Wallet System Design

Introduction

The wallet system provides a mechanism for storing and retrieving security-sensitive data such as system credentials and private keys from a secure host, applying ACLs to that data, and automatically creating of certain types of security data on demand (such as Kerberos keytabs).

The initial implementation of the wallet is targetted at Kerberos keytab distribution and the replacement of Stanford University's legacy Kerberos-v4-based sysctl system for distributing srvtabs and keytabs. After that initial implementation, additional data types will be added. SSL certificates, ssh private keys, and database passwords are likely early candidates. The implementation of keytabs is described in detail below, and similar detailed designs for other data types will be added as part of the later phases of the design.

This design document is not entirely complete in the area of exact protocol commands, supported arguments, and the details of the ACL manipulation protocol. These areas of the design are still being developed.

Assumptions

For the rest of this document, the term "object" will be used for a piece of security-sensitive data stored in the wallet. The object "metadata" is the authorization and history information around that object. Be aware that an object "stored" in the wallet may not be physically present on the wallet server; instead, the object may be a type of object that can be dynamically generated on demand by the wallet server or retrieved from elsewhere. For example, most Kerberos keytabs stored in the wallet will exist in the wallet only in the form of metadata and will be generated dynamically on demand when requested.

Wallet uses remctl for its network protocol, which provides Kerberos v5 GSS-API authentication and encryption of all data. The rest of this design document will assume the connection from a wallet client to the wallet server is handled via the remctl protocol, that the wallet server therefore knows the authenticated Kerberos principal of the client, and that data passed between the server and the client is encrypted. For more information about the remctl protocol, see:

<http://www.eyrie.org/~eagle/software/remctl/protocol.html>

remctl requires Kerberos v5 authentication, and therefore all clients using the wallet to retrieve data will use Kerberos v5 authentication.

We assume the wallet server is itself a secure host. Compromise of this host will compromise all stored data it has and will allow an attacker to perform any operation possible via the wallet. In order to limit the effectiveness of such an attack, certain keys may be excluded from the wallet's management purview (via kadm5.acl rules on the KDCs, for example) and require management by Kerberos administrators with kadmin. However, compromise of the wallet system would have a significant security impact and the system should be managed with the sort of security precautions as one would apply to a Kerberos KDC.

Server Design

Protocol Operations

The wallet server supports the following protocol operations on an object:

    autocreate      Create an object with default ACLs
    check           Determine whether an object exists
    create          Create a new wallet entry for an object
    destroy         Delete the wallet entry for a given object
    owner           Set the owner of an object
    acl             Set an ACL on an object
    expires         Set the expiration of an object
    flag            Set or clear flags on an object
    show            Show the metainformation about the object
    get             Retrieve the named object from the wallet
    store           Store the named object in the wallet

The first nine operations manipulate or display the metadata of the object. The next two operations store or retrieve the object itself.

The create, owner, acl, and expires operations are only available to wallet administrators. Even if one is the listed owner of an object, one may not change the owner, ACL, or expiration date on that object. (This may be reconsidered later to permit the owner to set the ACLs on an object. This design mirrors the existing Stanford University srvtab distribution system and is maximally conservative.)

Objects are created with an autocreate or a create command (which creates the metadata without storing any data). Administrators can create objects with the create command. Object creation via autocreate. When someone attempts to get or store an object that doesn't already exist, as determined by the check call, the wallet client attempts autocreate. On the server, the type and name of the object and the operation is passed to a policy function, which returns an ACL. If the user is authorized by that ACL, the object is created and that ACL becomes the object's new owner.

ACLs

Each ACL consists of zero or more lines, each of which has a scheme and an identifier. Initially, two schemes will be supported: krb5 and netdb.

An ACL line of scheme krb5 will have a single fully-qualified principal name as an identifier; only that principal will be authorized by that ACL line.

An ACL line of scheme netdb will have an identifier naming a specific machine. The user will be authorized if that user has a role of "admin" or "team" for that machine. See netdb-role-api for the specific remctl API for performing that query.

For all ACLs, each ACL line is tried against the user principal. If any ACL line authorizes the user, that user is authorized. If no ACL line authorizes the user, permission to perform the operation is refused.

