CACert's OCSP responder signs its OCSP responses with an interesting
certificate, which has an extendedKeyUsage extension permitting OCSP signing
but has no keyUsage extension at all. RFC2560 says absolutely nothing about the
responder certificate requiring a keyUsage extension, it only says that it must
have an extendedKeyUsage extension with the OCSP signing OID in it. But GPGSM,
in certlist.c function cert_usage_p(), requires that (use != ~0) in order to
accept a certificate for an OCSP responder (otherwise it spits out "Certificate
should have not been used for OCSP response signing"), even though the
extendedKeyUsage extension is present and hence sets have_ocsp_signing to true
(as long as ksba_cert_get_key_usage() fails, "use" is set to ~0 and at that
point nothing else matters: the cert is rejected).
I have attached CACert's OCSP responder certificate for your reference.