Lenstra was also involved in the development of the number field sieve.[3] With coauthors, he showed the great potential of the algorithm early on by using it to factor the ninth Fermat number, which was far out of reach by other factoring algorithms of the time.[4] He has since been involved with several other number field sieve factorizations including the current record, RSA-768.
On 1 March 2005, Arjen Lenstra, Xiaoyun Wang, and Benne de Weger of Eindhoven University of Technology demonstrated[6] construction of two X.509 certificates with different public keys and the same MD5 hash, a demonstrably practical hash collision. The construction included private keys for both public keys.
^Arjen K. Lenstra and Mark S. Manasse. "Factoring by electronic mail", in Proceedings of Eurocrypt '89, Lecture Notes in Computer Science, Volume 434, pp. 355-371, 1990.
^A. K. Lenstra, H. W. Lenstra, Jr., M. S. Manasse, and J. M. Pollard, “The Number
Field Sieve,” in A. K. Lenstra and H. W. Lenstra, Jr. (eds.) The Development of
the Number Field Sieve, Lecture Notes in Mathematics 1554, Springer-Verlag, New
York, pp. 11–42, 1993.
^A. K. Lenstra, H. W. Lenstra, Jr., M. S. Menasse, and J. M. Pollard, “The Factorization of the Ninth Fermat Number,” Math. Comp. 61, 319–349, 1993.
^A.K. Lenstra, H.W. Lenstra, Jr., and L. Lovász, "Factoring polynomials with rational coefficients". Mathematische Annalen 261 (4): pp. 515–534, 1982.