git.distorted.org.uk Git - u/mdw/putty/blob - sshdss.c

   1 #include <stdio.h>
   2 #include <stdlib.h>
   3
   4 #include "ssh.h"
   5
   6 #define GET_32BIT(cp) \
   7     (((unsigned long)(unsigned char)(cp)[0] << 24) | \
   8     ((unsigned long)(unsigned char)(cp)[1] << 16) | \
   9     ((unsigned long)(unsigned char)(cp)[2] << 8) | \
  10     ((unsigned long)(unsigned char)(cp)[3]))
  11
  12 #define PUT_32BIT(cp, value) { \
  13     (cp)[0] = (unsigned char)((value) >> 24); \
  14     (cp)[1] = (unsigned char)((value) >> 16); \
  15     (cp)[2] = (unsigned char)((value) >> 8); \
  16     (cp)[3] = (unsigned char)(value); }
  17
  18 #if 0
  19 #define DEBUG_DSS
  20 /*
  21  * Condition this section in for debugging of DSS.
  22  */
  23 static void diagbn(char *prefix, Bignum md) {
  24     int i, nibbles, morenibbles;
  25     static const char hex[] = "0123456789ABCDEF";
  26
  27     printf("%s0x", prefix ? prefix : "");
  28
  29     nibbles = (3 + ssh1_bignum_bitcount(md))/4; if (nibbles<1) nibbles=1;
  30     morenibbles = 4*md[0] - nibbles;
  31     for (i=0; i<morenibbles; i++) putchar('-');
  32     for (i=nibbles; i-- ;)
  33         putchar(hex[(bignum_byte(md, i/2) >> (4*(i%2))) & 0xF]);
  34
  35     if (prefix) putchar('\n');
  36 }
  37 #else
  38 #define diagbn(x,y)
  39 #endif
  40
  41 static void getstring(char **data, int *datalen, char **p, int *length) {
  42     *p = NULL;
  43     if (*datalen < 4)
  44         return;
  45     *length = GET_32BIT(*data);
  46     *datalen -= 4; *data += 4;
  47     if (*datalen < *length)
  48         return;
  49     *p = *data;
  50     *data += *length; *datalen -= *length;
  51 }
  52 static Bignum getmp(char **data, int *datalen) {
  53     char *p;
  54     int i, j, length;
  55     Bignum b;
  56
  57     getstring(data, datalen, &p, &length);
  58     if (!p)
  59         return NULL;
  60     if (p[0] & 0x80)
  61         return NULL;                   /* negative mp */
  62     b = newbn((length+1)/2);
  63     for (i = 0; i < length; i++) {
  64         j = length - 1 - i;
  65         if (j & 1)
  66             b[j/2+1] |= ((unsigned char)p[i]) << 8;
  67         else
  68             b[j/2+1] |= ((unsigned char)p[i]);
  69     }
  70     while (b[0] > 1 && b[b[0]] == 0) b[0]--;
  71     return b;
  72 }
  73
  74 static Bignum get160(char **data, int *datalen) {
  75     char *p;
  76     int i, j, length;
  77     Bignum b;
  78
  79     p = *data;
  80     *data += 20; *datalen -= 20;
  81
  82     length = 20;
  83     while (length > 0 && !p[0])
  84         p++, length--;
  85     b = newbn((length+1)/2);
  86     for (i = 0; i < length; i++) {
  87         j = length - 1 - i;
  88         if (j & 1)
  89             b[j/2+1] |= ((unsigned char)p[i]) << 8;
  90         else
  91             b[j/2+1] |= ((unsigned char)p[i]);
  92     }
  93     return b;
  94 }
  95
  96 static Bignum dss_p, dss_q, dss_g, dss_y;
  97
  98 static void dss_setkey(char *data, int len) {
  99     char *p;
 100     int slen;
 101     getstring(&data, &len, &p, &slen);
 102
 103 #ifdef DEBUG_DSS
 104     {
 105         int i;
 106         printf("key:");
 107         for (i=0;i<len;i++)
 108             printf("  %02x", (unsigned char)(data[i]));
 109         printf("\n");
 110     }
 111 #endif
 112
 113     if (!p || memcmp(p, "ssh-dss", 7)) {
 114         dss_p = NULL;
 115         return;
 116     }
 117     dss_p = getmp(&data, &len);
 118     dss_q = getmp(&data, &len);
 119     dss_g = getmp(&data, &len);
 120     dss_y = getmp(&data, &len);
 121 }
 122
