From: MenTaLguY <mental@...>
Date: 2008-03-27T06:33:27+09:00
Subject: Re: Encode as alphanumeric

On Thu, 27 Mar 2008 05:44:12 +0900, Trans <transfire@gmail.com> wrote:
> On Mar 26, 4:27 pm, MenTaLguY <men...@rydia.net> wrote:
>> On Thu, 27 Mar 2008 05:13:23 +0900, Trans <transf...@gmail.com> wrote:
>> > I need to take an arbitrary byte string and encode it as alphanumeric
>> > (and decode it back again). Any pointers on a easy way to do this?
>>
>> What sort of encoding do you have in mind?  Hexadecimal?  Base64?
> 
> I'm using a two-way encryption algorithm to create a registration key.
> The encryption algorithm produces an arbitrary byte string. I need to
> encode that into something I can give to a end-user, ie. an
> alphanumeric string.
> 
> Base64 is close but not quite [a-zA-Z0-9].

I see.  Base 62 then?  The most direct way is to get things as a bignum
and then convert bases using divmod.

 DIGIT_CHARS = ["0".."9", "a".."z", "A".."Z"].map { |r| r.to_a }.flatten
 BASE = DIGIT_CHARS.size
 DIGIT_VALUES = Hash[*(0...BASE).map { |i| [ DIGIT_CHARS[i], i ] }.flatten]

 def convert_base(digits, from_base, to_base)
   bignum = 0
   digits.each { |digit| bignum = bignum * from_base + digit }
   converted = []
   until bignum.zero?
     bignum, digit = bignum.divmod to_base
     converted.push digit
   end
   converted.reverse
 end

 def encode(byte_string)
   convert_base(byte_string.unpack("C*"), 256, BASE).map { |d|
     DIGIT_CHARS[d]
   }.join('')
 end

 def decode(encoded)
   convert_base(encoded.split('').map { |c|
     DIGIT_VALUES[c]
   }, BASE, 256).pack("C*")
 end

There are more efficient ways of accomplishing this, of course.

Note that with this implementation, you'd probably want to zero-fill to
whatever standard number of characters and bytes you're using, and of
course a key of all zeroes will result in an empty string.

As a usability thing, I'd also suggest non-uniformly adding dashes or
spaces in standard places to the encoded form, to help users visually
"chunk" the keys if they ever might have to type them in.

-mental