1 files changed, 169 insertions, 0 deletions
diff --git a/libcrypto_utils/android_pubkey.c b/libcrypto_utils/android_pubkey.c
new file mode 100644
index 000000000..3052e52bf
--- /dev/null
+++ b/libcrypto_utils/android_pubkey.c
@@ -0,0 +1,169 @@
+/*
+ * Copyright (C) 2016 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include <crypto_utils/android_pubkey.h>
+
+#include <assert.h>
+#include <stdlib.h>
+#include <string.h>
+
+#include <openssl/bn.h>
+
+// Better safe than sorry.
+#if (ANDROID_PUBKEY_MODULUS_SIZE % 4) != 0
+#error RSA modulus size must be multiple of the word size!
+#endif
+
+// Size of the RSA modulus in words.
+#define ANDROID_PUBKEY_MODULUS_SIZE_WORDS (ANDROID_PUBKEY_MODULUS_SIZE / 4)
+
+// This file implements encoding and decoding logic for Android's custom RSA
+// public key binary format. Public keys are stored as a sequence of
+// little-endian 32 bit words. Note that Android only supports little-endian
+// processors, so we don't do any byte order conversions when parsing the binary
+// struct.
+typedef struct RSAPublicKey {
+    // Modulus length. This must be ANDROID_PUBKEY_MODULUS_SIZE.
+    uint32_t modulus_size_words;
+
+    // Precomputed montgomery parameter: -1 / n[0] mod 2^32
+    uint32_t n0inv;
+
+    // RSA modulus as a little-endian array.
+    uint8_t modulus[ANDROID_PUBKEY_MODULUS_SIZE];
+
+    // Montgomery parameter R^2 as a little-endian array of little-endian words.
+    uint8_t rr[ANDROID_PUBKEY_MODULUS_SIZE];
+
+    // RSA modulus: 3 or 65537
+    uint32_t exponent;
+} RSAPublicKey;
+
+// Reverses byte order in |buffer|.
+static void reverse_bytes(uint8_t* buffer, size_t size) {
+  for (size_t i = 0; i < (size + 1) / 2; ++i) {
+    uint8_t tmp = buffer[i];
+    buffer[i] = buffer[size - i - 1];
+    buffer[size - i - 1] = tmp;
+  }
+}
+
+bool android_pubkey_decode(const uint8_t* key_buffer, size_t size, RSA** key) {
+  const RSAPublicKey* key_struct = (RSAPublicKey*)key_buffer;
+  bool ret = false;
+  uint8_t modulus_buffer[ANDROID_PUBKEY_MODULUS_SIZE];
+  RSA* new_key = RSA_new();
+  if (!new_key) {
+    goto cleanup;
+  }
+
+  // Check |size| is large enough and the modulus size is correct.
+  if (size < sizeof(RSAPublicKey)) {
+    goto cleanup;
+  }
+  if (key_struct->modulus_size_words != ANDROID_PUBKEY_MODULUS_SIZE_WORDS) {
+    goto cleanup;
+  }
+
+  // Convert the modulus to big-endian byte order as expected by BN_bin2bn.
+  memcpy(modulus_buffer, key_struct->modulus, sizeof(modulus_buffer));
+  reverse_bytes(modulus_buffer, sizeof(modulus_buffer));
+  new_key->n = BN_bin2bn(modulus_buffer, sizeof(modulus_buffer), NULL);
+  if (!new_key->n) {
+    goto cleanup;
+  }
+
+  // Read the exponent.
+  new_key->e = BN_new();
+  if (!new_key->e || !BN_set_word(new_key->e, key_struct->exponent)) {
+    goto cleanup;
+  }
+
+  // Note that we don't extract the montgomery parameters n0inv and rr from
+  // the RSAPublicKey structure. They assume a word size of 32 bits, but
+  // BoringSSL may use a word size of 64 bits internally, so we're lacking the
+  // top 32 bits of n0inv in general. For now, we just ignore the parameters
+  // and have BoringSSL recompute them internally. More sophisticated logic can
+  // be added here if/when we want the additional speedup from using the
+  // pre-computed montgomery parameters.
+
+  *key = new_key;
+  ret = true;
+
+cleanup:
+  if (!ret && new_key) {
+    RSA_free(new_key);
+  }
+  return ret;
+}
+
+static bool android_pubkey_encode_bignum(const BIGNUM* num, uint8_t* buffer) {
+  if (!BN_bn2bin_padded(buffer, ANDROID_PUBKEY_MODULUS_SIZE, num)) {
+    return false;
+  }
+
+  reverse_bytes(buffer, ANDROID_PUBKEY_MODULUS_SIZE);
+  return true;
+}
+
+bool android_pubkey_encode(const RSA* key, uint8_t* key_buffer, size_t size) {
+  RSAPublicKey* key_struct = (RSAPublicKey*)key_buffer;
+  bool ret = false;
+  BN_CTX* ctx = BN_CTX_new();
+  BIGNUM* r32 = BN_new();
+  BIGNUM* n0inv = BN_new();
+  BIGNUM* rr = BN_new();
+
+  if (sizeof(RSAPublicKey) > size ||
+      RSA_size(key) != ANDROID_PUBKEY_MODULUS_SIZE) {
+    goto cleanup;
+  }
+
+  // Store the modulus size.
+  key_struct->modulus_size_words = ANDROID_PUBKEY_MODULUS_SIZE_WORDS;
+
+  // Compute and store n0inv = -1 / N[0] mod 2^32.
+  if (!ctx || !r32 || !n0inv || !BN_set_bit(r32, 32) ||
+      !BN_mod(n0inv, key->n, r32, ctx) ||
+      !BN_mod_inverse(n0inv, n0inv, r32, ctx) || !BN_sub(n0inv, r32, n0inv)) {
+    goto cleanup;
+  }
+  key_struct->n0inv = (uint32_t)BN_get_word(n0inv);
+
+  // Store the modulus.
+  if (!android_pubkey_encode_bignum(key->n, key_struct->modulus)) {
+    goto cleanup;
+  }
+
+  // Compute and store rr = (2^(rsa_size)) ^ 2 mod N.
+  if (!ctx || !rr || !BN_set_bit(rr, ANDROID_PUBKEY_MODULUS_SIZE * 8) ||
+      !BN_mod_sqr(rr, rr, key->n, ctx) ||
+      !android_pubkey_encode_bignum(rr, key_struct->rr)) {
+    goto cleanup;
+  }
+
+  // Store the exponent.
+  key_struct->exponent = (uint32_t)BN_get_word(key->e);
+
+  ret = true;
+
+cleanup:
+  BN_free(rr);
+  BN_free(n0inv);
+  BN_free(r32);
+  BN_CTX_free(ctx);
+  return ret;
+}