Security in communications and
networks
PA018
Vašek Matyáš
Major security enablers ­ critical
(infrastructure) applications
* Kerberos
* Public-key crypto based ­ certificates,
typically X.509 ­ SSH, SSL/TLS
* Shared-key crypto based ­ symmetric key
ciphers, hash functions
Key distribution (with indirect
authentication)
* Direct distribution
* Key distribution center (also generates the
key ­ following slide)
* Key transport center
A BEKAB(KS,...)
TTP - C
A B
EKBC(KS)
EKAC(KS)
Indirect authentication ­ key
distribution topologies.
TTP-managed
TTP - C
A B
Direct (pull/push)
TTP - C
A B
EKAC(KS)
EKBC(KS)
EKAC(KS, EKBC(KS))
EKBC(KS)
Kerberos
* Greek mythology ­ guardian to the entrance of
Hades (master of the Underground)
* MIT project Athena ­ MIŤs UNIX-based campuswide
academic computing facility
Kerberos ­ threat model
* Users reading messages of other users
* Users replaying messages of other users
* Users altering a workstation network address
* Users impersonating themselves
Kerberos ­ approach
* Centralised authentication server
authenticating both users and machines
* Using symmetric-key techniques, no
public-key techniques
Kerberos
* Trusted third-party authentication service
* Key Distribution Center (KDC) grants
authentication tokens ("tickets") to users
­ Trusted, dedicated machine
* Applications can use Kerberos for:
­ Data authentication
­ Data integrity
­ Data confidentiality
Kerberos used for applications
* telnet, rlogin, rcp, FTP, etc.
* Use Kerberos Protocol to exchange
authentication information
* Client application uses Ticket-GrantingTicket
to obtain service tickets from KDC
* May use session key to encrypt data
checksums (data integrity) or encrypt data
(data confidentiality)
Kerberos ­ simple authentication
* C  AS: IDC || PC || IDV
* AS  C: Ticket
* C  V: IDC || Ticket
Ticket = EKv (IDC || PC || IDV)
Kerberos Tickets
* Ticket-Granting-Ticket
­ Used to obtain further tickets
­ Requires password or additional authentication from
user
­ Lifetime in hours
* Service Tickets
­ Issued to user from KDC
­ User can not decrypt ticket
­ User passes ticket to authenticate to server
Kerberos
* Simplified version of the protocol
­ L ­ ticket lifetime
­ Def.: ticketB = EKBT(k, "A", L), auth = Ek("A", TA)
­ (1) A  T: "A", "B", nA
­ (2) A  T: ticketB, EKAT(k, nA, L, "B")
­ (3) A  B: ticketB, auth
­ (4) A  B: Ek(TA)
(1)
(2)
(3)
A B
¨T
(4)
Kerberos Tickets (Credentials)
* Partly encrypted data structures
­ client ID
­ server ID
­ timestamp
­ session key
­ encrypted part (session key, client info, timestamp)
* Passed the way KDC  client  server
* Encrypted with the key of intended recipient
Kerberos ­ important terms
­ C = Client
­ AS = authentication server
­ V = server
­ IDC = identifier of user on C
­ IDV = identifier of V
­ PC = password of user on C
­ ADC = network address of C
­ KV = secret encryption key shared by AS an V
­ TS = timestamp
­ || = concatenation
Kerberos ­ time vs. replay issue
* The threat: an opponent steals a ticket and
uses it before its expiry time
* Lifetime of the ticket-granting ticket
­ Too short  frequent ticket requests
­ Too long  greater risk of replay attack
Tickets
* Ticket-Granting Ticket ­ get once per logon
* Service-Granting Ticket ­ get then once
before first use of a service (usually in a
given logon session)
* Authenticated Service Request ­ once per
(service) session
Kerberos(v4) Authentication Process
Authentication Service Exhange ­ To obtain the Ticket-Granting Ticket
1) C  AS: IDC || IDTGS || TS1
2) AS  C: EKc (KC,TGS|| IDTGS || TS2 || Lifetime2 || TicketTGS)
Ticket-Granting Service Echange ­ To obtain the Service-Granting Ticket
3) C  TGS: IDV || TicketTGS || AuthenticatorC
4) TGS  C: EKc (KC,V|| IDV || TS4 || TicketV)
Client/Server Authentication Exhange: To Obtain Service
5) C  V: TicketV || AuthenticatorC
6) V  C: EKc,v (TS5 +1)
Kerberos today
* Currently two broadly used versions:
* 4 - restricted to a single realm (domain)
* 5 - allows inter-realm authentication
* Kerberos v5 is an Internet standard (RFC1510)
* MSFT implementation (since Windows 2000)
X.509 based authentication
* X.509 specifies the format for public-key
certificates.
