If SHA-2 is not broken, why would one switch to SHA-3 and not just stay with SHA-2? In this post, we highlight another argument why Keccak/SHA-3 is a better choice than SHA-2, namely openness, in analogy with open-source versus closed-source in software development and deployment.
Software has two sides: its executable and its source code. The former is used as a black box by the users, while the latter is of interest to developers who want to extend it, to understand its inner workings or to make sure there is no obvious malicious code. As an analogy, we see the specification of the cryptographic primitive, mode or algorithm in a (proposed) cryptographic standard as the counterpart of the software executable: It allows everyone to include the cryptographic object, as is, in his/her project. The counterpart of the source code in cryptography would be the design rationale, preliminary cryptanalysis and evidence of extensive third-party cryptanalysis: These are the elements that give insight into the inner workings and ultimately trust.
The transition of cryptography from a proprietary activity to a scientific one in the last 50 years can be seen as a move from closed-source to open-source in this analogy. Surprisingly, there are exceptions and we still see closed-source cryptography today.
The SHA-1 and SHA-2 NIST standard hash functions were designed behind closed doors at NSA. The standards were put forward in 1995 and 2001 respectively, without public scrutiny of any significance, despite the fact that at time of publication there was already a considerable cryptographic community doing active research on this subject. Even the 2015 update of FIPS 180, the standard that specifies SHA-2, does not contain, nor refer to, a design rationale.
In contrast, SHA-3 is the result of an open call of NIST to the cryptographic community for hash function proposals. There was no restriction on who could participate, so submissions were open in the broadest possible sense. Every submitted candidate algorithm had to contain a description, a design rationale and preliminary cryptanalysis. The authors of the 64 submissions included the majority of people active in open symmetric crypto research at the time. NIST solicited the symmetric crypto community for performing and publishing research in cryptanalysis, implementations, proofs and comparisons of the candidates and based its decision on the results. After a three-round process involving hundreds of people in the community for several years, NIST finally announced that Keccak was selected to become the SHA-3 standard.
The open effort of the symmetric crypto community did not stop there. Since then, Keccak has remained under public scrutiny and new papers appear regularly. Paper after paper confirms the large safety margin of Keccak. What is important, is that these papers reach a high degree of sophistication as research can start from the preliminary cryptanalysis that we provided in our SHA-3 submission document.
It is true that cryptanalysis of MD5, SHA-1 and SHA-2 has also reached a high degree of sophistication. However, this took longer to develop due to the absence of rationale and preliminary cryptanalysis, but also due to the adoption of the ARX design methodology.
SHA-2 is essentially a security patch of SHA-1 while SHA-3 is its open-source alternative, much in the same way that Triple-DES is a security patch for DES and AES the open-source alternative. In retrospect, even if Triple-DES is not broken, would you still recommend not to switch to AES?