PGP 5.X puts a marker packet at the beginning of any binary message which
includes new-style packet tag octets (ones with bit 6 set). This
causes PGP 2.X to recognize that the message is a PGP message which
requires a newer version of PGP. Without the marker packet, PGP 2.X
thinks it is not a PGP message.
The binary parser for PGP 5.X also looks for an old-style tag octet at
the beginning of the message. This is in pgpFileTypePGP in the
pgpFileType.c module.
In order for PGP 5.X to recognize that a binary message which uses
new-format tags is a PGP message, you should put a marker packet at
the front. That's why it is not reading the SKESK example properly.
Hal Finney
hal(_at_)rain(_dot_)org
hal(_at_)pgp(_dot_)com