Key Distribution, Encryption, and Wrapping

One important aspect of key management is to ensure that the same key used in encrypting a given message by a sender is the same key used to decrypt the message by the intended receiver. The problem is how to securely exchange the necessary keys or information without allowing others to obtain or deduce them. This is referred to as the key distribution problem. One solution is to protect the symmetric session key using a special-purpose long-term key known as a key-encrypting key (KEK). KEKs can thus be used in key distribution or key exchange processes.

In cryptography, the process of using a KEK to protect session keys is appropriately called key wrapping.

Key wrapping uses symmetric ciphers to securely encrypt (thus encapsulating) a plaintext key along with any associated integrity information and data.

Key wrapping can be used when protecting session keys in untrusted storage or when sending over an untrusted transport mechanism. Key wrapping or encapsulation using a KEK can be accomplished using either symmetric or asymmetric ciphers. If the cipher is a symmetric KEK, both the sender and receiver will need a copy of the same key. If using an asymmetric cipher with public and private key properties to encapsulate a session key, both the sender and receiver will need each other's public keys.

In today's applications, protocols such as Transport Layer Security (TLS), Pretty Good Privacy (PGP), and Secure/Multipurpose Internet Mail Extensions (S/MIME) use the services of KEKs to provide session key confidentiality and integrity.

These protocols may also use KEKs to authenticate the binding of the session key originator and the session key itself, assuring that the session key came from the real sender and not someone pretending to be authorized individuals.

Asymmetric key encryption provides a similar means to allow members of a group to conduct secure transactions spontaneously. The receiver's public key certificate, which contains the receiver's public key, is retrieved by the sender from the key server and used as part of a publickey encryption scheme, such as S/MIME, PGP, or even TLS, to encrypt a message and send it. The digital certificate is the medium that contains the public key of each member of the group; this makes the key portable, scalable, and easier to manage than an outof-band method of key exchange.