How to make gpg-agent forget the passphrase automatically

You can use the technique described on this page:

http://fvue.nl/wiki/Debian_4.0:_Installing_gpg-agent

Here's the gist:

1. Install gpg-agent and pinentry program:

   sudo apt-get install gnupg-agent pinentry-curses
   

2. Add the lines below to ~/.profile. Any POSIX-confirming shell should include this file.

   # Invoke GnuPG-Agent the first time we login.
   # Does `~/.gpg-agent-info' exist and points to gpg-agent process accepting signals?
   if test -f $HOME/.gpg-agent-info && \
       kill -0 `cut -d: -f 2 $HOME/.gpg-agent-info` 2>/dev/null; then
       GPG_AGENT_INFO=`cat $HOME/.gpg-agent-info | cut -c 16-`
   else
       # No, gpg-agent not available; start gpg-agent
       eval `gpg-agent --daemon --no-grab --write-env-file $HOME/.gpg-agent-info`
   fi
   export GPG_TTY=`tty`
   export GPG_AGENT_INFO
   

   This little script will be activated when you login. If the agent is not running, it will be started. When the agent is started, it shows how to set environment variables in order to connect to it. The script saves these values in ~/.gpg-agent-info, so that when you start another login session the script can setup the variables correctly and thus use the agent.

You will only have to enter your passphrase once per boot. The agent will store your keys in memory, so you don't have to enter the passphrase again.

You probably don't have the environment variable $GPG_AGENT_INFO set.

What you can do as a temporary measure is run gpg-agent bash (or the shell of your choice) and retry decryption.

On the long run this might just be solved by first logging out and then log back in to your system, then check echo $GPG_AGENT_INFO again. If it is there chances are good that you no longer have to retype your passphrase every time.

Most Linux distributions have hooks for this in place and I expect OSX to be as user friendly, the requirement, of course being that gpg/gpg-agent is available (installed) before starting your session.