Generating unique, hard-to-guess "coupon" codes

Question

My Rails app needs to generate electronic coupons for users. Each coupon given should have a unique coupon code that can be redeemed on our system.

For example a coupon for a free burrito. User A receives a coupon for a free burrito and then User B receives a coupon for a free burrito. The 2 coupons should have unique coupon codes.

What is the best way to generate a code like this that is not easily forged? I don't want users to have a high success rate of typing in random numbers and redeeming other peoples coupons.

I guess thinking about it like a gift card with a unique number on the back is what I am looking for.

Answer 1

The code needs to be unguessable, because the only verification you can perform before giving the user their reward is to check whether the code they entered exists in your list of "issued" codes.

• That means the number of all possible codes in that format is much larger than the number of codes you want to issue,. Depending on how easy it is to simply try codes (think of a script trying repeatedly), then you might need all possible codes to outnumber issued codes by a factor of a million or a billion or more. This sounds high, but is possible in relatively short strings.

• It also means that the codes that you use must be chosen as randomly as possible within all possible codes. This is necessary to avoid users figuring out that most valid codes start with "AAA" for example. More sophisticated users might spot that your "random" codes use a hackable random number generator (Ruby's default rand() is fast and statistically good for random data, but is hackable in this way, so don't use it).

The starting point for such a secure code would be the output from a cryptographic PRNG. Ruby has the securerandom library, which you can use to get a raw code like this:

require 'securerandom'
SecureRandom.hex
# => "78c231af76a14ef9952406add6da5d42"

This code is long enough to cover any realistic number of vouchers (millions each for everyone on the planet), without any meaningful chance of repetition or being easy to guess. However, it is a bit awkward to type from a physical copy.

Once you know how to generate a random, practically unguessable code, your next problem is understanding user experience and deciding how much you can realistically compromise security in the name of usability. You need to bear in mind the value to the end user, and therefore how hard someone might try to get a valid code. I cannot answer that for you, but can make some general points about usability:

• Avoid ambiguous characters. In print, it is sometimes difficult to see the difference between 1, I and l for example. We often understand what it is supposed to be from context, but a randomised string of characters does not have this context. It would be a bad user experience to have to try several variations of a code by testing 0 vs O, 5 vs S etc.

• Use either lower case or upper case letters but not both. Case sensitivity will not be understood or followed by some %age of your users.

• Accept variations when matching codes. Allow spaces and dashes. Perhaps even allow 0 and O to mean the same thing. This is best done by processing the input text so it is in the right case, strip separator characters etc.

• In print, separate the code into a few small parts, it will be easier for the user to find their place in the string and type a few characters at once.

• Don't make the code too long. I would suggest 12 characters, in 3 groups of 4.

• Here's an interesting one - you may want to scan the code for possible rude words, or avoid the characters that would generate them. If your code contained only the characters K, U, F, C, then there would be a high chance of offending a user. This isn't usually a concern because users do not see most computer secure codes, but these ones will be in print!

Putting that all together, this is how I might generate a usable code:

# Random, unguessable number as a base20 string
#  .reverse ensures we don't use first character (which may not take all values)
raw_string = SecureRandom.random_number( 2**80 ).to_s( 20 ).reverse
# e.g. "3ecg4f2f3d2ei0236gi"


# Convert Ruby base 20 to better characters for user experience
long_code = raw_string.tr( '0123456789abcdefghij', '234679QWERTYUPADFGHX' )
# e.g. "6AUF7D4D6P4AH246QFH"


# Format the code for printing
short_code = long_code[0..3] + '-' + long_code[4..7] + '-' + long_code[8..11]
# e.g. "6AUF-7D4D-6P4A"

There are 20**12 valid codes in this format, which means you can issue a billion of your own codes, and there would be one in four million chance of a user simply guessing a correct one. In cryptography circles that would be very bad (this code is insecure against a fast local attack), but for a web form offering free burritos to registered users, and where you would notice a someone trying four million times with a script, it is ok.