29 April 2021
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. Received monthly by more than 50,000 subscribers. Written by Hanno Böck.
On April 23, well-known security researcher Dan Kaminsky passed away. Kaminsky was most famously known for finding security flaws in the Domain Name System (DNS), but he also discovered flaws in the TLS certificate infrastructure and X.509.
In a talk Kaminsky gave in 2009 at several conferences (a recording is available from CCC), he presented an issue with MD2 hashes. Back then, a certificate from Verisign was listed in certificate root stores that had a self-signature with an MD2 hash.
With a preimage attack on MD2, this could theoretically have allowed for creating an intermediate certificate that has the same hash and is signed by that root. A theoretical preimage attack against MD2 had been known since 2008, with a complexity of 2^73. This is an impractical attack, but it comes close to what is practically breakable.
In response to this research, MD2 support was removed from all major TLS libraries, making this an example of a crypto algorithm being deprecated before it could become a problem. No improved preimage attacks on MD2 have been published since then, yet it could be speculated that the removal of MD2 support made the research less interesting.
Kaminsky presented further attacks affecting X.509 certificate parsing. The most notable one, which was codiscovered by Moxie Marlinspike, enabled issuing a certificate with a null byte that would be interpreted differently by CAs and browsers.
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