decrypt

This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.

#include <stdio.h>
#include <stdlib.h>
#include <gcrypt.h>
#include <errno.h>

int hex2bin_c(unsigned int c)
{
if ((c >= '0')&&(c <= '9'))
return c - '0';
if ((c >= 'A')&&(c <= 'F'))
return c - 'A' + 10;
if ((c >= 'a')&&(c <= 'f'))
return c - 'a' + 10;
return -1;
}

int hex2bin(const char *str, char **bin, int *len)
{
char *p;
int i, l;

if (!bin)
return EINVAL;

for (i = 0; str[i] != '\0'; i++)
if (hex2bin_c(str[i]) == -1)
return EINVAL;

l = i;
if ((l & 1) != 0)
return EINVAL;
l /= 2;

p = malloc(l);
if (p == NULL)
return ENOMEM;

for (i = 0; i < l; i++)
p[i] = hex2bin_c(str[i*2]) << 4 | hex2bin_c(str[i*2+1]);

*bin = p;
if (len)
*len = l;

return 0;
}

int c_decrypt(char *ct, int len, char **resp, char *reslenp)
{
const char *h1 = ct;
const char *h4 = ct + 20;
const char *enc = ct + 40;

char ht[20], h2[20], h3[20], key[24];
const char *iv = h1;
char *res;
gcry_cipher_hd_t ctx;
int reslen;

if (len < 48)
return 0;
len -= 40;

memcpy(ht, h1, 20);

ht[19]++;
gcry_md_hash_buffer(GCRY_MD_SHA1, h2, ht, 20);

ht[19] += 2;
gcry_md_hash_buffer(GCRY_MD_SHA1, h3, ht, 20);

memcpy(key, h2, 20);
memcpy(key+20, h3, 4);
/* who cares about parity anyway? */

gcry_md_hash_buffer(GCRY_MD_SHA1, ht, enc, len);

if (memcmp(h4, ht, 20) != 0)
return -1;

res = malloc(len);
if (res == NULL)
return -1;

gcry_cipher_open(&ctx, GCRY_CIPHER_3DES, GCRY_CIPHER_MODE_CBC, 0);
gcry_cipher_setkey(ctx, key, 24);
gcry_cipher_setiv(ctx, iv, 8);
gcry_cipher_decrypt(ctx, (unsigned char *)res, len, (unsigned char *)enc, len);
gcry_cipher_close(ctx);

reslen = len - res[len-1];
res[reslen] = '\0';

if (resp)
*resp = res;
if (reslenp)
*reslenp = reslen;
return 0;
}

int main(int argc, char *argv[])
{
int i, len, ret = 0;
char *bin, *pw;

gcry_check_version(NULL);

for (i = 1; i < argc; i++) {
ret = hex2bin(argv[i], &bin, &len);
if (ret != 0) {
perror("decoding input");
continue;
}
ret = c_decrypt(bin, len, &pw, NULL);
free(bin);
if (ret != 0) {
perror("decrypting input");
continue;
}
printf("%s\n", pw);
free(pw);
}

exit(ret != 0);
}

Download: cisco-decrypt