bitcoin/bitcoin — pull request #3473 by tarbell (2013-12-29 10:46:02+00:00)
Title: Added extractpubkey rpc call, allows extraction of public key from a message and signature.

Can be useful for logging into websites, without requiring the user to enter a username when they log in. User signs a message and the site recovers the public key from the known message and signature and checks the public key belongs to a user in the database.