7. Passwords
Can be made secure in a stand-
alone environment
Subject to sniffing attacks when
used over a network
Network password solutions often
include encryption techniques
8. Encryption Techniques
Symmetric - Secret Key: the same key
for encryption and decryption. Tends to be
fast and is good for data encryption.
However, the key management issues
associated with secret key can be
significant.
e.g. DES = Data Encryption Standard
9. Encryption Techniques
Asymmetric - Public/Private Key: a publicly
known key for encryption and a private key
for decryption (or vice versa). Tends to be
slow and is generally only useful for
encrypting small amounts of data (such as
passwords, PINs and symmetric keys.)
e.g. RSA = Rivest, Shamir, Adleman
PGP = Pretty Good Privacy
(Phil Zimmerman)
11. Anyone can read the message.
Non-repudiation - can only have come from
User A.
Decrypt
User A’s
Private
Key
User A’s
Public
Key
Message
or data
Confirmed
message or
data
Digital
Signature
User A User B
Encrypt
Digital signatures
12. Kerberos - What Is It?
Authentication service developed by MIT
to allow users and services to
authenticate
Designed for client/server environments
Uses secret key cryptography - data
encryption standard (DES)
13. Why Is It Needed?
Authentication across a network to normal
services sends clear-text passwords,
capable of being discovered in a sniffing
attack
Users are annoyed at having to type
passwords in often
Services were developed, such as rlogin,
rsh, IDENT which used “authentication by
assertion”
14. Kerberos Authentication
Kerberos Authentication server issues
user a “ticket”
User requests a remote service
Remote service looks at ticket to verify
who the user is
15. Kerberos - How It Works
Both user and service must have “keys”
registered with the Kerberos
Authentication Server
User’s key is derived from a password he
chooses
16. Kerberos Session
kinit - call to initially set up ticket
prompt for password
telnet - call to kerberized client
17. Client
Key Distribution Center
Authentication
Server
Ticket Granting
Server
Request: User login name
IP address
Client
kinit
Auth
info
Service
Client
Auth info
Auth
info
Kerberos key
User key
Server session key
TGT key
Service secret key
18. Kerberos - How It Works
Initialization
User requests a Kerberos “Ticket
Granting Ticket” (TGT) by running kinit
kinit builds a request which has:
user login name
client machine IP address
name of ticket - here it is krbtgt, the
Kerberos ticket-granting ticket
Kerberos looks in its database to see is
user is allowed to request a TGT on this
host
19. Kerberos - How It Works
Initialization
Kerberos sends user a message which
contains two copies of the ticket:
One copy is encrypted with Kerberos’
secret key
One copy is in plain text
Entire message is encrypted with user’s
key
kinit client process receives message
and decrypts it based on the password
the user typed in
20. Kerberos - How It Works
Initialization
If the message decrypts correctly, kinit
puts the TGT into /tmp/tktuid where uid is
user’s user ID
kinit uses session key in the TGT to
encrypt an “authenticator” consisting of
principal name, IP address of client
machine and current time
21. Kerberos - How It Works
Service Request
User requests service, telnet, for
example
kerberized telnet client sends a request
to Kerberos server containing the TGT
stored in /tmp/tgtuid and the
authenticator
Kerberos uses its secret key to decrypt
the TGT, extracts the session key from
the TGT and decrypts the authenticator
22. Kerberos - How It Works
Service Request
To validate the user:
Kerberos compares the contents of the
authenticator to the contents of the TGT
Kerberos compares the expiration
timestamp in the authenticator to the
current time
Kerberos builds a session key for the
telnet session, and makes two copies
one encrypted with TGT
one encrypted with telnetd’s key
23. Kerberos - How It Works
Service Request
Session key sent to user
telnet client uses the TGT key to decrypt
the session key, and adds ticket to
Kerberos ticket file
telnet client builds an authenticator for
the ticket, encrypts it with the session key
and sends ticket which was encrypted
with telnetd key and authenticator to
telnetd service
24. Kerberos - How It Works
Service Request
telnetd service decrypts ticket with its
secret key to get the session key
telnetd service uses session key to
decrypt authenticator
if information in ticket and authenticator
agree, telnetd sends back a message to
the user and the session begins
25. Kerberos Limitations
Bad passwords are still subject to
a dictionary attack
Kerberos V4 subject to cracker
attack (worse than some standard
Unix security)
Kerberos V5 subject to sniffer
attack
Passwords still subject to host
security
Trojan horses in Kerberos client
software can divulge passwords
26. Kerberos Limitations
Security over Kerberos database
containing users’ and services’
encryption keys must be strictly enforced
Security over master Kerberos password
must be kept
27. Other Security Enhancements
One-time Passwords
Device - SecurID
List of passwords - SKey
Public-key Cryptography
28. Today
Code available for free from MIT
Some vendor support:
Cygnus
OpenVision
DEC
IBM
Many Universities and some government
institutions have implemented Kerberos
Not too many businesses have
implemented it
29. Benefits of Kerberos
No Clear Text Passwords Across
Internet
Users Do Not Need to Enter Password
Multiple Times
30. Future
Kerberos will use public-key
cryptography for the initial TGT request
Windows 2000 (formerly called NT 5.0)
will have a Kerberos implementation
31. References
The Moron's Guide to Kerberos, Version 1.2.2
http://gost.isi.edu/brian/security/kerberos.html
Kerberos: An Authentication Service for Computer
Networks
http://nii.isi.edu/publications/kerberos-neuman-tso.html
Kerberos References
http://sol.usc.edu/~laura/kerb_refs.html
RFC 1510
http://www.faqs.org/rfcs/rfc1510.html
