Assuming you trust the metadata registration practices of the InCommon Federation, you will want to verify the XML signature on each and every metadata aggregate you consume. Failure to do so will seriously compromise your metadata refresh process.
To verify the XML signature on a SAML metadata aggregate, you need an authentic copy of the metadata signing certificate, that is, the certificate that contains the public key corresponding to the private metadata signing key. The certificate must be obtained securely since all subsequent operations depend on it.
You may check the integrity of the downloaded certificate in a variety of ways. For example, on a GNU/Linux system, you could use curl and openssl to check the integrity of the metadata signing certificate as follows:
# get the metadata signing certificate on md.incommon.org # and display the HTTP response header $ CERT_PATH=/path/to/inc-md-cert.pem $ /usr/bin/curl --silent --dump-header /dev/tty http://md.incommon.org/certs/inc-md-cert.pem > $CERT_PATH HTTP/1.1 200 OK Date: Thu, 19 Dec 2013 14:01:00 GMT Server: Apache Last-Modified: Wed, 18 Dec 2013 21:08:31 GMT ETag: "150037-4fd-4edd5727611c0" Accept-Ranges: bytes Content-Length: 1277 Connection: close Content-Type: text/plain; charset=UTF-8 # compute the SHA-1 and SHA-256 fingerprints of the metadata signing certificate $ /usr/bin/openssl x509 -sha1 -in $CERT_PATH -noout -fingerprint SHA1 Fingerprint=7D:B4:BB:28:D3:D5:C8:52:E0:80:B3:62:43:2A:AF:34:B2:A6:0E:DD $ /usr/bin/openssl x509 -sha256 -in $CERT_PATH -noout -fingerprint SHA256 Fingerprint=2F:9D:9A:A1:FE:D1:92:F0:64:A8:C6:31:5D:39:FA:CF:1E:08:84:0D:27:21:F3:31:B1:70:A5:2B:88:81:9F:5B
Once the certificate file is locally installed, you can use it to verify the signature on the metadata file.