Transcript 02 extra reading Kerberos

Network Security:
Kerberos
Tuomas Aura
T-110.5240 Network security
Aalto University, Nov-Dec 2014
Outline
Kerberos authentication
Kerberos in Windows domains
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Kerberos authentication
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Kerberos
Shared-key protocol for user login authentication
Uses passwords as shared keys
Solves security and scalability problems in password-based
authentication in large domains
Based loosely on the Needham-Schroeder secret-key protocol
Kerberos v4 1988- at MIT
Kerberos v5 1993- [RFC 4120]
Updated protocol and algorithms
ASN.1 BER message encoding
Implemented in Windows 2000 and later
Used in intranets: e.g. university Unix systems, corporate Windows
domains
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Kerberos architecture
KDC
4. KRB_TGS_REP
Application
server B
5. KRB_AP_REQ
6. KRB_AP_REP
Client A
ap_client.exe
ap_server.exe
3. KRB_TGS_REQ
TGS
2. KRB_AS_REP
1. KRB_AS_REQ
AS
1.–2. Authentication
3.–4. Ticket for a specific service
5.–6. Authentication to the service
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Kerberos architecture (some details)
KDC
Service ticket, KAB
4. KRB_TGS_REP
TGT
3. KRB_TGS_REQ
TGT, KAT
2. KRB_AS_REP
1. KRB_AS_REQ
krbtgt@RealmY
Application
server B
5. KRB_AP_REQ
Service ticket
Client A
A@RealmY
6. KRB_AP_REP
ap_client.exe
B@RealmY
TGS
ap_server.exe
AS
1.–2. Authentication with password
→ client gets TGT and KAT
3.–4. Authentication with TGT and KAT
→ client gets
service ticket and KAB
5.–6. Authentication with service
ticket and KAB
→ client gets service access
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Kerberos ticket
Message type, version
FLAGS
KEY
CNAME, CREALM
Client name and realm
TRANSITED
transit realms
AUTH-TIME, END-TIME
CADDR
Client IP address (optional)
AUTORIZATION-DATA
App-specific access constraints
Encrypted with server’s master key
REALM, SNAME
Server name and realm
Same format for both TGT and
service ticket
Credentials = ticket + key
ASN.1 encoding in Kerberos v5
“Encryption” also protects
integrity, actually encryption
and a MAC
Flags:
FORWARDABLE, FORWARDED,
PROXIABLE, PROXY, MAY-POST-DATE,
POSTDATED, INVALID, RENEWABLE,
INTINIAL, PRE-AUTHENT, HW-AUTHENT
INITIAL flag indicates TGT
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Kerberos protocol (more details)
Initial login of user A:
1.
2.
A → AS:
AS → A:
Ticket request:
3.
4.
A → TGS:
TGS → A:
Preauthentication, A, TGS, NA1, AddrA
A, TGT, EKA (KA-TGS, NA1, TGS, AddrA)
TGT, AuthenticatorA-TGS, B, NA2, AddrA
A, Ticket, EKA-TGS (KAB, NA2, B, AddrA)
Authentication to server B:
5.
6.
A → B:
B → A:
Ticket, AuthenticatorAB
AP_REP
KA , KTGS, KB = master keys of A, TGS and B
KA-TGS = shared key for A and TGS
KAB = shared key for A and B
TGT = B, EKTGS (INITIAL, KA-TGS, A, Tauth, Texpiry1, AddrA))
Ticket = B, EKB(KAB, A, Tauth, Texpiry2, AddrA))
Preauthentication = EKA (1 TA)
AuthenticatorA-TGS = EKA-TGS (2 TA)
AuthenticatorAB = EKAB (3 TA)
AP_REP = EKAB(4 TA)
A, B = principal names
Tx = timestamp
AddrA = A’s IP addresses
Notes:
1234)
ASN.1 encoding
adds type tags to all
messages
Encryption mode also
protects message
integrity
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Kerberos realms
Realm X
Realm Y
Cross-realm trust
User registration
User A
Server B
Users and services registered to one KDC form a realm
name@realm, e.g. A@X, [email protected]
Cross-realm trust:
Two KDCs X and Y share a key (krbtgt@Y is registered in KDC X and krbtgt@X in KDC Y)
KDCs believe each other to be honest and competent to name users in their own realm
Cross-realm authentication:
Client A@X requests from TGS at realm X a ticket for TGS at realm Y
The ticket is encrypted for krbtgt@Y (i.e. TGS at realm Y)
Client A@X requests from TGS at realm Y a ticket for server B@Y
Access control at several steps:
Local policy at each KDC about when to honor tickets from other realms
Local policy at B@Y about whether to allow access to users from other realms
ACLs at B@Y determine whether the users is allowed to access the particular resources
Possible to transit multiple realms → TRANSITED field in the ticket lists
intermediate realms
Local policy at each server about which transit realms are allowed
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Realm hierarchy
contoso.com
dev.contoso.com
sales.contoso.com
Charlie
euro.sales.contoso.com asia.sales.contoso.com
Cross-realm trust
Bob
David
Alice
User registration
Large organizations can have a realm hierarchy
Hierarchy follows internet names
→ easy to find a path between realms
→ can filter cross-realm requests based on the names
Can add shortcut links or create even a fully connected graph
between KDCs
E.g. Windows domain hierarchy
Compare with X.509 certification hierarchy: similarities,
differences?
