An Internet Certificate Authority’s Secret Key Was Four

A private cryptographic key is meant to be difficult to guess. One of the compromised keys at certificate authority eMudhra was 4. Not four words. Not a four-digit PIN. Four.

Ars Technica’s October 20 report describes eight weak private-key values associated with intermediate TLS certificates. A normal key of the kind involved has an enormous space of possible values.

Here is why that does not remain one company’s small-number problem. An intermediate certificate can authorize other certificates that browsers and applications trust. Get its private key and you can potentially sign credentials for somebody else’s site. The system can reject an impostor with the wrong certificate and accept the same impostor once four has supplied the right one.

That is not evidence that anyone has intercepted your traffic. At publication, Ars reported no transparency-log evidence of malicious use. Exploiting the keys also requires an attacker to get into the path between communicating parties; knowing the key alone does not make every encrypted conversation available. The danger is that an impostor in that position could arrive carrying credentials the system recognizes.

Blocking the bad certificates sounds like a neat way to finish. Most browsers and some major applications had done so by the time of the report. But the trust also lives in Linux systems, embedded devices, containers and tools that bring their own stored lists of trusted authorities. A browser update does not travel around your network personally informing every machine that its old acquaintance is no longer respectable.

Cryptography engineer Filippo Valsorda warned Ars of a long cleanup reaching forgotten and unmaintained systems. The cause of the weak keys was not established, and eMudhra was not immediately available to answer questions.

The secret was easy to discover. Now the Internet has to discover all the places it trusted it.