1 /*
2 * Copyright (C) 2016 The Android Open Source Project
3 *
4 * Licensed under the Apache License, Version 2.0 (the "License");
5 * you may not use this file except in compliance with the License.
6 * You may obtain a copy of the License at
7 *
8 * http://www.apache.org/licenses/LICENSE-2.0
9 *
10 * Unless required by applicable law or agreed to in writing, software
11 * distributed under the License is distributed on an "AS IS" BASIS,
12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13 * See the License for the specific language governing permissions and
14 * limitations under the License.
15 */
16
17 #include <crypto_utils/android_pubkey.h>
18
19 #include <assert.h>
20 #include <stdlib.h>
21 #include <string.h>
22
23 #include <openssl/bn.h>
24
25 // Better safe than sorry.
26 #if (ANDROID_PUBKEY_MODULUS_SIZE % 4) != 0
27 #error RSA modulus size must be multiple of the word size!
28 #endif
29
30 // Size of the RSA modulus in words.
31 #define ANDROID_PUBKEY_MODULUS_SIZE_WORDS (ANDROID_PUBKEY_MODULUS_SIZE / 4)
32
33 // This file implements encoding and decoding logic for Android's custom RSA
34 // public key binary format. Public keys are stored as a sequence of
35 // little-endian 32 bit words. Note that Android only supports little-endian
36 // processors, so we don't do any byte order conversions when parsing the binary
37 // struct.
38 typedef struct RSAPublicKey {
39 // Modulus length. This must be ANDROID_PUBKEY_MODULUS_SIZE.
40 uint32_t modulus_size_words;
41
42 // Precomputed montgomery parameter: -1 / n[0] mod 2^32
43 uint32_t n0inv;
44
45 // RSA modulus as a little-endian array.
46 uint8_t modulus[ANDROID_PUBKEY_MODULUS_SIZE];
47
48 // Montgomery parameter R^2 as a little-endian array of little-endian words.
49 uint8_t rr[ANDROID_PUBKEY_MODULUS_SIZE];
50
51 // RSA modulus: 3 or 65537
52 uint32_t exponent;
53 } RSAPublicKey;
54
55 // Reverses byte order in |buffer|.
reverse_bytes(uint8_t * buffer,size_t size)56 static void reverse_bytes(uint8_t* buffer, size_t size) {
57 for (size_t i = 0; i < (size + 1) / 2; ++i) {
58 uint8_t tmp = buffer[i];
59 buffer[i] = buffer[size - i - 1];
60 buffer[size - i - 1] = tmp;
61 }
62 }
63
android_pubkey_decode(const uint8_t * key_buffer,size_t size,RSA ** key)64 bool android_pubkey_decode(const uint8_t* key_buffer, size_t size, RSA** key) {
65 const RSAPublicKey* key_struct = (RSAPublicKey*)key_buffer;
66 bool ret = false;
67 uint8_t modulus_buffer[ANDROID_PUBKEY_MODULUS_SIZE];
68 RSA* new_key = RSA_new();
69 if (!new_key) {
70 goto cleanup;
71 }
72
73 // Check |size| is large enough and the modulus size is correct.
74 if (size < sizeof(RSAPublicKey)) {
75 goto cleanup;
76 }
77 if (key_struct->modulus_size_words != ANDROID_PUBKEY_MODULUS_SIZE_WORDS) {
78 goto cleanup;
79 }
80
81 // Convert the modulus to big-endian byte order as expected by BN_bin2bn.
82 memcpy(modulus_buffer, key_struct->modulus, sizeof(modulus_buffer));
83 reverse_bytes(modulus_buffer, sizeof(modulus_buffer));
84 new_key->n = BN_bin2bn(modulus_buffer, sizeof(modulus_buffer), NULL);
85 if (!new_key->n) {
86 goto cleanup;
87 }
88
89 // Read the exponent.
90 new_key->e = BN_new();
91 if (!new_key->e || !BN_set_word(new_key->e, key_struct->exponent)) {
92 goto cleanup;
93 }
94
95 // Note that we don't extract the montgomery parameters n0inv and rr from
96 // the RSAPublicKey structure. They assume a word size of 32 bits, but
97 // BoringSSL may use a word size of 64 bits internally, so we're lacking the
98 // top 32 bits of n0inv in general. For now, we just ignore the parameters
99 // and have BoringSSL recompute them internally. More sophisticated logic can
100 // be added here if/when we want the additional speedup from using the
101 // pre-computed montgomery parameters.
102
103 *key = new_key;
104 ret = true;
105
106 cleanup:
107 if (!ret && new_key) {
108 RSA_free(new_key);
109 }
110 return ret;
111 }
112
android_pubkey_encode_bignum(const BIGNUM * num,uint8_t * buffer)113 static bool android_pubkey_encode_bignum(const BIGNUM* num, uint8_t* buffer) {
114 if (!BN_bn2bin_padded(buffer, ANDROID_PUBKEY_MODULUS_SIZE, num)) {
115 return false;
116 }
117
118 reverse_bytes(buffer, ANDROID_PUBKEY_MODULUS_SIZE);
119 return true;
120 }
121
android_pubkey_encode(const RSA * key,uint8_t * key_buffer,size_t size)122 bool android_pubkey_encode(const RSA* key, uint8_t* key_buffer, size_t size) {
123 RSAPublicKey* key_struct = (RSAPublicKey*)key_buffer;
124 bool ret = false;
125 BN_CTX* ctx = BN_CTX_new();
126 BIGNUM* r32 = BN_new();
127 BIGNUM* n0inv = BN_new();
128 BIGNUM* rr = BN_new();
129
130 if (sizeof(RSAPublicKey) > size ||
131 RSA_size(key) != ANDROID_PUBKEY_MODULUS_SIZE) {
132 goto cleanup;
133 }
134
135 // Store the modulus size.
136 key_struct->modulus_size_words = ANDROID_PUBKEY_MODULUS_SIZE_WORDS;
137
138 // Compute and store n0inv = -1 / N[0] mod 2^32.
139 if (!ctx || !r32 || !n0inv || !BN_set_bit(r32, 32) ||
140 !BN_mod(n0inv, key->n, r32, ctx) ||
141 !BN_mod_inverse(n0inv, n0inv, r32, ctx) || !BN_sub(n0inv, r32, n0inv)) {
142 goto cleanup;
143 }
144 key_struct->n0inv = (uint32_t)BN_get_word(n0inv);
145
146 // Store the modulus.
147 if (!android_pubkey_encode_bignum(key->n, key_struct->modulus)) {
148 goto cleanup;
149 }
150
151 // Compute and store rr = (2^(rsa_size)) ^ 2 mod N.
152 if (!ctx || !rr || !BN_set_bit(rr, ANDROID_PUBKEY_MODULUS_SIZE * 8) ||
153 !BN_mod_sqr(rr, rr, key->n, ctx) ||
154 !android_pubkey_encode_bignum(rr, key_struct->rr)) {
155 goto cleanup;
156 }
157
158 // Store the exponent.
159 key_struct->exponent = (uint32_t)BN_get_word(key->e);
160
161 ret = true;
162
163 cleanup:
164 BN_free(rr);
165 BN_free(n0inv);
166 BN_free(r32);
167 BN_CTX_free(ctx);
168 return ret;
169 }
170