X9 X9.143-2021 describes a method consistent with the requirements of ANSI X9.24 Retail Financial Services Symmetric Key Management Part 1 for the secure exchange of keys between SCDs that share a symmetric key to wrap keys and other relevant data. This could be host to host or host to transaction-originating SCD. This method may also be used for the storage of keys under a symmetric key. Interoperability may be less of a factor when storing keys for use with a given implementation. The symmetric key used for storage need not be shared when using this method for key storage. This document establishes the security requirements for an interoperable Key Block, which is a structure containing the encrypted key and its attributes wrapped together.