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ECE454/599 Computer and Network Security

Dr. Jinyuan (Stella) SunDept. of Electrical Engineering and Computer ScienceUniversity of Tennessee Fall 2012

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Kerberos V4• Tickets and ticket-granting tickets• Configuration• Replicated KDCs• Realms and interrealm authentication• Encryption for integrity only/privacy and integrity• Message formats

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Many-to-Many Authentication

How do users prove their identities when requesting services from servers on the network?

Naïve solution: every server knows every user’s password:- insecure: compromise of any server will compromise all users- inefficient: a user must contact every server to change password

Users Servers

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Using Trusted Third Party

Trusted authentication service on the network:knows all passwords, can grant access to any serverconvenient, but also the single point of failure requires high level of physical security

User Servers

User requests ticket for someservice; proves his identity

User receives ticket

Ticket is used to accessdesired network service

Knows all users’ andservers’ passwords

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KerberosNetwork authentication protocolProvides strong authentication for

client/server applications, using secret-key cryptography

A user types in a password and logs into a workstation. On behalf of the user, the workstation authenticates and accesses resources seamlessly

Developed at MITKerberos V4 and V5 are widely deployedKDC: a database of <principal, key> and a

library of subroutines

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Configuration• Kerberos server: KDC• Each principal has its master key, KAlice, shared with KDC

- human user: key is derived from password- machine: key is pre-configured

• KDC has a master key, KKDC, known only by itself, to encrypt user master keys and ticket-granting tickets• KDC keeps a database of <principal, key>, where “key” for each user is encrypted by KKDC• Based on secret-key cryptography: DES, V5 theoretically can use other encryption algorithms

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Session Keys• For login sessions• Should we use the master key KAlice (long-term key)?• Use KAlice at the beginning of each session to negotiate a session key SA

- eavesdropping - database reading - password guessing: online, offline (dictionary attack)

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Ticket-Granting Ticket (TGT)• Recall: ticket

• TGT: KKDC{Alice, SA, expiration time,…}, for assigning the session key

Obtaining a TGT

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Obtaining Services from A Remote Node• After obtaining TGT, the credential for Alice… • Step 1: Alice uses TGT to obtain a ticket• Step 2: Alice uses ticket to log into remote node

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Step 1: Obtaining A Ticket to Bob

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Step 2: Accessing Bob

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Overview of Kerberos

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Replicated KDCs• Purposes: - Prevent single point failure - Prevent performance bottleneck • Multiple KDCs - One master copy for read/write - Multiple replicas for read only - All having the same database and the same master key• Updating KDC database - KDC’s database is transferred in clear - Privacy: keys are stored as ciphertext encrypted by KKDC - Integrity: a cryptographic hash of the database file and a timestamp

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Realms• To scale to a large network including multiple

administrations, the principals are divided into realms. Each realm has its own KDC.

• The KDCs of other realms are treated as resources (principals) of a local realm.

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Interrealm Authentication

• Kerberos V4 does not allow authentication through a chain of KDCs: a rogue KDC can impersonate other realms • Kerberos V5 does: hierarchy of realms

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Interrealm Authentication (2)

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Key Version Numbers• If Bob changes his master key, Alice’s ticket

for Bob will be invalidated without Alice knowing it

• Solution: Each key has a version number. Old keys are maintained for a period of time. Different keys are identified by their version numbers. Tickets are sent together with the key version numbers.

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Other Uses of Kerberos• Besides authentication…• Integrity only: perform an undocumented

arithmetic based on mod 2^63 – 1 on the message concatenated with the session key, which results in a checksum. Send the message with the checksum

• Privacy and integrity

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Privacy and Integrity• Plaintext Cipher Block Chaining (PCBC)

- Modify any cipher block will garble the rest of the message- Put recognizable data at the end of a message- Does not prevent swapping attack (why?)