 123 static char *dss_fmtkey(void) {
 124     char *p;
 125     int len, i, pos, nibbles;
 126     static const char hex[] = "0123456789abcdef";
 127     if (!dss_p)
 128         return NULL;
 129     len = 8 + 4 + 1;                   /* 4 x "0x", punctuation, \0 */
 130     len += 4 * (dss_p[0] + dss_q[0] + dss_g[0] + dss_y[0]);   /* digits */
 131     p = malloc(len);
 132     if (!p) return NULL;
 133
 134     pos = 0;
 135     pos += sprintf(p+pos, "0x");
 136     nibbles = (3 + ssh1_bignum_bitcount(dss_p))/4; if (nibbles<1) nibbles=1;
 137     for (i=nibbles; i-- ;)
 138         p[pos++] = hex[(bignum_byte(dss_p, i/2) >> (4*(i%2))) & 0xF];
 139     pos += sprintf(p+pos, ",0x");
 140     nibbles = (3 + ssh1_bignum_bitcount(dss_q))/4; if (nibbles<1) nibbles=1;
 141     for (i=nibbles; i-- ;)
 142         p[pos++] = hex[(bignum_byte(dss_q, i/2) >> (4*(i%2))) & 0xF];
 143     pos += sprintf(p+pos, ",0x");
 144     nibbles = (3 + ssh1_bignum_bitcount(dss_g))/4; if (nibbles<1) nibbles=1;
 145     for (i=nibbles; i-- ;)
 146         p[pos++] = hex[(bignum_byte(dss_g, i/2) >> (4*(i%2))) & 0xF];
 147     pos += sprintf(p+pos, ",0x");
 148     nibbles = (3 + ssh1_bignum_bitcount(dss_y))/4; if (nibbles<1) nibbles=1;
 149     for (i=nibbles; i-- ;)
 150         p[pos++] = hex[(bignum_byte(dss_y, i/2) >> (4*(i%2))) & 0xF];
 151     p[pos] = '\0';
 152     return p;
 153 }
 154
 155 static char *dss_fingerprint(void) {
 156     struct MD5Context md5c;
 157     unsigned char digest[16], lenbuf[4];
 158     char buffer[16*3+40];
 159     char *ret;
 160     int numlen, i;
 161
 162     MD5Init(&md5c);
 163     MD5Update(&md5c, "\0\0\0\7ssh-dss", 11);
 164
 165 #define ADD_BIGNUM(bignum) \
 166     numlen = (ssh1_bignum_bitcount(bignum)+8)/8; \
 167     PUT_32BIT(lenbuf, numlen); MD5Update(&md5c, lenbuf, 4); \
 168     for (i = numlen; i-- ;) { \
 169         unsigned char c = bignum_byte(bignum, i); \
 170         MD5Update(&md5c, &c, 1); \
 171     }
 172     ADD_BIGNUM(dss_p);
 173     ADD_BIGNUM(dss_q);
 174     ADD_BIGNUM(dss_g);
 175     ADD_BIGNUM(dss_y);
 176 #undef ADD_BIGNUM
 177
 178     MD5Final(digest, &md5c);
 179
 180     sprintf(buffer, "%d ", ssh1_bignum_bitcount(dss_p));
 181     for (i = 0; i < 16; i++)
 182         sprintf(buffer+strlen(buffer), "%s%02x", i?":":"", digest[i]);
 183     ret = malloc(strlen(buffer)+1);
 184     if (ret)
 185         strcpy(ret, buffer);
 186     return ret;
 187 }
 188
 189 static int dss_verifysig(char *sig, int siglen, char *data, int datalen) {
 190     char *p;
 191     int i, slen;
 192     char hash[20];
 193     Bignum qm2, r, s, w, i1, i2, i3, u1, u2, sha, v;
 194     int ret;
 195
 196     if (!dss_p)
 197         return 0;
 198
 199 #ifdef DEBUG_DSS
 200     {
 201         int i;
 202         printf("sig:");
 203         for (i=0;i<siglen;i++)
 204             printf("  %02xf", (unsigned char)(sig[i]));
 205         printf("\n");
 206     }
 207 #endif
 208     /*
 209      * Commercial SSH (2.0.13) and OpenSSH disagree over the format
 210      * of a DSA signature. OpenSSH is in line with the IETF drafts:
 211      * it uses a string "ssh-dss", followed by a 40-byte string
 212      * containing two 160-bit integers end-to-end. Commercial SSH
 213      * can't be bothered with the header bit, and considers a DSA
 214      * signature blob to be _just_ the 40-byte string containing
 215      * the two 160-bit integers. We tell them apart by measuring
 216      * the length: length 40 means the commercial-SSH bug, anything
 217      * else is assumed to be IETF-compliant.
