2026 marks the 50th anniversary of public key infrastructure (PKI), a milestone built on decades of development in public-key cryptography. In recognition, we look back at the trailblazers who created PKI, modernized it for the internet era and continue to champion its use today.
The Founders of PKI (1970s)
In the early 1970s at the UK intelligence agency Government Communications Headquarters (GCHQ), James Ellis, Clifford Cocks and Malcolm Williamson secretly arrived at the same ideas later published independently as Rivest-Shamir-Adleman (RSA) and Diffie-Hellman. Whitfield Diffie and Martin Hellman then published “New Directions in Cryptography” (1976), which brought public-key cryptography into the open literature, introducing Diffie-Hellman key exchange. Ron Rivest, Adi Shamir and Len Adleman invented RSA in 1977, and Loren Kohnfelder’s 1978 MIT thesis proposed signed certificates binding a name to a public key.
The GCHQ Trio: James Ellis, Clifford Cocks and Malcolm Williamson
Notable Roles: GCHQ cryptographers (Cocks: Chief Mathematician)
Key Years: Ellis (1970), Cocks (1973), Williamson (1974)
Location: Cheltenham, England (GCHQ)
Focus of Work: Non-secret (public-key) encryption
Notable Honors: IEEE Milestone (2010); NSA Hall of Honor (2021)
PKI Achievements
Ellis, Cocks and Williamson arrived at the core concepts of public-key cryptography (PKC) at GCHQ. In 1970, Ellis argued two parties could communicate securely without sharing a secret key. Cocks essentially devised the scheme now known as RSA (1973), and Williamson devised the protocol now known as Diffie-Hellman key exchange (1974).
The Impact on PKI: Their work stayed classified until 1997, so the public versions of RSA and Diffie-Hellman key exchange were independent rediscoveries of schemes GCHQ already had.
Whitfield Diffie
Most Recent Roles: Chief technologist, Ziroh Labs; Stanford scholar
Years Active: 1970s-present
Education: B.S. in mathematics, MIT (1965)
Focus of Work: Public-key cryptography, cryptography policy
Notable Honors: Turing Award (2015); Levchin Prize (2026)
PKI Achievements
Diffie and Hellman authored “New Directions in Cryptography” (1976), the paper that brought public-key cryptography into the open literature. The paper is also the origin of Diffie-Hellman key exchange (the basis of today’s ephemeral Diffie-Hellman (DHE) key exchange).
The Impact on PKI: Diffie co-authored the 1976 paper that openly proposed public-key cryptography and introduced Diffie-Hellman key exchange, opening the path to public adoption of PKC and PKI.
Martin Hellman
Most Recent Roles: Professor emeritus of electrical engineering, Stanford
Years Active: 1969-present
Location: Stanford, California
Focus of Work: Public-key cryptography, nuclear risk analysis
Notable Honors: Turing Award (2015); Levchin Prize (2026)
PKI Achievements
Hellman co-authored “New Directions in Cryptography” (1976) with Diffie, which publicly introduced public-key cryptography and Diffie-Hellman key exchange. Hellman co-invented the Merkle-Hellman knapsack public-key cryptosystem (1978, with Merkle), one of the first public-key encryption schemes, later broken by Shamir (1982).
The Impact on PKI: Hellman, with Diffie, moved public-key cryptography into the open academic literature in 1976, where RSA and the first certificate schemes followed.
Ralph Merkle
Most Recent Roles: Nanofactory CBN; formerly Georgia Tech professor
Years Active: 1974-present
Location: San Francisco Bay Area, California
Focus of Work: Key distribution, hash trees, nanotechnology
Notable Honors: Levchin Prize (2020); Inventors Hall of Fame (2011)
PKI Achievements
Merkle’s Puzzles, published in 1978, was the first known scheme devised outside government for two parties to agree on a secret over an open channel. Diffie and Hellman cited it as a “partial solution.” Merkle also invented Merkle trees, which now underpin Certificate Transparency logs.
The Impact on PKI: Merkle’s hash tree structure now lets anyone verify that a certificate was logged in Certificate Transparency, and it underpins hash-based post-quantum signature standards.
