Why is the Porta cipher reciprocal?

Each of its 13 alphabets swaps letters in pairs, so applying the same substitution twice returns the original letter. That means encryption and decryption are the identical operation: feed the ciphertext back in with the same keyword and you get the plaintext.

How does the keyword choose an alphabet?

Key letters are grouped in pairs: A or B selects the first tableau row, C or D the second, and so on up to Y or Z for the thirteenth. The keyword repeats over the message, so each plaintext letter is enciphered with the alphabet for the matching key letter.

How is it different from the Vigenere cipher?

Both are polyalphabetic and keyword-driven, but Vigenere uses 26 shifting alphabets and is not reciprocal, so encryption and decryption differ. Porta uses 13 reciprocal alphabets, always maps the first half of the alphabet to the second half and back, and is its own inverse.

Is the Porta cipher secure today?

No. Dating from 1563, it is a historical polyalphabetic cipher broken easily with modern techniques such as Kasiski examination and frequency analysis once the key length is found. Use it for learning and puzzles, never for real protection.

What happens to spaces, digits, and punctuation?

Only letters are enciphered. Spaces, digits, and punctuation pass through unchanged and do not consume a key letter, so the keyword stays aligned with the letters of your message. Letter case is preserved in the output.

Porta Cipher — Gera Tools

Giovanbattista della Porta described this cipher in 1563, and it was one of the first practical polyalphabetic systems. Its defining trait is that it is reciprocal: there is no separate decrypt mode, because running ciphertext back through with the same keyword restores the plaintext.

How it works

Porta uses 13 alphabets, one for each pair of key letters (A/B, C/D, … Y/Z). Within every alphabet the first half of the letters (A–M) always maps into the second half (N–Z) and vice versa, with a shift determined by the key-letter pair:

row r = floor((keyLetter - 'A') / 2)        (0..12)
if plaintext index i < 13:  cipher = ((i + r) mod 13) + 13
else:                       cipher = (i - 13 - r) mod 13

Because A–M and N–Z are always exchanged, the function is its own inverse — the same key encrypts and decrypts.

Tips and example

With the keyword LEMON, the message ATTACK AT DAWN enciphers to SEAUVP PA XTEL; feeding that ciphertext back in with LEMON returns the original. Choose a long, non-dictionary keyword to make the repeating-key pattern harder to spot, though be clear this is a historical cipher: it falls quickly to classical cryptanalysis and is meant for puzzles and study, not real secrets.