flutter_eddsa 0.0.3

Flutter FFI plugin for Ed25519 digital signatures and X25519 Diffie-Hellman key exchange. High-performance Curve25519 cryptography via native C.

flutter_eddsa

Elliptic-curve cryptography for Flutter — Ed25519 digital signatures and X25519 Diffie-Hellman key exchange, delivered through a direct native FFI bridge with no platform-channel overhead.

The same algorithms that secure Signal, WireGuard, OpenSSH, and TLS 1.3 — now available as a clean, cross-platform Flutter plugin.

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Why Curve25519?

Curve25519 is the modern standard for high-speed, high-security elliptic-curve cryptography:

• Ed25519 — deterministic digital signatures. Fast to sign, fast to verify, no random number generator required at signing time, and immune to the class of fault attacks that affect ECDSA.
• X25519 — Diffie-Hellman key agreement. Two parties can establish a shared secret over an untrusted channel without ever transmitting a private key.

Both primitives are built on the same underlying curve, which means an Ed25519 key pair can be converted to X25519 format — useful when a single long-term key pair needs to serve both signing and key-exchange roles.

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Features

• Ed25519 public key derivation, signing, and verification
• X25519 Diffie-Hellman key agreement
• Key conversion — Ed25519 ↔ X25519 for both public and secret keys
• Pure native performance via dart:ffi — no method channels, no serialisation overhead
• Cryptographically secure random key generation
• Android · iOS · Linux · macOS · Windows

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Installation

dependencies:
  flutter_eddsa: ^0.0.1

flutter pub get

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Usage

Generate a key pair

import 'package:flutter_eddsa/flutter_eddsa.dart';

// Generate a random 32-byte secret key
final secret = EddsaUtils.generateRandom32();

// Derive the corresponding Ed25519 public key
final publicKey = Ed25519.derivePublicKey(secret);

Sign a message

final message   = EddsaUtils.bytesFromString('Hello, flutter_eddsa!');
final signature = Ed25519.signMessage(secret, publicKey, message);

Verify a signature

final isValid = Ed25519.verifySignature(signature, publicKey, message);
print(isValid); // true

X25519 Diffie-Hellman key exchange

Diffie-Hellman lets two parties establish a shared secret over a public channel without ever transmitting a private key. The idea: each party combines their own secret with the other party's public key, and the mathematics of the elliptic curve guarantees both arrive at the same result.

Alice                              Bob
─────                              ───
aliceSecret (private)              bobSecret (private)
alicePublic = secret × G           bobPublic  = secret × G
         ──── exchange public keys ────▶
sharedSecret = aliceSecret × bobPublic
                                   sharedSecret = bobSecret × alicePublic
         (same value on both sides — proven by the associativity of scalar multiplication)

In code:

// Each party generates a key pair
final aliceSecret = EddsaUtils.generateRandom32();
final alicePublic  = Ed25519.generateX25519PublicKey(aliceSecret);

final bobSecret = EddsaUtils.generateRandom32();
final bobPublic  = Ed25519.generateX25519PublicKey(bobSecret);

// They exchange public keys (safe to send over untrusted network)
// Each computes the shared secret independently
final aliceShared = Ed25519.diffieHellman(aliceSecret, bobPublic);
final bobShared   = Ed25519.diffieHellman(bobSecret,   alicePublic);

// Both arrive at the same 32-byte value
assert(EddsaUtils.hexFromBytes(aliceShared) ==
       EddsaUtils.hexFromBytes(bobShared));

// Use the shared secret to derive an encryption key (e.g. with HKDF or SHA-256)

The shared secret is typically passed through a key-derivation function (KDF) before use as a symmetric encryption key.

Convert an Ed25519 key pair to X25519

final xSecret = Ed25519.secretKeyToX25519(secret);
final xPublic  = Ed25519.publicKeyToX25519(publicKey);

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API reference

Ed25519

All methods are static. Keys are 32 bytes; signatures are 64 bytes.

Method	Input	Output	Description
derivePublicKey(secret)	32 B secret	32 B public key	Derives an Ed25519 public key
signMessage(secret, publicKey, message)	keys + arbitrary message	64 B signature	Signs a message
verifySignature(signature, publicKey, message)	64 B sig + key + message	bool	Verifies a signature
generateX25519PublicKey(scalar)	32 B scalar	32 B public key	Scalar × base point (X25519 key generation)
scalarMultiply(scalar, point)	32 B scalar + 32 B point	32 B result	Raw X25519 scalar multiplication
diffieHellman(secret, peerPublicKey)	own secret + peer public	32 B shared secret	X25519 key agreement
publicKeyToX25519(edPublicKey)	32 B Ed25519 public key	32 B X25519 public key	Key format conversion
secretKeyToX25519(edSecretKey)	32 B Ed25519 secret key	32 B X25519 secret key	Key format conversion

EddsaUtils

Method	Description
generateRandom32()	Cryptographically secure 32-byte random value
bytesFromHex(hex)	Decode a lowercase hex string to Uint8List
hexFromBytes(bytes)	Encode Uint8List as a lowercase hex string
bytesFromString(text)	Encode a UTF-8 string as bytes
stringFromBytes(bytes)	Decode bytes to a UTF-8 string

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Platform support

Platform	Status
Android	✅
iOS	✅
macOS	✅
Linux	✅
Windows	✅
Web	❌ (not yet supported)

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Native cryptographic core

The cryptographic implementation is provided by libeddsa — a compact, public-domain C library by Philipp Lay with the following properties:

• Written in portable C99
• Constant-time scalar multiplication (timing-attack resistant)
• Stack-cleaning after secret key operations
• No external dependencies
• Under 90 KB compiled

The C source is vendored directly into src/crypto/ and compiled as part of the Flutter build for each platform — no pre-built binaries, no external downloads.

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Security notes

• Keys and signatures are passed as raw Uint8List — never log or persist secret keys.
• generateRandom32() uses Dart's Random.secure(), which draws from the OS entropy pool.
• This plugin provides primitives only. For a complete secure channel you will also need a symmetric cipher and message authentication (e.g. AES-GCM or ChaCha20-Poly1305).

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License

This plugin is licensed under the GNU General Public License v3.0 — see LICENSE.

The bundled C cryptographic library (src/crypto/) is in the public domain (original work by Philipp Lay).