Given a String plaintext and a keyword of length n, arrange the plaintext into rows of length n. Then, the plaintext columns are re-arranged into the alphabetical order of the letters in the key. The output is created by going down the resequenced columns, one character at a time. Read a far more complete description on this page:

https://en.wikipedia.org/wiki/Transposition_cipher#Columnar_transposition

Note that this is the irregular version with no "filler" letters to complete the rectangle if it is not completely filled. It is quite possible that the last row of plaintext, therefore, will be a different length than the other rows. Having unequal length rows also makes for an additional challenge writing the columns into the output.

The first test case uses only letters for plaintext and key, but actually, in theory, this cipher can function with any visible characters.