A foaf:mbox_sha1sum of a foaf:Person is a textual representation of the result of applying the SHA1 mathematical functional to a 'mailto:' identifier (URI) for an Internet mailbox that they stand in a foaf:mbox relationship to.

In other words, if you have a mailbox (foaf:mbox) but don't want to reveal its address, you can take that address and generate a foaf:mbox_sha1sum representation of it. Just as a foaf:mbox can be used as an indirect identifier for its owner, we can do the same with foaf:mbox_sha1sum since there is only one foaf:Person with any particular value for that property.

Many FOAF tools use foaf:mbox_sha1sum in preference to exposing mailbox information. This is usually for privacy and SPAM-avoidance reasons. Other relevant techniques include the use of PGP encryption (see Edd Dumbill's documentation) and the use of FOAF-based whitelists for mail filtering.

Code examples for SHA1 in C#, Java, PHP, Perl and Python can be found in Sam Ruby's weblog entry. Remember to include the 'mailto:' prefix, but no trailing whitespace, when computing a foaf:mbox_sha1sum property.