Messages agent wants to use your confidential information stored in “<key>” in your keychain

After upgradering to Mavericks, I have been bombarded repeatedly with the following prompt: > Messages agent wants to use your confidential information stored in “” in your keychain > [Allow] [Always allow] [Deny] No matter how many times each option is used, the popup appears again after a very short while (usually less than 15 seconds). Erasing the Messages.app/iChat-related preference files from ~/Library/Preferences/ did *not* work.