Strong Authentication – System Design and Deployment

Matt CrawfordFermilab

Computer Security Team

Outline Motivation and Requirements Components and Configuration Technical Factors Human Factors

Why? Reduce effort spent on intrusions &

recovery Regulatory climate is demanding increased

attention to access controls Management has agreed with the goals

outlined in SLCCC-TWG white paper: Alternatives to Reusable Passwords: Robust Authentication

Requirements Significant improvement in security. Acceptable to the user community.

– Carrots:• Single sign-on for users.• Central account & password administration for

sysadmins. Schedule

– Implementation may be staged but must offer meaningful improvement for Run II.

Components and Methods

Why not ssh? Ssh addresses encryption, but misses

several other key issues:– Performance -- why encrypt everything?– Account management– Password management– Password/certificate vulnerability– “Illusory security”

Illusory Security

Remote Site(or local!)

Desktop Server

Server

Supposedly Secure Realm

clear encrypted

Four Realms Strengthened

– Kerberos authentication required for all network logins.

Untrusted– Hosts, on- or off-site, from which logins to

Strengthened realm are not permitted. Portal

– Gateway between Untrusted and Strengthened. Trusted

– An outside Kerberos realm with which we cross-authenticate.

Kerberos Kerberos version 5 is a protocol for

authentication of users and services (collectively called principals.)– Created at MIT, circa 1987.– Designed for use over insecure networks.– Still under active development.– Several commercial products are built on it.– Many Universities and Labs use it.

AFS uses the Kerberos version 4 protocol. DCE uses Kerberos 5.

Kerberos Keys Each principal has a symmetric (secret) key.

– Users’ keys are hashed passwords.– Service keys are random bit-strings.

All keys are known by the Key Distribution Center (KDC)

Keys are used to decrypt short messages from the KDC. Knowledge of a key proves identity.

Kerberos does not send passwords over the network. Session keys are sent, encrypted under user and service keys.

Kerberos KDC KDC is replicated - one master per realm

and N 0 slaves.– Manual intervention needed to turn slave into

master, but all data is present on slaves. Addition, deletion and change of principals,

including password changes, require communication with master KDC.

Kerberos Key Servers KDCs must be on dedicated, secured

machines. Physical security is important. CPU, storage and network requirements are

moderate. Rough [O(5 min)] clock synchronization is

required between clients and KDC. Kerberos administrative functions may be

performed remotely.

Application Servers Any system which provides a Kerberos-

authenticated service over the network.– Telnet, rlogin, FTP, POP, CVS, etc.

• Application must have a Kerberos key to receive and decrypt tickets prepared by the KDC.

Authorization is done locally, as now.– Example: A user in the Kerberos realm must also

be listed in the local or NIS password file to log in, although no password needs to be recorded locally.

Ticket Delivery{ Foo }K(X) denotes data Foo encrypted under X’s key.

Service ticket:[ Svc, { User, SessKey, OtherInfo }K(SVC) ]

Ticket-Granting Ticket is just a service ticket with Scv=“krbtgt”.

Ticket-Granting Service reply:[ PA_data, User, Ticket, { SessKey, OtherInfo,

Svc}K(USER) ]

Overall Schematic

Strengthened RealmPortal

Untrusted Realm

On-Site Off-Site

Application Servers

One-timepasswords

Kerberos

KerberosKDC

Trusted Realm

KDC

Desktops

Kerberos-Secured Access

Strengthened RealmPortal

Untrusted Realm

On-Site Off-Site

Application Servers

One-timepasswords

Kerberos

KerberosKDC

Trusted Realm

KDC

Desktops

Cross-Authenticated Access

Strengthened RealmPortal

Untrusted Realm

On-Site Off-Site

Application Servers

One-timepasswords

Kerberos

KerberosKDC

Trusted Realm

KDC

Desktops

Access through Portal

Strengthened RealmPortal

Untrusted Realm

On-Site Off-Site

Application Servers

One-timepasswords

Kerberos

KerberosKDC

Trusted Realm

KDC

Desktops

Remote Access without Kerberos To prevent disclosure of passwords, initial

entry to Kerberos system must not be allowed by typing a password over a clear network connection!

