# Alphanumeric Substitution Cipher

An alphanumeric substitution cipher is any code or cipher that represents numbers through letters and/or letters through numbers, especially by assigning numbers to letters of the Latin alphabet.

## Puzzle Application

The most basic version of an alphanumeric substitution cipher is one that presents a message with each letter encoded into its own position in the Latin alphabet. Therefore a message like "THE ANSWER TO THIS PUZZLE IS..." would be encoded as such:

T-H-E A-N-S-W-E-R T-O T-H-I-S P-U-Z-Z-L-E I-S...                             20-8-5 1-14-19-23-5-18 20-15 20-8-9-19 16-21-26-26-12-5 9-19...

This cipher is commonly known as A1Z26, as A is represented by the number 1 and Z is represented by 26. Puzzles may also present this A1Z26 substitution via an ordered series of 26 items (such as the playable character in Super Smash Bros. Melee) or the first 26 entries in a larger ordered series (such as the first 26 in a book series, like The Hardy Boys). By presenting solvers with members of these lists, the cipher can still function if they take the ordinal number of each of the given entries, and perform the decryption as normal.

Alternative versions of this cipher may utilize other alphabets, such as the Greek alphabet (which has 24 letters) or the Hawaiian alphabet (which has only 12 letters), or use collections of symbols that have numbers associated with them already (such as Unicode).

These ciphers can also be used in the opposite direction, translating numbers into letters and requiring solvers to identify the numbers that originally belonged in their place. This application is what allows Cryptarithm puzzles to function. This application also does not commonly use the A1Z26 system, as representing individual letters in creative ways is less possible, and the alphametics-style of puzzle does not allow for extraneous clues.

It's also possible for an alphanumeric puzzle to not conform to the A1Z26 model at all, instead using either a complete substitution (with "random" letter/number pairings), or introduce more complicated mathematical operations to represent letters in sequence, such as adding a series of numbers together after going through the A1Z26 conversion. These methods are far more useful for sending actual secret messages, or creating puzzles that are significantly more cryptographically-focused.

## Strategy

Depending on how the constructor has applied an alphanumeric substitution, identifying its use may be very easy or very difficult. On the easy end is if solvers are presented with numbers within the range of 1 and 26 directly. While it may not always be an A1Z26 cipher, it's almost certain that translating the letters in that fashion will either result in a useful string of text, or gibberish. The latter would imply that it's either not A1Z26, or that it is, and there's multiple steps to decoding it (in which case, start looking for more directions).

If a puzzle does not have numbers within this range (but still has strings of numbers), consider the number of unique numbers. If that is still within 26, it could still be this cipher, and it may just be a simple cryptogram. Alternatively, it could be A1Z26 but with the numbers needing to be taken modulo 26, providing new numbers within the correct range.

If there are no numbers at all, but there are letters arranged in mathematical equations, then you're looking at an cryptarithm puzzle, and should bring over the best mathematician on your team.

If there are no numbers or mathematic equations, you could still be dealing with an alphanumeric substitution cipher. However, it would show up later in the puzzle, likely for extraction. If you finish most of a puzzle and are left with a bunch of numbers, repeat the check above. Additionally, if you go through a puzzle and are left with a series of names or items that seem to belong to the same set, consider checking if they're all within the first 26 members of that set, and perform the decryption based on their position.

## Notable Examples

• Big Musical Number (MITMH 2008) (web) - Replaces numbers in a large math equation with letters from A to T. Click to revealThe trick is that each of the letters represents a number found in the lyrics of the songs in the audio file, meaning that the actual replaced numbers range from 10 to 40,000,307.
• Flags o1 6ur 10the15 (MITMH 2018) (web) - Click to revealNot only involves the A1Z26 cipher in the final extraction, but uses a mathematical alphanumeric substitution cipher based on the last-used instances of particular letters, and adding together the numbers of those positions for any cases of consecutive replacements.