I had a small discussion with my friend on combining two encryption algorithms based on 64-bit keys.
Say I have a plain text of 64 bits which I want to encrypt. My Key size is 128 bits. I will use the standard DES algorithm with the first 64 bits of the key to encrypt plaintext. With the remaining 64 bits of key, I will apply the Blowfish algorithm on ciphertext that DES produces. Assume this is my “encryption algorithm”.
Now, what I want to know is: will key size is considered as 120 bits (56-bit for DES + 64 bit for Blowfish), or will it be considered as 2 individual key sizes (56 bit and 64 bit) with two different encryption algorithms? As 64 bit keys based algorithms are weak with current computational environments, can cryptanalysis attack or break the security easily?
Personally I believe double-encryption algorithms provide more security on the plain text. But somehow I am not able to convince my friend that combining two weak 64-bit algorithms will provide a double key strength. Please share your thoughts to help me understand.