Generate random looking code from consecutive integers

Question

I would like to generate a 6-character alphanumeric code (eg. A3SJ8D) from a 32-bit positive integer where each code in the sequence does not appear similar to the previous.

A3SJ8D
G54FGS
...

This code must be reversible so that G54FGS could be translated back to 2, for example (1:1 mapping).

This "randomness" is not for security purposes but for very simple obfuscation. In other words, the method need not be "secure".

Edit

To clarify, I understand that the max possible value for an unsigned 32-bit integer, (2^32)-1, exceeds the max possible value of a 6-character alpha-numeric code using 10 digits and 26 letters, (36^6)-1. So, the positive integer to be encoded must not overflow the bounds established by the number of characters available to the code set.

Answered!

Example

Here is a simple code example in Javascript based on @nwellnhof's accepted answer below.


var Skip32 = require('skip32').Skip32,
    key = "0123456789".split("").map(function(c) { return c.charCodeAt(0) }),
    cipher = new Skip32(key),
    codelen = 6,
    radix = 36,
    max = Math.pow(radix,codelen);

function numToCode(num) {
    while ((num = cipher.encrypt(num)) >= max) {}
    return num.toString(radix).toUpperCase();
}

function codeToNum(code) {
    var num = parseInt(code,radix);
    while ((num = cipher.decrypt(num)) >= max) {}
    return num;
}

1-1 mapping? A given integer can have more than one alphanumeric code? What about negative integers? — जलजनक, Feb 25 '13 at 18:41
`"Simplest implementation wins"` sounds more like something for [CodeGolf](http://codegolf.stackexchange.com/). Voting to close. — Bernhard Barker, Feb 25 '13 at 18:43
What have you tried? What do you mean by wins? is it a sort of competition? — jurgenreza, Feb 25 '13 at 18:44
unique 32 bit Intergers = 4,294,967,296 & unique six-char alphanums = ((26+10)^6) = 2,176,782,336... Simply put, impossible. — जलजनक, Feb 25 '13 at 18:47
@jurgenreza Presumably OP means the winner gets the accepted answer, but this doesn't fit on SO. — Bernhard Barker, Feb 25 '13 at 18:47
What you're looking for is essentially a simple encryption algorithm. — nwellnhof, Feb 25 '13 at 18:50
@Dukeling I know, it is a bit insulting though. Voting to close. — jurgenreza, Feb 25 '13 at 19:02
@DoSparKot you may want to post that as an answer to "win" :) — jurgenreza, Feb 25 '13 at 19:07
Possible duplicate [Simple Integer Encryption](http://stackoverflow.com/questions/2769539/simple-integer-encryption) — जलजनक, Feb 25 '13 at 19:46
Ok. Removed the "win" line since people like to get touchy. @DoSparKot Yes, I understand not all 32-bits can be used. Ok to truncate at 30. And yes, I am looking for a 1-1 mapping. Positive integers only. Thanks. — Jonathan Hawkes, Feb 25 '13 at 19:50
@jurgenreza No. Not a competition. I only meant that there is obviously more than one way to skin this cat, and I'm looking for a simple answer. — Jonathan Hawkes, Feb 25 '13 at 19:52

score 2 · Accepted Answer · answered Feb 25 '13 at 19:09

2

I'd go with the SKIP32 cipher which is a 32-bit block cipher based on Skipjack. Simply choose a random key, encrypt the integer, and output the result in base 36. You can find a implementation in C here.

answered Feb 25 '13 at 19:09

nwellnhof

32,319
7
89
113

A 32-bit block cipher looks to be exactly what I'm looking for. Any variation should do. And since I really only need the first 30 bits (so I don't overflow the 6 characters), I can use the folding method described here. http://blog.notdot.net/2007/9/Damn-Cool-Algorithms-Part-2-Secure-permutations-with-block-ciphers – Jonathan Hawkes Feb 25 '13 at 20:36

Joni · Answer 2 · 2013-03-02T01:14:24.503

1

You'll get a sequence of fairly random looking codes if you multiply 1,2,3,... by a fairly large odd integer and use base 36 to convert to string.

For example, if you multiply by 123456789 you get the following sequence for 1, 2, 3, 4:

1: 21i3v9
2: 4307qi
3: 64iblr
4: 860fh0

To reverse the operation multiply by the multiplicative inverse, for example 102505021 in the case of 123456789.

To make the sequence look a bit more "random" you can further scramble the numbers with an xor or addition.

In fact, this is how the linear congruential generator of pseudorandom numbers works.

edited Mar 02 '13 at 01:14

answered Feb 25 '13 at 20:07

Joni

108,737
14
143
193

I thought of doing something like this, but then I limit the number of 6-char codes I can generate. I wanted at least a billion possible codes. Thanks for answering though. – Jonathan Hawkes Feb 25 '13 at 20:45
Multiplication by an odd integer modulo a power of two is one-to-one. There are no limitations. – Joni Feb 25 '13 at 21:41

Generate random looking code from consecutive integers

Answered!

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