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SerialKeyGenerator

SerialKeyGenerator is an Objective-C library for Mac OSX for creating and validating serial keys (to be used, for instance, in software licensing). The output is serial keys in the form XXXXX-XXXXX-XXXXX-XXXXX-XXXXX made from numbers [0-9] and upper case characters [A-Z] (essentially, a base36 number).

Features:

  • Encrypted/decrypted using a secret key with Skipjack block cipher
  • Serial key contains: date of generation, sequence number and a 32 bit custom data
  • Serials keys are randomized
  • Supports both ARC and non ARC applications

Internal packed (unencoded) format

Byte 0: Identifier, always 0xAA
Byte 1:	Random byte
Bytes 2-5: 32 bit sequence
Bytes 6-9: 32 bit user data
Bytes 10-13: 32 bit date
Byte 14: Random byte
Byte 16: Identifier, always 0xAA

Usage

Add the following files to your project:

- skipjack.h
- skipjack.m
- SerialKeyGenerator.h
- SerialKeyGenerator.m

And #import "SerialKeyGenerator.h" in your source.

Generation Sample

NSString *secret = @"mysecret";
// Create the SerialKeyGenerator with your secret
SerialKeyGenerator *skg = [SerialKeyGenerator serialKeyGeneratorWithSecret:secret];
// Set count, start sequence and user data
NSUInteger count = 50;
UInt32 start = 1;
UInt32 data = 0;

// generate ONE serial key with given sequence number and user data
NSString * serial = [skg generateSerialWithSequence:start data:data];

// generate a LIST of 50 serial keys with given sequence start number, count and user data
NSArray *serials = [skg generateSerials:count start:start data:data];

Validation Sample

// don't actually save your secret like this
NSString *secret = @"mysecret";
NSString *serial = @"CR70H-64SLH-DY3R1-UDN96-VDK6B";
// Create the SerialKeyGenerator with your secret
SerialKeyGenerator *skg = [SerialKeyGenerator serialKeyGeneratorWithSecret:secret];
// decode the serial and get the info structure
id<SerialKeyInfo> serialInfo = [skg decodeSerial:serial];

UInt32 sequence = [serialInfo sequence];
UInt32 data = [serialInfo data];
NSString *dateString = [serialInfo.date description];

About Security

Skipjack block cipher is not bullet proof and has known attacks (see wikipedia article on Skipjack)

Another problem is that if the secret key needs to be included in the client software in order to decrypt the entered serial key that makes it potentially susceptible to reverse engineering and discovering your secret key. This is not a problem if you add some online serial validation scheme (not a feature included with this library). But if you include your secret in the client, don't add it as plain text like this:

NSString *secret = @"mysecretkey";

Rather, try to mangle it a bit. It won't stop a determined hacker but it won't be just plainly seen with a hex editor:

// create a the secret string: *$#12345abcABC
NSString *part = @"ABC";
NSString *secret = [[NSString stringWithFormat:@"%@%i%@%@", @"*$#", 12345, @"abc", part]];

Or be creative.