Inline AES 256 / CBC Implementation

Today I like to share some encryption code, which can be used in various situations – and is also useful for embedded systems.

It is a unique implementation of the AES 256 / CBC crypto algorithm. The goal of this implementation was neither speed nor size. It was written to inline it into existing code.

The compiler will create a new instance of the algorithm on the fly, at the place where you insert it into the code. Depending of used optimization flags, this inlined code can get very compact.

Goals

I had numerous goals and constraints while I created this implementation:

  • Completely inline.
  • Generated by the compiler on the fly.
  • Easy to read and understand.
  • Suitable for embedded processors.
  • No static tables (S-Box) in memory or the binary.
  • Minimal dependencies (just stdint and cstddef headers).
  • Compatible with other AES 256 / CBC implementation.

The following things were not relevant to me:

  • Speed
  • Size
  • Multiple algorithms
  • Error checking of inputs

Requirements and Usage

The requirements are very minimal, all what you need is a C++17 compatible compiler which provides the stdint and cstddef headers for the uint32_t types and the size_t type.

Using the code is very simple because you just add an include to it:

#include "../InlineAES256/CBC.hpp"

#include <iostream>
#include <iomanip>
#include <string>


using namespace lrcrypto;


void hexBlock(const uint8_t *data, size_t size) {
    for (size_t i = 0; i < size; ++i) {
        std::cout << std::internal << std::setw(2) << std::setfill('0')
            << std::hex << static_cast<int>(data[i]);
        std::cout << " ";
    }
    std::cout << std::endl;
}


int main(int argc, const char * argv[]) {
	// Do _not_ store your key in a real application!
    const uint8_t key[aes256::cKeySizeBytes] = {
        0x4e, 0x89, 0x5a, 0xb7, 0xaf, 0x3d, 0xf6, 0xcf,
        0x6d, 0x22, 0x86, 0x01, 0x32, 0x0a, 0x8a, 0xa7,
        0x7b, 0x61, 0x2d, 0xdc, 0x92, 0xe0, 0xc9, 0x8a,
        0xd2, 0x43, 0x43, 0xb9, 0x35, 0x57, 0xad, 0x80
    };
    // Do _not_ use a static IV in a real application!
    const uint8_t iv[aes256::cBlockSizeBytes] = {
        0x44, 0xa9, 0x48, 0x66, 0x3d, 0xb9, 0x92, 0xb8,
        0x2b, 0xe7, 0xd5, 0xed, 0xf0, 0x6d, 0x71, 0x4d
    };
    // The buffer size has to be a multiple of the block size
    // which is 16 bytes.
    const int bufferSize = (aes256::cBlockSizeBytes * 4);
    uint8_t buffer[bufferSize];
    std::memset(buffer, 0, bufferSize);
    std::strcpy(reinterpret_cast<char*>(buffer), "Hello World!");
    std::cout << "Original: " << buffer << std::endl;
    hexBlock(buffer, bufferSize);
    aes256::SubstitutionBox box;
    box.initialize();
    aes256::encryptDataCBC(box, buffer, bufferSize, key, iv);
    std::cout << "Encrypted..." << std::endl;
    hexBlock(buffer, bufferSize);
    aes256::decryptDataCBC(box, buffer, bufferSize, key, iv);
    std::cout << "Decrypted: " << buffer << std::endl;
    hexBlock(buffer, bufferSize);
    return 0;
}

Source and License

I license the source using a very permissive MIT license. You can find it on GitHub:

https://github.com/LuckyResistor/InlineAES256CBC

If you have questions, miss some information or just have any feedback, feel free to add a comment below.

Have fun!

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