About 10 years ago, it was actually possible to look people up by their e-mail address online. You could also find a persons e-mail by searching for his or her name. Back then there where a lot of e-mail directories that acted like the yellow pages but for e-mail addresses. Very handy, but when spam became a problem, no one was willing to publicise their e-mail address and the e-mail search quickly died out.

Years passed and nobody thinks seriously about searching for people by their e-mail address anymore. It was tossed out of our toolbox – abandoned and forgotten.

Then a few years ago, something wonderful started to happen with the web. Community sites, forums, blog platforms etc. stated publishing FOAF and SIOC documents. Both documents contain e-mail addresses of people but not in the traditional sense. They publish SHA1 hashed e-mail addresses.

You can hash an e-mail using the SHA1 algorithm but you can never reverse it. That means the hashed e-mail addresses are secured from spam bots, but they are also left public for all of us to search for. All you need to do is to hash an e-mail address and do a Google search with the hashed value. Try searching for my hashed e-mail address on Google or go hash your own e-mail.

Here is a quick way of using SHA1 algorithm to hash any string value in C#.

public static string CalculateSHA1(string value)


  value = value.ToLowerInvariant().Trim();

  return FormsAuthentication.HashPasswordForStoringInConfigFile(value, "sha1").ToLowerInvariant();


The limitations of the SHA1 e-mail search is that you can only find people that have an online profile or blog, participate in online discussions or comments on blogs. The number of searchable people will rise as more and more sites start supporting FOAF and SIOC.


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