For more details and other ACL types that will be supported in the future, see design-acl.

There will be one general system ACL with the special name ADMIN that identifies wallet administrators. Administrators are permitted to perform any operation except get and store, and can add themselves to ACLs to get and store objects. (Requiring that they add themselves to the ACLs first is not a security measure but a requirement to not be sloppy with ACLs when one is an administrator.)

Metadata

Each object stored in the wallet has the following metadata associated with it (with examples for a keytab for host/windlord.stanford.edu that should be retrievable by the Kerberos principal rra/root@stanford.edu):

Type-Specific Attributes

Object types may support additional attributes, which are keys and lists of values. The acceptable keys and values are determined by the individual object implementations. For many object types, no attributes are supported.

Objects of type keytab support two attributes: an optional list of enctypes for which keys should be generated for that principal, and an optional list of external systems with which the keytab is synchronized. The enctype list list can be used to restrict the Kerberos enctypes for a particular keytab to only those supported by that application. In the absence of a list associated with a keytab, the default enctype list in the KDC will be used. The sync attribute currently only supports a value of kaserver, which means that the DES key in the keytab is set as the key for a corresponding principal in an AFS kaserver.

Flags

Each object can have flags set on it. Currently, the only defined flags are:

    locked          Nothing permitted regardless of ACL except show
    unchanging      Use existing data, don't regenerate

The unchanging flag will only have meaning for those types where the backend can support either generating new data or using the existing stored data.

History

The wallet server will keep a history log of every operation performed against an object, keyed by object type and name (for object changes). A remctl interface will be provided so that wallet administrators can query this log and see the history of a given object. The wallet server will also keep a history log of every operation performed against an ACL, keyed by the ACL ID.

In addition, the wallet server will log to syslog every operation performed, not only on objects but on ACLs, Kerberos principal groups, keytab enctype metadata, and so forth.

Keytab Server Backend

Basic Operation

The keytab backend will not support the store operation, only the get operation. Normally, a get will result in the generation of a new keytab (possibly constrained by the list of enctypes for that keytab) and hence the invalidation of any existing keytabs for that principal.

The wallet server will only have kadmin ACLs to manage a specific set of principals to prevent the wallet from being used to change core Kerberos keys or to change user accounts.

NetDB Default ACLs

For Stanford's purposes, if a user attempts to autocreate keytab for which no entry had previously been created with create and that keytab is one of a specific set of host-bound principals as configured by the local wallet server deployment (generally things like host/*), we will check the principal against an ACL of scheme netdb and identifier equal to the host name for the principal. If that ACL authorizes the user, we will automatically create a wallet entry for this host, owned by an ACL of scheme netdb and identifier equal to the fully qualified name of the system. This will allow anyone with NetDB ownership of the system to manage the keytabs.

This is implemented via a Stanford-specific wallet configuration file that uses the server autocreate support. See examples/stanford.conf to see how this is implemented.

Retrieving Existing Keytabs

The flag unchanging can be set on keytabs to indicate that, rather than generating a new key on a get operation on that keytab object, the existing key should be extracted from the KDC and returned. This removes some protection around abuse of the wallet system since it allows one to get access to an existing key without invalidating the system key and then forge authentication to that service. Accordingly, this flag may only be set by wallet administrators unless a flag ACL is created on that object, and as a matter of policy it should only be granted when there's a compelling reason for it.

When a keytab with the unchanging flag set is retrieved with get, rather than generating a new keytab, the wallet server requests the current keys in keytab form from the KDC via a separate interface. The KDC will return only keys for principals matching a set specifically configured on the KDC. All strongly privileged keytabs should be excluded from this (and ideally, only those keytabs known to require caching should be listed here). The keys will be extracted from the KDC using kadmin.local with the -norandkey option, added with a Stanford-local patch (but expected to be in MIT Kerberos 1.7).

Client Design

Basic Operation

The client will use the remctl libraries for all communication with the wallet server. The wallet server name will be determined by a compile-time default, overridden by configuration in krb5.conf or by a command-line option. It should support the get, store, and show operations (although we will skip store for the initial implementation since it's not required for keytabs), and in general should pass any command on to the server so that we can add new commands later without modifying the client code. When called with a get or store command, it should check whether the object already exists with the check command and, if not, attempt autocreation with the autocreate command.

