android_pubkey.c revision b62146dcabedcacf99349793c1947797a173f0e6
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// Better safe than sorry. 24#if (ANDROID_PUBKEY_MODULUS_SIZE % 4) != 0 25#error RSA modulus size must be multiple of the word size! 26#endif 27 28// Size of the RSA modulus in words. 29#define ANDROID_PUBKEY_MODULUS_SIZE_WORDS (ANDROID_PUBKEY_MODULUS_SIZE / 4) 30 31// This file implements encoding and decoding logic for Android's custom RSA 32// public key binary format. Public keys are stored as a sequence of 33// little-endian 32 bit words. Note that Android only supports little-endian 34// processors, so we don't do any byte order conversions when parsing the binary 35// struct. 36typedef struct RSAPublicKey { 37 // Modulus length. This must be ANDROID_PUBKEY_MODULUS_SIZE. 38 uint32_t modulus_size_words; 39 40 // Precomputed montgomery parameter: -1 / n[0] mod 2^32 41 uint32_t n0inv; 42 43 // RSA modulus as a little-endian array. 44 uint8_t modulus[ANDROID_PUBKEY_MODULUS_SIZE]; 45 46 // Montgomery parameter R^2 as a little-endian array of little-endian words. 47 uint8_t rr[ANDROID_PUBKEY_MODULUS_SIZE]; 48 49 // RSA modulus: 3 or 65537 50 uint32_t exponent; 51} RSAPublicKey; 52 53// Reverses byte order in |buffer|. 54static void reverse_bytes(uint8_t* buffer, size_t size) { 55 for (size_t i = 0; i < (size + 1) / 2; ++i) { 56 uint8_t tmp = buffer[i]; 57 buffer[i] = buffer[size - i - 1]; 58 buffer[size - i - 1] = tmp; 59 } 60} 61 62bool android_pubkey_decode(const uint8_t* key_buffer, size_t size, RSA** key) { 63 const RSAPublicKey* key_struct = (RSAPublicKey*)key_buffer; 64 bool ret = false; 65 uint8_t modulus_buffer[ANDROID_PUBKEY_MODULUS_SIZE]; 66 RSA* new_key = RSA_new(); 67 if (!new_key) { 68 goto cleanup; 69 } 70 71 // Check |size| is large enough and the modulus size is correct. 72 if (size < sizeof(RSAPublicKey)) { 73 goto cleanup; 74 } 75 if (key_struct->modulus_size_words != ANDROID_PUBKEY_MODULUS_SIZE_WORDS) { 76 goto cleanup; 77 } 78 79 // Convert the modulus to big-endian byte order as expected by BN_bin2bn. 80 memcpy(modulus_buffer, key_struct->modulus, sizeof(modulus_buffer)); 81 reverse_bytes(modulus_buffer, sizeof(modulus_buffer)); 82 new_key->n = BN_bin2bn(modulus_buffer, sizeof(modulus_buffer), NULL); 83 if (!new_key->n) { 84 goto cleanup; 85 } 86 87 // Read the exponent. 88 new_key->e = BN_new(); 89 if (!new_key->e || !BN_set_word(new_key->e, key_struct->exponent)) { 90 goto cleanup; 91 } 92 93 // Note that we don't extract the montgomery parameters n0inv and rr from 94 // the RSAPublicKey structure. They assume a word size of 32 bits, but 95 // BoringSSL may use a word size of 64 bits internally, so we're lacking the 96 // top 32 bits of n0inv in general. For now, we just ignore the parameters 97 // and have BoringSSL recompute them internally. More sophisticated logic can 98 // be added here if/when we want the additional speedup from using the 99 // pre-computed montgomery parameters. 100 101 *key = new_key; 102 ret = true; 103 104cleanup: 105 if (!ret && new_key) { 106 RSA_free(new_key); 107 } 108 return ret; 109} 110 111static bool android_pubkey_encode_bignum(const BIGNUM* num, uint8_t* buffer) { 112 if (!BN_bn2bin_padded(buffer, ANDROID_PUBKEY_MODULUS_SIZE, num)) { 113 return false; 114 } 115 116 reverse_bytes(buffer, ANDROID_PUBKEY_MODULUS_SIZE); 117 return true; 118} 119 120bool android_pubkey_encode(const RSA* key, uint8_t* key_buffer, size_t size) { 121 RSAPublicKey* key_struct = (RSAPublicKey*)key_buffer; 122 bool ret = false; 123 BN_CTX* ctx = BN_CTX_new(); 124 BIGNUM* r32 = BN_new(); 125 BIGNUM* n0inv = BN_new(); 126 BIGNUM* rr = BN_new(); 127 128 if (sizeof(RSAPublicKey) > size || 129 RSA_size(key) != ANDROID_PUBKEY_MODULUS_SIZE) { 130 goto cleanup; 131 } 132 133 // Store the modulus size. 134 key_struct->modulus_size_words = ANDROID_PUBKEY_MODULUS_SIZE_WORDS; 135 136 // Compute and store n0inv = -1 / N[0] mod 2^32. 137 if (!ctx || !r32 || !n0inv || !BN_set_bit(r32, 32) || 138 !BN_mod(n0inv, key->n, r32, ctx) || 139 !BN_mod_inverse(n0inv, n0inv, r32, ctx) || !BN_sub(n0inv, r32, n0inv)) { 140 goto cleanup; 141 } 142 key_struct->n0inv = (uint32_t)BN_get_word(n0inv); 143 144 // Store the modulus. 145 if (!android_pubkey_encode_bignum(key->n, key_struct->modulus)) { 146 goto cleanup; 147 } 148 149 // Compute and store rr = (2^(rsa_size)) ^ 2 mod N. 150 if (!ctx || !rr || !BN_set_bit(rr, ANDROID_PUBKEY_MODULUS_SIZE * 8) || 151 !BN_mod_sqr(rr, rr, key->n, ctx) || 152 !android_pubkey_encode_bignum(rr, key_struct->rr)) { 153 goto cleanup; 154 } 155 156 // Store the exponent. 157 key_struct->exponent = (uint32_t)BN_get_word(key->e); 158 159 ret = true; 160 161cleanup: 162 BN_free(rr); 163 BN_free(n0inv); 164 BN_free(r32); 165 BN_CTX_free(ctx); 166 return ret; 167} 168