最优非对称加密填充(OAEP)

2012-11-19 23:55 Rudrj2 阅读(20008) 评论(1) 编辑收藏举报

下面是从OPENSSL上截取的rsa_oaep.c的源码：

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  1 #if !defined(OPENSSL_NO_SHA) && !defined(OPENSSL_NO_SHA1)
  2 #include <stdio.h>
  3 #include "cryptlib.h"
  4 #include <openssl/bn.h>
  5 #include <openssl/rsa.h>
  6 #include <openssl/evp.h>
  7 #include <openssl/rand.h>
  8 #include <openssl/sha.h>
  9 
 10 static int MGF1(unsigned char *mask, long len,
 11     const unsigned char *seed, long seedlen);
 12 
 13 int RSA_padding_add_PKCS1_OAEP(unsigned char *to, int tlen,
 14     const unsigned char *from, int flen,
 15     const unsigned char *param, int plen)
 16     {
 17     int i, emlen = tlen - 1;
 18     unsigned char *db, *seed;
 19     unsigned char *dbmask, seedmask[SHA_DIGEST_LENGTH];
 20 
 21     if (flen > emlen - 2 * SHA_DIGEST_LENGTH - 1)
 22         {
 23         RSAerr(RSA_F_RSA_PADDING_ADD_PKCS1_OAEP,
 24            RSA_R_DATA_TOO_LARGE_FOR_KEY_SIZE);
 25         return 0;
 26         }
 27 
 28     if (emlen < 2 * SHA_DIGEST_LENGTH + 1)
 29         {
 30         RSAerr(RSA_F_RSA_PADDING_ADD_PKCS1_OAEP, RSA_R_KEY_SIZE_TOO_SMALL);
 31         return 0;
 32         }
 33 
 34     to[0] = 0;
 35     seed = to + 1;
 36     db = to + SHA_DIGEST_LENGTH + 1;
 37 
 38     if (!EVP_Digest((void *)param, plen, db, NULL, EVP_sha1(), NULL))
 39         return 0;
 40     memset(db + SHA_DIGEST_LENGTH, 0,
 41         emlen - flen - 2 * SHA_DIGEST_LENGTH - 1);
 42     db[emlen - flen - SHA_DIGEST_LENGTH - 1] = 0x01;
 43     memcpy(db + emlen - flen - SHA_DIGEST_LENGTH, from, (unsigned int) flen);
 44     if (RAND_bytes(seed, SHA_DIGEST_LENGTH) <= 0)
 45         return 0;
 46 #ifdef PKCS_TESTVECT
 47     memcpy(seed,
 48        "\xaa\xfd\x12\xf6\x59\xca\xe6\x34\x89\xb4\x79\xe5\x07\x6d\xde\xc2\xf0\x6c\xb5\x8f",
 49        20);
 50 #endif
 51 
 52     dbmask = OPENSSL_malloc(emlen - SHA_DIGEST_LENGTH);
 53     if (dbmask == NULL)
 54         {
 55         RSAerr(RSA_F_RSA_PADDING_ADD_PKCS1_OAEP, ERR_R_MALLOC_FAILURE);
 56         return 0;
 57         }
 58 
 59     if (MGF1(dbmask, emlen - SHA_DIGEST_LENGTH, seed, SHA_DIGEST_LENGTH) < 0)
 60         return 0;
 61     for (i = 0; i < emlen - SHA_DIGEST_LENGTH; i++)
 62         db[i] ^= dbmask[i];
 63 
 64     if (MGF1(seedmask, SHA_DIGEST_LENGTH, db, emlen - SHA_DIGEST_LENGTH) < 0)
 65         return 0;
 66     for (i = 0; i < SHA_DIGEST_LENGTH; i++)
 67         seed[i] ^= seedmask[i];
 68 
 69     OPENSSL_free(dbmask);
 70     return 1;
 71     }
 72 
 73 int RSA_padding_check_PKCS1_OAEP(unsigned char *to, int tlen,
 74     const unsigned char *from, int flen, int num,
 75     const unsigned char *param, int plen)
 76     {
 77     int i, dblen, mlen = -1;
 78     const unsigned char *maskeddb;
 79     int lzero;
 80     unsigned char *db = NULL, seed[SHA_DIGEST_LENGTH], phash[SHA_DIGEST_LENGTH];
 81     unsigned char *padded_from;
 82     int bad = 0;
 83 
 84     if (--num < 2 * SHA_DIGEST_LENGTH + 1)
 85         /* 'num' is the length of the modulus, i.e. does not depend on the
 86          * particular ciphertext. */
 87         goto decoding_err;
 88 
 89     lzero = num - flen;
 90     if (lzero < 0)
 91         {
 92         /* signalling this error immediately after detection might allow
 93          * for side-channel attacks (e.g. timing if 'plen' is huge
 94          * -- cf. James H. Manger, "A Chosen Ciphertext Attack on RSA Optimal
 95          * Asymmetric Encryption Padding (OAEP) [...]", CRYPTO 2001),
 96          * so we use a 'bad' flag */
 97         bad = 1;
 98         lzero = 0;
 99         flen = num; /* don't overflow the memcpy to padded_from */
100         }
101 
102     dblen = num - SHA_DIGEST_LENGTH;
