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SHA-1 Hash Algorithm “SHAttered”

SHAtteredFebruary 23, 2017, Google announced they had successfully exploited a known vulnerability in the widely-used SHA-1 hash algorithm.

This is important because hash algorithms are used to create digital signatures.

Digital signatures are the basis for two important things:

  1. Proof that content – such as an application – has not been changed since it was signed, and
  2. Non-repudiation on the internet

Digital signatures are used not only for electronic document signatures, but also for establishing trust in Web servers (HTTPS certificates), version control, the validity of backups, and so on.

So, this could be a shattering announcement for those still using SHA-1. But…before hyperventilating, let’s get some background. Perhaps several deep breaths are a more appropriate response to this news.

The most important property of a secure hash algorithm is the likelihood of two different documents producing the same hashed result – known as a “hash collision.” If this probability is infinitesimally small, then the hash algorithm is suitably secure for use in the Internet – which has, essentially, an infinitesimally large number of things that can be signed.

Since 2005, we have known about a theoretical collision attack against SHA-1.  However, nobody had publicly demonstrated the ability to exploit this vulnerability.

Security pros have long been urging organizations to move to alternative algorithms such as SHA-256, and the SHA-3 hash family.  TLS 1.3 is removing the ability to use SHA-1 this month. Some popular browsers will begin marking SHA-1 based certificates as untrusted.

Due to this announcement, your organization should now make it a priority to move to SHA-1 alternatives, too.

Having said all that, the Google blog post referenced above provides some interesting information about the number of computations required to compute the collision:

  • 9 quintillion (9,223,372,036,854,775,808) SHA-1 computations
  • 6,500 years of CPU computation to complete the first phase of the attack
  • 110 years of GPU computation to complete the second phase

Google states that “While those numbers seem very large, the SHA-1 shattered attack is still more than 100,000 times faster than a brute force attack which remains impractical.”

On the other hand, recent successful distributed denial of service (DDS) attacks have proven the ability of attackers to subvert 100’s of thousands of devices to do their bidding.  There is no technical reason they couldn’t use these devices to distribute the computations needed to generate SHA-1 collisions.

Mitigation of this vulnerability requires moving to a more secure hash algorithm such as those mentioned above.

In my opinion, it isn’t time to get the Heartbleed bug respirator out of the closet. However, you DO need to immediately start building a plan to complete migration of your organization to a stronger hash algorithm within the next six months or so.

If you don’t, you might want to make sure that respirator still works.

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