One-Time Pad Cipher

Why
The one-time pad cipher is important because it is an unconditionally secure cipher when the necessary conditions are met

Two Conditions for Secure Substitution Ciphers
Any substitution cipher can be a one-time pad cipher if the usage of the key satisfies these two conditions for secure substitution ciphers: In practice however, the one-time pad cipher refers to the cipher using a bitwise XOR (Vernam cipher)
 * The number of bits in the key >= The number of bits in the message
 * The key is used once