Transcript Introduction

FIREWALLS & NETWORK SECURITY with
Intrusion Detection and VPNs, 2nd ed.
9
Encryption and Firewalls
By Whitman, Mattord, & Austin
© 2008 Course Technology
Learning Objectives
 Describe the role encryption plays in a firewall
architecture
 Explain how digital certificates work and why
they are important security tools
 Analyze the workings of SSL, PGP, and other
popular encryption schemes
 Discuss Internet Protocol Security (IPSec) and
identify its protocols and modes
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Slide 2
Firewalls and Encryption
 Hackers take advantage of a lack of encryption
 Encryption:
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Preserves data integrity
Increases confidentiality
Is relied upon by user authentication
Plays a fundamental role in enabling VPNs
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Slide 3
Hacker and an Unencrypted Packet
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Slide 4
Hacker and an Encrypted Packet
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The Cost of Encryption
 CPU resources and time
 Bastion host that hosts the firewall should be
robust enough to manage encryption and other
security functions
 Encrypted packets may need to be padded to
uniform length to ensure that some algorithms
work effectively
 Can result in slowdowns
 Monitoring can burden system administrator
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Slide 6
Preserving Data Integrity
 Even encrypted sessions can go wrong as a
result of man-in-the-middle attacks
 Encryption can perform nonrepudiation using a
digital signature
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Maintaining Confidentiality
 Encryption conceals information to render it
unreadable to all but intended recipients
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Authenticating Network Clients
 Firewalls need to trust that the person’s claimed
identity is genuine
 Firewalls that handle encryption can be used to
identify individuals who have “digital ID cards”
that include encrypted codes
– Digital signatures
– Public keys
– Private keys
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Slide 9
Enabling Virtual Private Networks
(VPNs)
 As an integral part of VPNs, encryption:
– Enables the firewall to determine whether the
user who wants to connect to the VPN is actually
authorized to do so
– Encodes payload of information to maintain
privacy
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Slide 10
Principles of Cryptography
 Encryption: the process of converting an original
message into a form that cannot be understood
by unauthorized individuals
 Cryptology, the science of encryption,
encompasses two disciplines:
– Cryptography: describes the processes involved
in encoding and decoding messages so that
others cannot understand them
– Cryptanalysis: the process of deciphering the
original message (plaintext) from an encrypted
message (ciphertext) without knowing the
algorithms and keys used to perform the
encryption
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Encryption Definitions
 Algorithm: the mathematical formula or method
used to convert an unencrypted message into
an encrypted message
 Cipher: the transformation of the individual
components (characters, bytes, or bits) of an
unencrypted message into encrypted
components
 Ciphertext or cryptogram: the unintelligible
encoded message resulting from an encryption
 Cryptosystem: the set of transformations
necessary to convert an unencrypted message
into an encrypted message
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Encryption Definitions (continued)
 Decipher: to decrypt or convert ciphertext to
plaintext
 Encipher: to encrypt or convert plaintext to
ciphertext
 Key or cryptovariable: the information used in
conjunction with the algorithm to create the
ciphertext from the plaintext; it can be a series
of bits used in a mathematical algorithm or the
knowledge of how to manipulate the plaintext
 Keyspace: the entire range of values that can
possibly be used to construct an individual key
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Encryption Definitions (continued)
 Plaintext: the original unencrypted message that
is encrypted and results from successful
decryption
 Steganography: the process of hiding
messages, usually within graphic images
 Work factor: the amount of effort (usually
expressed in units of time) required to perform
cryptanalysis on an encoded message
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Cryptographic Notation
M represents original message; C represents
ciphertext; E represents encryption process; D
represents the decryption process; K represents
a key
So…
E(M) = C encrypting a message results in
cyphertext
D(C) = M and D[E(M)] = M
E(M,K) = C specifies encrypting the message with
a key; keys can be annotated K1, K2, etc. in
the case of multiple keys
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Common Ciphers
 In encryption, the most commonly used
algorithms include three functions: substitution,
transposition, and XOR
 In a substitution cipher, you substitute one value
