wiki:TIEDCredentials

Version 7 (modified by faber@isi.edu, 7 years ago) (diff)

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In Progress - please ignore

GENI-Compatible ABAC credentials

GENI is currently working toward making ABAC the primary authorization system for the GENI AM API. A first step in that direction is to use the ABAC prover library, libabac, to make authorization decisions based on the current GENI policy. A first step in this direction is to implement current GENI policy - including a new "speaks-for" feature using ABAC without changing the existing AM API calls or credential formats. This document describes our approach to do so.

The workflow for an aggregate manager is that when it receives a request through the GENI AM API it will initialize an ABAC prover with its current policy (to allow dynamic policy changes), import the information in the GENI Credentials in the request, and ask the prover if the caller has the privileges needed to make the call. Specifically, the AM will check the same privilege strings it currently checks but using the ABAC prover to get the allowed/denied answers.

That leaves two problems to address:

  • Encoding the policy for import into the ABAC prover
  • Importing the information from GENI credentials into the ABAC prover

Because we are encoding the current static, global GENI policies, storing and importing them is relatively easy. We can encode the generic parts of the policy into a configuration file on the aggregate manager once. For dynamic parts of the policy - specializing the generic policy for a specific target - a short code stub will be inserted into the AM. This is consistent with using an RT1-lite template. Note that none of these policy encodings need to be exchanged, so they can be stored in legacy libabac formats.

Importing the information in a GENI credential is accomplished by modifying libabac's credential input routines to recognize GENI credentials and convert them into valid ABAC rules. The ABAC rules need not be exchanged; they are only used in generating the answer to the authorization question. Our plan is to use existing GENI credential formats in the short term, but to add new formats in the long term. The new formats will support the full expressiveness of ABAC and can be used to store policies in more human readable formats as well as to pass more expressive ABAC information around in the future.

GENI components issuing GENI credentials and making requests using them will continue to pass the same credentials around, and require no changes. Parties making authorization decisions based on GENI credentials will add code provided by ISI to interface to the ABAC prover and rely on it to check access. This will add speaks-for to GENI.

The rest of this document discussed the semantics of GENI credentials including delegation and the "speaks-for" semantics, explains how those will be encoded into ABAC logic, and discusses the dynamic policy insertion. Readers will want to read farther the more they are interested in ABAC.

GENI Privilege Credentials

A GENI privilege credential encodes a set of statements of the form "The issuer of this credential (a principal) gives the owner of it (a principal) these privileges (strings) with respect to the target (a principal)." The privilege strings are defined with respect to the GENI APIs. For each of the privileges, a boolean is given and is true if the privilege can be passed to others.

The issuer is the principal that signed the credential. The target and owner are given explicitly as X.509 certificates. We can identify each of these principals by their public key from the X.509 certificates. In the short term that binding is stable. In the long term the ABAC policy can be take changing keys into account.

When the credential is delegated, the credential assigning the delegatable privilege (the base credential) is included in a new credential signed by the owner of the base credential. The owner of the base credential is the issuer of the new credential. The new credential assigns privileges to an new owner. The new credential is valid if the base one is, if the delegated rights are marked delegatable, and the expiration time of the new credential is no longer than the base. This recurses.

GENI Privilege Credentials and "Speaks-for"

The "speaks-for" privilege is intended to be used as follows. A user wishes to use a hosted tool to access aggregate managers but does not want to give its identity certificate and private key to that tool, which may be a web service. The user can issue a GENI credential granting a "speaks-for" right to the tool (a principal). The tool includes that credential in its requests and the aggregate manager (and other parties making authorization decisions) will treat these requests as though they came from the user.

This differs from delegation in two ways:

  • Credential issuers need not issue delegatable privileges
  • A tool need not receive and manipulate all the delegated credentials

"Speaks-for" makes all existing GENI credentials delegatable by "speaks-for". To make future credentials immune to this delegation will require a format change to the privilege credentials.

GENI Policy in ABAC

Here we describe the ABAC rules that describe the GENI policy as it stands. The policy and credential formats are somewhat entwined, and we cannot speak of one completely without the other. This section assumes one knows the ABAC definitions, and the RT1-lite encoding of RT1 single parameter attributes. (In a nutshell, an RT1-lite encoding of the RT1 credential Principal.Attribute(parameter) is Principal.attribute_parameter).

We give the ABAC as RT1, and explain how the RT1-lite conversion will happen later.

Unlike the description above, we first describe how to encode a policy that supports "speaks-for" and then add delegation. Speaks for is actually the simpler rule.