* The certificate contains the public key of a
user and is signed with the private key of a
Certification Authority (CA).
* Distributed environment using a database with
certificate (user) information.
* Used in S/MIME, IP Security, SSL/TLS, SET.
Secure Sockets Layer / Transport Layer Security
What is SSL/TLS?
* Protocols providing security and reliability
* Protecting communication of two applications
* Running over standard protocols like TCP
* SSL ­ developed by Netscape, supported also
by Microsoft...
* TLS ­ IETF standard (sometimes called SSL
v3.1)
* Transparent for higher-level protocols like
HTTP
* Using PKI and X.509 certificates
What security SSL/TLS provide?
Three basic security services:
* Entity authentication ­ the entities are authenticated
using server and client certificates.
* Integrity ­ message authentication code (MAC) which
ensures the data received is same as the data sent.
* Confidentiality ­ after the initial "handshake", a symmetric
key is defined and used to encrypt all subsequent
communication (even checked passwords, etc.).
Concepts of SSL/TLS
* Record Protocol
­The basic layer of the protocol.
­Works over TCP/IP (or other transport protocol).
­Allows for encapsulation of different higher level
protocols (HTTP, FTP, telnet, etc.) which run unmodified.
* Handshake Protocol
­Allows the server and client to authenticate each other.
­By default, server authentication is mandatory, client
authentication optional.
­Authentication through presentation of digital certificates.
* And verification of the ability to use the related private key!
... more detail
* Establish Session
­ Send random challenge value, accept public key.
­ Verify signed challenge.
­ Deliver session key protected by recipienťs public key.
* Communicate Protected Data
­ Encrypt data using agreed cipher and the session key.
­ Produce hash regularly to protect integrity.
­ Data packed into sequenced records.
* (Change Cipher - optional)
* Finish Session
* http://www.ietf.org/rfc/rfc2246.txt
Typical network attacks
* Holes in software (sendmail, RPC, NFS, or
the firewall itself )
* Network snooping ­ search for gold
* IP/DNS spoofing ­ masquerade
* Holes in new (higher-level) protocols
* Denial of Service (or even DDoS)
* ...
Intrusion Detection Systems
* Intrusion ­ activity aimed at disrupting or
circumventing a service within an
organization's system
* Also penetration, breach (, attack)
* Social engineering 
* Technical methods
IDS Principles
* Anomaly detection
­ Unusual pattern (as
compared to typical
user/system behavior).
­ False positives!
* Misuse detection
­ Pattern of intrusion(like)
behavior
­ False negatives!
Combine these two approaches!
IDS Topologies
Network-based
* Checking network traffic
* Use raw network packets.
* Typically a network
adapter running in
promiscuous - monitoring
and analyzing all traffic.
* Responses like admin
notification, connection
termination, session
recording (for forensic
analysis), other detailed
evidence collection.
Host-based
* Checking machines (log files,
etc.).
* Started in 80s ­ log file
review.
* Typically monitor system,
event, and security logs on
WinNT and syslog on Unix.
* Also critical file checksum
control, response time, port
activities.
* Responses analogous...
Combine these two approaches!