for another; a monoalphabetic substitution uses
only one alphabet and a polyalphabetic
substitution use two or more alphabets
 The transposition cipher (or permutation cipher)
simply rearranges the values within a block to
create the ciphertext; this can be done at the bit
level or at the byte (character) level
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Common Ciphers (continued)
 In the XOR cipher conversion, the bit stream is
subjected to a Boolean XOR function against
some other data stream, typically a key stream
 XOR works as follows:
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‘0’
‘0’
‘1’
‘1’
XOR’ed
XOR’ed
XOR’ed
XOR’ed
with ‘0’ results in a ‘0’ (0  0 = 0)
with ‘1’ results in a ‘1’ (0  1 = 1)
with ‘0’ results in a ‘1’ (1  0 = 1)
with ‘1’ results in a ‘0’ (1  1 = 0)
 Simply put, if the two values are the same, you
get “0”; if not, you get “1”
 This process is reversible; that is, if you XOR
the ciphertext with the key stream, you get the
plaintext
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Vernam Cipher
 Also known as the one-time pad, the Vernam
cipher was developed at AT&T and uses a set
of characters that are used for encryption
operations only one time and then discarded
 The values from this one-time pad are added to
the block of text, and the resulting sum is
converted to text
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Book or Running Key Cipher
 Another method, used in the occasional spy
movie, is the use of text in a book as the
algorithm to decrypt a message
 The key relies on two components:
– Knowing which book to use
– A list of codes representing the page number,
line number, and word number of the plaintext
word
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Symmetric Encryption
 The previous methods of encryption/decryption
require the same algorithm and key be used to
both encipher/decipher the message
 This is known as private key encryption or
symmetric encryption
 In this approach, the same key—a secret key—
is used to encrypt and decrypt the message
 Usually extremely efficient, requiring simple
processing to encrypt or decrypt the message
 Main challenge is getting a copy of the key to
the receiver, a process that must be conducted
out-of-band to avoid interception
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Symmetric Encryption (continued)
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The Technology of Symmetric
Encryption
 Data Encryption Standard (DES)
– Developed in 1977 by IBM
– Based on the Data Encryption Algorithm (DEA),
which uses a 64-bit block size and a 56-bit key
– Federally approved standard for nonclassified
data
– Cracked in 1997 when developers of a new
algorithm, Rivest-Shamir-Aldeman, offered
$10,000 to whomever was first to crack it
– Fourteen thousand users collaborated over the
Internet to finally break the encryption
 Triple DES (3DES) was developed as an
improvement to DES and uses as many as
three keys in succession
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The Technology of Symmetric
Encryption (continued)
 Advanced Encryption Standard (AES)
– Successor to 3DES
– Based on Rijndael Block Cipher, which features
a variable block length and a key length of either
128, 192, or 256 bits
 In 1998, it took a special computer designed by
the Electronic Freedom Frontier more than 56
hours to crack DES; it would take the same
computer approximately 4,698,864 quintillion
years to crack AES
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Asymmetric Encryption
 Also known as public key encryption
 Uses two different but related keys
 Either key can be used to encrypt or decrypt
 If Key A is used to encrypt message, then only
Key B can decrypt; if Key B is used to encrypt
message, then only Key A can decrypt
 This technique is most valuable when one of the
keys is private and the other is public
 Problem: it requires four keys to hold a single
conversation between two parties, and the
number of keys grows geometrically as parties
are added
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Public Key Encryption
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Digital Signatures
 When asymmetric process is reversed, that the
message was sent by organization owning the
private key cannot be refuted (nonrepudiation)
 Digital signatures: encrypted messages verified
as authentic by independent facility (registry)
 Digital certificate: electronic document, similar to
digital signature, attached to file certifying that
file is from the organization it claims to be from
and has not been modified from original format
 Certificate Authority (CA): agency that manages
issuance of certificates and serves as electronic
notary public to verify their origin and integrity
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Digital Signatures (continued)
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Public Key Infrastructure
 Public key infrastructure (PKI) is the entire set of
hardware, software, and cryptosystems necessary
to implement public key encryption
 Systems are based on public key cryptosystems
and include digital certificates and certificate
