--pgp6 flag. This is not nearly as involved as --pgp2. In short, it

turns off force_mdc, turns on no_comment, escape_from, and force_v3_sigs,
and sets compression to 1.  It also restricts the user to IDEA (if
present), 3DES, CAST5, MD5, SHA1, and RIPEMD160. See the comments above
algo_available() for lots of discussion on why you would want to do this.

g10/encode.c

@@ -624,20 +624,3 @@ encode_crypt_files(int nfiles, char **files, STRLIST remusr)
         }
     }
 }
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