When retrieving a keytab, the client should support either creating a new keytab file or adding the keys from the downloaded keytab to an existing keytab so that multiple keys can be merged into the same keytab. This is useful for services that expect all their keys to be in krb5.keytab, or for adding keys for all a host's aliases to its krb5.keytab.

Srvtab Generation

For backward compatibility with a Kerberos v4 realm, the wallet client, when downloading a keytab, should also be able to optionally create a srvtab with the DES key extracted from that keytab (if any). In order to get the Kerberos v4 kvno for the key (which may differ from the Kerberos v5 kvno), it will obtain a Kerberos v4 service ticket for that principal and extract the kvno from that service ticket.

Similarly, whenever a keytab is created or changed, the server needs to synchronize the key with Kerberos v4 (using the sync attribute on keytab objects).

Security Considerations

System Compromise

By its nature, the wallet is an obvious attack target target. It has access to generate arbitrary keytabs for many different service principals, it will eventually store a variety of high-value privileged data, and it has to be accessible over the wallet protocol to clients.

This risk can be reduced by running minimal accessible services on this system, co-located this service only with other high-security applications if anything, and closely monitored for security issues. In addition, the wallet should only have access on the KDC to those principal classes that are managed by the wallet, specifically not including any core Kerberos administrative principals, any user accounts, and any administrator accounts.

The system should use iptables and similar firewall mechanisms to limit all access only to those ports providing known public services, and there to as few IP addresses as possible.

The wallet database should be stored locally on the system running the wallet server and not be accessible from any other system.

Administration of the wallet should be done over protocol. Logging on to the wallet system should be done only in the case of emergency, upgrade, or system maintenance and should not be done for any routine task.

Protocol Compromise

The security of the wallet is dependent on the security of the underlying remctl protocol. The remctl code has been carefully audited for security issues and is already in widespread use and should be treated as a core security component.

Most communications between the wallet and the KDC are done over the kadmin protocol, which is Kerberos-authenticated and encrypts the network communications. The other communication with the KDC is done via remctl. Extracting existing keys from the KDC can only be done on the KDC using kadmin.local and access can therefore be tightly controlled by the KDC remctl interface.

Retrieving Existing Keytabs

One key security concern is the wallet's ability to retrieve existing keytabs. The normal Kerberos key management system has some built-in defense against an attacker obtaining a keytab for a service principal: since that invalidates the existing keytab, the attacker will still not be able to authenticate to a service that uses that service principal, and any authentication done by the service principal will start failing. Compromise attacks are therefore often converted to denial of service attacks, and it's very difficult to launch a silent attack.

This changes when access is granted to existing keys. An attacker who obtains the existing keys can silently forge authentication to a service protected by that service principal, or can silently impersonate that service principal to other services without interfering with normal operations.

The unchanging flag should therefore only be set when there is a clear need, and the backend on the KDC that allows the wallet system to retrieve existing keys should be as restrictive as possible about which keys can be retrieved. Using this system is, however, better from a security standpoint than saving a copy of a keytab and installing it as part of a build process or copying it between multiple systems since the wallet at least maintains an audit trail of downloads and doesn't keep a local copy of the keytab, only the means for retrieving it. So, for example, a compromise that makes available only the backup image of the wallet server and not its credentials cannot obtain existing keytabs since they're not stored on its disk.

Auditing

The wallet server audits operations in three ways. First, an authenticated principal, hostname, and timestamp is kept up to date for each object for the last creation, modification, and retrieval date for that object. Second, an audit trail is kept for all operations on an object to allow retrieval of the complete history of an object. Third, all wallet operations are logged to syslog and therefore suitable for archiving, analysis, and forensics.

License

Copyright 2007, 2008, 2013

The Board of Trustees of the Leland Stanford Junior University

Copying and distribution of this file, with or without modification, are permitted in any medium without royalty provided the copyright notice and this notice are preserved. This file is offered as-is, without any warranty.

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