Gone in 60 Months or Less

by Gavin Hill, Director, Product Marketing & Threat Research Center at Venafi

For years, cybercriminals have been taking advantage of the blind trust organizations and users place in cryptographic keys and digital certificates. Only now are vendors starting to respond to the use of keys and certificates as an attack vector.

Last month, for example, Google announced that as of Q1 2014 Google Chrome and the Chromium browser will not accept digital certificates with a validity period of more than 60 months. Certificates with a longer validity period will be considered invalid.[i] Mozilla is considering implementing the same restrictions, however no decision has been announced yet. But are the responses from vendors enough in the constant battle against compromised keys and certificates as an attack vector?

The Certificate Authority Browser (CA/B) Forum, a volunteer organization that includes leading Certificate Authorities (CAs) and software vendors, has issued some baseline requirements for keys and certificates, which include reducing the certificate’s validity period. By 1 April 2015 CAs should not issue certificates that have a validity period greater than 39 months.[ii] The CA/B Forum makes some—very few—exceptions whereby CAs are allowed to issue certificates that have a 60-month validity period.

The National Institute of Standards and Technology (NIST) has disallowed the use of 1024-bit keys after 31 December 2013 because they are insecure. Rapid advances in computational power and cloud computing make it easy for cybercriminals to break 1024-bit keys. When a researcher from Ecole Polytechnique Fédérale de Lausanne (EPFL) in Switzerland cracked a 700-bit RSA key[iii] in 2007, he estimated that 1024-bit key lengths would be exploitable 5 to 10 years from then. Not even three years later, in 2010, researchers cracked a 1024-bit RSA key.[iv]

Last week Symantec responded to the NIST’s recommendation in a Symantec blog, stating that on 1 October 2013 Symantec will automatically revoke all certificates that have a key length of less than 2048 bits. The only exception is certificates that are set to expire before 31 December 2013. Symantec responded quickly because the company wants to help customers avoid potential disruptions to their websites and internal systems during the holiday period.[v]

Both the certificate’s validity period and the key’s length are paramount in any security strategy. The deprecation of vulnerable key lengths is the first step in mitigating against keys and certificates as an attack vector, but reducing the validity period of certificates is an important second step. Longer validity periods offer an inviting open door to cybercriminals who can take advantage of advances in computational power and cloud computing to launch more sophisticated attacks. No one knows when 2048-bit keys will be broken, but enforcing a 60-month validity period will help organizations adhere to best practices, rotating certificates on a regular basis and when doing so potentially replacing older certificates with ones that have better cypher strengths. Who knows, in 60 months companies may need to move to 4096-bit keys to achieve adequate security.

Symantec’s move to revoke all 1024-bit certificates with expiration dates after 31 December 2013 on the 1 October 2013 is a bold move, which is most certainly in the right direction. With such a short amount of time before the certificates become invalid, however, it will be very challenging for many organizations to replace the certificates in time. Most organizations—more than 50%--don’t have a clue how many keys and certificates they have in their inventory.[vi] Moreover, they manage their certificate inventories manually, making it difficult to respond quickly to new guidelines or actual attacks.

Cyber-attacks continue to advance in complexity and speed and increasingly target the keys and certificates used to establish trust—from the data center to the cloud. With the advances in technology, is a 60-month, or even a 39-month, validity period for certificates short enough to reduce risk? Perhaps certificates should be ephemeral, with a lifespan of only a few seconds? Reducing the lifespan of certificates to only a few seconds may drastically limit the exploitation of certificates as an attack vector.