If we assume that the encryption algorithm is just simple static encryption using xor, we can just search for plaintext inside ciphertext, like this:
for i in ciphertext:
key = ciphertext[i:i+4] xor "This"
if decrypt(ciphertext[i:<length of plaintext>], key) == " program cannot run in DOS mode":
we found it!!!
else:
continue searching
It's as simple as that—we assume the key from the result of the xoring ciphertext and the first few bytes of the plaintext and then test this key with the remaining plain text. If this key works it will reveal the remaining plain text of the ciphertext, which means that you will have found the secret key and can decrypt the remaining data.
