What is a Private Key? What is a Public Key?

Public key cryptography is a form of cryptography which generally allows users to communicate securely without having prior access to a shared secret key. This is done by using a pair of cryptographic keys, designated as public key and private key, which are related mathematically.

In public key cryptography, the private key is kept secret, while the public key may be widely distributed. In a sense, one key "locks" a lock; while the other is required to unlock it. It should not be feasible to deduce the private key of a pair given the public key, and in high quality algorithms no such technique is known.

One analogy is that of a locked store front door with a mail slot. The mail slot is exposed and accessible to the public; its location (the street address) is in essence the public key. Anyone knowing the street address can go to the door and drop a written message through the slot. However, only the person who possesses the matching private key, the store owner in this case, can open the door and read the message.

The term asymmetric key cryptography is a synonym for public key cryptography though a somewhat misleading one. There are asymmetric key encryption algorithms that do not have the public key-private key property noted above. For these algorithms, both keys must be kept secret, that is both are private keys.

There are many forms of public key cryptography, including:

* public key encryption, which keeps a message secret from anyone that does not possess a specific private key;
* public key digital signature, which allows anyone to verify that a message was created with a specific private key; and,
* key agreement, which generally, allows two parties that may not initially share a secret key to agree on one.

(From Wikipedia, the free encyclopedia.)
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