Introduction
The main function of eduGAIN is to act as a trusted exchange service of information required for interfederation to work. This document describes the methods used to facilitate interfederation based on SAML and must be seen as an addition to the eduGAIN SAML Profile document [eduGAIN-Profile].
The current mode of operations of the eduGAIN SAML profile is to collect entities from participant federations provided in the form of federation metadata feeds, combine them into a single eduGAIN aggregate and republish. The aggregation process also serves as a validation service in order to ensure that the resulting global eduGAIN matadata aggregate conforms to all required standards.
This central component of eduGAIN SAML service is called Metadata Distribution Service (MDS).
Technical operational details about metadata signing, publication and other procedures can be found in the eduGAIN Operational Practice statement Document [eduGAIN-OPS].
Terms
The terms defined below are a required extension of the terminology defined in [eduGAIN-Profile]. The reader should consult both dictionaries for a complete picture.
| federation metadata feed | A SAML metadata file originating from a participant federation acting as a SAML Metadata Producer |
| eduGAIN matadata aggregate | A SAML metadata file obtained as an aggregate of federation metadata feeds according to the procedures described in this document |
| federation metadata channel | A location (in the form of http/https URL) pointing to the distribution source of the federation metadata feed |
Source of metadata
MDS bases its aggregation function on information provided by each participant Federation as specified in [eduGAIN-Profile]:
a federation metadata channel,
an RSA / EC public key with which the metadata metadata feed document will be signed. This will normally be made available in the form of an X.509 certificate,
the registrationAuthority attribute value to be associated with the federation metadata feed.
This information needs to be registered with eduGAIN OT in a trust preserving way as described in [eduGAIN-OPS].
In order to eliminate unnecessary traffic, the http/https server serving the federation metadata feed location SHOULD support the Conditional GET Request, this way signalling that the federation metadata feed has not been changed.
Metadata acquisition and validation
General
After a successful verification (as described further down), each federation metadata feed is saved locally for possible future use.
If a saved federation metadata feed copy exists and it also follows from the Conditional GET Request that the feed has not changed, the saved copy is being used for further processing.
A federation metadata channel which cannot deliver a document (fetched or from cache) that passes all of the required tests is regarded as empty.
Verification of origin
As specified by the [eduGAIN-Profile] in order to assure metadata integrity and originality, each federation metadata feed MUST be signed as specified in [SAMLMeta]. This signature made with the key matching the one supplied to the eduGAIN OT is the only element on which trust is based. In particular MDS does not use trust that might be derived from an https endpoint details.
Metadata signature verification is done against the public key alone. If the public key for the federation metadata feed channel is supplied in the form of an X.509 certificate, other aspects of the certificate such as its expiry date do not form part of signature verification. This approach is borrowed from the SAML metadata interoperability profile. In particular an expired certificate will still be used for the verification purpose.
condition evaluated | reason | |
|---|---|---|
S1 | The signature exists and is valid | eduGAIN-profile section 4 |
S2 | The signature can be validated with the public key configured for the federation metadata channel | eduGAIN-profile section 4 |
| S3 | The signature was made using an explicit ID reference, not an empty reference | eduGAIN-profile section 4 |
| S4 | The signature reference refers to the document element | eduGAIN-profile section 4 |
| S5 | The signature's digest algorithm is at least as strong as SHA-256, and does not use MD5 | eduGAIN-profile section 4 |
| S6 | The signature's signature method is RSA with an associated digest at least as strong as | eduGAIN-profile section 4 |
| S7 | The signature's transforms contain only these permissible values:
| eduGAIN-profile section 4 |
Verification of metadata validity
After a positive verification of integrity and originality (as described in the previous section), the following validity verification steps are performed.
Verification of the document as a whole:
Condition Evaluated | Reason | |
|---|---|---|
A1 | the document root element is md:EntitiesDescriptor | |
A2 | all required namespaces are declared, that is xmlns:md, xmlns:mdrpi, xmlns:ds. | |
A3 | md:EntitiesDescriptor contains md:Extensions element with mdrpi:PublicationInfo element in which the publisher and creationInstant attributes exist | eduGAIN Profile |
| A4 | the creationInstant attribute uses the dateTime format required by SAMLMeta and does not point to the future | SAMLMeta sec. 2.2.1 |
A5 | validUntil attribute in EntitiesDescriptor element exists, can be converted to a time value and it does not point to the past | SAML lines: 348; 316 |
A6 | validUntil attribute with a value not earlier than 120 hours (5 days) and not later than 2304 hours (28 days) after the creationInstant | eduGAIN-profile |
A7 | the fetched document schema-validates against following SAML metadata schemas:
|
For each md:EntityDescriptor element the following verification is performed:
Condition Evaluated | Reason | |
E1 | entityID attribute value has no space characters, starts with http:// or https:// or urn: and must be unique within given feed | SAMLmeta, ^anyURI |
E2 | md:Extensions element with mdrpi:RegistrationInfo is defined and registrationAuthority attribute matches the value registered with the eduGAIN OT for a given federation | eduGAIN-profile |
E3 | if within md:ContactPerson element any of the following elements is declared: GivenName, Surname, EmailAddress, TelephoneNumber - its values must not be empty | SAMLmeta, ^string |
E4 | if md:Organization element is declared with md:OrganizationDisplayName and/or md:OrganizationName and/or md:OrganizationURL elements then values of these elements must not be empty | SAMLmeta, ^anyURI ^string |
^anyURI - see [SAML] 1.3.2
^string - see [SAML] 1.3.1
For each role descriptor element declared under md:EntityDescriptor the following verification is performed:
Condition Evaluated | Reason | |
R1 | md:IDPSSODescriptor element must have a signing certificate (ds:KeyDescriptor/ds:KeyInfo/ds:X509Data/ds:X509Certificate) | |
R2 | if md:Extentions element with md:UIInfo exists:
| |
R3 | if md:Extentions element with md:DiscoHints exist:
|
Resulting eduGAIN metadata aggregate
Federation metadata feeds are combined into a single collection - the eduGAIN metadata aggregate as described in detail later. If an md:EntityDescriptor/@entityID value appears in more than one federation metadata feed, the resulting collection will contain only one of these entities; the others will be discarded. MDS does not attempt to merge or otherwise combine the clashing entity descriptions. See the technical details for a description of the collision handling algorithm.
