dsa_sig.c revision f03f8485e7d1a5258e16e0e376229f431d2c53ac 
1/*
2 * Copyright 2013 The Android Open Source Project
3 *
4 * Redistribution and use in source and binary forms, with or without
5 * modification, are permitted provided that the following conditions are met:
6 *     * Redistributions of source code must retain the above copyright
7 *       notice, this list of conditions and the following disclaimer.
8 *     * Redistributions in binary form must reproduce the above copyright
9 *       notice, this list of conditions and the following disclaimer in the
10 *       documentation and/or other materials provided with the distribution.
11 *     * Neither the name of Google Inc. nor the names of its contributors may
12 *       be used to endorse or promote products derived from this software
13 *       without specific prior written permission.
14 *
15 * THIS SOFTWARE IS PROVIDED BY Google Inc. ``AS IS'' AND ANY EXPRESS OR
16 * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
17 * MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO
18 * EVENT SHALL Google Inc. BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
19 * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
20 * PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS;
21 * OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY,
22 * WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR
23 * OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF
24 * ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
25 */
26
27#include <string.h>
28
29#include "mincrypt/dsa_sig.h"
30#include "mincrypt/p256.h"
31
32/**
33 * Trims off the leading zero bytes and copy it to a buffer aligning it to the end.
34 */
35static inline int trim_to_p256_bytes(unsigned char dst[P256_NBYTES], unsigned char *src,
36        int src_len) {
37    int dst_offset;
38    while (*src == '\0' && src_len > 0) {
39        src++;
40        src_len--;
41    }
42    if (src_len > P256_NBYTES || src_len < 1) {
43        return 0;
44    }
45    dst_offset = P256_NBYTES - src_len;
46    memset(dst, 0, dst_offset);
47    memcpy(dst + dst_offset, src, src_len);
48    return 1;
49}
50
51/**
52 * Unpacks the ASN.1 DSA signature sequence.
53 */
54int dsa_sig_unpack(unsigned char* sig, int sig_len, p256_int* r_int, p256_int* s_int) {
55    /*
56     * Structure is:
57     *   0x30 0xNN  SEQUENCE + s_length
58     *     0x02 0xNN  INTEGER + r_length
59     *       0xAA 0xBB ..   r_length bytes of "r" (offset 4)
60     *     0x02 0xNN  INTEGER + s_length
61     *       0xMM 0xNN ..   s_length bytes of "s" (offset 6 + r_len)
62     */
63    int seq_len;
64    unsigned char r_bytes[P256_NBYTES];
65    unsigned char s_bytes[P256_NBYTES];
66    int r_len;
67    int s_len;
68
69    memset(r_bytes, 0, sizeof(r_bytes));
70    memset(s_bytes, 0, sizeof(s_bytes));
71
72    /*
73     * Must have at least:
74     * 2 bytes sequence header and length
75     * 2 bytes R integer header and length
76     * 1 byte of R
77     * 2 bytes S integer header and length
78     * 1 byte of S
79     *
80     * 8 bytes total
81     */
82    if (sig_len < 8 || sig[0] != 0x30 || sig[2] != 0x02) {
83        return 0;
84    }
85
86    seq_len = sig[1];
87    if ((seq_len <= 0) || (seq_len + 2 != sig_len)) {
88        return 0;
89    }
90
91    r_len = sig[3];
92    /*
93     * Must have at least:
94     * 2 bytes for R header and length
95     * 2 bytes S integer header and length
96     * 1 byte of S
97     */
98    if ((r_len < 1) || (r_len > seq_len - 5) || (sig[4 + r_len] != 0x02)) {
99        return 0;
100    }
101    s_len = sig[5 + r_len];
102
103    /**
104     * Must have:
105     * 2 bytes for R header and length
106     * r_len bytes for R
107     * 2 bytes S integer header and length
108     */
109    if ((s_len < 1) || (s_len != seq_len - 4 - r_len)) {
110        return 0;
111    }
112
113    /*
114     * ASN.1 encoded integers are zero-padded for positive integers. Make sure we have
115     * a correctly-sized buffer and that the resulting integer isn't too large.
116     */
117    if (!trim_to_p256_bytes(r_bytes, &sig[4], r_len)
118            || !trim_to_p256_bytes(s_bytes, &sig[6 + r_len], s_len)) {
119        return 0;
120    }
121
122    p256_from_bin(r_bytes, r_int);
123    p256_from_bin(s_bytes, s_int);
124
125    return 1;
126}
127