Keystream

In cryptography, a keystream is a stream of random or pseudorandom characters that are combined with a plaintext message to produce an encrypted message (the ciphertext).

Thus we can not encrypt numbers, commas, spaces and other symbols.

To encrypt we add the keystream numbers to the plaintext.

And to decrypt we subtract the same keystream numbers from the ciphertext to get the plaintext.

Here the plaintext message "attack at dawn" is combined by addition with the keystream "kjcngmlhylyu" and produces the ciphertext "kcvniwlabluh".