A string containing information that binds the ECDH derived key to the two parties involved or to the context of the key. It may include details like identities of the two parties deriving the key, context of the operation, session IDs, and optionally a nonce. It must not contain zero bytes. It is not recommended to reuse shared information for multiple ECDH key derivations, as it could result in derived key material being the same across different derivations.