 218      */
 219     if (siglen != 40) {                /* bug not present; read admin fields */
 220         getstring(&sig, &siglen, &p, &slen);
 221         if (!p || memcmp(p, "ssh-dss", 7)) {
 222             return 0;
 223         }
 224         sig += 4, siglen -= 4;             /* skip yet another length field */
 225     }
 226     diagbn("p=", dss_p);
 227     diagbn("q=", dss_q);
 228     diagbn("g=", dss_g);
 229     diagbn("y=", dss_y);
 230     r = get160(&sig, &siglen);
 231     diagbn("r=", r);
 232     s = get160(&sig, &siglen);
 233     diagbn("s=", s);
 234     if (!r || !s)
 235         return 0;
 236
 237     /*
 238      * Step 1. w <- s^-1 mod q.
 239      */
 240     w = newbn(dss_q[0]);
 241     qm2 = copybn(dss_q);
 242     decbn(qm2); decbn(qm2);
 243     diagbn("qm2=", qm2);
 244     /* Now qm2 is q-2, and by Fermat's Little Theorem, s^qm2 == s^-1 (mod q).
 245      * This is a silly way to do it; may fix it later. */
 246     modpow(s, qm2, dss_q, w);
 247     diagbn("w=", w);
 248
 249     /*
 250      * Step 2. u1 <- SHA(message) * w mod q.
 251      */
 252     u1 = newbn(dss_q[0]);
 253     SHA_Simple(data, datalen, hash);
 254     p = hash; slen = 20; sha = get160(&p, &slen);
 255     diagbn("sha=", sha);
 256     modmul(sha, w, dss_q, u1);
 257     diagbn("u1=", u1);
 258
 259     /*
 260      * Step 3. u2 <- r * w mod q.
 261      */
 262     u2 = newbn(dss_q[0]);
 263     modmul(r, w, dss_q, u2);
 264     diagbn("u2=", u2);
 265
 266     /*
 267      * Step 4. v <- (g^u1 * y^u2 mod p) mod q.
 268      */
 269     i1 = newbn(dss_p[0]);
 270     i2 = newbn(dss_p[0]);
 271     i3 = newbn(dss_p[0]);
 272     v = newbn(dss_q[0]);
 273     modpow(dss_g, u1, dss_p, i1);
 274     diagbn("gu1p=", i1);
 275     modpow(dss_y, u2, dss_p, i2);
 276     diagbn("yu2p=", i2);
 277     modmul(i1, i2, dss_p, i3);
 278     diagbn("gu1yu2p=", i3);
 279     modmul(i3, One, dss_q, v);
 280     diagbn("gu1yu2q=v=", v);
 281     diagbn("r=", r);
 282
 283     /*
 284      * Step 5. v should now be equal to r.
 285      */
 286
 287     ret = 1;
 288     for (i = 1; i <= v[0] || i <= r[0]; i++) {
 289         if ((i > v[0] && r[i] != 0) ||
 290             (i > r[0] && v[i] != 0) ||
 291             (i <= v[0] && i <= r[0] && r[i] != v[i]))
 292             ret = 0;
 293     }
 294
 295     freebn(w);
 296     freebn(qm2);
 297     freebn(sha);
 298     freebn(i1);
 299     freebn(i2);
 300     freebn(i3);
 301     freebn(v);
 302     freebn(r);
 303     freebn(s);
 304
 305     return ret;
 306 }
 307
 308 struct ssh_hostkey ssh_dss = {
 309     dss_setkey,
 310     dss_fmtkey,
 311     dss_fingerprint,
 312     dss_verifysig,
 313     "ssh-dss",
 314     "dss"
 315 };