GMEllipticCurveCrypto

Elliptic Curve Crypto

An Objective-C library for Elliptic Curve Digital Signing Algorithm (ECDSA) and for Elliptic Curve Diffie-Hellman (ECDH).

ECDSA allows signatures to be generated using a private key and validated using a public key.

ECDH allows two identities to use their own private keys and each other's public key to generate a shared secret, which can then be used for encryption.

This library is largely based on the easy-ecc library (https://github.com/kmackay/easy-ecc).

Features

• Supports: secp128r1, secp192r1, secp256r1, secp384r1
• Automatically detects curve based on private or public key
• Supports keys as raw bytes or as base64 encoded strings
• BSD 2-clause license

[](https://travis-ci.org/Ankit Thakur/GMEllipticCurveCrypto)

Usage

To run the example project, clone the repo, and run pod install from the Example directory first.

Requirements

Installation

GMEllipticCurveCrypto is available through CocoaPods. To install it, simply add the following line to your Podfile:

pod "GMEllipticCurveCrypto"

Author

Ankit Thakur, ankitthakur85@gmail.com

License

GMEllipticCurveCrypto is available under the BSD 2-Clause license. See the LICENSE file for more info.

API

Generate a new ECC key pair

GMEllipticCurveCrypto *crypto = [GMEllipticCurveCrypto generateKeyPairForCurve:
                                                         GMEllipticCurveSecp192r1];
NSLog(@"Public Key: %@", crypto.publicKeyBase64);
NSLog(@"Private Key: %@", crypto.privateKeyBase64);

Using keys

Keys can be accessed and set interchangably in either raw bytes or as base64 encoded strings.

crypto.publicKeyBase64 = @"AtF8hCxh9h1zlExuOZutuw+tRzmk3zVdfA==";
NSLog(@"Public Key: base64=%@, rawBinary=%@", crypto.publicKeyBase64, crypto.publicKey);

char bytes[] = { 2, 209, 124, 132, 44, 97, 246, 29, 115, 148, 76, 110, 57, 155, 173, 
                 187, 15, 173, 71, 57, 164, 223, 53, 93, 124 };
crypto.publicKey = [NSData dataWithBytes:bytes length:25];
NSLog(@"Public Key: base64=%@, rawBinary=%@", crypto.publicKeyBase64, crypto.publicKey);

Generate a signature for a message

The signing operations require a message the same length as the curve; so generally, a hash algorithm is used to fix the original message's length.

Signatures using ECDSA will be twice the curve size. So, the 192 bit curve will produce a signature that is 48 bytes (384 bits) long.

Also note that the signature is intentionally different each time because ECDSA uses a random k value.

// The first 24 bytes of the SHA-256 hash for "Hack the Planet!"
char bytes[] = { 56, 164, 34, 250, 121, 21, 2, 18, 65, 4, 161, 90, 126, 145, 111, 204, 
                 151, 65, 181, 4, 231, 177, 117, 154 };
NSData *messageHash = [NSData dataWithBytes:bytes length:24];
        
GMEllipticCurveCrypto *crypto = [GMEllipticCurveCrypto cryptoForCurve:
                                                GMEllipticCurveSecp192r1];
crypto.privateKeyBase64 = @"ENxb+5pCLAGT88vGmE6XLQRH1e8i/0rz";
NSData *signature = [crypto signatureForHash:messageHash];
NSLog(@"Signature: %@", signature);

Verify a signature

// messageHash and signature from above

crypto = [GMEllipticCurveCrypto cryptoForCurve:GMEllipticCurveSecp192r1];
crypto.publicKeyBase64 = @"AtF8hCxh9h1zlExuOZutuw+tRzmk3zVdfA==";;
BOOL valid = [crypto verifySignature:signature forHash:messageHash];
NSLog(@"Valid Signature: %@", (valid ? @"YES": @"NO"));

Shared secret

Shared secrets using ECDH are the same length as the curve. So, the 192 bit curve will produce a shared secret that is 24 bytes (192 bits) long.

NSString *alicePublicKey = @"A9N+XWIjLCYAwa8Hb7T6Rohttqo91CF8HQ==";
NSString *alicePrivateKey = @"frs4puAKipcbevvwJb7l77xACgB/FyBv";

NSString *bobPublicKey = @"A35aoteno4wnAdJgV8AXKKl1AfPVRrSZQA==";
NSString *bobPrivateKey = @"LP83qv81MsXVyPOFV7V5jKVOoU4DKPUS";


// Alice performs...
GMEllipticCurveCrypto *alice = [GMEllipticCurveCrypto cryptoForCurve:
                                               GMEllipticCurveSecp192r1];
alice.privateKeyBase64 = alicePrivateKey;
NSData *aliceSharedSecret = [alice sharedSecretForPublicKeyBase64:bobPublicKey];
NSLog(@"Shared Secret Alice: %@", aliceSharedSecret);

// Bob performs...
GMEllipticCurveCrypto *bob = [GMEllipticCurveCrypto cryptoForCurve:
                                             GMEllipticCurveSecp192r1];
bob.privateKeyBase64 = bobPrivateKey;
NSData *bobSharedSecret = [bob sharedSecretForPublicKeyBase64:alicePublicKey];
NSLog(@"Shared Secret Bob: %@", bobSharedSecret);

// And now both parties have the same secret!
NSLog(@"Shared secrets equal? %d", [aliceSharedSecret isEqualToData:bobSharedSecret]);

Convenience functions

Automatically detects curve and sets up the private or public key

+ (GMEllipticCurveCrypto*)cryptoForKey: (NSData*)privateOrPublicKey;
+ (GMEllipticCurveCrypto*)cryptoForKeyBase64: (NSString*)privateOrPublicKey;

Automatically hash and compute the signature for a message

Include the GMEllipticCurveCrypto+hash.h category to hash data automatically before signing and verifying. The hash algorithm used must be at least the length of the curve. The hash will have the right-most bytes truncated, if necessary.

- (NSData*)hashSHA256AndSignData: (NSData*)data;
- (BOOL)hashSHA256AndVerifySignature: (NSData*)signature forData: (NSData*)data;

- (NSData*)hashSHA384AndSignData: (NSData*)data;
- (BOOL)hashSHA384AndVerifySignature: (NSData*)signature forData: (NSData*)data;

Why?

Kenneth MacKay's easy-ecc is an awesome, simple-to-use implementation of essential Elliptic Curve Cryptographic functions, however, the curve used is specified as a compile-time constant, so it cannot be changed at runtime.

This library allows any and as many different curves to be used at once.

Donations?

Sure! :-)

Bitcoin - 1LNdGsYtZXWeiKjGba7T997qvzrWqLXLma