I am current learning the internal working of several Symmetric key Cryptographic algorithms, and came across DES and AES.
DES takes in input a 64 bit key and AES-128 takes in input a 128 bit key
My question is, since the user provides these algorithms inputs of variable size (or bits), then how is it able to upscale/downscale it to lower bits in order to feed it to the algorithm. Does it uses KDF (key Derivation function)? If yes, then is the KDF capable of decreasing the size of bits too?
By decreasing I mean, If the User Inputs a key = APPLE PIES, and gives it as an input to DES. Then how is DES able to convert this (80 bit) key into a 64 bit one?