User talk:Sandy Harris/Sandbox

Public key systems

Public key or asymmetric key cryptography was first proposed, in the open literature, in 1976 by Whitfield Diffie and Martin Hellman.. The historian David Kahn described public-key cryptography as "the most revolutionary new concept in the field since polyalphabetic substitution emerged in the Renaissance". . There are two reasons public key cryptography is so important. One is that it solves the key management problem described in the preceding section; the other is that public key techniques are the basis for digital signatures.

In a public key system, keys are created in matched pairs, such that when one of a pair is used to encrypt, the other must be used to decrypt. The system is designed so that calculation of one key from knowledge of the other is computationally infeasible, even though they are necessarily related. Keys are generated secretly, in interrelated pairs. One key from a pair becomes the public key and can be published. The other is the private key and is kept secret, never leaving the user's computer.

One big payoff is that two users (traditionally, A and B or Alice and Bob) need not share a secret key in order to communicate securely. When used for content confidentiality, the public key is typically used for encryption, while the private key is used for decryption. If Alice has (a trustworthy, verified copy of) Bob's public key, then she can encrypt with that and know that only Bob can read the message since only he has the matching private key. He can reply securely using her public key. This solves the key management problem. The difficult question of how to communicate secret keys securely does not need to even be asked; the private keys are never communicated and there is no requirement that communication of public keys be done securely.

Moreover, key management on a single system becomes much easier. In a system based on secret keys, if Alice communicates with $N$ people, her system must manage $N$ secret keys all of which change periodically, all of which must sometimes be communicated, and each of which must be kept secret from everyone except the one person it is used with. For a public key system, the main concern is managing her own private key; that generally need not change and it is never communicated to anyone.

Of course, she must also manage the public keys for her correspondents. In some ways, this is easier; they are already public and need not be kept secret. However, it is absolutely necessary to authenticate each public key. Consider a philandering husband sending passionate messages to his mistress. If the wife creates a public key in the mistress' name and he does not check the key's origins before using it to encrypt messages, he may get himself in deep trouble.

The other big payoff is that, given a public key cryptosystem, digital signatures are a straightforward application. The basic principle is that if Alice uses her private key to encrypt some known data then anyone can decrypt with her public key and, if they get the right data, they know (assuming the system is secure and her private key unknown to others) that it was her who did the encryption. In effect, she can use her private key to sign a document. The details are somewhat more complex and are dealt with in a later section.

Practical use of asymmetric cryptography, on any sizable basis, requires a public key infrastructure (PKI). A public key will normally be embedded in a digital certificate that is issued by a certification authority. In the event of compromise of the private key, the certification authority can revoke the key by adding it to a certificate revocation list. Digital certificates, like passports or other identification documents, usually have expiration dates, and a means of verifying both the validity of the certificate and of the certificate issuer.

In many applications, public keys are widely published — on the net, in the phonebook, on business cards, on key server computers which provide an index of public keys. However, it is also possible to use public key technology while restricting access to public keys.

Public key exchanges are used to open up secure secret key channels between strangers across the internet.

The early asymmetric techniques were vulnerable to some forms of cryptanalysis, until Diffie and Hellman showed that public-key cryptography was practical by presenting the Diffie-Hellman key exchange protocol. In 1978, Ronald Rivest, Adi Shamir, and Len Adleman invented Rivest-Shamir-Adleman (RSA), another public-key system Previously released as an MIT "Technical Memo" in April 1977, and published in Martin Gardner's Scientific American "Mathematical Recreations" column</ref>.

In 1997, it finally became publicly known that asymmetric cryptography had been invented by James H. Ellis at GCHQ, a British intelligence organization, in the early 1970s, and that both the Diffie-Hellman and RSA algorithms had been previously developed (by Malcolm J. Williamson and Clifford Cocks, respectively).