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One-time pad

One-time pad (OTP) is an encryption algorithm in which the plaintext is combined with a secret random key or pad, which is used only once. A modular addition is typically used to combine plaintext elements with pad elements. For binary data, the operation XOR amounts to the same thing as modular addition.
A one time pad is the only currently known unconditionally secure encryption system. Other encryption systems are cryptographically secure which means that they have a cost associated with breaking, this cost will be very high, but it would theoretically be possible to break if enough compute time could be gathered.

To be effective, the random key must be at least the same size as the message and be used only once. In OTP is difficult to distribute the pads of random numbers to all the necessary parties.