File Coverage

src/rsa/rsa_pkcs1_sig_unpad.c
Criterion Covered Total %
statement 27 34 79.4
branch 9 20 45.0
condition n/a
subroutine n/a
pod n/a
total 36 54 66.6


line stmt bran cond sub pod time code
1             /*
2             * Copyright (c) 2017 Thomas Pornin
3             *
4             * Permission is hereby granted, free of charge, to any person obtaining
5             * a copy of this software and associated documentation files (the
6             * "Software"), to deal in the Software without restriction, including
7             * without limitation the rights to use, copy, modify, merge, publish,
8             * distribute, sublicense, and/or sell copies of the Software, and to
9             * permit persons to whom the Software is furnished to do so, subject to
10             * the following conditions:
11             *
12             * The above copyright notice and this permission notice shall be
13             * included in all copies or substantial portions of the Software.
14             *
15             * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
16             * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
17             * MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
18             * NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS
19             * BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN
20             * ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
21             * CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
22             * SOFTWARE.
23             */
24              
25             #include "inner.h"
26              
27             /* see bearssl_rsa.h */
28             uint32_t
29 2           br_rsa_pkcs1_sig_unpad(const unsigned char *sig, size_t sig_len,
30             const unsigned char *hash_oid, size_t hash_len,
31             unsigned char *hash_out)
32             {
33             static const unsigned char pad1[] = {
34             0x00, 0x01, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF
35             };
36              
37             unsigned char pad2[43];
38             size_t u, x2, x3, pad_len, zlen;
39              
40 2 50         if (sig_len < 11) {
41 0           return 0;
42             }
43              
44             /*
45             * Expected format:
46             * 00 01 FF ... FF 00 30 x1 30 x2 06 x3 OID [ 05 00 ] 04 x4 HASH
47             *
48             * with the following rules:
49             *
50             * -- Total length is that of the modulus and the signature
51             * (this was already verified by br_rsa_i31_public()).
52             *
53             * -- There are at least eight bytes of value 0xFF.
54             *
55             * -- x4 is equal to the hash length (hash_len).
56             *
57             * -- x3 is equal to the encoded OID value length (so x3 is the
58             * first byte of hash_oid[]).
59             *
60             * -- If the "05 00" is present, then x2 == x3 + 4; otherwise,
61             * x2 == x3 + 2.
62             *
63             * -- x1 == x2 + x4 + 4.
64             *
65             * So the total length after the last "FF" is either x3 + x4 + 11
66             * (with the "05 00") or x3 + x4 + 9 (without the "05 00").
67             */
68              
69             /*
70             * Check the "00 01 FF .. FF 00" with at least eight 0xFF bytes.
71             * The comparison is valid because we made sure that the signature
72             * is at least 11 bytes long.
73             */
74 2 50         if (memcmp(sig, pad1, sizeof pad1) != 0) {
75 0           return 0;
76             }
77 262 50         for (u = sizeof pad1; u < sig_len; u ++) {
78 262 100         if (sig[u] != 0xFF) {
79 2           break;
80             }
81             }
82              
83             /*
84             * Remaining length is sig_len - u bytes (including the 00 just
85             * after the last FF). This must be equal to one of the two
86             * possible values (depending on whether the "05 00" sequence is
87             * present or not).
88             */
89 2 50         if (hash_oid == NULL) {
90 0 0         if (sig_len - u != hash_len + 1 || sig[u] != 0x00) {
    0          
91 0           return 0;
92             }
93             } else {
94 2           x3 = hash_oid[0];
95 2           pad_len = x3 + 9;
96 2           memset(pad2, 0, pad_len);
97 2           zlen = sig_len - u - hash_len;
98 2 50         if (zlen == pad_len) {
99 0           x2 = x3 + 2;
100 2 50         } else if (zlen == pad_len + 2) {
101 2           x2 = x3 + 4;
102 2           pad_len = zlen;
103 2           pad2[pad_len - 4] = 0x05;
104             } else {
105 0           return 0;
106             }
107 2           pad2[1] = 0x30;
108 2           pad2[2] = x2 + hash_len + 4;
109 2           pad2[3] = 0x30;
110 2           pad2[4] = x2;
111 2           pad2[5] = 0x06;
112 2           memcpy(pad2 + 6, hash_oid, x3 + 1);
113 2           pad2[pad_len - 2] = 0x04;
114 2           pad2[pad_len - 1] = hash_len;
115 2 50         if (memcmp(pad2, sig + u, pad_len) != 0) {
116 0           return 0;
117             }
118             }
119 2           memcpy(hash_out, sig + sig_len - hash_len, hash_len);
120 2           return 1;
121             }