protected override void Decode ()
{
keyPurpose = new ArrayList ();
ASN1 sequence = new ASN1 (extnValue.Value);
if (sequence.Tag != 0x30)
throw new ArgumentException ("Invalid ExtendedKeyUsage extension");
// for every policy OID
for (int i=0; i < sequence.Count; i++)
keyPurpose.Add (ASN1Convert.ToOid (sequence [i]));
}