how does gpg know which cipher is needed (in this case AES256 instead of the default CAST5?
The OpenPGP Symmetric-Key Encrypted Session Key Packet (RFC 4880, §5.3) says which algorithm.
wouldn't it be "better" to not tell anyone what encryption type was used?
The standard security goal for an unauthenticated cipher is IND-CPA, short for ...
Symmetry is something that is inherent in the design of SPN ciphers. It doesn't explicitly make the layout of the cipher smaller, but it definitely helps. You also see this in AES-NI where you can pipeline AES so each round is an instruction as you can just put latches on the boundaries. For Feistel Ciphers, or Simon at least, I just run the ciphers as ...
S & P boxes are not random permutations.
Changing them is not easy.
The position of each element of an such a box is chosen following principles such as Strict Avalanche criterion, Bit independence criterion, non linearity, xor table distribution and maximum expected linear probability.
Not following these makes it prone to differential cryptanalysis.
Yes, Vigenère cipher is vulnerable to frequency analysis.
BUT! It requires some pre-processing first.
I propose to walk us through a small example of how frequency analysis can help decrypting Vigenère cipher in order to get a better idea of the process.
Firstly, it is important to notice that if you were to choose a key length of 1, then Vigenère cipher ...
It's actually rather easy to break the Vigenère cipher.
The first published method to decrypt Vigenère without having knowledge of the key (breaking the cipher) was done in 1863 and this could be done by hand.
Since the keyword is always repeating you would find a lot of repetitions in a ciphertext.
For example as described in Wikipedia:
If the ...
For example when I encrypt message A with key K and it outputs B, can somehow A and B be put together, so it will output K?
Except for encryption messages where you are specifically restricted to use a specifc key K only once (e.g. OTP), then no, it is infeasible to recover K from A and B.
This is a fairly fundamental requirement on encryption methods (...