Cryptographic protocols
Richard A. DeMillo, Nancy Ann Lynch, Michael Merritt · 1982
A cryptographic transformation is a mapping f from a set of cleartext messages, M, to a set of ciphertext messages. Since for m e M, f(m) should hide the contents of m from an enemy, f-1 should, in a certain technical sense, be difficult to infer from f(m) and public knowledge about f.