How to get gpg-agent to cache the passphrases I enter when reading/saving *.gpg files?

Question

I want to work with gpg-encrypted files using Emacs, and would like to be able to enter the passphrase for each encrypted file only once per Emacs session.

I used to be able to do this by setting the variable epa-file-cache-passphrase-for-symmetric-encryption to t, but this no longer works. The explanation can be found in the last line of the documentation for the variable epa-file-cache-passphrase-for-symmetric-encryption:

(defcustom epa-file-cache-passphrase-for-symmetric-encryption nil
  "If non-nil, cache passphrase for symmetric encryption.

For security reasons, this option is turned off by default and
not recommended to use.  Instead, consider using gpg-agent which
does the same job in a safer way.  See Info node `(epa) Caching
Passphrases' for more information.

Note that this option has no effect if you use GnuPG 2.0."
  :type 'boolean
  :group 'epa-file)

After a recent system update, the gpg in my system is now version 2.1.8, which renders epa-file-cache-passphrase-for-symmetric-encryption ineffective.

The references mentioned in the documentation above lead to a page that just tells you, again, to use gpg-agent, but says nothing about how to do it. I have not been able to find clear instructions on how to set up gpg-agent for the use-case I described at the beginning of this post. Any help would be appreciated.

FWIW, ps tells me that /usr/bin/gpg-agent --supervised is indeed running. What else must I do to get it to cache the passphrases I enter when working with *.gpg files?

Also, the value of my epg-gpg-program variable is /usr/bin/gpg, which refers to the installed version 2.1.8 instance of gpg mentioned earlier.

The versions of Emacs I am using are 26.0.50 and 27.0.50. For both cases, the version of the epa package is 23.1.

Answer 1

https://www.gnupg.org/documentation/manuals/gnupg/Agent-Options.html#Agent-Options

In this documentation, the options you might be interested in

• default-cache-ttl n, Set the time a cache entry is valid to n seconds. The default is 600 seconds. Each time a cache entry is accessed, the entry’s timer is reset. To set an entry’s maximum lifetime, use max-cache-ttl. Note that a cached passphrase may not evicted immediately from memory if no client requests a cache operation. This is due to an internal housekeeping function which is only run every few seconds.
• max-cache-ttl n Set the maximum time a cache entry is valid to n seconds. After this time a cache entry will be expired even if it has been accessed recently or has been set using gpg-preset-passphrase. The default is 2 hours (7200 seconds).

you could set this in the config file ~/.gnupg/gpg-agent.conf

eg: default-cache-ttl 86400

Seems gpg-agent doesn't support caching passwords for entire session