Ronald Linn Rivest (; born May 6, 1947) is an American cryptographer and computer scientist whose work has spanned the fields of algorithms and combinatorics, cryptography, machine learning, and election integrity. He is an Institute Professor at the Massachusetts Institute of Technology (MIT), and a member of MIT's Department of Electrical Engineering and Computer Science and its Computer Science and Artificial Intelligence Laboratory. Along with Adi Shamir and Len Adleman, Rivest is one of the inventors of the RSA algorithm, for which they won the 2002 ACM Turing Award. He is also the inventor of the symmetric key encryption algorithms RC2, RC4, and RC5, and co-inventor of RC6. He also devised the MD2, MD4, MD5 and MD6 cryptographic hash functions.
Education Rivest earned a bachelor's degree in mathematics from Yale University in 1969, and a Ph.D. degree in computer science from Stanford University in 1974 for research supervised by Robert W. Floyd.
Career At MIT, Rivest is a member of the Theory of Computation Group, and founder of MIT CSAIL's Cryptography and Information Security Group. Rivest was a founder of RSA Data Security (now merged with Security Dynamics to form RSA Security), Verisign, and of Peppercoin. His former doctoral students include Avrim Blum, Benny Chor, Sally Goldman, Burt Kaliski, Anna Lysyanskaya, Margrit Betke, Ron Pinter, Robert Schapire, Alan Sherman, and Mona Singh.
Research Rivest is especially known for his research in cryptography. He has also made significant contributions to algorithm design, to the computational complexity of machine learning, and to election security.
Cryptography Rivest, jointly with Adi Shamir and Leonard Adleman, introduced the RSA cryptosystem in 1978,[C1] which revolutionized modern cryptography by providing the first usable and publicly described method for public-key cryptography. Rivest had reportedly thought of the key idea behind the cryptosystem after having drunk large amounts of wine while celebrating Passover with Shamir and Adleman at a student's house. The three won the 2002 Turing Award for "their ingenious contribution to making public-key cryptography useful in practice." The same paper was also the first to introduce Alice and Bob, the fictional heroes of many subsequent cryptographic protocols. In the same year, Rivest, Adleman, and Michael Dertouzos first formulated homomorphic encryption and its applications in secure cloud computing,[C2] an idea that would not come to fruition until over 40 years later when secure homomorphic encryption algorithms were finally developed. Rivest was one of the inventors of the GMR public signature scheme, published with Shafi Goldwasser and Silvio Micali in 1988,[C3] and of ring signatures, an anonymized form of group signatures invented with Shamir and Yael Tauman Kalai in 2001.[C7] He designed the MD4 and MD5 cryptographic hash functions, published in 1990 and 1992 respectively,[C4][C5] and a sequence of symmetric key block ciphers that include RC2, RC4, RC5, and RC6.[C6][C8] Other contributions of Rivest to cryptography include chaffing and winnowing, the interlock protocol for authenticating anonymous key-exchange, cryptographic time capsules such as LCS35 based on anticipated improvements to computation speed through Moore's law, key whitening and its application through the xor–encrypt–xor key mode in extending the Data Encryption Standard to DES-X, and the Peppercoin system for cryptographic micropayments.
Algorithms In 1973, Rivest and his coauthors published the first selection algorithm that achieved linear time without using randomization.[A1] Their algorithm, the median of medians method, is commonly taught in algorithms courses. Rivest is also one of the two namesakes of the Floyd–Rivest algorithm, a randomized selection algorithm that achieves a near-optimal number of comparisons.[A2] Rivest's 1974 doctoral dissertation concerned the use of hash tables to quickly match partial words in documents; he later published this work as a journal paper.[A3] His research from this time on self-organizing lists[A4] became one of the important precursors to the development of competitive analysis for online algorithms. In the early 1980s, he also published well-cited research on two-dimensional bin packing problems,[A5] and on channel routing in VLSI design.[A6] He is a co-author of Introduction to Algorithms (also known as CLRS), a standard textbook on algorithms, with Thomas H. Cormen, Charles E. Leiserson and Clifford Stein. First published in 1990, it has extended into four editions, the latest in 2022.[A7]
Learning In the problem of decision tree learning, Rivest and Laurent Hyafil proved that it is NP-complete to find a decision tree that identifies each of a collection of objects through binary-valued questions (as in the parlor game of twenty questions) and that minimizes the expected number of questions that will be asked.[L1] With Avrim Blum, Rivest also showed that even for very simple neural networks it can be NP-complete to train the network by finding weights that allow it to solve a given classification task correctly.[L3] Despite these negative results, he also found methods for efficiently inferring decision lists,[L2] decision trees,[L4] and finite automata.[L5]
Elections A significant topic in Rivest's more recent research has been election security, based on the principle of software independence: that the security of elections should be founded on physical records, so that hidden changes to software used in voting systems cannot result in undetectable changes to election outcomes. His research in this area includes improving the robustness of mix networks in this application,[V1] the 2006 invention of the ThreeBallot paper ballot based end-to-end auditable voting system (which he released into public domain in the interest of promoting democracy),[V2] and the development of the Scantegrity security system for optical scan voting systems.[V3] He was a member of the Election Assistance Commission's Technical Guidelines Development Committee.
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