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/*
* Cryptobox Message Routines
* (C) 2013 Jack Lloyd
*
* Distributed under the terms of the Botan license
*/
#include <botan/cryptobox_psk.h>
#include <botan/pipe.h>
#include <botan/lookup.h>
#include <botan/loadstor.h>
namespace Botan {
namespace CryptoBox {
namespace {
const u32bit CRYPTOBOX_MAGIC = 0x571B0E4F;
const std::string CRYPTOBOX_CIPHER = "AES-256/CBC";
const std::string CRYPTOBOX_MAC = "HMAC(SHA-256)";
const std::string CRYPTOBOX_KDF = "KDF2(SHA-256)";
const size_t MAGIC_LENGTH = 4;
const size_t KEY_KDF_SALT_LENGTH = 10;
const size_t MAC_KEY_LENGTH = 32;
const size_t CIPHER_KEY_LENGTH = 32;
const size_t CIPHER_IV_LENGTH = 16;
const size_t MAC_OUTPUT_LENGTH = 32;
}
std::vector<byte> encrypt(const byte input[], size_t input_len,
const SymmetricKey& master_key,
RandomNumberGenerator& rng)
{
std::unique_ptr<KDF> kdf(get_kdf(CRYPTOBOX_KDF));
const secure_vector<byte> cipher_key_salt =
rng.random_vec(KEY_KDF_SALT_LENGTH);
const secure_vector<byte> mac_key_salt =
rng.random_vec(KEY_KDF_SALT_LENGTH);
SymmetricKey cipher_key =
kdf->derive_key(CIPHER_KEY_LENGTH,
master_key.bits_of(),
cipher_key_salt);
SymmetricKey mac_key =
kdf->derive_key(MAC_KEY_LENGTH,
master_key.bits_of(),
mac_key_salt);
InitializationVector cipher_iv(rng, 16);
std::unique_ptr<MessageAuthenticationCode> mac(get_mac(CRYPTOBOX_MAC));
mac->set_key(mac_key);
Pipe pipe(get_cipher(CRYPTOBOX_CIPHER, cipher_key, cipher_iv, ENCRYPTION));
pipe.process_msg(input, input_len);
secure_vector<byte> ctext = pipe.read_all(0);
std::vector<byte> out(MAGIC_LENGTH);
store_be(CRYPTOBOX_MAGIC, &out[0]);
out += cipher_key_salt;
out += mac_key_salt;
out += cipher_iv.bits_of();
out += ctext;
mac->update(out);
out += mac->final();
return out;
}
secure_vector<byte> decrypt(const byte input[], size_t input_len,
const SymmetricKey& master_key)
{
const size_t MIN_CTEXT_SIZE = 16; // due to using CBC with padding
const size_t MIN_POSSIBLE_LENGTH =
MAGIC_LENGTH +
2 * KEY_KDF_SALT_LENGTH +
CIPHER_IV_LENGTH +
MIN_CTEXT_SIZE +
MAC_OUTPUT_LENGTH;
if(input_len < MIN_POSSIBLE_LENGTH)
throw Decoding_Error("Encrypted input too short to be valid");
if(load_be<u32bit>(input, 0) != CRYPTOBOX_MAGIC)
throw Decoding_Error("Unknown header value in cryptobox");
std::unique_ptr<KDF> kdf(get_kdf(CRYPTOBOX_KDF));
const byte* cipher_key_salt = &input[MAGIC_LENGTH];
const byte* mac_key_salt = &input[MAGIC_LENGTH + KEY_KDF_SALT_LENGTH];
SymmetricKey mac_key = kdf->derive_key(MAC_KEY_LENGTH,
master_key.bits_of(),
mac_key_salt,
KEY_KDF_SALT_LENGTH);
std::unique_ptr<MessageAuthenticationCode> mac(get_mac(CRYPTOBOX_MAC));
mac->set_key(mac_key);
mac->update(&input[0], input_len - MAC_OUTPUT_LENGTH);
secure_vector<byte> computed_mac = mac->final();
if(!same_mem(&input[input_len - MAC_OUTPUT_LENGTH], &computed_mac[0], computed_mac.size()))
throw Decoding_Error("MAC verification failed");
SymmetricKey cipher_key =
kdf->derive_key(CIPHER_KEY_LENGTH,
master_key.bits_of(),
cipher_key_salt, KEY_KDF_SALT_LENGTH);
InitializationVector cipher_iv(&input[MAGIC_LENGTH+2*KEY_KDF_SALT_LENGTH],
CIPHER_IV_LENGTH);
const size_t CTEXT_OFFSET = MAGIC_LENGTH + 2 * KEY_KDF_SALT_LENGTH + CIPHER_IV_LENGTH;
Pipe pipe(get_cipher(CRYPTOBOX_CIPHER, cipher_key, cipher_iv, DECRYPTION));
pipe.process_msg(&input[CTEXT_OFFSET],
input_len - (MAC_OUTPUT_LENGTH + CTEXT_OFFSET));
return pipe.read_all();
}
}
}
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