User must log in to Portal realm first, with some other non-disclosing password scheme.

No change in accessing Untrusted Realm systems.

Technical Factors

AFS Integration AFS uses Kerberos v4 protocol with a

modified password key algorithm. A Kerberos v5 KDC in possession of the

master key for an AFS cell can generate a service ticket which is convertible to an AFS token for that cell.– Code from ANL & NRL, tested.– User with TGT runs “aklog”, no password.

• Transparent through krb5.conf app-defaults.

Enforcing Password Security To avoid exposing Kerberos passwords, non-

Kerberos network logins must be disabled - initial tickets must be obtained locally!– Easily configured.– May be verified by network scan.– Anonymous FTP is still allowed.

Password policies (quality, cracklib check, expiration, history) are enforced by the master KDC.

Portal Realm Provides authentication for users who lack

Kerberos software or secure network channels, and obtains their initial tickets.– One-time passwords– Hardware tokens– Java ssh applet?

KDC and portal kinit/login must be modified to use host keys in place of user keys for TGT passing.

Portal Realm Features Telnet and ftp are supported through the

portal realm.– ssh/scp may be desirable.– File transfer by pass-through ftp or AFS.– No strong desires expressed for additional

services (CVS? HTTPS? XDM?) Clearly preferable to move to strengthened

realm rather than use portal on a regular basis.

Portal Realm Features “Real” remote users use telnet more than X.

– Increased interactive load on portal realm.– Increased consumption of one-time passwords.– Change sociology?

Keyboard mappings through portal realm may be hopeless, or may be unimportant.

“Foreign” token systems will not be supported.

Portal Realm Features Ticket expiration during portal session may

expose Kerberos password.– E.g. a login session left overnight.– Users should log out and in again to portal

realm to get new tickets.– Strong temptation to simply re-kinit in

strengthened realm.– Train users “don’t do that.”– Could be mostly prevented by software means.

Human Factors

How to “get Kerberized”...

UNIX– Get user key– Install UPS product– Get host key

Win32– Get user key– Get software– Run setup

Users’ View - with Kerberos With a desktop in the strengthened realm

and the login.krb5 program which obtains initial tickets, the users’ experience changes only slightly:– Auto-login available with telnet & ftp.– Tickets will expire if session is very long.

• Established connections will not be terminated.• Tickets may be renewable, or new ones may be

obtained when needed to establish new sessions.– Must know kpasswd, klist and kinit commands.

Users’ View - w/o Kerberos Non-Kerberos logins to Strengthened realm

hosts will be refused without asking for a password.– telnet “Authentication failed.”– rlogin, rsh “Connection refused.”– ftp “Access denied: authentication required.”

Users’ View - Portal Realm From non-Kerberos desktops, users must log

in to one of a special set of hosts, with a one-time authenticator.

Procedures may be unfamiliar to the occasional user.

From a Portal host, log in to a Strengthened system or ftp files to/from AFS space.

X (and other) connections back to Untrusted systems are allowed.

Sysadmins’ View Must install Kerberized user applications

and servers. Must disable non-Kerberized versions.

– Remove servers from inetd.conf, put clients later in $PATH or hide them.

Maintenance effort is roughly equivalent to maintaining a locally-installed product, plus modification of one system file.– S/W update frequency is very low.

Sysadmins’ View (2) No Kerberos tickets for local user “root”. ksu can replace or supplement su, with

added flexibility.– Special principals such as user/root@REALM

can be given general root access, or be restricted to certain commands.

– Possible savings in distribution and typing of root passwords, and quicker answers to “Who has root access to this host?”

Account Administration Special account types are needed/requested:

– Group accounts• Access by several (many) individuals• Best accommodated by entering individual

principals in .k5login file.– “Generic” accounts

• Aren’t assigned to an individual (e.g. operator consoles).

– Transient accounts• Technically easy — policy issues?

For Developers GSSAPI (Generic Security Services

Application Programming Interface) provides calls to create a Kerberos-authenticated session, with optional– Integrity– Privacy– Mutual Authentication

Bindings exist for C, Java, Python, Perl, and other languages.

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