What is the Trithemius cipher?

Johannes Trithemius described this cipher in 1508. The first letter is not shifted, the second moves one place, the third two places, and so on through the alphabet. It works like a Vigenère cipher whose key is the whole alphabet in order.

Some puzzles start at a different shift or step by a different amount, even backwards, so you can set both. The cipher identifier checks the common versions automatically.

Example

Tig fehnl qb rpdr decodes to The cache is here.