1/* challenge.c --- Generate a CRAM-MD5 challenge string.
2 * Copyright (C) 2002-2012 Simon Josefsson
3 *
4 * This file is part of GNU SASL Library.
5 *
6 * GNU SASL Library is free software; you can redistribute it and/or
7 * modify it under the terms of the GNU Lesser General Public License
8 * as published by the Free Software Foundation; either version 2.1 of
9 * the License, or (at your option) any later version.
10 *
11 * GNU SASL Library is distributed in the hope that it will be useful,
12 * but WITHOUT ANY WARRANTY; without even the implied warranty of
13 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
14 * Lesser General Public License for more details.
15 *
16 * You should have received a copy of the GNU Lesser General Public
17 * License along with GNU SASL Library; if not, write to the Free
18 * Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor,
19 * Boston, MA 02110-1301, USA.
20 *
21 */
22
23#include <stdio.h>
24#include <string.h>
25#include <assert.h>
26
27/* Get prototype. */
28#include "challenge.h"
29
30/* Get gc_nonce. */
31#include <gc.h>
32
33/*
34 * From draft-ietf-sasl-crammd5-02.txt:
35 *
36 * The data encoded in the challenge contains a presumptively
37 * arbitrary string of random digits, a time-stamp, and the
38 * fully-qualified primary host name of the server.
39 * ...
40 * challenge = "<" 1*DIGIT "." 1*DIGIT "@" hostname ">"
41 * hostname = 1*(ALPHA / DIGIT) *("." / "-" / ALPHA / DIGIT)
42 *
43 * This implementation avoid the information leakage by always using 0
44 * as the time stamp and a fixed host name. This should be
45 * unproblematic, as any client that try to validate the challenge
46 * string somehow, would violate the same specification:
47 *
48 * The client MUST NOT interpret or attempt to validate the
49 * contents of the challenge in any way.
50 *
51 */
52
53/* The sequence of X in TEMPLATE must be twice as long as NONCELEN. */
54#define NONCELEN 10
55#define TEMPLATE "<XXXXXXXXXXXXXXXXXXXX.0@localhost>"
56
57/* The probabilities for each digit are skewed (0-5 is more likely to
58 occur than 6-9), but it is just used as a nonce anyway. */
59#define DIGIT(c) (((c) & 0x0F) > 9 ? \
60 '0' + ((c) & 0x0F) - 10 : \
61 '0' + ((c) & 0x0F))
62
63int
64cram_md5_challenge (char challenge[CRAM_MD5_CHALLENGE_LEN])
65{
66 char nonce[NONCELEN];
67 size_t i;
68 int rc;
69
70 assert (strlen (TEMPLATE) == CRAM_MD5_CHALLENGE_LEN - 1);
71
72 memcpy (challenge, TEMPLATE, CRAM_MD5_CHALLENGE_LEN);
73
74 rc = gc_nonce (nonce, sizeof (nonce));
75 if (rc != GC_OK)
76 return -1;
77
78 for (i = 0; i < sizeof (nonce); i++)
79 {
80 challenge[1 + i] = DIGIT (nonce[i]);
81 challenge[11 + i] = DIGIT (nonce[i] >> 4);
82 }
83
84 return 0;
85}
86