On the Subject of the Slidefair CM

A machine that knows no bounds when it comes to ciphers.

The key below will be referred to as the Slidefair Key:

ABCDEFGHIJKLMNOPQRSTUVWXYZ
ABCDEFGHIJKLMNOPQRSTUVWXYZ

If there is a number on screen A, take out the letter at that position in the encrypted word. Set aside this letter to be put back in at a later step. Split the encrypted word into pairs of letters.

For each letter pair, do the following:

• Shift the bottom alphabet of the Slidefair Key so that the leftmost letter is the Nth letter shown on screen 1.
• Take the Nth letter pair and find the 1st letter in the top row and the 2nd letter in the bottom row.
• If the 2 letters share the same column, replace each letter with the letter that is on the opposite end of their row.
• Otherwise, replace each of them with the letter on the same row but in the column of the other letter in the original pair.

Finally, if you took out a letter, insert that letter back into position equal to the number on screen A. You should now have a decrypted word.

Example

Encrypted Word: HEYDG
Screen 1: XG
Screen A: 4

HEYDG + 4 → HEYG

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HE → SP

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YG → AE

Decrypted Word: SPAE + 4 + D → SPADE