Ben-Zion Chor (1956–2021): A Pioneer of Privacy and Biological Computation
Ben-Zion "Benny" Chor was a seminal figure in theoretical computer science and bioinformatics, known for his infectious intellectual curiosity and his ability to bridge disparate scientific worlds. Over a career spanning four decades, Chor moved from the abstract rigors of cryptography and distributed computing to the complex, data-driven challenges of evolutionary biology. He is perhaps most widely recognized as a co-inventor of Private Information Retrieval (PIR), a concept that remains a cornerstone of modern privacy-preserving technology.
1. Biography: From the Technion to Global Prominence
Ben-Zion Chor was born on March 31, 1956, in Israel. His academic journey began at the Technion – Israel Institute of Technology, where he earned his B.Sc. in Mathematics (1980) and an M.Sc. in Computer Science (1982).
For his doctoral studies, Chor moved to the Massachusetts Institute of Technology (MIT), where he worked under the supervision of Ronald Rivest (the "R" in RSA). He completed his Ph.D. in 1985 with a dissertation titled "Two Issues in Public Key Cryptography," which laid the groundwork for his future contributions to secure communication.
After a postdoctoral fellowship at MIT and Harvard, Chor returned to Israel in 1987 to join the faculty of the Technion. In 2001, he moved to Tel Aviv University (TAU), where he became a Professor in the School of Computer Science. He remained at TAU until his untimely passing in 2021, serving as a pillar of the Israeli scientific community and a mentor to dozens of graduate students.
2. Major Contributions: Cryptography and Bioinformatics
Chor’s intellectual output can be divided into two major phases: his early work in theoretical computer science and his later focus on computational biology.
Private Information Retrieval (PIR)
Chor’s most enduring contribution to computer science is the invention of Private Information Retrieval (with co-authors Goldreich, Kushilevitz, and Sudan). PIR addresses a fundamental paradox: how can a user retrieve an item from a database without the database owner learning which item was requested? This work established a new sub-field of cryptography, influencing everything from anonymous web browsing to secure cloud computing.
The Chor-Rivest Cryptosystem
During his time at MIT, he co-developed the Chor-Rivest cryptosystem, a public-key encryption scheme based on the "subset sum" (knapsack) problem. While many knapsack-based systems were eventually broken, the Chor-Rivest system remained remarkably resilient for over a decade, proving Chor’s deep intuition for mathematical structures in security.
Phylogenetics and Evolutionary Computation
In the late 1990s, Chor pivoted toward Bioinformatics. He applied his expertise in algorithms to the study of evolution, specifically focusing on:
- Phylogenetic Reconstruction: Developing algorithms to build "trees of life" from DNA sequences.
- Maximum Likelihood: He proved that finding the most likely evolutionary tree is an "NP-hard" problem, meaning it is computationally exhaustive, which helped guide how biologists approach genomic data.
- Metabolic Networks: Analyzing the robustness and evolution of chemical pathways within cells.
3. Notable Publications
Chor’s bibliography includes over 100 peer-reviewed papers. His most influential works include:
- "Private Information Retrieval" (1995/1998, Journal of the ACM): Co-authored with O. Goldreich, E. Kushilevitz, and M. Sudan. This paper introduced the PIR protocol and is one of the most cited works in cryptographic history.
- "A Knapsack Type Public Key Cryptosystem Based on Arithmetic in Finite Fields" (1988, IEEE Transactions on Information Theory): Co-authored with Ronald Rivest, detailing their unique encryption method.
- "On the Power of Multi-Party Protocols" (1986): A foundational paper in distributed computing that explored how multiple parties can compute a function while keeping their inputs private.
- "Maximum Likelihood of Evolutionary Trees is Hard" (2005, Journal of Computational Biology): A critical proof that defined the computational limits of biological modeling.
4. Awards & Recognition
Though Chor prioritized research over accolades, his work received significant institutional recognition:
- STOC Test of Time Award (2020): Awarded by the ACM Symposium on Theory of Computing for his 1995 paper on Private Information Retrieval, recognizing its lasting impact 25 years later.
- IACR Fellow: He was a prominent member of the International Association for Cryptologic Research.
- Teaching Excellence: Chor was repeatedly recognized as one of the top lecturers at both the Technion and Tel Aviv University, known for his clarity and humor.
5. Impact & Legacy
Chor’s legacy is twofold. In the realm of Privacy, his work on PIR provided the theoretical framework for "Oblivious RAM" and modern differential privacy. Today’s privacy-enhancing technologies (PETs) owe a direct debt to his 1995 breakthroughs.
In Bioinformatics, he was a "bridge-builder." At a time when computer scientists and biologists rarely spoke the same language, Chor translated complex biological questions into rigorous algorithmic problems. His work helped transform biology from a descriptive science into a computational one.
Furthermore, his influence lives on through his students. Many of his former PhD candidates are now leading researchers at top-tier universities and tech giants like Google and Microsoft.
6. Collaborations
Chor was a highly social researcher who thrived on collaboration. His key partners included:
- Ronald Rivest: His mentor and co-author on knapsack cryptography.
- Oded Goldreich: A long-term collaborator on foundations of cryptography.
- The "PIR Team": Eyal Kushilevitz and Madhu Sudan.
- Biological Collaborators: He frequently worked with David Penny and Michael Hendy (evolutionary biologists) to ensure his algorithms were biologically grounded.
7. Lesser-Known Facts
- The "Benny-isms": Chor was famous for his sharp wit and self-deprecating humor. He often started his lectures with a joke or an observation that would disarm the most intimidating mathematical topics.
- An Avid Hiker: Despite his heavy academic load, Chor was a passionate outdoorsman. He spent significant time hiking the trails of Israel and the Alps, often using these walks to mentally untangle complex proofs.
- A "Late" Bloomer in Biology: Unlike many who specialize early, Chor didn't deeply engage with biology until his 40s. He famously took undergraduate-level biology courses while already a tenured professor of computer science to ensure he truly understood the field he was entering.
- Public Intellectual: He was deeply concerned with the ethical implications of technology, often speaking about the balance between national security and individual privacy long before it became a mainstream political issue.
Ben-Zion Chor passed away in 2021, leaving behind a body of work that continues to protect the privacy of digital users and illuminate the evolutionary history of life on Earth.