Expressing GENI privileges with speaks-for in ABAC

For a given request at an Aggregate Manager, the Manager knows the principal making the request, the target of the request, and which privilege is required to execute it. It has a prover initialized with its policy (an ABAC context in libabac terms). It adds any credentials in the request and asks the prover a yes/no question - "does the principal making this request have the proper privilege?"

The first thing to encode in ABAC is "the proper privilege." The ABAC attribute AM.privilege(Target) means that AM believes principals in that set have privilege with respect to Target. For example, the resolve on a slice S would be AM.resolve(S). When a principal (P) requests an operation that requires resolve rights on slice S, the Manager (AM) asks the prover if P is a member of AM.resolve(S) (or in other words, if P has the AM.resolve(S) attribute).

Aggregate Managers do not issue these credentials to users. They are elements of the local policy, encoded in a configuration file. While ABAC allows delegation here, for now lets assume a series of simple delegations. There are a set of issuers that each Aggregate Manager believes. For each Issuer that AM believes, it has a rule like this:

AM.privilege(Target) <- Issuer.privilege(Target)

There is one of those rules for each privilege it understands

AM.resolve(Target) <- Issuer.resolve(Target)
AM.info(Target) <- Issuer.info(Target)

Issuers, such as Slice Authorities and Clearinghoses, issue credentials to users. Here we describe how libabac will translate a credential into multiple ABAC statements inside the prover.

Because GENI allows all privileges to be passed around by "speaks-for" the ABAC for issuing a privilege to a user (P) looks like:

Issuer.privilege(Target) <- Issuer.speaks_for(P)
Issuer.speaks_for(P) <- P
Issuer.speaks_for(P) <- P.speaks_for(P)

The Issuer says that anyone that speaks for P has the privilege. The Issuer says P speaks for itself. The Issuer says that anyone P says speaks for P speaks for P. When an Issuer hands out a GENI credential assigning privilege with respect to Target, it is making those three statements in ABAC. The first line is repeated for each privilege in the credential; the last two are added to the prover once per credential.

When a user (P) issues a speaks-for credential for a tool (T), that credential encodes the following ABAC rule:

P.speaks_for(P) <- T

Note that this interprets the same credential format (GENI privilege credential) differently depending on the privilege being assigned (speaks-for). This is because of the implicit semantics of "speaks-for" buried in its definition.

To be concrete: if (in GENI terms) AM trusts Issuer about resolve on Target, Issuer has handed P a GENI credential assigning resolve on Target, and P has issued a "speaks-for" credential to tool T, and T makes a request including both credentials, AM has the following ABAC rules in its prover:

AM.resolve(Target) <- Issuer.resolve(Target)                 /* from static policy and RT1 lite expansion */
Issuer.resolve(Target) <- Issuer.speaks_for(P)               /* From Issuer GENI privilege credential */
Issuer.speaks_for(P) <- P                                    /* From Issuer GENI privilege credential */
Issuer.speaks_for(P) <- P.speaks_for(P)                      /* From Issuer GENI privilege credential */
P.speaks_for(P) <- T                                         /* From User GENI privilege credential (speaks_for) */

When AM asks its prover if T can proceed (if T is in AM.resolve(Target), the proof chain looks like:

AM.resolve(Target) <- Issuer.resolve(Target) <- Issuer.speaks_for(P) <- P.speaks_for(P) <- T

Adding Delegation

Each privilege in a GENI privilege credential includes a flag indicating it is delegatable. For each privilege they assign to user P, such a credential adds the following two rules:

Issuer.privilege(Target) <- Issuer.can_delegate_privilege(Target).privilege(Target)
Issuer.can_delegate_privilege(Target) <- Issuer.speaks_for(P)

The issuer says that anyone in the Issuer.can_delegate_privilege(Target) set can assign Issuer.privilege(Target) and that any principal who speaks for P (including P) is in that set. This formulation implies that "speaks-for" works for delegation as well. A tool that can speak for a user can delegate on that user's behalf. If the second line is:

Issuer.can_delegate_privilege(Target) <- P

then only the user can delegate.

RT1-lite templates for AMs

One of the things RT1-lite must do is to generate RT0 statements for new parameterizations of the RT1 statements. In this system, an RT0 statement of the form

AM.privilege_Target <- Issuer.privilege_Target

Needs to be created and added to the ABAC context the first time each (privilege, Target) pair is encountered by AM. This is the code stub mentioned in the introduction. Similar stubs will be needed by other resource controllers.

Conclusions

This has described the implementation strategy for GENI privilege credentials using existing credential formats and implementing "speaks-for." Most of the development is focused on accepting existing GENI privilege credentials and translating them into ABAC. We have also laid out the ABAC that reflects the existing GENI authorization strategy at AMs.