Computer Security and Cryptography

10.1 IN THE BEGINNING …

For centuries, encipherment was provided exclusively by conventional or single key cryptosystems. A class of transformations = {T_k : k ∈ } was defined with y = T_k(x) denoting the ciphertext resulting from the encipherment of x using the key k. Knowledge of k permitted the computation of and the recovery of the plaintext . Each party to an enciphered communication either agreed in advance to key k or a third party delivered the key over an alternative secure path. The secrecy proffered by the encipherment data depended on whether the cryptosystem T would resist cryptanalysis. Could the key k or plaintext be recovered from {y_i = T_k(x_i)} under suitable conditions?

All of this changed in 1976 with the appearance of papers by Whitfield Diffie (then a graduate student) and Martin Hellman [Diffie and Hellman 1976a,b]. They invented public key cryptography (PKC) in response to the expanded role of information processing technology in our society, coupled with access to public data networks. Encipherment would not only be needed by governments, but also to protect

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