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So, Xbox redeemable codes are 25 character case insensitive strings of letters and numbers. However, it has been suggested that not all letter/number combinations might appear (eg. 0/O). In the interest of fraud detection, what is the definitive (read as: {{Citation needed}}) list of possible characters which might make up this string?

Bonus points: given these additional rules, what are the number of possible strings that could be formed.

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While gaming related, I don't think this is a gaming question. Any 25-character string with the same restrictions will have the same possibilities. – Matthew Read May 12 '11 at 15:48
@Matt the restrictions are sort of an aside (mostly to make the question more fun), it is the rules I am interested in. Gaming.SE has long been established as a source for questions about consoles and their services (cite). – tzenes May 12 '11 at 15:50
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@Matt if you think this is a math question, please re-read it. – tzenes May 12 '11 at 15:53
@tzenes The way you used "rules" and "restrictions" differently in your comment confused me. They're not different :P If the question's "What are the restrictions/rules for forming an Xbox code?" then yeah, not a math question. – Matthew Read May 12 '11 at 15:58
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@Grace Note: Ah, nevermind—It's to resolve a debate in the comments here. – SevenSidedDie May 12 '11 at 18:00
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1 Answer

up vote 11 down vote accepted

I went straight to the source and just e-mailed Xbox Support. I didn't think they would answer, but to my surprise, they did respond (screenshot of email is the best proof i can give) with the invalid characters.

There are 8 invalid letters and 3 invalid digits:

A E I O U L S N      0 1 5

That leaves them with 18 valid letters and 7 valid digits:

B C D F G H J K M P Q R T V W X Y Z     2 3 4 6 7 8 9

With 25 characters needed for a code and 25 possible letters and digits, if my math is right, and it's entirely possible it is not, that's (18+7)25 which my calculator says is 8.8817842 × 1034, which equals a whole lot of different combinations.

Screenshot of the e-mail from Xbox Support: enter image description here

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Shouldn't that be (18+7)^25? I don't have my math-brain on right now, but by analogy to binary-number size calculations: an 8-bit binary number gives 2^8 possible permutations, which is digits^length. In this case we have (18+7) available digits, and a length of 25: (18+7)^25 or 25^25… which, oh hey, in this specific case it doesn't matter which way around they are. ;) – SevenSidedDie May 13 '11 at 16:47
@SevenSidedDie You are right, thanks for the heads up. – Doozer Blake May 13 '11 at 17:41
Wonder why they don't use S or N. – Raven Dreamer♦ May 13 '11 at 18:31
@Raven Dreamer S is too much like the 5 they don't include. But I can't figure out why the N – Doozer Blake May 13 '11 at 19:09
I would guess N because of M. B and 8 are both acceptable, and that surprises me a little. – Dave DuPlantis May 13 '11 at 21:21

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