The Caramba team from the Loria has been awarded the Levchin Prize for its CADO-NFS software

Emmanuel Thomé, Pierrick Gaudry, Paul Zimmermann - Caramba

 

Our congratulations to Pierrick Gaudry, Emmanuel Thomé and Paul Zimmermann, CNRS and Inria researchers with the Loria and winners of the Levchin 2025 Prize!

The Levchin Prize is awarded for high-impact cryptography innovations and their use in real-world systems. It was presented to our researchers on March 26th at Real-World Crypto, a major annual international conference on applied cryptography that attracts around 600 academic and industrial professionals working in this field.

This prestigious award rewards the three researchers for their CADO-NFS computer security software developed by the Caramba team since 2007. CADO-NFS is now the benchmark software for testing the robustness of encryption keys.

Encryption is based on complex mathematical problems.

For credit card payments, surfing on the Internet or even just using your car keys, the confidentiality of your exchanges is guaranteed by public key cryptography or asymmetric cryptography. This method was first published in 1976 by Whitfield Diffie and Martin Hellman. It is based on the exchange of a public key, the encryption code, and a private key, the decryption code. One of the most common examples used today is RSA encryption which was named after its three authors Ronald Rivest, Adi Shamir and Leonard Adleman and first published in 1977.

These security protocols are based on complex mathematical problems like integer factorisation and discrete logarithm.

Of course, it is easy to say that 51 equals 3 x 17 but this problem becomes more difficult when much larger numbers are involved. Integer factorisation is the foundation for cryptographic systems and its basic principle involves the multiplication of two integers to create an encryption key. The challenge for cryptanalysts is to use sophisticated algorithms to find these two original numbers. This means that the longer the key is, the more robust the encryption which of course makes it more difficult to break. The other mathematical problem at the core of encryption systems is the discrete logarithm which serves to find the inverse calculation of the exponential for certain integers in a mathematical structure known as ‘finite fields’.

CADO-NFS – a software for testing the robustness of encryption keys.

Back in the 1980s, a group of researchers developed the number field sieve (NFS) – an algorithm for factoring prime numbers and calculating discrete logarithms.

In 2007, Pierrick Gaudry, Emmanuel Thomé and Paul Zimmermann began to work on implementing this algorithm in software which led to the creation of CADO-NFS (Crible Algébrique – Distribution, Optimisation – Number Field Sieve). This unique open-source software combines the two mathematical problems and features over 250,000 lines of code. The team makes daily updates to the software which has also been enriched by many various contributors over the years.

“CADO-NFS has become the reference tool for testing the robustness of cryptographic systems and calibrating recommended key sizes,” explain the three researchers. “It’s used in the academic sphere, for proof-of-concept experiments and by government departments like the ANSSI[1], for example, who use it to develop cybersecurity recommendations.”

The Levchin Prize is in recognition of the 18 years of development and enhancement of this software with many research results produced and records broken in the process. ”In cryptanalysis, we work on attacking systems to check their security. In 2015, our work enabled us to discover the major Logjam flaw in the TLS protocol which secures internet connections,’ the team explains. “In 2019 and 2020, we also broke records for breaking encryption keys with the help of CADO-NFS. Several thousand computer cores ran calculations for several months to achieve these results, notably on the GRID5000 platform.” These record-breaking calculations were used to factor the integers RSA-240 (795 bits) and RSA 250 (829 bits) during the RSA challenge and the team’s record is still standing…

Find out more:

[1] French Cybersecurity Agency