out-of-band

Information transfer using a channel or method that is outside (i.e., separate from or different from) the main channel or normal method.

Tutorial: Out-of-band mechanisms are often used to distribute shared secrets (e.g., a symmetric key) or other sensitive information items (e.g., a root key) that are needed to initialize or otherwise enable the operation of cryptography or other security mechanisms. Example: Using postal mail to distribute printed or magnetic media containing symmetric cryptographic keys for use in Internet encryption devices. (See: key distribution.)