From 816435a86097a6609cb6e5ad422083bc49b19632 Mon Sep 17 00:00:00 2001
From: Mark Haines <mark.haines@matrix.org>
Date: Thu, 11 Jun 2015 14:20:35 +0100
Subject: Move AES specific details behind a cipher interface

---
 src/cipher.cpp | 125 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 125 insertions(+)
 create mode 100644 src/cipher.cpp

(limited to 'src/cipher.cpp')

diff --git a/src/cipher.cpp b/src/cipher.cpp
new file mode 100644
index 0000000..86cde88
--- /dev/null
+++ b/src/cipher.cpp
@@ -0,0 +1,125 @@
+#include "axolotl/cipher.hh"
+#include "axolotl/crypto.hh"
+#include "axolotl/memory.hh"
+#include <cstring>
+
+axolotl::Cipher::~Cipher() {
+
+}
+
+namespace {
+
+static const std::size_t SHA256_LENGTH = 32;
+
+struct DerivedKeys {
+    axolotl::Aes256Key aes_key;
+    std::uint8_t mac_key[SHA256_LENGTH];
+    axolotl::Aes256Iv aes_iv;
+};
+
+
+static void derive_keys(
+    std::uint8_t const * kdf_info, std::size_t kdf_info_length,
+    std::uint8_t const * key, std::size_t key_length,
+    DerivedKeys & keys
+) {
+    std::uint8_t derived_secrets[80];
+    axolotl::hkdf_sha256(
+        key, key_length,
+        NULL, 0,
+        kdf_info, kdf_info_length,
+        derived_secrets, sizeof(derived_secrets)
+    );
+    std::memcpy(keys.aes_key.key, derived_secrets, 32);
+    std::memcpy(keys.mac_key, derived_secrets + 32, 32);
+    std::memcpy(keys.aes_iv.iv, derived_secrets + 64, 16);
+    axolotl::unset(derived_secrets);
+}
+
+static const std::size_t MAC_LENGTH = 8;
+
+} // namespace
+
+
+axolotl::CipherAesSha256::CipherAesSha256(
+    std::uint8_t const * kdf_info, std::size_t kdf_info_length
+) : kdf_info(kdf_info), kdf_info_length(kdf_info_length) {
+
+}
+
+
+std::size_t axolotl::CipherAesSha256::mac_length() const {
+    return MAC_LENGTH;
+}
+
+
+std::size_t axolotl::CipherAesSha256::encrypt_ciphertext_length(
+    std::size_t plaintext_length
+) const {
+    return axolotl::aes_encrypt_cbc_length(plaintext_length);
+}
+
+
+std::size_t axolotl::CipherAesSha256::encrypt(
+    std::uint8_t const * key, std::size_t key_length,
+    std::uint8_t const * plaintext, std::size_t plaintext_length,
+    std::uint8_t * ciphertext, std::size_t ciphertext_length,
+    std::uint8_t * output, std::size_t output_length
+) const {
+    if (encrypt_ciphertext_length(plaintext_length) < ciphertext_length) {
+        return std::size_t(-1);
+    }
+    struct DerivedKeys keys;
+    std::uint8_t mac[SHA256_LENGTH];
+
+    derive_keys(kdf_info, kdf_info_length, key, key_length, keys);
+
+    axolotl::aes_encrypt_cbc(
+        keys.aes_key, keys.aes_iv, plaintext, plaintext_length, ciphertext
+    );
+
+    axolotl::hmac_sha256(
+        keys.mac_key, SHA256_LENGTH, output, output_length - MAC_LENGTH, mac
+    );
+
+    std::memcpy(output + output_length - MAC_LENGTH, mac, MAC_LENGTH);
+
+    axolotl::unset(keys);
+    return output_length;
+}
+
+
+std::size_t axolotl::CipherAesSha256::decrypt_max_plaintext_length(
+    std::size_t ciphertext_length
+) const {
+    return ciphertext_length;
+}
+
+std::size_t axolotl::CipherAesSha256::decrypt(
+     std::uint8_t const * key, std::size_t key_length,
+     std::uint8_t const * input, std::size_t input_length,
+     std::uint8_t const * ciphertext, std::size_t ciphertext_length,
+     std::uint8_t * plaintext, std::size_t max_plaintext_length
+) const {
+    DerivedKeys keys;
+    std::uint8_t mac[SHA256_LENGTH];
+
+    derive_keys(kdf_info, kdf_info_length, key, key_length, keys);
+
+    axolotl::hmac_sha256(
+        keys.mac_key, SHA256_LENGTH, input, input_length - MAC_LENGTH, mac
+    );
+
+    std::uint8_t const * input_mac = input + input_length - MAC_LENGTH;
+    if (!axolotl::is_equal(input_mac, mac, MAC_LENGTH)) {
+        axolotl::unset(keys);
+        return std::size_t(-1);
+    }
+
+    std::size_t plaintext_length = axolotl::aes_decrypt_cbc(
+        keys.aes_key, keys.aes_iv, ciphertext, ciphertext_length, plaintext
+    );
+
+    axolotl::unset(keys);
+    return plaintext_length;
+}
-- 
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