The RSA Trio: Ron Rivest, Adi Shamir and Len Adleman
Most Recent Roles: Professors at MIT, the Weizmann Institute and USC
Years Active: 1970s-present
Locations: Cambridge, Massachusetts; Rehovot, Israel; Los Angeles, California
Focus of Work: Public-key cryptography, cryptanalysis
Notable Honors: ACM A.M. Turing Award (2002, shared)
PKI Achievements
Rivest, Shamir and Adleman co-created the Rivest-Shamir-Adleman algorithm (RSA algorithm), first described in 1977. Adleman, acting as cryptanalyst, broke 42 earlier designs before the 43rd, based on factoring, held. RSA was instrumental in the development of secure, verifiable digital signatures, a cornerstone of modern PKI.
The Impact on PKI: RSA was the first publicly described public-key cryptosystem for both encryption and digital signatures, and it served as the dominant signature algorithm for X.509 certificates for decades.
Loren Kohnfelder
Most Recent Roles: Author; Google and Microsoft security (retired)
Years Active: 1978-2020s
Education: S.B., MIT (1978)
Focus of Work: Digital certificates, secure software design
Known For: Coining “certificate” (1978); co-creating STRIDE
PKI Achievements
Kohnfelder’s 1978 MIT undergraduate thesis introduced digital certificates: signed statements binding a name to a public key. In the thesis, certificates are signed by the “Public File,” a trusted directory that foreshadowed today’s certificate authorities (CAs). Security author Roger Grimes notes that some consider Kohnfelder the “Father of PKI.”
The Impact on PKI: Kohnfelder proposed signed certificates binding a name to a public key, the model X.509 certificates follow today, and coined the term “certificate” in this sense.
Early Adopters and Advocates (1980s-2000s)
These decades produced the standards that today’s PKI runs on. Taher Elgamal’s 1984 paper introduced the ElGamal signature scheme, which became the basis for the Digital Signature Algorithm (DSA) from the National Institute of Standards and Technology (NIST). Neal Koblitz and Victor Miller independently proposed elliptic curve cryptography in 1985. Phil Zimmermann released Pretty Good Privacy (PGP) in 1991 and introduced the Web of Trust in 1992. Warwick Ford, Russ Housley, Tim Polk and David Solo wrote the Internet X.509 certificate profile (RFC 2459, revised as RFC 3280), and Carl Ellison and Bruce Schneier published “Ten Risks of PKI” in 2000.
Taher Elgamal
Most Recent Roles: General partner, Evolution Equity Partners
Years Active: 1984-present
Education: Ph.D., Stanford (1984)
Focus of Work: Discrete-log cryptography, SSL/TLS
Notable Honors: Marconi Prize (2019); National Academy of Engineering
PKI Achievements
Elgamal’s 1984 paper introduced the ElGamal encryption and signature schemes, based on the discrete logarithm problem. That signature scheme is the basis for NIST’s DSA. As Netscape’s chief scientist, he drove Secure Sockets Layer (SSL) and supported its widespread adoption as an open standard.
The Impact on PKI: Elgamal’s signature scheme became the basis for NIST’s DSA and, through DSA, its elliptic-curve variant ECDSA, and his work at Netscape drove SSL toward the open standard that became Transport Layer Security (TLS).
The ECC Inventors: Neal Koblitz and Victor Miller
Most Recent Roles: Univ. of Washington (Koblitz); Nexus Labs (Miller)
Years Active: 1985-present (in cryptography)
Education: Ph.D.s from Princeton (Koblitz) and Harvard (Miller)
Focus of Work: Elliptic curve and pairing-based cryptography
Notable Honors: Levchin Prize (2021, shared)
PKI Achievements
Koblitz and Miller independently proposed elliptic curve cryptography (ECC) in 1985, an alternative to RSA that relies on the elliptic curve discrete logarithm problem instead of integer factorization. Miller’s paper “Use of Elliptic Curves in Cryptography” proposed an elliptic-curve Diffie-Hellman key exchange. ECC offers comparable security with much smaller keys.
The Impact on PKI: Their independent 1985 proposals of ECC gave cryptography a small-key alternative to RSA that underpins ECDSA and ECDHE.
Phil Zimmermann
Most Recent Roles: TU Delft (emeritus); co-founder, Silent Circle
Years Active: 1991-present
Known As: Creator of PGP and the Web of Trust
Focus of Work: Email and voice encryption, privacy
Notable Honors: Internet Hall of Fame (2012); EFF Pioneer Award (1995)
PKI Achievements
Zimmermann created Pretty Good Privacy (PGP), an encryption program for public use, which launched in 1991. PGP was one of the first widely available public-key encryption programs. Zimmermann introduced the Web of Trust in the PGP 2.0 manual in 1992, a decentralized alternative to centralized, certificate authority (CA)-based PKI.
