# Botan

# Message Authentication Codes (MAC)

A Message Authentication Code algorithm computes a tag over a message utilizing a shared secret key. Thus a valid tag confirms the authenticity and integrity of the message. Only entities in possession of the shared secret key are able to verify the tag.

Note

When combining a MAC with unauthenticated encryption mode, prefer to first encrypt the message and then MAC the ciphertext. The alternative is to MAC the plaintext, which depending on exact usage can suffer serious security issues. For a detailed discussion of this issue see the paper “The Order of Encryption and Authentication for Protecting Communications” by Hugo Krawczyk

The Botan MAC computation is split into five stages.

1. Instantiate the MAC algorithm.

2. Set the secret key.

3. Process IV.

4. Process data.

5. Finalize the MAC computation.

class MessageAuthenticationCode
  std::string name() const

    Returns a human-readable string of the name of this algorithm.

  void clear()

    Clear the key.

  std::unique_ptr<MessageAuthenticationCode> new_object() const

    Return a newly allocated object of the same type as this one. The new object is unkeyed.

  void set_key(const uint8_t *key, size_t length)

    Set the shared MAC key for the calculation. This function has to be called before the data is processed.

  bool valid_keylength(size_t length) const

    This function returns true if and only if length is a valid keylength for the algorithm.

  size_t minimum_keylength() const

    Return the smallest key length (in bytes) that is acceptable for the algorithm.

  size_t maximum_keylength() const

    Return the largest key length (in bytes) that is acceptable for the algorithm.

  void start(const uint8_t *nonce, size_t nonce_len)

    Set the IV for the MAC calculation. Note that not all MAC algorithms require an IV. If an IV is required, the function has to be called before the data is processed. For algorithms that don’t require it, the call can be omitted, or else called with `nonce_len` of zero.

  void update(const uint8_t *input, size_t length)

    Process the passed data.

  void update(const secure_vector<uint8_t> &in)

    Process the passed data.

  void update(uint8_t in)

    Process a single byte.

  void final(uint8_t *out)

    Complete the MAC computation and write the calculated tag to the passed byte array.

  secure_vector<uint8_t> final()

    Complete the MAC computation and return the calculated tag.

  bool verify_mac(const uint8_t *mac, size_t length)

    Finalize the current MAC computation and compare the result to the passed `mac`. Returns `true`, if the verification is successful and false otherwise.

## Code Examples

The following example computes an HMAC with a random key then verifies the tag.

    #include <botan/auto_rng.h>
    #include <botan/hex.h>
    #include <botan/mac.h>

    #include <assert.h>

    namespace {

    std::string compute_mac(std::string_view msg, std::span<const uint8_t> key) {
       auto hmac = Botan::MessageAuthenticationCode::create_or_throw("HMAC(SHA-256)");

       hmac->set_key(key);
       hmac->update(msg);

       return Botan::hex_encode(hmac->final());
    }

    }  // namespace

    int main() {
       Botan::AutoSeeded_RNG rng;

       const auto key = rng.random_vec(32);  // 256 bit random key

       // "Message" != "Mussage" so tags will also not match
       const std::string tag1 = compute_mac("Message", key);
       const std::string tag2 = compute_mac("Mussage", key);
       assert(tag1 != tag2);

       // Recomputing with original input message results in identical tag
       const std::string tag3 = compute_mac("Message", key);
       assert(tag1 == tag3);

       return 0;
    }

The following example code computes a AES-256 GMAC and subsequently verifies the tag. Unlike most other MACs, GMAC requires a nonce which must not repeat or all security is lost.

    #include <botan/hex.h>
    #include <botan/mac.h>

    #include <iostream>

    int main() {
       const auto key = Botan::hex_decode_locked("1337133713371337133713371337133713371337133713371337133713371337");
       const auto nonce = Botan::hex_decode("FFFFFFFFFFFFFFFFFFFFFFFF");
       const auto data = Botan::hex_decode_locked("6BC1BEE22E409F96E93D7E117393172A");

       const auto mac = Botan::MessageAuthenticationCode::create_or_throw("GMAC(AES-256)");
       mac->set_key(key);
       mac->start(nonce);
       mac->update(data);
       const auto tag = mac->final();
       std::cout << mac->name() << ": " << Botan::hex_encode(tag) << '\n';

       // Verify created MAC
       mac->start(nonce);
       mac->update(data);
       std::cout << "Verification: " << (mac->verify_mac(tag) ? "success" : "failure");
       return 0;
    }

The following example code computes a valid AES-128 CMAC tag and modifies the data to demonstrate a MAC verification failure.

