SHA-1 Collision Probability

Posted on September 25, 2009 by Dave Grijalva

Git uses SHA-1 hashes of the contents of commits as their ids. I’ve wondered before what the probability is of a collision. According to Pro Git: (emphasis mine)

If all 6.5 billion humans on Earth were programming, and every second, each one was producing code that was the equivalent of the entire Linux kernel history (1 million Git objects) and pushing it into one enormous Git repository, it would take 5 years until that repository contained enough objects to have a 50% probability of a single SHA-1 object collision. A higher probability exists that every member of your programming team will be attacked and killed by wolves in unrelated incidents on the same night.

Good enough for me. The book also discusses what will happen in the event of a collision, but it’s less interesting.

Metonymy

Posted on September 8, 2009 by Dave Grijalva

Zamboni is a company that makes electronic ice resurfacers. Calling the machine a zamboni is incorrect. Using a company’s name in place of the name of the product is a very common type of metonymy that may seem like a corporation’s dream, but it can actually cause them problems. In 1965, the Duncan Yo-Yo Company lost it’s trademark on the term “yo-yo” because it had become the common term to describe that sort of toy. This fear of becoming a genericized trademark is why you see companies refer to their products with such silly names as “BAND-AID® Brand Adhesive Bandages.”

Here are a few examples of these. The wikipedia entry lists a few more. Can you think of any others?