What is the Gronsfeld cipher?

Gronsfeld is handled by the Vigenère tool: type a number as the key.

Gronsfeld is Vigenère with digits instead of letters. Each digit of the key says how far to shift one letter, and the number repeats as needed. A key of 31415 shifts the first letter by 3, the second by 1, the third by 4, and so on.

Puzzle caches like it because the key can be a number found somewhere else in the puzzle: a year, a phone number, part of a coordinate, or the digits of pi.

Example

KFPMT with the key 31415 decodes to HELLO.