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We modified the encryption algorithm so it will work despite losing the password... Probably.
Files given:
The encryption method is pretty simple, convert the plaintext to a number , find the length of the file, compute the ciphertext and save this as the encrypted file.
Notice that file length, as a mathmatical function, is simply , and since , if and , the file size of the encrypted file, being , is almost the same as the plaintext file as is a very small number.
We can numerically approximate the length of the file with your favourite numerical root finding algorithm, for instance Newton-Raphson to roughly an just brute force values near it and check if is divisible by , if it is, then compute and save it. With this, we get the flag:
The script to get the flag can be found at solve.sage