It is easy to crack a ciphertext encrypted by Vigenere cipher, if you know the plaintext is in a natural language like English. There are two methods to discover the key used to encrypt the plaintext. Either you know frequencies of certain characters that will occur more often than others (
e for English) or you know a word that will occur in the plaintext (
a, etc. for English).
Cryptographically secure random
What if the plaintext is actually CSR (cryptographically secure random) data, and you encrypt it using Vigenere cipher with a CSR key.
Let's say we reuse the key four times (to account for flaws if random is not CSR?). Also, the key is sufficiently large so that brute forcing it is not an option (for example, the key may be 1KB, and therefore the cipher and plaintext is 4KB).
Would it be possible to crack that, even though you are reusing the same key (the key is also CSR) but not as large as the plaintext data?
or: how secure is this relative to other methods for encryption/decryption?
In case you ask yourself: 'why would you want to encrypt random data?':
Well, if the answer is no, and thus it is impossible to crack it. That would mean that once a shared secret is established between two persons, you could establish an information-theoretically secure connection, right? Because once you have a shared secret key 'A' of 1KB. Then you could create a shared secret key 'B' of 2KB using 'A' two times on newly random generated data. Then you could use 1KB of 'B' as a one-time pad on your actual plaintext communications in English. And you could use the other 1KB of 'B' for safely communicating the next 2KB sized key and so forth.