Secret Key Cryptography

Secret key cryptography is the classical form of cryptography. Two candidates A and B that want to share secure information use the same key for encryption and decryption, which requires prior communication between A and B over a secure channel. 

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In contrast with secret key cryptography, public key cryptography is based on separate encryption and decryption keys, where one of them can be published. Anyone can use that public key to encrypt a message that only the owner of the private key... 

Often, Ke = that is, the encryption and decryption keys are identical, and in this case we refer to it simply as the key or the secret key. This is called symmetric or secret key cryptography. In contrast, in asymmetric or public key cryptography, the encryption keys and decryption keys are different from each other, and only the decryption key needs to be kept secret. In public key cryptography, the decryption key is also sometimes called the secret key. 

As an analogy, think of the following scenario You manufacture multiple locks and an only key for those locks then you distribute the locks to your friends but keep the only key for yourself. When one of your friends wants to send you a secret letter, she will put the letter in a box and lock the box with your lock. Once it is locked, even your friend no longer can open the box because she does not have the key for the lock and because you have the only key for the lock. Therefore the box can be delivered safely to you even if you use some untrusted third party (e.g., courier, postman) for delivery. Only you can open the box with the only key you keep. Conversely, if you want to send a secret letter to your friend, you put your letter in a box and lock it with your friend s lock. Once locked, even you cannot unlock the box because your friend has the only key for the box. Consequently you are assured that only your friend can read your secret letter. Even if the box is misdelivered to someone else, the content of your letter is still kept secret because the box can be only opened by your friend, the friend you intended to access the secret letter. The key advantage of this asymmetric-key cryptography or public-key cryptography is that secret keys are kept with their owners and do not ever need to be shared with anyone else. 

Traditionally, all cryptography was symmetric key cryptography. In a symmetric key cryptosystem, the encryption key Ke and the decryption key Kd are the same, denoted simply by K. The key K must be kept secret, and it is also important that an eavesdropper who sees repeated encryptions using the same key can not learn the key. The simple substitution cipher described earlier is an example of a symmetric key cryptosystem. 

The concept of public key cryptography was originally proposed in 1976 by Whitfield Diffie and Martin Heilman, and independently by Ralph Merkle. In 1997, Britain s Government Communications Headquarters (GCHQ) released previously classified documents revealing three British government employees, James Ellis, Clifford Cocks, and Malcolm Williamson, developed these same ideas several years earlier, but kept them secret for reasons of national security. There is some evidence that the United States National Security Agency (NSA)... 

With public key cryptography, some of the key management problems are solved. However, in order for Alice s public key to be useful, it is important that others know that it is her key, and not someone else masquerading as her for the purpose of receiving her secret messages. Hence, it is important that the binding between a public key and an identity is authenticated. Wide-scale methods for doing this are called public key infrastructures, and are also described below. 

Cipher key Secret cryptography key that is used by the Key Expansion routine to generate a set of round keys. 

In conventional cryptography there is only one key used for encryption and decryption. The entire process for sending secret messages has two major components ... 

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