Tweak AES cbc to add pcks7 padding bytes

include/axolotl/crypto.hh

@@ -42,17 +42,11 @@ struct Aes256Iv {
 };
 
 
-std::size_t aes_pkcs_7_padded_length(
+std::size_t aes_encrypt_cbc_length(
     std::size_t input_length
 );
 
 
-void aes_pkcs_7_padding(
-    std::uint8_t const * input, std::size_t input_length,
-    std::uint8_t * output
-);
-
-
 void aes_encrypt_cbc(
     Aes256Key const & key,
     Aes256Iv const & iv,
@@ -61,7 +55,7 @@ void aes_encrypt_cbc(
 );
 
 
-void aes_decrypt_cbc(
+std::size_t aes_decrypt_cbc(
     Aes256Key const & key,
     Aes256Iv const & iv,
     std::uint8_t const * input, std::size_t input_length,

include/axolotl/list.hh

@@ -15,6 +15,11 @@ public:
     T const * begin() const { return _data; }
     T const * end() const { return _end; }
 
+    /**
+     * Is the list empty?
+     */
+    bool empty() { return _end == _data; }
+
     /**
      * The number of items in the list.
      */

src/crypto.cpp

@@ -107,26 +107,13 @@ void axolotl::curve25519_shared_secret(
 }
 
 
-std::size_t axolotl::aes_pkcs_7_padded_length(
+std::size_t axolotl::aes_encrypt_cbc_length(
     std::size_t input_length
 ) {
     return input_length + AES_BLOCK_LENGTH - input_length % AES_BLOCK_LENGTH;
 }
 
 
-void axolotl::aes_pkcs_7_padding(
-    std::uint8_t const * input, std::size_t input_length,
-    std::uint8_t * output
-) {
-    std::memcpy(output, input, input_length);
-    std::size_t padded_length = axolotl::aes_pkcs_7_padded_length(input_length);
-    std::uint8_t padding = padded_length - input_length;
-    for (std::size_t i = input_length; i < padded_length; ++i) {
-        output[i] = padding;
-    }
-}
-
-
 void axolotl::aes_encrypt_cbc(
     axolotl::Aes256Key const & key,
     axolotl::Aes256Iv const & iv,
@@ -137,17 +124,28 @@ void axolotl::aes_encrypt_cbc(
     ::aes_key_setup(key.key, key_schedule, 256);
     std::uint8_t input_block[AES_BLOCK_LENGTH];
     std::memcpy(input_block, iv.iv, AES_BLOCK_LENGTH);
-    for (std::size_t i = 0; i < input_length; i += AES_BLOCK_LENGTH) {
-        xor_block<AES_BLOCK_LENGTH>(input_block, &input[i]);
-        ::aes_encrypt(input_block, &output[i], key_schedule, 256);
-        std::memcpy(input_block, &output[i], AES_BLOCK_LENGTH);
+    while (input_length >= AES_BLOCK_LENGTH) {
+        xor_block<AES_BLOCK_LENGTH>(input_block, input);
+        ::aes_encrypt(input_block, output, key_schedule, 256);
+        std::memcpy(input_block, output, AES_BLOCK_LENGTH);
+        input += AES_BLOCK_LENGTH;
+        output += AES_BLOCK_LENGTH;
+        input_length -= AES_BLOCK_LENGTH;
+    }
+    std::size_t i = 0;
+    for (; i < input_length; ++i) {
+        input_block[i] ^= input[i];
+    }
+    for (; i < AES_BLOCK_LENGTH; ++i) {
+        input_block[i] ^= AES_BLOCK_LENGTH - input_length;
     }
+    ::aes_encrypt(input_block, output, key_schedule, 256);
     std::memset(key_schedule, 0, sizeof(key_schedule));
     std::memset(input_block, 0, sizeof(AES_BLOCK_LENGTH));
 }
 
 
-void axolotl::aes_decrypt_cbc(
+std::size_t axolotl::aes_decrypt_cbc(
     axolotl::Aes256Key const & key,
     axolotl::Aes256Iv const & iv,
     std::uint8_t const * input, std::size_t input_length,
@@ -164,6 +162,8 @@ void axolotl::aes_decrypt_cbc(
         }
     }
     std::memset(key_schedule, 0, sizeof(key_schedule));
+    std::size_t padding = output[input_length - 1];
+    return (padding > input_length) ? std::size_t(-1) : (input_length - padding);
 }
 
 

tests/test_crypto.cpp

@@ -73,24 +73,27 @@ TestCase test_case("AES Test Case 1");
 
 axolotl::Aes256Key key = {};
 axolotl::Aes256Iv iv = {};
-std::uint8_t input[32] = {};
+std::uint8_t input[16] = {};
 
 std::uint8_t expected[32] = {
     0xDC, 0x95, 0xC0, 0x78, 0xA2, 0x40, 0x89, 0x89,
     0xAD, 0x48, 0xA2, 0x14, 0x92, 0x84, 0x20, 0x87,
-    0x08, 0xC3, 0x74, 0x84, 0x8C, 0x22, 0x82, 0x33,
-    0xC2, 0xB3, 0x4F, 0x33, 0x2B, 0xD2, 0xE9, 0xD3
+    0xF3, 0xC0, 0x03, 0xDD, 0xC4, 0xA7, 0xB8, 0xA9,
+    0x4B, 0xAE, 0xDF, 0xFC, 0x3D, 0x21, 0x4C, 0x38
 };
 
+std::size_t length = axolotl::aes_encrypt_cbc_length(sizeof(input));
+assert_equals(std::size_t(32), length);
+
+
 std::uint8_t actual[32] = {};
 
 axolotl::aes_encrypt_cbc(key, iv, input, sizeof(input), actual);
-
 assert_equals(expected, actual, 32);
 
-axolotl::aes_decrypt_cbc(key, iv, expected, sizeof(expected), actual);
-
-assert_equals(input, actual, 32);
+length = axolotl::aes_decrypt_cbc(key, iv, expected, sizeof(expected), actual);
+assert_equals(std::size_t(16), length);
+assert_equals(input, actual, length);
 
 } /* AES Test Case 1 */