Key distribution

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In symmetric key cryptography, both parties must possess a secret key which they must exchange prior to using any encryption. Distribution of secret keys has been problematic until recently, because it involved face-to-face meeting, use of a trusted courier, or sending the key through an existing encryption channel. The first two are often impractical and always unsafe, while the third depends on the security of a previous key exchange.

In public key cryptography, the key distribution of public keys is done through public key servers. When a person creates a key-pair, he keeps one key private and the other, public-key, is uploaded to a server where it can be accessed by anyone to send the user a private, encrypted, message. Disclosure of these public keys is not only not a problem, but is actively encouraged. The private keys are never transmitted, and can therefore be physically secured.

Secure Sockets Layer (SSL) uses Diffie-Hellman key exchange if the client does not have a public-private key pair and a published certificate in the Public Key Infrastructure, and Public Key Cryptography if the user does have both the keys and the credential.

In secret sharing a secret (password, key, trade secret,...) is used as a seed to generate a number of distinct secrets, and the pieces are distributed so that some subset of the recipients can jointly authenticate themselves and use the secret information without learning what it is. Secret sharing is also called secret splitting, key splitting, and split knowledge.

We want to share N secrets among M people so that any M < N of them (M of N) can regenerate the original information, but no smaller group up to M − 1 can do so. There are mathematical problems of this type, such as the number of points needed to identify a polynomial of a certain degree (used in Shamir's scheme), or the number of intersecting hyperplanes needed to specify a point (used in Blakley's scheme). We can hand out data specifying any number of points on the curve, or hyperplanes through the point, without altering the number needed to solve the problem and, in our application, access the protected resource.