103     db = OPENSSL_malloc(dblen + num);
104     if (db == NULL)
105         {
106         RSAerr(RSA_F_RSA_PADDING_CHECK_PKCS1_OAEP, ERR_R_MALLOC_FAILURE);
107         return -1;
108         }
109 
110     /* Always do this zero-padding copy (even when lzero == 0)
111      * to avoid leaking timing info about the value of lzero. */
112     padded_from = db + dblen;
113     memset(padded_from, 0, lzero);
114     memcpy(padded_from + lzero, from, flen);
115 
116     maskeddb = padded_from + SHA_DIGEST_LENGTH;
117 
118     if (MGF1(seed, SHA_DIGEST_LENGTH, maskeddb, dblen))
119         return -1;
120     for (i = 0; i < SHA_DIGEST_LENGTH; i++)
121         seed[i] ^= padded_from[i];
122   
123     if (MGF1(db, dblen, seed, SHA_DIGEST_LENGTH))
124         return -1;
125     for (i = 0; i < dblen; i++)
126         db[i] ^= maskeddb[i];
127 
128     if (!EVP_Digest((void *)param, plen, phash, NULL, EVP_sha1(), NULL))
129         return -1;
130 
131     if (memcmp(db, phash, SHA_DIGEST_LENGTH) != 0 || bad)
132         goto decoding_err;
133     else
134         {
135         for (i = SHA_DIGEST_LENGTH; i < dblen; i++)
136             if (db[i] != 0x00)
137                 break;
138         if (i == dblen || db[i] != 0x01)
139             goto decoding_err;
140         else
141             {
142             /* everything looks OK */
143 
144             mlen = dblen - ++i;
145             if (tlen < mlen)
146                 {
147                 RSAerr(RSA_F_RSA_PADDING_CHECK_PKCS1_OAEP, RSA_R_DATA_TOO_LARGE);
148                 mlen = -1;
149                 }
150             else
151                 memcpy(to, db + i, mlen);
152             }
153         }
154     OPENSSL_free(db);
155     return mlen;
156 
157 decoding_err:
158     /* to avoid chosen ciphertext attacks, the error message should not reveal
159      * which kind of decoding error happened */
160     RSAerr(RSA_F_RSA_PADDING_CHECK_PKCS1_OAEP, RSA_R_OAEP_DECODING_ERROR);
161     if (db != NULL) OPENSSL_free(db);
162     return -1;
163     }
164 
165 int PKCS1_MGF1(unsigned char *mask, long len,
166     const unsigned char *seed, long seedlen, const EVP_MD *dgst)
167     {
168     long i, outlen = 0;
169     unsigned char cnt[4];
170     EVP_MD_CTX c;
171     unsigned char md[EVP_MAX_MD_SIZE];
172     int mdlen;
173     int rv = -1;
174 
175     EVP_MD_CTX_init(&c);
176     mdlen = EVP_MD_size(dgst);
177     if (mdlen < 0)
178         goto err;
179     for (i = 0; outlen < len; i++)
180         {
181         cnt[0] = (unsigned char)((i >> 24) & 255);
182         cnt[1] = (unsigned char)((i >> 16) & 255);
183         cnt[2] = (unsigned char)((i >> 8)) & 255;
184         cnt[3] = (unsigned char)(i & 255);
185         if (!EVP_DigestInit_ex(&c,dgst, NULL)
186             || !EVP_DigestUpdate(&c, seed, seedlen)
187             || !EVP_DigestUpdate(&c, cnt, 4))
188             goto err;
189         if (outlen + mdlen <= len)
190             {
191             if (!EVP_DigestFinal_ex(&c, mask + outlen, NULL))
192                 goto err;
193             outlen += mdlen;
194             }
195         else
196             {
197             if (!EVP_DigestFinal_ex(&c, md, NULL))
198                 goto err;
199             memcpy(mask + outlen, md, len - outlen);
200             outlen = len;
201             }
202         }
203     rv = 0;
204     err:
205     EVP_MD_CTX_cleanup(&c);
206     return rv;
207     }
208 
209 static int MGF1(unsigned char *mask, long len, const unsigned char *seed,
210          long seedlen)
211     {
212     return PKCS1_MGF1(mask, len, seed, seedlen, EVP_sha1());
213     }
214 #endif

RSA中的一个短信息可以导致密文易遭受短信息攻击。也已经说明简单地给信息填充伪数据(填充位)也许就会使敌手的工作变得十分困难，但是通过更进一步的努力，她还可以对密文进行攻击。解决这种攻击的办法就是应用称为最优非对称加密填充(OAEP)的过程。下图，就是该过程的简单版本，执行时也许要用更为完善的版本。