Work out one time pad length instantly with clear inputs, formula shown and shareable results.
A one-time pad is the only cipher with a proof of perfect secrecy, but the proof requires a truly random key at least as long as the message, used exactly once and then destroyed. That is why it is impractical: a year of modest traffic needs 75 GB of pre-shared random key, and the key distribution problem is strictly harder than the encryption problem it solves.
Pad requirement
key bytes = message bytes; total = message bytes x messages per day x days
XORing two ciphertexts made with the same pad cancels the key and leaves the XOR of the two plaintexts, which is readily solvable. This is exactly how the VENONA decryptions succeeded.
That is precisely what a stream cipher is — a pseudorandom pad generated from a short key. It gives computational rather than perfect secrecy, but the key is 32 bytes instead of gigabytes.