Private-key encryption (also called symmetric encryption or secret-key encryption) involves using the same key for encryption and decryption. Encryption involves applying an operation (an algorithm) to the data to be encrypted using the private key to make them unintelligible. The slightest algorithm (such as an exclusive OR) can make the system nearly tamper proof (keep in mind that there is no such thing as absolute security).
However, in the 1940s, Claude Shannon proved that to be completely secure, private-key systems need to use keys that are at least as long as the message to be encrypted. Moreover, symmetric encryption requires the use of a secure channel in order to exchange the key, which seriously diminishes the usefulness of this kind of encryption system.
The main disadvantage of a secret-key cryptosystem is related to the exchange of keys, making the following problem of key distribution arise:
A user wanting to communicate with several people while ensuring separate confidentiality levels has to use as many private keys as there are people. For a group of N people using a secret-key cryptosystem, it is necessary to distribute a number of keys equal to N * (N-1) / 2.
In the 1920s, Gilbert Vernam and Joseph Mauborgne developed the One-Time Pad method (sometimes called One-Time Password and abbreviated OTP), based on a randomly generated private key that is used only once and is then destroyed. During the same period, the Kremlin and the White House were connected by the famous red telephone - a telephone where calls were encrypted thanks to a private key following the one-time pad method. The private key was exchanged thanks to the diplomatic bag (playing the role of the secure channel).