Consider a password hash function that works as follows on a system where the password must contain only letters:

Step $1.$ Take each letter in the password and replace it with a number representing its place in the alphabet.

Step $2.$ Take each number from Step $1,$ multiply it by $2,$ and add $1.$

Step $3.$ Combine the resulting numbers, separated by $0s,$ into a single string. This string is the encrypted password.