One-Time Pad is an encryption system that is unbreakable providing certain conditions are met.
The conditions that make a one-time pad impossible to crack are also ones that make the technique impractical for many modern applications. Plaintext is paired with a random secret key that is also called a one-time pad. Each bit or character of the plaintext is then encrypted by combining it with the corresponding bit or character from the pad using modular addition. The resulting ciphertext will be uncrackable if the key is truly random, of stipulated length, is never reused wholly or partly, and is an airtight secret. This presents secret key distribution challenges, hence a one-time pad’s un-scalability in the modern sphere.
One-time pad takes its name from the printing of a secret key onto a pad of piper whose top sheet could be torn off once used, limiting its use to just a single instance. One-time pad was used in concert with, or rather added to, Vernam’s Cipher. One-time pad has been used for diplomatic communiques and by the KGB with exotic means of distributing, securing, and discarding secret keys, all of which speak to the obstacle of sensible key sharing with one-time pad.
One-Time Pad can be abbreviated OTP however this may create confusion with one-time password (OTP).
“The use of one-time pad for encryption is unbreakable but the system imposes an almost unreasonable demand on people to exchange the elaborate secret-key scheme. It’s real cloak and dagger stuff: it is (almost) essential for people to share the keys that are used for encryption in person, making the system unscalable for modern use cases such as online services.”