I'm trying to PGP-encrypt data, by importing a standard text block:

-----BEGIN PGP PUBLIC KEY BLOCK-----
data ...
-----END PGP PUBLIC KEY BLOCK-----

#!/usr/bin/perl -w
use Crypt::OpenPGP;
use Crypt::OpenPGP::KeyRing;

my($keyfile, $email) = @ARGV;
my $ring = Crypt::OpenPGP::KeyRing->new(Filename => $keyfile);
my $pgp = Crypt::OpenPGP->new(PubRing => $ring);
my $crypt = $pgp->encrypt(
    Data => 'squeamish ossifrage',
    Recipients => [ $email ],
    Armour => 1,
    Cipher => 'CAST5',
);
defined($crypt) ? print $crypt : die $pgp->errstr;
[download]

This works fine for encrypting with my colleague's key, but only after adding the Cipher => 'CAST5' - without that, his decrypt function complains of a 'bad session key'.

What I don't understand is how I can determine the appropriate cipher given the public key: I don't see anything in the Crypt::OpenPGP interface that would allow me to ask that from the KeyRing or KeyBlock objects once I've determined them.

I suspect I'm missing a fundamental concept somewhere; clues gratefully received.

Hugo