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Message Types• AS_REQ • TGS_REQ • AS_REP (also TGS_REP)• AP_REQ• AP_REP• AS_REQ_MUTUAL• AS_ERR• AP_ERR• PRIV• SAFE

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Use of AS_REQ

Obtaining a TGT

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Tickets

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AS_REQ

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AS_REP and TGS_REP

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Credentials

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Use of TGS_REQ

Obtaining a ticket to remote server Bob

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TGS_REQ

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AS_REP and TGS_REP

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Authenticators

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Use of AP_REQ and AP_REP

Accessing the remote server Bob

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AP_REQ

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AP_REP and Encrypted Data PRIV

Decrypted Data

• AP_REP is application specific, when mutual authentication is needed, it takes the format of encrypted data PRIV

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SAFE: Integrity-Checked Data

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AS_ERR

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AP_ERR

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Kerberos V4 vs. V5• Encryption system: V4 requires DES, V5 can

use any• Internet protocol: V4 requires IP, V5 can use

other types• Message byte ordering: V4 uses B BIT, all

message structures are defined using Abstract Syntax Notation One (ASN.1) and Basic Encoding Rules (BER) in V5 providing unambiguous byte ordering

• Ticket lifetime: 21 hours in V4 (encoded in a 1-octet quantity), V5 tickets include explicit start and end time allowing arbitrary lifetimes

• Master keys are different in different realms in V5

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Kerberos V4 vs. V5 (Cont’d)• Authentication forwarding/delegation: V4

does not allow and V5 allows• Interrealm authentication: no chaining in V4

(N realms require O(N2) Kerberos-to-Kerberos relationships), V5 supports KDC hierarchy

• Session keys: negotiation of subsession keys is supported in V5 for different sessions of the same service type

• Privacy + integrity: V4 uses PCBC, V5 uses explicit integrity mechanisms (e.g., hash) with CBC encryption

• Password attacks: both versions are vulnerable

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Exercises (1)[Kaufman] 13.5:

With CBC, if one ciphertext block is lost, how many plaintext blocks are lost? With PCBC, why do things get back in sync if cn and cn+1 are switched? How about if a ciphertext block is lost? How about if ciphertext block n is switched with ciphertext block n+2? How about any permutation of the first n blocks?

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Exercises (1): Answer[Kaufman] 13.5:

In CBC decryption, each ciphertext block affects two plaintext blocks, one through decryption and one through XOR. In PCBC decryption, each ciphertext block affects the corresponding plaintext block by XORing its decryption, while it affects all following plaintext blocks by XORing the XOR of it and its decryption. Thus, a set of ciphertext blocks affects the following plaintext blocks in a manner independent of the order of ciphertext blocks within the set—the effect is just an XOR of the XOR of all the ciphertext blocks and their decryptions.

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Exercises (2)[Kaufman] 14.4:

Design a different method of Bob authenticating Alice when Bob does not remember his own master key, which places the work on Bob instead of Alice. In other words, Alice will act as if Bob was an ordinary civilized thing that does not remember its own master key, and Bob interacts appropriately with the KDC so that Alice will be unaware that Bob didn't know his own master key.

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Exercises (2): Answer[Kaufman] 14.4:

We assume that Bob has a valid TGT and still remembers the session key SB. With the cooperation of the KDC, Bob can still decrypt messages encrypted with his master key, and thus authenticate Alice as follows. Bob gets the KDC to decrypt a message encrypted with his master key by sending the encrypted message and his TGT (which contains Bob’s name and the session key encrypted with the KDC’s master key) to the KDC. The KDC (which knows Bob’s master key) decrypts the message and sends it back to Bob encrypted with the session key. Since Bob knows the session key, he can now decrypt the message.

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Exercises (3)Alice wants to send Bob a large data file containing confidential data. She wants to make sure the file cannot be modified undetected during transmission. All Alice and Bob have is their public/private key pair PUBA/PRVA and PUBB/PRVB, respectively. Show how Alice will construct the message to be transmitted in a secure and efficient way. Show also how Bob will extract the data file from the received message. You can draw diagrams or write down the message construction/extraction using notations.

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Exercises (3): AnswerThe message in the signature (the last item) should be hashed first for better efficiency

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Reading Assignment• [Kaufman] Chapter 13