    #include <botan/hex.h>
    #include <botan/mac.h>

    #include <iostream>

    int main() {
       const auto key = Botan::hex_decode_locked("2B7E151628AED2A6ABF7158809CF4F3C");
       auto data = Botan::hex_decode("6BC1BEE22E409F96E93D7E117393172A");

       const auto mac = Botan::MessageAuthenticationCode::create_or_throw("CMAC(AES-128)");
       mac->set_key(key);
       mac->update(data);
       const auto tag = mac->final();

       // Corrupting data
       data.back()++;

       // Verify with corrupted data
       mac->update(data);
       std::cout << "Verification with malformed data: " << (mac->verify_mac(tag) ? "success" : "failure");

       return 0;
    }

## Available MACs

Currently the following MAC algorithms are available in Botan. In new code, default to HMAC with a strong hash like SHA-256 or SHA-384.

### Blake2B MAC

Available if `BOTAN_HAS_BLAKE2BMAC` is defined.

Algorithm specification name: `BLAKE2b(<optional output bits>)` (reported name) / `Blake2b(<optional output bits>)`

- Output bits defaults to 512.

- Examples: `BLAKE2b(256)`, `BLAKE2b`

### CMAC

A modern CBC-MAC variant that avoids the security problems of plain CBC-MAC. Approved by NIST. Also sometimes called OMAC.

Available if `BOTAN_HAS_CMAC` is defined.

Algorithm specification name: `CMAC(<BlockCipher>)` (reported name) / `OMAC(<BlockCipher>)`, e.g. `CMAC(AES-256)`

### GMAC

GMAC is related to the GCM authenticated cipher mode. It is quite slow unless hardware support for carryless multiplications is available. A new nonce must be used with each message authenticated, or otherwise all security is lost.

Available if `BOTAN_HAS_GMAC` is defined.

Warning

Due to the nonce requirement, GMAC is exceptionally fragile. Avoid it unless absolutely required.

Algorithm specification name: `GMAC(<BlockCipher>)`, e.g. `GMAC(AES-256)`

### HMAC

A message authentication code based on a hash function. Very commonly used.

Available if `BOTAN_HAS_HMAC` is defined.

Algorithm specification name: `HMAC(<HashFunction>)`, e.g. `HMAC(SHA-512)`

### KMAC

Added in version 3.2.

A SHA-3 derived message authentication code defined by NIST in SP 800-185.

There are two variants, `KMAC-128` and `KMAC-256`. Both take a parameter which specifies the output length in bits, for example `KMAC-128(256)`.

Available if `BOTAN_HAS_KMAC` is defined.

Algorithm specification names:

- `KMAC-128(<output size>)`, e.g. `KMAC-128(256)`

- `KMAC-256(<output size>)`, e.g. `KMAC-256(256)`

### Poly1305

A polynomial mac (similar to GMAC). Very fast, but tricky to use safely. Forms part of the ChaCha20Poly1305 AEAD mode. A new key must be used for each message, or all security is lost.

Available if `BOTAN_HAS_POLY1305` is defined.

Warning

Due to the nonce requirement, Poly1305 is exceptionally fragile. Avoid it unless absolutely required.

Algorithm specification name: `Poly1305`

### SipHash

Deprecated since version 3.8.0.

SipHash is primarily designed for hash table randomization and, while not known to be insecure for message authentication, is not advisable for this use due to the small output size (just 64 bits).

Defaults to “SipHash(2,4)” which is the recommended configuration, using 2 rounds for each input block and 4 rounds for finalization.

Available if `BOTAN_HAS_SIPHASH` is defined.

Algorithm specification name: `SipHash(<optional C>,<optional D>)`

- C defaults to 2

- D defaults to 4

- Examples: `SipHash(2,4)`, `SipHash(2)`, `SipHash`

### X9.19-MAC

Deprecated since version 3.7.0.

A CBC-MAC variant sometimes used in finance. Always uses DES. Sometimes called the “DES retail MAC”, also standardized in ISO 9797-1.

It is slow and has known attacks. Avoid unless required.

Available if `BOTAN_HAS_X919_MAC` is defined.

Algorithm specification name: `X9.19-MAC`