Email Security
* Postcard-like service
* PGP (Pretty Good Privacy)
* S/MIME (Secure Multipurpose Internet
Mail Extension)
* (X.400)
S/MIME messages
Combinations of two separately defined formats
­ (1) MIME entities
­ (2) Cryptographic Message Syntax (CMS) objects
S/MIME entity formats
­ one for enveloped (i.e., encrypted) ­ provides confidentiality
and key distribution services
­ two for signed ­ each provides integrity and data origin
authentication services
­ nested combinations of signed and encrypted formats
­ may nest in any order to any "reasonable" depth
­ multiple nesting is used to construct S/MIME Enhanced
Security Services
S/MIME version 2
RFC 2311 ­ S/MIME Version 2 Message Specification,
which is based on . . .
RFC 2315 ­ PKCS #7: Cryptographic Message Syntax
Version 1.5
­ Public-Key Cryptography Standards (PKCS): specifications
begun in 1991 by RSA Laboratories and other industry and
academic participants
­ PKCS #7: a general syntax for data that may have cryptography
applied to it, e.g., digital signatures
­ defines a "digital envelope for a recipient" :
(1) data encrypted in a content encryption key (CEK)
(2) CEK encrypted in a second key, known to the recipient
S/MIME version 3
RFC 2633 ­ S/MIME Version 3 Message Specification,
which is based on the following:
RFC 2630 ­ Cryptographic Message Syntax
­ enhancements to PKCS #7
­ adds attribute certificates, key agreement methods
­ adds encapsulation syntax for data protection
­ adds multiple, nested encapsulations
S/MIME uses three of the CMS data types
­ enveloped data
­ signed data
­ just plain data
S/MIME adds signed and unsigned attributes
IPSEC
* Authentication Header (AH), RFC-1826
­ Authenticity & integrity
* Encapsulating Security Payload (ESP),
RFC-1827
­ Confidentiality (non-repudiation of origin)
­ Tunneling mode (encapsulation incl. headers)
­ Transport mode (data encapsulation)
* Security Associations (SA)
IPSEC ­ Security Associations
* Set of security features for a given session
between two or more systems.
* Identifiable by Security Parameter Index
(SPI) and the IP address.
* SPI depends on its Domain of Interpretation
(DOI), this defines format, type of keyexchange,
naming conventions, etc. One
system can support more DOIs.
Parameters of Security Associations
* For AH ­ key authentication alg.
* For ESP ­ encryption alg., crypto
synchronization, initiation vector.
* Both for AH and ESP ­ level of security,
key lifetime, support of certificates, etc.
Firewalls
* Protect against attacks from the outside
(across the firewall)
* Attacks against internal data
* Denial-of-service attacks
* Communication options:
1. Allow
2. Deny
3. Translate (Proxy)
Basic options ­ firewalls
TCP/UDP Allow/Deny Packet filtering
(routers)
TCP A/D/Translate Circuit-gateway
(trust inside)
HTTP, FTP... A/D/T Applic.-gateway
Secure SHell
* SSH
* http://www.ssh.com/
* Non-commercial
downloads
* WinSCP
* http://winscp.sou
rceforge.net/eng/
* WinSCP
Closely related topics ­ to be
discussed later.
* Firewalls and network security
­ Guest lecture next week ­ Josef Pojsl,
Technical Director, Trusted Network Solutions
Course reading ­ week 6
* The Evolution of the Kerberos Authentication
System ­ Kohl, Neumann, Ts'o; 1991
ftp://athena-dist.mit.edu/pub/kerberos/doc/krb_evol.PS
­ Limitations of Kerberos 4, and changes made in
Kerberos 5.
Questions?
Term project presentations April 17 & 24!
(And then May 15, of course.  )
Schedule follows...
Term project presentations!!!
April 17:
* Barányi
* Halabica
* Červenka
* Drašar
* Folkman
* Gerguri
April 24:
* Benkovský
* Henzl
* Hubr
* Hulán
* Kocian
* Ondrák
Reminders: the presentation is worth (up to) 5 points from your course
score; it should last at most 15 minutes (time for questions &
discussion will be provided); laptop with AcroRead and PowerPoint
will be available. Rehearse!!!!!
May 15:
* Ashurova
* Honus
* Puškár
* Soběslavský
* Svoboda
* Synak
* Štverák