For each entity document the following are removed:
*/@xml:base
md:EntityDescriptor/@ID
md:EntityDescriptor/@validUntil
md:EntityDescriptor/@cacheDuration
The eduGAIN metadata aggregate's md:EntitiesDescriptor element sets the following attributes:
name is set to http://edugain.org
validUntil is set 96 hours into the future
cacheDuration is set to 6h
- ID is based on the time of its generation and has the format “eduGAIN” followed by the complete UTC date/time value (YYYYMMDDThhmmssZ)
The eduGAIN metadata aggregate is signed in conformance to the signature profile described in section 3.1 of [SAMLMeta]. In particular the signature:
- is enveloped - http://www.w3.org/2000/09/xmldsig#enveloped-signature
- contains ds:Reference containing a URI reference to the document element's ID attribute
- uses SHA-256 digest method - http://www.w3.org/2001/04/xmlenc#sha256
- uses RSA + SHA-256 signature method - http://www.w3.org/2001/04/xmldsig-more#rsa-sha256
- uses exclusive canonicalisation - http://www.w3.org/TR/2001/REC-xml-c14n-20010315
Alerts and information
In the case when
a federation metadata feed is unavailable (the corresponding federation feed channel is not responding)
a federation metadata feed does not validate correctly
an alert is raised and delivered to the Operational Team. An error status is set on the eduGAIN status page https://technical.edugain.org/status and the cause of the error is displayed in the details section. The remaining cache time is also displayed. The status is also available through the eduGAIN access API, as described on: https://technical.edugain.org/monitoring. If the error condition persists reminder messages are sent in the intervals of 6 hours. If the federation metadata feed can be accessed/validated again, a recovery message is delivered to the eduGAIN OT.
During every aggregation run the validUntil timer for each of the federation metadata feeds is performed.
- If the remaining validity period is below 96 and above 12 hours an alert is raised once a day at 14 hour UTC,
- If the remaining validity period is below 12 and above 6 hours an alert is raised every second hour,
- If the remaining validity period is below 6 hours an alert is raised every hour.
Detailed technical description
Metadata acquisition
Federation public keys, federation feed channel locations (metadata URL), registrationAuthority strings are stored in the eduGAIN database.
Aggregation process is performed in the following steps:
all federations with the status “in production” are selected from the eduGAIN database
for each federation its metadata URL is used to access federation metadata feed
the metadata URL is contacted by presenting If-None-Match and If-Modified-Since header values from the last successful metadata fetching process (conditional GET support)
the response 304 means that metadata was not modified - in this case the latest saved copy is used in aggregation process
the response 200 means that a new metadata feed is available
the eduGAIN validator is run against any new metadata feed
any feed error generated by the eduGAIN validator triggers the appropriate report, the offending metadata is rejected and the last successful saved copy is used instead if it is still valid
any successfully checked metadata feed is saved locally
Metadata validation
Each freshly downloaded federation metadata feed is processed in order to verify integrity and originality and the adherence to all required standards and policy conditions.
Signature verification is handled with the Shibboleth Metadata Aggregator v. 0.9.2
Schema conformance validation is handled with the Shibboleth Metadata Aggregator v. 0.9.2
Additional conditions, in particular those defined by the [eduGAIN-Profile] are handled by eduGAIN specific code in the eduGAIN validator implemented in Python with lxm and OpenSSL modules.
Metadata combination and collision handling
All valid federation metadata feeds are passed to the aggregator in a sequence ordered according to the date when federations have started to supply data to eduGAIN. During aggregation the first occurrence of a given entityID will be used in the resulting eduGAIN metadata aggregate, any of the following occurrences will be discarded.
It should be noted that this algorithm makes it possible that an entity being served by one federation will be later replaced by its version from another federation if this latter federation comes first in the processing order.
Metadata aggregation is performed with pyFF (currently 0.10.0dev)
Acknowledgment
This document borrows heavily from Ian Young’s https://gist.github.com/iay/7486653
References
[SAML] https://docs.oasis-open.org/security/saml/v2.0/saml-core-2.0-os.pdf
[SAMLMeta] https://docs.oasis-open.org/security/saml/v2.0/saml-metadata-2.0-os.pdf
[eduGAIN-Profile]
[eduGAIN-OPS]