Modern day cryptography involves a lot of number theory. Passwords and other sensitive data need to be encrypted before being transmitted via the internet, there can be several encryption schemes, here are two simple encryption schemes
Password : "secret" and if the encryption scheme is a shift of 1 letter in the alphabet then the encrypted password = tfdsfu. If there is a key (key value abcdef) used along with our sample password "secret" then each of the letters in the password is shifted by 1,2,,3,4,5 and 6 letters respectively so the encrypted password is "tgfvjz"
So if the encrypted password is "bvrf" and the key is "adeg" what is the original password ?..