authorities
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Public Key Infrastructure (continued)
 Can increase an organization’s ability to protect
its information assets by providing:
– Authentication: digital certificates authenticate
identity of each party in an online transaction
– Integrity: digital certificate asserts content signed
by the certificate has not been altered in transit
– Confidentiality: keeps information confidential by
ensuring it is not intercepted during transmission
– Authorization: digital certificates can replace user
IDs and passwords, enhance security, and
reduce overhead
– Nonrepudiation: certificates validate actions
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Hybrid Systems
 Pure asymmetric key encryption not widely used
except in area of certificates; instead, typically
employed in conjunction with symmetric key
encryption, creating a hybrid system
 Hybrid process currently in use is based on
Diffie-Hellman key exchange, which provides
method to exchange private keys using public
key encryption without exposure to third parties
 In this method, asymmetric encryption is used to
exchange symmetric keys, so two entities can
conduct quick, efficient, secure communications
based on symmetric encryption; Diffie-Hellman
provided the foundation for subsequent
developments in public key encryption
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Hybrid Encryption
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Using Cryptographic Controls
 Generation of unbreakable ciphertext is possible
only if proper key management infrastructure
has been constructed and cryptosystems are
operated and managed correctly
 Cryptographic controls can be used to support
several aspects of business:
– Confidentiality and integrity of e-mail and its
attachments
– Authentication, confidentiality, integrity, and
nonrepudiation of e-commerce transactions
– Authentication and confidentiality of remote
access through VPN connections
– Higher standard of authentication when used to
supplement access control systems
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E-mail Security
 Secure Multipurpose Internet Mail Extensions
(S/MIME) builds on Multipurpose Internet Mail
Extensions (MIME); adds encryption and
authentication via digital signatures
 Privacy Enhanced Mail (PEM) proposed by
Internet Engineering Task Force (IETF) as a
standard that will function with public key
cryptosystems; uses 3DES and RSA for key
exchanges and digital signatures
 Pretty Good Privacy (PGP): uses IDEA Cipher, a
128-bit symmetric key block encryption algorithm
with 64-bit blocks for message encoding; RSA for
symmetric key exchange and digital signatures
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Securing the Web
 Secure Electronic Transactions (SET)
– Developed by MasterCard and VISA in 1997 to
provide protection from electronic payment fraud
– Encrypts credit card transfers with DES and uses
RSA for key exchange
 Secure Sockets Layer (SSL)
– Developed by Netscape in 1994 to provide
security for online electronic commerce
transactions
– Uses several algorithms; mainly relies on RSA
for key transfer and IDEA, DES, or 3DES for
encrypted symmetric key-based data transfer
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Securing the Web (continued)
 Secure Hypertext Transfer Protocol (SHTTP)
– An encrypted version of HTTP
– Provides secure e-commerce transactions and
encrypted Web pages for secure data transfer
over the Web, using several different algorithms
 Secure Shell (SSH)
– Uses tunneling to provide security for remote
access connections over public networks
– Provides authentication services between a client
and a server
– Used to secure replacement tools for terminal
emulation, remote management, and file transfer
applications
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Securing the Web (continued)
 IP Security (IPSec): primary and now dominant
cryptographic authentication and encryption product
of IETF’s IP Protocol Security Working Group
 IPSec combines several different cryptosystems:
– Diffie-Hellman key exchange for deriving key material
between peers on a public network
– Public key cryptography for signing the Diffie-Hellman
exchanges to guarantee the identity of the two parties
– Bulk encryption algorithms for encrypting the data
– Digital certificates signed by a certificate authority to
act as digital ID cards
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Securing the Web (continued)
 IPSec has two components:
– The IP Security protocol itself, which specifies
the information to be added to an IP packet and
indicates how to encrypt packet data
– The Internet Key Exchange, which uses
asymmetric key exchange and negotiates the
security associations
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Securing the Web (continued)
 IPSec works in two modes of operation:
– Transport mode: only IP data is encrypted—not
the IP headers themselves; allows intermediate
nodes to read source and destination addresses
– Tunnel mode: entire IP packet is encrypted and
inserted as payload in another IP packet
