FIREWALLS & NETWORK SECURITY with
Intrusion Detection and VPNs, 2nd ed.
Chapter 9 Encryption and Firewalls
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
Firewalls & Network Security, 2nd ed. - Chapter 9 Slide 2
Firewalls and Encryption
Hackers take advantage of a lack of encryption
Encryption:
– Preserves data integrity
– Increases confidentiality
– Is relied upon by user authentication
– Plays a fundamental role in enabling VPNs
Slide 3 Firewalls & Network Security, 2nd ed. - Chapter 9
Hacker and an Unencrypted Packet
Slide 4 Firewalls & Network Security, 2nd ed. - Chapter 9
Hacker and an Encrypted Packet
Slide 5 Firewalls & Network Security, 2nd ed. - Chapter 9
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
Slide 6 Firewalls & Network Security, 2nd ed. - Chapter 9
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
Slide 7 Firewalls & Network Security, 2nd ed. - Chapter 9
Maintaining Confidentiality
Encryption conceals information to render it
unreadable to all but intended recipients
Slide 8 Firewalls & Network Security, 2nd ed. - Chapter 9
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
Slide 9 Firewalls & Network Security, 2nd ed. - Chapter 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
Slide 10 Firewalls & Network Security, 2nd ed. - Chapter 9
Firewalls & Network Security, 2nd ed. - Chapter 9 11
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
Firewalls & Network Security, 2nd ed. - Chapter 9 12
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
Firewalls & Network Security, 2nd ed. - Chapter 9 13
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
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
Firewalls & Network Security, 2nd ed. - Chapter 9 14
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 Firewalls & Network Security, 2nd ed. - Chapter 9 Slide 15
Firewalls & Network Security, 2nd ed. - Chapter 9 16
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
Firewalls & Network Security, 2nd ed. - Chapter 9 17
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: – ‗0‘ XOR‘ed with ‗0‘ results in a ‗0‘ (0 0 = 0)
– ‗0‘ XOR‘ed with ‗1‘ results in a ‗1‘ (0 1 = 1)
– ‗1‘ XOR‘ed with ‗0‘ results in a ‗1‘ (1 0 = 1)
– ‗1‘ XOR‘ed 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
Firewalls & Network Security, 2nd ed. - Chapter 9 18
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
Firewalls & Network Security, 2nd ed. - Chapter 9 19
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
Firewalls & Network Security, 2nd ed. - Chapter 9 20
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
Firewalls & Network Security, 2nd ed. - Chapter 9 21
Symmetric Encryption (continued)
Firewalls & Network Security, 2nd ed. - Chapter 9 22
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
Firewalls & Network Security, 2nd ed. - Chapter 9 23
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
Firewalls & Network Security, 2nd ed. - Chapter 9 24
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
Firewalls & Network Security, 2nd ed. - Chapter 9 25
Public Key Encryption
Firewalls & Network Security, 2nd ed. - Chapter 9 26
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
Firewalls & Network Security, 2nd ed. - Chapter 9 27
Digital Signatures (continued)
Firewalls & Network Security, 2nd ed. - Chapter 9 28
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
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 Firewalls & Network Security, 2nd ed. - Chapter 9 29
Firewalls & Network Security, 2nd ed. - Chapter 9 30
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
Firewalls & Network Security, 2nd ed. - Chapter 9 31
Hybrid Encryption
Firewalls & Network Security, 2nd ed. - Chapter 9 32
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
Firewalls & Network Security, 2nd ed. - Chapter 9 33
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
Firewalls & Network Security, 2nd ed. - Chapter 9 34
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
Firewalls & Network Security, 2nd ed. - Chapter 9 35
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
Firewalls & Network Security, 2nd ed. - Chapter 9 36
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
Firewalls & Network Security, 2nd ed. - Chapter 9 37
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
Firewalls & Network Security, 2nd ed. - Chapter 9 38
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
Firewalls & Network Security, 2nd ed. - Chapter 9 39
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
Firewalls & Network Security, 2nd ed. - Chapter 9 40
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
Kerberos (continued)
Firewalls & Network Security, 2nd ed. - Chapter 9 Slide 41
Kerberos (continued)
Firewalls & Network Security, 2nd ed. - Chapter 9 Slide 42
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
Firewalls & Network Security, 2nd ed. - Chapter 9 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
Firewalls & Network Security, 2nd ed. - Chapter 9 Slide 44
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
Firewalls & Network Security, 2nd ed. - Chapter 9 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 inter-
keystroke timings to discern sensitive session
information
Firewalls & Network Security, 2nd ed. - Chapter 9 Slide 46
Defending from Attacks
Firewalls & Network Security, 2nd ed. - Chapter 9 Slide 47
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
Firewalls & Network Security, 2nd ed. - Chapter 9 48
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
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 Firewalls & Network Security, 2nd ed. - Chapter 9 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
Firewalls & Network Security, 2nd ed. - Chapter 9 Slide 50