m511chacha20 0.0.3

End-to-End-encryption Suite with curve M-511 for ECDH and ChaCha20 for files, strings and maps.

This package provides curve M-511 for ECDH and ChaCha20 for symmetric encryption. Together, they provide the building blocks for end-to-end encryption.

Introduction

Elliptic Curve Diffie-Hellman (ECDH) allows two parties to establish a shared secret over an insecure channel, even in the presence of an eavesdropper. This package implements ECDH based on the elliptic curve M-511, as specified in Bernstein et al. (2013) https://eprint.iacr.org/2013/647.pdf and considered safe according to https://safecurves.cr.yp.to. It provides 256-bit security, matching the 256-bit key size used by ChaCha20.

ChaCha20 is a widely used, fast, and secure symmetric encryption algorithm, implemented according to RFC 7539 (https://datatracker.ietf.org/doc/html/rfc7539). ChaCha20 uses a secret key and a nonce to generate a pseudorandom keystream. Encryption is performed by XORing this keystream with the plaintext. Therefore, it acts like a key expansion mechanism that expands a secret key and nonce into a pseudorandom keystream that can be used like an infinite One-Time Pad.

Features

• ECDH with curve M-511
• Chacha20 encryption

Getting started

Add the package by writing

flutter pub add m511chacha20

Full E2EE rundown

First, Alice and Bob each create a Keypair.

    KeyPair alice = KeyPair.fresh();
    KeyPair bob = KeyPair.fresh();

Each KeyPair has a public key hex that is safe to be published.

    alice.publicKeyHex
    bob.publicKeyHex

Alice and Bob each compute the same shared key. This key can be used to derive a shared key.

    BigInt aliceSharedSecret = KeyPair.computeSharedSecret(alice.privateKeyHex, bob.publicKeyHex);
    BigInt bobSharedSecret = KeyPair.computeSharedSecret(bob.privateKeyHex, alice.publicKeyHex);

    expect(aliceSharedSecret, bobSharedSecret);

You can then use this key, to create a ChaCha20 encrypter:

ChaCha20 chaCha20 = ChaCha20(ChaCha20Key.fromBigInt(commonSecret));

To encrypt or decrypt a message, you must provide a nonce; You can imagine a nonce as an "extra piece of randomness". While for a key it is important to be entirely secret, for a nonce it is important to be entirely random.

Nonce.fromUUID(...) or
Nonce.fromString(..)

will hash the string using sha256 and take the 12 leading bytes (96 bits) to create a nonce from a string. Once you have a nonce, you can finally encrypt and decrypt using chacha20 like

chaCha20.encrypt(plainText, nonce)
chaCha20.decrypt(cipherText, nonce)

File encryption

If you want to encrypt or decrypt Files, you can use

await computeFileListEncryptionPromise(
      FileListEncryptionPromise(
          filePromises: [
            FileEncryptionPromise(
                inputFile: "test/test_files/plaintext_example.txt",
                nonce: nonce,
                key: key,
                outputFile: "test/test_files/encrypted_example.txt"
            )
          ]
      ),
      onCipherFinished: (CipherResult cipherResult) {
        finishCalled = true;
      }
    )

Full example

KeyPair alice = KeyPair.fresh();
KeyPair bob = KeyPair.fresh();

BigInt aliceSharedSecret = KeyPair.computeSharedSecret(
    alice.privateKeyHex,
    bob.publicKeyHex
);
BigInt bobSharedSecret = KeyPair.computeSharedSecret(
    bob.privateKeyHex,
    alice.publicKeyHex
);
BigInt commonSecret = KeyPair.computeSharedSecret(bob.privateKeyHex,alice.publicKeyHex);
Nonce nonce = Nonce.fromUUID(Uuid().v1());
ChaCha20 chacha20 = ChaCha20(ChaCha20Key.fromBigInt(commonSecret)); 
String cipherText = chacha20.encrypt(plainText, nonce);
String plainText = chacha20.decrypt(cipherText,nonce);

File Encryption

bool wasSuccess = await computeFileListEncryptionPromise(
  FileListEncryptionPromise(
      filePromises: [
        FileEncryptionPromise(
            inputFile: "test/test_files/plaintext_example.txt",
            nonce: nonce,
            key: key,
            outputFile: "test/test_files/encrypted_example.txt"
        )
      ]
  ),
  onCipherFinished: (CipherResult cipherResult) {
    // ADD YOUR LOGIC HERE
  }
)

Additional information

You can contribute on github https://codeberg.org/LocalityMedia/m511chacha20