out, in, len))
147 /* Process blocks at the start - but leave the last 2 blocks */
148 len -= AES_BLOCK_SIZE + residue;
150 if (!ctx->hw->cipher(ctx, out, in, len))
155 /* Save the iv that will be used by the second last block */
156 memcpy(mid_iv.c, ctx->iv, AES_BLOCK_SIZE);
158 /* Decrypt the last block first using an iv of zero */
159 memset(ctx->iv, 0, AES_BLOCK_SIZE);
160 if (!ctx->hw->cipher(ctx, pt_last.c, in + residue, AES_BLOCK_SIZE))
164 * Rebuild the ciphertext of the second last block as a combination of
165 * the decrypted last block + replace the start with the ciphertext bytes
166 * of the partial second last block.
168 memcpy(ct_mid.c, in, residue);
169 memcpy(ct_mid.c + residue, pt_last.c + residue, AES_BLOCK_SIZE - residue);
171 * Restore the last partial ciphertext block.
172 * Now that we have the cipher text of the second last block, apply
173 * that to the partial plaintext end block. We have already decrypted the
174 * block using an IV of zero. For decryption the IV is just XORed after
175 * doing an AES block - so just XOR in the cipher text.
177 do_xor(ct_mid.c, pt_last.c, residue, out + AES_BLOCK_SIZE);
179 /* Restore the iv needed by the second last block */
180 memcpy(ctx->iv, mid_iv.c, AES_BLOCK_SIZE);
182 * Decrypt the second last plaintext block now that we have rebuilt the
185 if (!ctx->hw->cipher(ctx, out, ct_mid.c, AES_BLOCK_SIZE))
188 return len + AES_BLOCK_SIZE + residue;
191 static size_t cts128_cs3_encrypt(PROV_CIPHER_CTX *ctx, const unsigned char *in,
192 unsigned char *out, size_t len)
194 aligned_16bytes tmp_in;
197 if (len <= AES_BLOCK_SIZE) /* CS3 requires 2 blocks */
200 residue = len % AES_BLOCK_SIZE;
202 residue = AES_BLOCK_SIZE;
205 if (!ctx->hw->cipher(ctx, out, in, len))
211 memset(tmp_in.c, 0, sizeof(tmp_in));
212 memcpy(tmp_in.c, in, residue);
213 memcpy(out, out - AES_BLOCK_SIZE, residue);
214 if (!ctx->hw->cipher(ctx, out - AES_BLOCK_SIZE, tmp_in.c, AES_BLOCK_SIZE))
216 return len + residue;
221 * The cipher text (in) is of the form C(0), C(1), ., C(n), C(n-1)* where
222 * C(n) is a full block and C(n-1)* can be a partial block
223 * (but could be a full block).
224 * This means that the output plaintext (out) needs to swap the plaintext of
225 * the last two decoded ciphertext blocks.
227 static size_t cts128_cs3_decrypt(PROV_CIPHER_CTX *ctx, const unsigned char *in,
228 unsigned char *out, size_t len)
230 aligned_16bytes mid_iv, ct_mid, pt_last;
233 if (len <= AES_BLOCK_SIZE) /* CS3 requires 2 blocks */
236 /* Process blocks at the start - but leave the last 2 blocks */
237 residue = len % AES_BLOCK_SIZE;
239 residue = AES_BLOCK_SIZE;
240 len -= AES_BLOCK_SIZE + residue;
243 if (!ctx->hw->cipher(ctx, out, in, len))
248 /* Save the iv that will be used by the second last block */
249 memcpy(mid_iv.c, ctx->iv, AES_BLOCK_SIZE);
251 /* Decrypt the Cn block first using an iv of zero */
252 memset(ctx->iv, 0, AES_BLOCK_SIZE);
253 if (!ctx->hw->cipher(ctx, pt_last.c, in, AES_BLOCK_SIZE))
257 * Rebuild the ciphertext of C(n-1) as a combination of
258 * the decrypted C(n) block + replace the start with the ciphertext bytes
259 * of the partial last block.
261 memcpy(ct_mid.c, in + AES_BLOCK_SIZE, residue);
262 if (residue != AES_BLOCK_SIZE)
263 memcpy(ct_mid.c + residue, pt_last.c + residue, AES_BLOCK_SIZE - residue);
265 * Restore the last partial ciphertext block.
266 * Now that we have the cipher text of the second last block, apply
267 * that to the partial plaintext end block. We have already decrypted the
268 * block using an IV of zero. For decryption the IV is just XORed after
269 * doing an AES block - so just XOR in the ciphertext.
271 do_xor(ct_mid.c, pt_last.c, residue, out + AES_BLOCK_SIZE);
273 /* Restore the iv needed by the second last block */
274 memcpy(ctx->iv, mid_iv.c, AES_BLOCK_SIZE);
276 * Decrypt the second last plaintext block now that we have rebuilt the
279 if (!ctx->hw->cipher(ctx, out, ct_mid.c, AES_BLOCK_SIZE))
282 return len + AES_BLOCK_SIZE + residue;
285 static size_t cts128_cs2_encrypt(PROV_CIPHER_CTX *ctx, const unsigned char *in,
286 unsigned char *out, size_t len)
288 if (len % AES_BLOCK_SIZE == 0) {
289 /* If there are no partial blocks then it is the same as CBC mode */
290 if (!ctx->hw->cipher(ctx, out, in, len))
294 /* For partial blocks CS2 is equivalent to CS3 */
295 return cts128_cs3_encrypt(ctx, in, out, len);
298 static size_t cts128_cs2_decrypt(PROV_CIPHER_CTX *ctx, const unsigned char *in,
299 unsigned char *out, size_t len)
301 if (len % AES_BLOCK_SIZE == 0) {
302 /* If there are no partial blocks then it is the same as CBC mode */
303 if (!ctx->hw->cipher(ctx, out, in, len))
307 /* For partial blocks CS2 is equivalent to CS3 */
308 return cts128_cs3_decrypt(ctx, in, out, len);
311 int ossl_aes_cbc_cts_block_update(void *vctx, unsigned char *out, size_t *outl,
312 size_t outsize, const unsigned char *in,
315 PROV_CIPHER_CTX *ctx = (PROV_CIPHER_CTX *)vctx;
318 if (inl < AES_BLOCK_SIZE) /* There must be at least one block for CTS mode */
328 * Return an error if the update is called multiple times, only one shot
331 if (ctx->updated == 1)
335 if (ctx->cts_mode == CTS_CS1)
336 sz = cts128_cs1_encrypt(ctx, in, out, inl);
337 else if (ctx->cts_mode == CTS_CS2)
338 sz = cts128_cs2_encrypt(ctx, in, out, inl);
339 else if (ctx->cts_mode == CTS_CS3)
340 sz = cts128_cs3_encrypt(ctx, in, out, inl);
342 if (ctx->cts_mode == CTS_CS1)
343 sz = cts128_cs1_decrypt(ctx, in, out, inl);
344 else if (ctx->cts_mode == CTS_CS2)
345 sz = cts128_cs2_decrypt(ctx, in, out, inl);
346 else if (ctx->cts_mode == CTS_CS3)
347 sz = cts128_cs3_decrypt(ctx, in, out, inl);
351 ctx->updated = 1; /* Stop multiple updates being allowed */
356 int ossl_aes_cbc_cts_block_final(void *vctx, unsigned char *out, size_t *outl,