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Password guessing attacks
Kerberos is vulnerable to password guessing:
Sniffed KRB_AS_REQ or KRB_AS_REP can be used to test
candidate passwords → offline brute-force password guessing
In Kerberos v4, anyone could request a password-encrypted
TGT from AS → easy to obtain material for password cracking
Preauthentication in Kerberos v5 prevents active attacks to
obtain material for password cracking → must sniff it
!
Note: active vs. passive attacks
Misleading thinking: active attacks (e.g. MitM) are more
difficult to implement than passive attacks (sniffing)
Reality: Active attacks can often be initiated by the attacker
while passive attacks require attacker to wait for something to
sniff → vulnerability to such active attacks is serious
!
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PKINIT
Goal: take advantage of an existing PKI to bootstrap
authentication in Kerberos
Replaces the KRB_AS_REQ / KRB_AS_REP exchange
with a public-key protocol
Public-key authentication and encryption to obtain TGT
Continue with standard Kerberos → transparent to TGS
and application servers
No password, so not vulnerable to password
guessing
Uses DSS signatures and ephemeral DH
Windows 2000 and later, now standard [RFC 4556]
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Delegation
Server may need to perform tasks on the client’s behalf
Recursive RPC; agents operating when the user is no longer logged in;
batch processing at night
Alice can give her TGT or service ticket and key to David
Controlling the use of delegated rights in applications:
Ticket may specify many allowed client IP addresses
Authorization-data field in ticket may contain app-specific restrictions
It is safer to delegate a service ticket than TGT
Ticket flags related to delegation:
FORWARDABLE flag in TGT: the TGT can be used to obtain a new TGT with
different IP addresses
PROXIABLE flag in TGT: the TGT can be used to obtain service tickets with
a different IP address
Kerberos delegation is identity delegation
When B has A’s ticket and key, B can act as A and nobody can tell the
difference → difficult to audit access; similar to sharing passwords
Other protocols delegate only access rights, and the delegate can be
identified
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Kerberos in Windows
domains
Thanks to Dieter Gollmann
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Windows access control summary
The O/S stores security attributes for each
processes (subject) in an access token
Token contains a list of privileges and SIDs (i.e. user
and group identifiers)
Permissions are decided by comparing the list of SIDs
against a DACLs on an object
The access token is local to the machine, created at
login time, and never sent over the network
How to authorize access to resources managed by a
Windows service (=daemon) on a remote server,
e.g. over remote procedure call (RPC)?
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Network credentials
Alice’s user name, SID and network credentials are
cached on the user workstation
username and password, or TGT and KA-TGS
Alice’s processes can use her network credentials
for remote login
Two authentication protocols: NTLM and Kerberos V5 (RFC
1510)
These authentication protocols do not reveal the
password to the server
Applications can also ask the user for a different
user name and credentials and store them
separately
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Tokens and remote access
Access tokens are meaningful only to the local machine and
cannot be sent over network
The server does not trust the client machine to tell who Alice is and
which groups she belongs to
Instead, the client authenticates Alice to the server using her
network credentials. The server creates a new login session
and a new token (on the server) for Alice
The service may now assign the token to a process or thread
(=impersonation)
The authentication protocols also
provide the server with Alice’s user and group SIDs
produce a session key for protecting data between the client and
server
Encryption and authentication of session data is controlled
by applications
Different secure session protocol exist for network logon, RPC, COM
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Kerberos ticket in Windows
Message type, version
FLAGS
KEY
Username, domain
CNAME, CREALM
Client name and realm
TRANSITED
transit realms
AUTH-TIME, END-TIME
User and group SIDs
CADDR
Client IP address (optional)
AUTORIZATION-DATA
App-specific access constrains
Encrypted with server’s master key
REALM, SNAME
Server name and realm
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Related reading
William Stallings. Network security essentials:
applications and standards, 3rd ed. chapter 4.1; 4th ed.
chapter 4.1–4.2 (Kerberos v5 only)
William Stallings. Cryptography and Network Security,
4th ed.: chapters 14.1 (Kerberos v5)
Dieter Gollmann. Computer Security, 2nd ed.: chapter
12.4; 3rd ed. chapter 15.4
Kaufmann, Perlman, Speciner. Network security, 2nd
ed.: chapter 14
Online:
How the Kerberos Version 5 Authentication Protocol Works,
http://technet.microsoft.com/en-us/library/cc772815(v=ws.10).aspx
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Exercises
How does Kerberos fix the flaw in Needham-Schroeder
secret-key protocol?
Find source code for a Kerberized client/server
application (e.g. telnet) and see how it accesses
Kerberos services
Why is Kerberos used on the intranets and TLS/SSL on
the Internet? Could it be the other way?
Learn about Encrypted Key Exchange (EKE) and other
similar password-based authentication protocols. Which
problem do they solve?
Should standard protocols include data fields or
messages for proprietary extensions? What are the
arguments for and against?
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