MidTerm Question 5

1
MidTerm Question 5

• Given the following security architecture,
  specify ways that the use of firewalls and/or
  intrusion detection systems might be used to
  improve the architecture. Due to cost
  constraints, you are limited to acquiring no more
  than two additional devices to support your
  security policy. Be specific on the use of
  filters and proxies. State any assumptions with
  regard to security policy and justify how the
  improved architecture supports your security
  policy. (20 points)
• You are the security manager for a company site
  consisting of 50 technical staff and a number of
  support and temporary staff. Most staff use
  Windows 2000 and/or Linux systems on their
  desktops. You have an Internet connection
  through a T-1 line into a two-port router that
  provides some packet filtering of ports
  recommended in previous CERT advisories. Your
  site is about to begin a new electronic ordering
  service to its customers through a web interface
  that will store and process customer credit card
  information. The current Beta test web site is
  on the desktop of one of your technical staff.
  There are no other specific security products in
  place at the site. While your technical staff
  has a lot of computer programming expertise, you
  have no one on staff that is familiar with
  current security products or their configuration.

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5 Part Answer

• Define the (delta) Threat
• Refine the policy
• Structure the architecture
• Address staff/training issues
• 5. Maintain the system

3
Initial Architecture
PC
PC with web Server and DB
PC
Office LAN
PC
PC
PC
Router
PC
T-1 Line to Internet
PC
PC
PC
PC
PC
PC
PC
PC
PC
PC
4
Improved Architecture
PC
PC
Office LAN
PC
PC
PC
Router
PC
T-1 Line to Internet
PC
PC
PC
PC
Firewall
PC
PC
PC
PC
PC
PC
IDS
PC with web Server and DB
5
Question 8

• You are the security manager of the web site for
  a major corporation. After the introduction of a
  controversial product, your web site is flooded
  with web traffic from all over the Internet, and
  the system supporting the web server crashes,
  destroying a full days cache of product ordering
  transactions. How would you determine whether
  this was simply an increase in normal traffic
  versus an attack on your company? How would you
  respond to ensure the survivability of your
  company? What steps would you take to ensure
  that future events of this type were handled more
  smoothly? (10 points)

6
PKI
Tom Longstaff

• CERT Coordination Center
• Software Engineering Institute
• Carnegie Mellon University
• Pittsburgh PA 1521

SM

• The CERT Coordination Center is sponsored by the
  Advanced Research Projects Agency (ARPA). The
  Software Engineering Institute is sponsored by
  the U.S. Department of Defense.

7
Types of Encryption Systems

• Two basic types
• Shared (or symmetric) key encryption
• Public (or asymmetric) key encryption
• Shared use of a single key for both encryption
  and decryption that both parties must share
• Tends to be more efficient
• Used for block ciphers
• Public different keys used for encryption and
  decryption
• Most popular form is based on RSA or Diffie
  Helman
• More computational intensive (uses
  exponentiation)
• Frequently used for symmetric key exchange

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Using Public Key for Signatures

• A digital signature is a cryptographically strong
  hash of a longer data set
• E.g., MD5 used by tripwire and others to verify
  the integrity of the information
• If you create a digital signature to a document,
  then encrypt it with your private key, anyone can
  verify two properties of this information
• Integrity (through the MD5 checksum)
• Source (only the owner of the private key could
  have encrypted the signature)
• A digital signature, signed with a private key on
  a public key becomes a trust verifier for that
  key

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Signed Keys

• Example
• Alice has an asymmetric key pair - creates an MD5
  checksum of the key and encrypts it with her
  private key
• Alice Kpublic,(KMD5public)Kprivate
• Bob (K) verifies the authenticity of the key,
  then encrypts the MD5 with his private key as
  well
• Kpubic,(KMD5public)Kprivate
  ,(KMD5public) Kprivate
• And so on

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Other properties in signed keys

• In addition to the MD5, other properties may be
  included in the private encrypted part of the
  public key record
• Level of trust
• Relationship with the key owner
• Link to other CA information
• Kpubic,(KMD5public,owner,email)Kprivate
  ,(KMD5public,moderate trust,email,authoritati
  ve CA) Kprivate

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Web of trust

• From these building blocks, a web of trust can be
  built
• Two users cross-sign each others public keys
• Alice signs Bob who signs Charlie who signs Dain
  who signs Alice
• A particularly trusted user signs many keys
• If you have a small number of individuals you
  trust, you can build a bridge to a new recipient
• This is the principle behind pgp

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Other pgp attributes

• Trust of a key you are signing
• Trust of a key you receive
• Key rings
• Key servers
• PGP designed to sign static documents, not live
  transactions but the PKI built up with PGP can be
  used to exchange a session key for a live block
  cypher
• E.g., pgp phone.

