Need to implement a gpg/gpg2-compatible tool to encrypt millions of files in unsupervised mode

[Robert J. Hansen]

> First, please allow me to define a bit of ad-hoc
> nomenclature.  I will use the uppercase tems "ENCRYPT",
> "ENCRYPTION", etc. as shorthands for "compress and
> AES256-encrypt", "compression and AES256 encryption", etc.
> Likewise, I will use "DECRYPT", etc. as shorthands for
> "[AES256] decrypt and decompress", etc.

For a straightforward purely-symmetric use case, consider using OpenSSL.
 Just use it wisely: specify your own key and initialization vector,
rather than trust OpenSSL's weak password derivation function.

$ tar -cJf - mydir | openssl aes-128-cbc -out foo.txz -K
DEADBEEFDEADBEEFDEADBEEFDEADBEEF -iv DEADBEEFDEADBEEFDEADBEEFDEADBEEF
-out mydir.txz.aes

Don't get me wrong: GnuPG can definitely do the job you want.  But
consider whether you really need RFC4880's baroque packet format, weird
CFB mode, and everything else.  Sometimes there's a lot to be said for
simplicity.

> [1] This gpg/gpg2 compatibility requirement is important, as
> an insurance that the files will be DECRYPTABLE in the
> "distant" future (10-15y), even the my tool is not properly
> enough maintained to be operational then.  This, of course,
> assumes that gpg will have greater longevity than a privately
> implemented, single-user tool like mine.

OpenSSL meets your requirement.