Decoder for password encoding of Cisco VPN client

/* This program is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version.

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.

You should have received a copy of the GNU General Public License along with this program; if not, write to the Free Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA*/


/* Requires libgcrypt version 1.1.90 or newer
Compile with: gcc -Wall -o cisco-decrypt cisco-decrypt.c $(libgcrypt-config --libs --cflags)
Usage: ./cisco-decrypt DEADBEEF...012345678 424242...7261 */


#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);
}