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An overview of various security case studies, including the needham-schroeder protocol, kerberos, secure socket layer (ssl), and public key encryption. Secret key encryption, scenarios for secure communication, authenticated communication with a server, and digital signatures. It also discusses popular encryption schemes and their performance.
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❚ Review scenarios ❚ Needham-Schroeder ❚ Brief summary of common algorithms ❚ Kerberos ❚ Secure Socket Layer
Encrypted message:
E(K, M) = {M}K
Decrypted message:
D(K, E(K, M)) = D(K, {M} (^) K ) = M
It is hard to get M from {M}K without knowing K
Alice and Bob share a secret key KAB. Alice wants to send a secret message M to Bob.
How can Bob and Alice safely get the shared key KAB?
How can Bob know that M wasn’t a replay?
Alice wants to access Bob’s files on a local file server. Sara is a trusted authentication server that holds passwords and current secret keys.
This is the simplified scenario for Kerberos. K (^) AB is the session key.
Alice wants to sign document M so that any recipient can verify it came from Alice. This assumes that Alice has a private-public key pair. A digest is like a checksum.
Instructor’s Guide for Coulouris, Dollimore and Kindberg© Addison-Wesley Publishers 2000 Distributed Systems: Concepts and Design Edn. 3
Figure 7. Alice’s bank account certificate
Instructor’s Guide for Coulouris, Dollimore and Kindberg© Addison-Wesley Publishers 2000 Distributed Systems: Concepts and Design Edn. 3
Figure 7. Public-key certificate for Bob’s Bank
Instructor’s Guide for Coulouris, Dollimore and Kindberg© Addison-Wesley Publishers 2000 Distributed Systems: Concepts and Design Edn. 3
Figure 7. X509 Certificate format
Subject (^) Distinguished Name, Public Key Issuer Distinguished Name, Signature Period of validity Not Before Date, Not After Date Administrative information Version, Serial Number Extended Information
Instructor’s Guide for Coulouris, Dollimore and Kindberg© Addison-Wesley Publishers 2000 Distributed Systems: Concepts and Design Edn. 3
Figure 7. Performance of encryption and secure digest algorithms
Key size/hash size (bits)
Extrapolated speed (kbytes/sec.)
PRB optimized (kbytes/s)
TEA 128 700 - DES 56 350 7746 Triple-DES 112 120 2842 IDEA 128 700 4469 RSA 512 7 - RSA 2048 1 - MD5 128 1740 62425 SHA 160 750 25162
Instructor’s Guide for Coulouris, Dollimore and Kindberg© Addison-Wesley Publishers 2000 Distributed Systems: Concepts and Design Edn. 3
Figure 7. The Needham–Schroeder secret-key authentication protocol
Header Message Notes
S returns a message encrypted in A’s secret key, containing a newly generated key KAB and a ‘ticket’ encrypted in B’s secret key. The nonce NA demonstrates that the message was sent in response to the preceding one. A believes that S sent the message because only S knows A’s secret key.
{ KAB , A } K (^) B { NB } K (^) AB { NB - 1} K (^) AB
❚ Follows Needham and Schroeder very closely
❚ Uses time values as nonces
❚ When user logs in, the login program sends user’s name to the kerberos authentication server
❚ If user is known, server replies with a session key and a nonce encrypted in the user’s password and a ticket for TGS
❚ After login program authenticates the information, it can erase the user’s password from memory
Instructor’s Guide for Coulouris, Dollimore and Kindberg© Addison-Wesley Publishers 2000 Distributed Systems: Concepts and Design Edn. 3
Figure 7. System architecture of Kerberos
Client Server DoOperation
Authenticationdatabase
Login session setup
grantingTicket- service T
Kerberos Key Distribution Centre
session setupServer
Authen-tication
functionService
Step B
Step A
Step C
C (^) S
❚ SSL Record Protocol layer implements a secure channel that encrypts and authenticates message through any connection- oriented protocol
❚ SSL Handshake Layer – has three modules: ❙ SSL handshake protocol ❙ SSL change cipher specification ❙ SSL alert protocol
❚ Implemented as application level libraries
❚ Widely uses as a session-layer protocol
❚ In web servers the https URLs initiate a SSL connection
❚ Provides a practical hybrid security scheme
❚ Requires public-key certificates issued by a recognized authority
Instructor’s Guide for Coulouris, Dollimore and Kindberg© Addison-Wesley Publishers 2000 Distributed Systems: Concepts and Design Edn. 3
Figure 7. SSL protocol stack
SSL Handshake protocol
SSL Change Cipher Spec
SSL Alert Protocol
Transport layer (usually TCP)
Network layer (usually IP)
SSL Record Protocol
HTTP Telnet
SSL protocols: Other protocols:
Instructor’s Guide for Coulouris, Dollimore and Kindberg© Addison-Wesley Publishers 2000 Distributed Systems: Concepts and Design Edn. 3
Figure 7. SSL handshake protocol
Client Server
ClientHello ServerHello Certificate Certificate Request ServerHelloDone Certificate Certificate Verify Change Cipher Spec Finished Change Cipher Spec Finished
Establish protocol version, session ID, cipher suite, compression method,exchange random values
Optionally send server certificate and request client certificate
S end client certificate response if requested
Change cipher suite and finish handshake
Instructor’s Guide for Coulouris, Dollimore and Kindberg© Addison-Wesley Publishers 2000 Distributed Systems: Concepts and Design Edn. 3
Figure 7. SSL handshake configuration options
Component Description Example Key exchange method
the method to be used for exchange of a session key
RSA with public-key certificates Cipher for data transfer
the block or stream cipher to be used for data
IDEA
Message digest function
for creating message authentication codes (MACs)
SHA