The Impact on PKI: PGP brought public-key cryptography to individuals and its Web of Trust challenged the idea that a central authority must vouch for keys, shaping the OpenPGP standard still used today.
The X.509 Profile Authors: Russ Housley, Warwick Ford, Tim Polk and David Solo
Notable Roles: Vigil Security, VeriSign, NIST and Citi, respectively
Key Years: RFC 2459 (1999), RFC 3280 (2002), RFC 5280 (2008)
Focus of Work: X.509 certificate profiles, PKI standards
Key Works: CMS (Housley); RFC 3647 (Ford); FIPS 201 (Polk)
Notable Honors: ACM Fellow (Housley, 2024 class)
PKI Achievements
Housley, Ford, Polk and Solo wrote RFC 2459 and RFC 3280, with Housley as first-named author, defining how X.509 certificates and certificate revocation lists (CRLs) are profiled and validated. Housley and Polk co-authored RFC 5280. Ford co-authored the certificate policy framework RFCs 2527 and 3647; Polk co-authored FIPS 201.
The Impact on PKI: Their X.509 profile, updated as RFC 5280, set the certificate and CRL baseline that public and private CAs still follow, making cross-platform certificate interoperability practical.
The "Ten Risks of PKI" Authors: Carl Ellison and Bruce Schneier
Notable Roles: Harvard (Schneier); Microsoft and Intel (Ellison)
Years Active: 1980s-2000s (Ellison); 1991-present (Schneier)
Known For: “Applied Cryptography” (Schneier); SPKI (Ellison)
Focus of Work: Applied cryptography, authorization certificates, trust
Key Works: “Ten Risks of PKI” (2000); SPKI RFCs 2692 and 2693
PKI Achievements
Ellison and Schneier authored “Ten Risks of PKI: What You’re Not Being Told About Public Key Infrastructure,” a 2000 paper arguing that PKI has been oversold as the answer to many network security problems. Ellison also led the SPKI RFCs 2692 and 2693. Schneier wrote “Applied Cryptography” (1994).
The Impact on PKI: Their paper pushed the field to question what CA-issued certificates actually prove, and Ellison’s SPKI RFCs showed how to separate authorization from identity.
Modern Web Era (2010s-Present)
The 2010s brought auditable certificate records, free automated issuance and stricter browser trust. Ben Laurie, Adam Langley and Emilia Käsper authored RFC 6962: Certificate Transparency in 2013. Josh Aas, Peter Eckersley, J. Alex Halderman and Eric Rescorla co-founded Let’s Encrypt, a free automated CA launched in 2015. Ryan Sleevi led Chrome’s response to Symantec’s certificate misissuance. Ryan Hurst led the Google team that created Google Trust Services. Dustin Moody leads NIST’s Post-Quantum Cryptography Standardization Project, which produced FIPS 203, 204 and 205 in August 2024.
The Certificate Transparency Authors: Ben Laurie, Adam Langley and Emilia Käsper
Notable Roles: Google security and TLS engineers when they wrote CT
Key Years: RFC 6962 (2013); RFC 9162, CT 2.0 (2021, Laurie)
Known For: OpenSSL (Laurie, Käsper); BoringSSL (Langley)
Focus of Work: Certificate Transparency, TLS, OpenSSL
Notable Honors: Levchin Prize (2024, shared)
PKI Achievements
Laurie, Langley and Käsper authored RFC 6962 Certificate Transparency (CT), a protocol published in 2013 for an auditable public record of TLS certificates. This framework challenged the notion that CAs were implicitly trustworthy, providing public visibility that makes fraudulent or rogue certificates easier to detect.
The Impact on PKI: CT gave the web an auditable record of TLS certificates, making rogue or fraudulent issuance far easier to detect.
The Let's Encrypt Founders: Josh Aas, Peter Eckersley, J. Alex Halderman and Eric Rescorla
Notable Roles: Mozilla, EFF, Univ. of Michigan and Mozilla, respectively
Key Years: ISRG founded (2013); Let’s Encrypt launched (2015)
Focus of Work: Free automated certificates, ACME, TLS
Notable Honors: Levchin Prize (2022, awarded to Let’s Encrypt)
In Memoriam: Peter Eckersley (died 2022)
PKI Achievements
Aas, Eckersley, Halderman and Rescorla co-founded Let’s Encrypt, a free automated CA launched in 2015. It automated issuance and renewal through the Automated Certificate Management Environment (ACME), making encrypted web communications more financially and technologically accessible to everyone. Eckersley led the ACME client that became Certbot. Rescorla edited TLS 1.3.
