Which is the principle of the encryption using a key?

[answerhappygod]

a) The key indicates which function is used for encryption. Thereby it is more difficult to decrypt an intercepted message as the function is unknown
b) The key contains the secret function for encryption including parameters. Only a password can activate the key
c) All functions are public, only the key is secret. It contains the parameters used for the encryption resp. decryption
d) The key prevents the user of having to reinstall the software at each change in technology or in the functions for encryption