Transcript No Slide Title

ECE-6612
http://www.csc.gatech.edu/copeland/jac/6612/
Prof. John A. Copeland
[email protected]
404 894-5177
fax 404 894-0035
Office: Klaus Bldg 3362
email or call for office visit.
Chapter 4a - Kerberos
Kerberos, v4 and v5
Provides a complete protocol for authentication and
secure communications for hosts connected by a data
communications network
•
Provides secure "tickets" to hosts that can be used
to initiate a secure message exchange
•
Standard message formats for encrypted and signed
messages, or signed plaintext messages
•
Formats for encoding expiration time, names, ...
•
Allows "read-only" slave KDC's (distributed KDCs)
Wikipedia: “KDC” or
http://www.zeroshell.org/kerberos/Kerberos-operation/
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Keberos uses Mediated Authentication
(with a Key Distribution Center, KDC
)
Bob
Jack
Alice
Kbob
Kalice
Mary
Tom
KDC
Paul
Peter
Dick
Jip
Trudi
Harry
KDC has unique Secret Keys with all legitimate hosts.
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Alice
(human)
logs on
to
Alice,
(PC)
Alice
Alice PC
hashes
Alice's
password
to get a
DES Key,
Kalice>Kak
(1)
{Ka,{TGT;Kk};
Kak}
Key
Distribut.
Ctr., KDC
Alice wants
Bob,{TGT;Kk}, generates
{time;Ka}
Kab Ka,
has
{Bob,Kab,Ticket
Kak, Kbob
-Bob; Ka}
{time; Kab},
Bob has
Shared
Secret Key
with KDC,
Kbob
{Kab,Alice; Kbob} ="Ticket"
{time + 1, Kab}
After the 1st exchange with the KDC, Alice has a
session key, Ka, and a "Ticket-Granting Ticket"
that she can use to request "Tickets" from KDC
(1) PC erases Alice's password and Kak from RAM (keeps session key Ka
in RAM). No keys ever stored on disk (what about virtual memory?).
(2)Time(stamp) is used as nonce (seconds after 1/1/1970)
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The Keys of Kerberos
1.
Password, Kalice - Only Alice knows it.
Alice’s PC can hash what Alice types in Kak = Hash(Kalice) .
KDC - also knows the hash, Kak. Uses Kak as key for encrypting TGT to Alice
(not used after that). TGT contains the daily session key Ka for use with the Ticket
Granting Server, TGS.
2. Ka: Session Key (KDC gave to Alice) Now Alice’s PC, as well as TGS know it.
3. Kab: Key for Alice & Bob - temporarily (daily) assigned by TGS
Given to Alice by TGS - encrypted with Ka and also encrypted
with Kb (Kkdc-Bob) inside a contact ticket. (Kb is Bob's daily session key from KDC)
Alice gives Bob the Ticket from KDC which has it (Kab)
encrypted with Kb (gotten by Bob from the TGS).
4. Kk: Key known only by KDC and TGS. Used to sign Ticket-Granting-Ticket for
verification. Alice can only replay {TGT;Kk} back to TGS to get a contact ticket.
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KDC
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Version 5
Host
Slave
KDC
Host
Host
Slave
KDC
Host
Host
Host
Master
KDC {db;Kmaster}
Host
Slave
KDC
Host
Host
Host
Realm
Slave
KDC
Slave
KDC
Host
• Replicated KDCs (slaves) are read only.
• Entire Host-KDC database is downloaded periodically
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KDC
(Hatter)
1
KDC
(Lion)
Lion
2
Alice
Realm
Wonderland
3
Dorothy
Lion can also be a
"principal" in
Wonderland (with the
Queen's OK)
Realm
Oz
Alice wants to talk to Dorothy
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Plaintext Cipher Block Chaining (P
PCBC)
IV
m1
m2
m3
(+)
(+)
(+)
E
E
E
c1
c2
c3
Key
The 1st 64-bit message segment is XOR'ed with
an initial vector (IV). Each following message
segment is XOR'ed with the preceding ciphertext
and plaintext segments-for privacy & integrity .
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Kerberos Message Integrity Check
(Message Digest)
MIC is Hash(<Ksession,message>)
The Hash algorithm was never published (but
source code can be obtained)
It is based on a checksum algorithm designed
by Juneman to use mod 2^31-1 (prime), but
changed to use 2^63-1 (not prime).
Cryptographers worry that it might be
breakable, or reversible (to get Ksession).
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Network Layer (IP) Addresses in Tickets
Only 4 bytes available, so limited to Internet
Protocol (Novel, IBM, Appletalk, IPv6... longer)
Makes "spoofing" harder, IP address must be
stolen from network as well as Ticket from Alice.
Prevents delegation, giving the ticket to another
host to represent you (which is allowed by
Kerberos V5)
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Why Study Kerberos v4
(Why doesn't everyone switch to v5)
Kerberos V4 is working well in many systems
Switching to V5 requires stopping the network
and upgrading every host at once before restart
Kerberos V5 is inefficient in some ways
compared to V4
•
Specified in ASN.1 (abstraction good and bad)
•
Example: 11 bytes required for 4-byte IP address.
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Kerberos v5 Cryptographic Algorithms
Kerberos v4 used Plaintext Cipher Block Chaining
and modified Juneman hash
Kerberos v5 can use a variety of encryptions
(DES in practice) and hashes (MD4, MD5).
Primary MIC (message integrity check) uses
• { confounder + MD5(confounder & message)}K'
•
K' = Kalice-bob (+) F0F0F0F0F0F0F0F0
A more modern MIC that is not used is
• MD5(Kalice-bob & message)
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Password security
Originally UNIX stored a hash of each User’s password in a
globally readable account. This can be attacked by hashing
all common words for a reverse lookup table.
• Do not send in clear except over short secure channels
(avoid using Telnet, FTP, http (for passwords), …)
• Choose had to guess passwords, enforce.
• Force changing passwords periodically
• Avoid keeping password in memory longer than
necessary to generate the user's key.
•
Send hash of (key+nonce) to KDC for authentication
• Add salt before hashing passwords for pw database
•
Add realm name to password before hashing for pw db
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Message Security and Integrity
Only exchange messages with authenticated hosts
Develop a session key and separate MIC key
using initial password exchange
Encrypt Diffie-Hellman exchanges to prevent
Bucket Brigade (man-in-middle) attacks.
Use MICs, especially with self-synchronizing
encryptions that survive permuting
message blocks (e.g., ECB) .
Get "random" numbers from true sources
Protect Master KDC Key and hashed-key database
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Concepts Used in Kerberos
Central Key Server (KDC) - n rather than n*(n-1)/2 sets of keys.
Could enforce “Connection Policy.”
Distributed KDCs (Slave KDCs) to prevent “Denial of Service” (DoS) Attack.
Use of password hashes, for verifying password without storing password.
“Dictionary Attack” - use of “salt” to improve security.
Message hashes for “Message Integrity Check” (MIC).
Authentication exchange - “nonce” to prevent “Replay Attack”.
Standard block encryption algorithm (DES) with unique “cipher feedback.”
Session keys to reduce exposure of primary keys.
Version 4 to 5 upgrade difficult. Newer systems (SSL, PGP, SSH) negotiate to
find the best common algorithms available to both.
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