The Impact on PKI: Let’s Encrypt grew into the largest CA by certificates issued, helping lift encrypted page loads (as measured by Firefox) from below 30% to around 80%.
Ryan Sleevi
Most Recent Roles: Apple; Google Chrome Root Program
Years Active: 2010s-present
Location: Raleigh-Durham, North Carolina
Focus of Work: Browser root programs, WebPKI policy
Key Works: RFC 7469 (2015); Chrome’s Symantec response (2015-2017)
PKI Achievements
Sleevi led Chrome’s response to Symantec’s misissuance, from requiring CT support for its new certificates (announced 2015) to the 2017 plan to distrust Symantec’s PKI. With Chris Evans and Chris Palmer, he co-authored RFC 7469 (HTTP Public Key Pinning), designed to counter Adversary in the Middle (AiTM) attacks; browsers later deprecated it.
The Impact on PKI: Sleevi led Chrome’s actions against Symantec’s misissuance, which ended with Chrome distrusting Symantec’s PKI and raised the bar for CA accountability.
Ryan Hurst
Most Recent Roles: CEO, Peculiar Ventures; Google (Trust Services)
Years Active: 1990s-present
Location: Seattle, Washington
Focus of Work: WebPKI, certificate authorities, transparency
Known For: Unmitigated Risk blog (since 2006); PKI.js contributor
PKI Achievements
Hurst is a PKI and TLS expert and Let’s Encrypt advisor who led the Google team that created Google Trust Services. According to his website, Unmitigated Risk, Hurst has “created, led, or advised the authorities behind roughly 60% of the certificates issued on the web.”
The Impact on PKI: Hurst led the team that created Google Trust Services, giving Google its own publicly trusted CA, after helping GlobalSign rebuild trust following DigiNotar.
Dustin Moody
Most Recent Roles: Mathematician, NIST; lead, NIST PQC standardization
Years Active: 2009-present
Location: Maryland
Focus of Work: Post-quantum cryptography standards
Key Works: NIST IR 8105 and IR 8413; lead author, NIST IR 8547
PKI Achievements
Moody leads NIST’s Post-Quantum Cryptography Standardization Project, which produced NIST’s first finalized post-quantum cryptography standards: FIPS 203 (ML-KEM), FIPS 204 (ML-DSA) and FIPS 205 (SLH-DSA) in August 2024. Rather than wait for Q-Day, Moody urged organizations to start migrating: “There is no need to wait for future standards.”
The Impact on PKI: Moody leads NIST’s PQC standardization project and is lead author of its migration roadmap, giving organizations concrete standards and timelines for quantum readiness.
Looking Ahead: PKI in a Post-Quantum World
Before PKI, organizations had to rely on trust: the assumption that if someone entered a username and password, that person must be the intended recipient of those credentials.
Today, we can issue cryptographically verifiable, certificate-based identities to a growing workforce of human and non-human entities. But the evolution of PKI is far from finished.
As we look to the future of PKI, three important themes emerge:
Certificate lifespans are shorter than ever. CA/Browser Forum’s Ballot SC-081v3 will reduce the maximum validity of public TLS certificates to just 47 days by March 2029. But with the rise of rogue AI agents, even that’s too long for many use cases.
PKI is the future of agentic security. Orphaned or improperly monitored agents can be compromised or misused quickly. Issuing, monitoring and revoking agent certificates at scale requires a PKI with robust capacity and fast revocation.
Quantum readiness requires PKI evolution. While NIST released its first post-quantum cryptography standards back in August 2024, ABI Research projected that just 2% of PKI solutions would support PQC algorithms in 2025, tripling to only 6% by 2030.
Quantum Day (Q-Day), when quantum computers can break today’s public-key algorithms such as RSA and ECC, will change the cybersecurity field forever. But thanks to PKI heroes like the ones we’ve discussed here, we already have the frameworks, algorithms and processes to withstand these attacks. As long as experts remain curious and committed to improvement, we’re optimistic that PKI will continue its reign as a gold-standard authentication solution for the foreseeable future, if not another 50 years.
