Extract modulus and exponent from public key

key_parts.m

// Create the keys with SecKeyGeneratePair
// Look at https://developer.apple.com/library/ios/samplecode/CryptoExercise/Introduction/Intro.html
NSData* keyData = [self _getPublicKeyBits];
NSLog(@"Public Key: %@", [keyData hexadecimalString]);

// http://en.wikipedia.org/wiki/Abstract_Syntax_Notation_One#Example_encoded_in_DER
const char *buffer = keyData.bytes;
NSData* modulus;
NSData* exponent;
// Check for correct ASN.1 DER start
if (buffer && buffer[0] == 0x30 && buffer[1] == keyData.length - 2) {
    if (buffer[2] == 0x02) {// Check for first int number,
                            // supposed to be the modulus
        NSUInteger length = buffer[3];
        modulus = [keyData subdataWithRange:NSMakeRange(4, length)];
        
        NSUInteger expOffset = 4+length;
        if (buffer[expOffset] == 0x02) {// Check for seconds integer
            length = buffer[expOffset + 1];
            exponent = [keyData subdataWithRange:NSMakeRange(expOffset+2, length)];
        }
    }
}
NSLog(@"Modulus: %@", [modulus hexadecimalString]);
NSLog(@"Exponent: %@", [exponent hexadecimalString]);

// Should be the same for a 512 bit rsa key
//  NSData* modulus = [keyData subdataWithRange:(NSRange){ 5, 64 }];
//  NSData* exponent = [keyData subdataWithRange:(NSRange){ 5 + 64 + 2, 3 }];