 IPSec and other cryptographic extensions to
TCP/IP often used to support a virtual private
network (VPN), a private, secure network
operated over a public, insecure network
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Securing Authentication
 A final use of cryptosystems is to provide
enhanced and secure authentication
 One approach to this issue is provided by
Kerberos, which uses symmetric key encryption
to validate an individual user’s access to various
network resources
 It keeps a database containing the private keys
of clients and servers that are in the
authentication domain that it supervises
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Kerberos
 Kerberos system knows these private keys and
can authenticate one network node (client or
server) to another
 Kerberos also generates temporary session
keys—that is, private keys given to the two
parties in a conversation
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Kerberos (continued)
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Kerberos (continued)
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Attacks on Cryptosystems
 Historically, attempts to gain unauthorized
access to secure communications have used
brute force attacks in which the ciphertext is
repeatedly searched for clues that can lead to
the algorithm’s structure (ciphertext attacks)
 This process, known as frequency analysis, can
be used along with published frequency of
occurrence patterns of various languages and
can allow an experienced attacker to quickly
crack almost any code if the individual has a
large enough sample of the encoded text
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Slide 43
Attacks on Cryptosystems (continued)
 Occasionally, an attacker may obtain duplicate
texts, one in ciphertext and one in plaintext,
which enable the individual to reverse-engineer
the encryption algorithm in a known-plaintext
attack scheme
 Alternatively, an attacker may conduct a
selected-plaintext attack by sending a potential
victim a specific text that they are sure the
victim will forward on to others; the attacker then
intercepts the encrypted message and
compares it to the original plaintext
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Attacks on Cryptosystems (continued)
 Man-in-the-middle attack: method used to
intercept the transmission of a public key or
even to insert a known key structure in place of
the requested public key
 Correlation attacks: collection of brute-force
methods that attempt to deduce statistical
relationships between the structure of the
unknown key and the ciphertext that is the
output of the cryptosystem
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Slide 45
Attacks on Cryptosystems (continued)
 In a dictionary attack, the attacker encrypts
every word in a dictionary using the same
cryptosystem as used by the target
 In a timing attack, the attacker eavesdrops
during a victim’s session and uses statistical
analysis of the user’s typing patterns and interkeystroke timings to discern sensitive session
information
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Defending from Attacks
 No matter how sophisticated encryption and
cryptosystems have become, however, they
have retained the same flaw that the first
systems contained thousands of years ago: if
you discover the key, that is, the method used
to perform the encryption, you can determine
the message
 Thus, key management is not so much the
management of technology but rather the
management of people
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Slide 47
Chapter Summary
 Encryption: process of rendering information
unreadable to all but the intended recipients;
purpose is to preserve the integrity and
confidentiality of information and/or make the
process of authenticating users more effective
 Firewalls use encryption both to provide
protection for data in transit and to help keep
firewall secure
 Encryption of data incurs costs since it requires
processing time to encrypt and decrypt the data
being protected
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Chapter Summary (continued)
 Cryptology: science of encryption
 Cryptography: complex process of making and
using codes
 Applying concealing techniques in encryption and
decoding ciphertext is called decryption
 Process used to decrypt data when the process
and/or keys are unknown is called cryptanalysis
 Cryptographic controls: techniques and tools used
to implement cryptographic protections; used to
secure e-mail, Web access, Web applications, file
transfers, remote access procedures like VPNs
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Slide 49
Chapter Summary (continued)
 Cryptographic control systems often subject to
attack
 Many methods of attack have evolved
– Brute computational approaches
– Use of weaknesses often found in
implementation of cryptographic controls
 Some attacks attempt to inject themselves
between the parties of a secured
communication channel
 Other attacks combine multiple brute-force
approaches into one correlation attack
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Slide 50