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 Public key AlgorithmsPublic key cryptosystems are based upon the following idea. In these systems the encryption and decryption keys (K and K-1) are not equal. So we use a pair of keys, one to perform the encryption and another to perform the decryption. The concept is very simple. We first generate a pair of keys and transmit one of them (the public key) which we will use for encryption to the party with whom we wish to communicate with. They then encrypt data using the key we gave them and we use the key we kept to decrypt that data. The public key cannot be used to decrypt and so no third party can decipher our data. The algorithm should be designed such that it is impossible to deduce the value of our private key from the value of the key we gave to the sender of the data. In practice no-one has ever designed an algorithm where it is actually impossible to deduce one key from the other since they are mathematically related. However, in practice it is possible to design an algorithm where it is so difficult to compute one key from the other that it becomes practically impossible. This is the principle that modern public key cryptosystems work under. Typically we’ll refer to this scheme like so. We write encryption as
and decryption as:
Where C is the ciphertext, EA is the encryption key (the public key), P is the plaintext and DA is the decryption key (the private key).
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