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PKI

• Mechanism to distribute and trust public keys
• Two types in common use Hierarchical and the Web
  of Trust
• Modified Hierarchical combines distinct
  Hierarchical PKIs with cross-realm authentication
• Common use of PKI refers to Hierarchical, but
  also covers Web of Trust and Modified Hierarchical

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Key and signature revocation

• What if a private key is compromised in the web
  of trust?
• First of all, need a mechanism to distribute this
  information
• Secondly, need to invalidate all signatures under
  this key
• May be able to limit the extent of revocation
  based on date of the revocation certificate

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Building up a hierarchy of keys

• In a hierarchical PKI, you need a root
  certificate whos security is above reproach
• Why?
• ROOTPublicKey,(ROOTPublicKeyMD5)ROOTPrivateKey
• CAPublicKey,(CAPublicKeyMD5)CAPrivateKey,(CAPubl
  icKeyMD5)ROOTPrivateKey
• UserPublicKey,(UserPublicKeyMD5)UserPrivateKey,(
  UserPublicKeyMD5)CAPrivateKey

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Distribution of Hierarchical Public Keys

• The root public key must be widely distributed in
  a variety of paths to everyone in the hierarchy
• Why multiple paths?
• What is the primary vulnerability here?
• If the root key is secure, the system can be
  consistent
• Root key is used to sign all revocation
  certificates for Cas
• Root servers do not need to sign keys lower in
  the hierarchy
• Why not?

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One versus Multiple Hierarchies

• What are the problems with a single root server
  for all PKI systems?
• If you want to trust users across hierarchies,
  you need cross-realm certification
• Combines Web-of-Trust with Hierarchical PKI
• Means that some root or CA public key is signed
  by one in the other hierarchy

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Problems in cross-realm certification

• Naming
• Different policies for inclusion in the hierarchy
• Different uses of keys
• Compatibly of algorithms and key records

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What does this have to do with operating system
security architectures?

• Application-level architecture
• Trust of users within the operating system
• Basic tool for linking users with processes
• Kerberos and related systems make use of these
  concepts to implement OS trust

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Kerberos

• Based on symmetric key encryption
• Solves the problems
• Untrusted client machines need to authenticate
  users
• Need data protection for applications
• Provides authentication and authorization for
  services
• Once the infrastructure is in place, applications
  must be instrumented to use kerberos
• Examples of Kerberoized applications
• Telnet
• BSD Rtools
• Email
• NFS, AFS, etc.

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The Key Distribution Center (KDC)

• Must be a physically secure host in the system
• Stores a shared key with each principal (each
  user and service that uses kerberos)
• The main job of the KDC is to create session keys
  and distribute them based on the shared secret
  key of the user
• Also known as an authentication server in the
  Kerberos documentation

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KDC Example

• Alice requests a session with Bob
• KDC encrypts a session key with Alices key and
  sends to Alice
• Also sends the session key and some info on Alice
  encrypted with Bobs key
• Now Alice can talk to Bob, Bob can decrypt the
  session key and open a comm with Alice

KDC
(Request Alice to Bob)Ak
(session key)Ak,(Alice, session key)Bk
Alice
Bob
Open comm, (Alice, session key)Bk
(data)SessionKey
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TicketGrantingServer (TGS)

• Really does the same job as the KDC, but in
  theory provides another layer of security
• Alice gets a ticket (session key) to talk to a
  TGS from the KDC
• Uses this ticket to request tickets to talk to
  Bob
• In practice, the KDC and TGS are the same system
  as the TGS has to have the same database of
  shared keys to create tickets for Bob

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Logging in to a Network

• You need to get a session key and a
  ticket-granting-ticket

AS_REQ, Alice need TGT
Alice
Workstation
Name, password
KDC
AS_REP Sa,TGTKa
Asks for Alices uid Gets the tgt Uses password
to decrypt the TGT If successful, discards Ka and
uses only the TGT to gain more tickets
Invents Sa Finds Alices master
key TGTAlice,SaKtgs
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Using the TGT and Sa

• After logging into the network, Alice asks to
  talk to Bob (e.g., rlogin to Bob the workstation)

TGS_REQ Alice rloign bob, TGT, timestampSa
Alice
Workstation Sa and TGT
rlogin bob
TGS
AS_REP bob, Kb, TbSa
AP_REQ Tb timestampSb
Invents Sb Decrypts TGT to get Sa Decrypts
timestamp to verify authenticity Finds Bobs
master key Creates ticket to bob Tb Alice, SbKb
AP_REP TimestampSb
Bob
Decrypts Tb to get Sb Decrypts timestamp Encrypts
new timestamp
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Kerberos V5

• In principle, the same as V4 but with a major
  overhaul of the implementation and addition of
  features.
• Allows for delegation of rights,
• renewable and postdated tickets,
• other cryptographic algorithms (V4 used only DES
  and Jueneman),
• allowed for a hierarchy of realms