Bulletproof TLS Newsletter 
Bulletproof TLS Newsletter is a free periodic newsletter bringing you commentary and news surrounding SSL/TLS and Internet PKI, designed to keep you informed about the latest developments in this space. Maintained by Hanno Böck.
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RFC 9155 deprecates MD5 and SHA-1 signatures in TLS handshake messages
30 Dec 2021
A new RFC officially deprecates the use of the MD5 and SHA-1 weak hash algorithms in TLS handshake messages. The deprecation of these old hash algorithms has a long history, dating back to research results from sixteen years ago.
In 2004 and 2005, Chinese cryptographer Xiaoyun Wang published practical collision attacks on MD5 and theoretical attacks on SHA-1. Yet today these algorithms are still in widespread use.
In 2008, researchers were able to use the weakness in MD5 to create a rogue CA certificate. But that attack relied on a number of conditions; notably, the CA used nonrandom serial numbers, which are no longer used by CAs today. Nevertheless, the attack showed the dangers of weak hash functions, and in the web PKI system, both MD5 and SHA-1 certificates have been deprecated.
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