DesfireCrypto

A small C++17 implementation of the cryptographic building blocks used by MIFARE DESFire integrations: AES-CBC encryption and decryption, AES-CMAC subkey generation, DESFire-style truncated CMAC output, CRC-32, and byte-vector helpers.

The repository is intentionally compact so the byte-level operations remain inspectable.

What it provides

• AES-128 encryption and decryption
• AES-CMAC generation with the first eight bytes returned for DESFire workflows
• CMAC support for empty, single-block, and multi-block messages
• Configurable session IV
• DESFire CRC-32 helper
• CMake library, example executable, and CTest target

Build and test

Requirements: a C++17 compiler and CMake 3.25 or newer.

cmake -S . -B build
cmake --build build
ctest --test-dir build --output-on-failure

The tests use the AES-CMAC examples from NIST SP 800-38B, truncated to the eight-byte value returned by getCMAC().

Example

#include <cstdint>
#include <vector>

#include "desfire_crypto/DesfireCrypto.h"

int main() {
    DesfireCrypto crypto;

    std::vector<uint8_t> key = {
        0x2b, 0x7e, 0x15, 0x16, 0x28, 0xae, 0xd2, 0xa6,
        0xab, 0xf7, 0x15, 0x88, 0x09, 0xcf, 0x4f, 0x3c,
    };
    std::vector<uint8_t> iv(16, 0x00);
    std::vector<uint8_t> message = {
        0x6b, 0xc1, 0xbe, 0xe2, 0x2e, 0x40, 0x9f, 0x96,
        0xe9, 0x3d, 0x7e, 0x11, 0x73, 0x93, 0x17, 0x2a,
    };

    crypto.initCMAC(key, iv);
    crypto.generateSubkeys();
    const auto cmac = crypto.getCMAC(message);
}

getCMAC() updates the object's IV as blocks are processed. Create a new instance or call setIv() when starting an independent calculation.

API overview

Method	Purpose
initCMAC(key, iv)	Initialize AES-CMAC state with a 16-byte key and IV
generateSubkeys()	Derive the two AES-CMAC subkeys
getCMAC(data)	Return the first eight bytes of the calculated CMAC
setIv(iv)	Replace the current session IV
encryptAes(data, key, iv)	AES-CBC encryption
decryptAes(data, key, iv)	AES-CBC decryption
crc32(data, length, output)	Calculate the four-byte CRC value

Security status

This implementation has not received an independent security audit. Validate it against the requirements and test vectors for your card/application profile before using it in production or for key-management operations. Do not log keys, derived subkeys, or session IVs.

License

MIT © Govind Yadav