Chapter 7 overview

1© 2009 Cisco Learning Institute.
CCNA Security
Chapter Seven
Cryptographic Systems

Lesson Planning
• This lesson should take 3-4 hours to present
• The lesson should include lecture,
demonstrations, discussions and assessments
• The lesson can be taught in person or using
remote instruction

Major Concepts
• Describe how the types of encryption, hashes,
and digital signatures work together to provide
confidentiality, integrity, and authentication
• Describe the mechanisms to ensure data
integrity and authentication
• Describe the mechanisms used to ensure data
confidentiality
• Describe the mechanisms used to ensure data
confidentiality and authentication using a public
key

Lesson Objectives
Upon completion of this lesson, the successful participant
will be able to:
1. Describe the requirements of secure communications including
integrity, authentication, and confidentiality
2. Describe cryptography and provide an example
3. Describe cryptanalysis and provide an example
4. Describe the importance and functions of cryptographic hashes
5. Describe the features and functions of the MD5 algorithm and of
the SHA-1 algorithm
6. Explain how we can ensure authenticity using HMAC
7. Describe the components of key management

Lesson Objectives
8. Describe how encryption algorithms provide confidentiality
9. Describe the function of the DES algorithms
10. Describe the function of the 3DES algorithm
11. Describe the function of the AES algorithm
12. Describe the function of the Software Encrypted Algorithm
(SEAL) and the Rivest ciphers (RC) algorithm
13. Describe the function of the DH algorithm and its supporting role
to DES, 3DES, and AES
14. Explain the differences and their intended applications
15. Explain the functionality of digital signatures
16. Describe the function of the RSA algorithm
17. Describe the principles behind a public key infrastructure (PKI)

Lesson Objectives
18. Describe the various PKI standards
19. Describe the role of CAs and the digital certificates that they
issue in a PKI
20. Describe the characteristics of digital certificates and CAs

Secure Communications
• Traffic between sites must be secure
• Measures must be taken to ensure it cannot be altered, forged, or
deciphered if intercepted
MARS
Remote Branch
VPN
VPN
Iron Port
Firewall
IPS
CSA
Web
Server
Email
Server DNS
CSA
CSA
CSA
CSA
CSA
CSA
CSA

Authentication
• An ATM Personal
Information Number (PIN)
is required for
authentication.
• The PIN is a shared
secret between a bank
account holder and the
financial institution.

Integrity
• An unbroken wax seal on an envelop ensures integrity.
• The unique unbroken seal ensures no one has read the
contents.

Confidentiality
• Julius Caesar
would send
encrypted
messages to his
generals in the
battlefield.
• Even if
intercepted, his
enemies usually
could not read, let
alone decipher,
the messages.
I O D Q N H D V W
D W W D F N D W G D Z Q

History
Scytale - (700 BC)
Jefferson encryption device
Vigenère table
German Enigma Machine

Transposition Ciphers
F...K...T...T...A...W.
.L.N.E.S.A.T.A.K.T.A.N
..A...A...T...C...D...
Ciphered Text
3
FKTTAW
LNESATAKTAN
AATCD
The clear text message would be
encoded using a key of 3.
1
FLANK EAST
ATTACK AT DAWN
Use a rail fence cipher and a
key of 3.
2
The clear text message would
appear as follows.
Clear Text

Substitution Ciphers
Caesar Cipher
Cipherered text
3
IODQN HDVW
DWWDFN DW GDZQ
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z A B C
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
1
FLANK EAST
ATTACK AT DAWN
Shift the top
scroll over by
three characters
(key of 3), an A
becomes D, B
becomes E, and
so on.
2
be encrypted as follows using a
key of 3.
Clear text

Cipher Wheel
Cipherered text
3
IODQN HDVW
DWWDFN DW GDZQ
1
FLANK EAST
ATTACK AT DAWN
Shifting the inner wheel by 3, then
the A becomes D, B becomes E,
and so on.
2
appear as follows using a key of 3.
Clear text

Vigenѐre Table
a b c d e f g h i j k l m n o p q r s t u v w x y z
A a b c d e f g h i j k l m n o p q r s t u v w x y z
B b c d e f g h i j k l m n o p q r s t u v w x y z a
C c d e f g h i j k l m n o p q r s t u v w x y z a b
D d e f g h i j k l m n o p q r s t u v w x y z a b c
E e f g h i j k l m n o p q r s t u v w x y z a b c d
F f g h i j k l m n o p q r s t u v w x y z a b c d e
G g h i j k l m n o p q r s t u v w x y z a b c d e f
H h i j k l m n o p q r s t u v w x y z a b c d e f g
I i j k l m n o p q r s t u v w x y z a b c d e f g h
J j k l m n o p q r s t u v w x y z a b c d e f g h i
K k l m n o p q r s t u v w x y z a b c d e f g h i j
L l m n o p q r s t u v w x y z a b c d e f g h i j k
M m n o p q r s t u v w x y z a b c d e f g h i j k l
N n o p q r s t u v w x y z a b c d e f g h i j k l m
O o p q r s t u v w x y z a b c d e f g h i j k l m n
P p q r s t u v w x y z a b c d e f g h i j k l m n o
Q q r s t u v w x y z a b c d e f g h i j k l m n o p
R r s t u v w x y z a b c d e f g h i j k l m n o p q
S s t u v w x y z a b c d e f g h i j k l m n o p q r
T t u v w x y z a b c d e f g h i j k l m n o p q r s
U u v w x y z a b c d e f g h i j k l m n o p q r s t
V v w x y z a b c d e f g h i j k l m n o p q r s t u
W w x y z a b c d e f g h i j k l m n o p q r s t u v
X x y z a b c d e f g h i j k l m n o p q r s t u v w
Y y z a b c d e f g h i j k l m n o p q r s t u v w x
Z z a b c d e f g h i j k l m n o p q r s t u v w x y

Stream Ciphers
•Invented by the Norwegian Army Signal
Corps in 1950, the ETCRRM machine
uses the Vernam stream cipher method.
•It was used by the US and Russian
governments to exchange information.
•Plain text message is eXclusively OR'ed
with a key tape containing a random
stream of data of the same length to
generate the ciphertext.
•Once a message was enciphered the
key tape was destroyed.
•At the receiving end, the process was
reversed using an identical key tape to
decode the message.

Defining Cryptanalysis
Cryptanalysis is from the Greek words kryptós (hidden), and
analýein (to loosen or to untie). It is the practice and the study of
determining the meaning of encrypted information (cracking the
code), without access to the shared secret key.
Allies decipher secret
NAZI encryption code!

Cryptanalysis Methods
Known Ciphertext
Brute Force Attack
With a Brute Force attack, the attacker has some portion of
ciphertext. The attacker attempts to unencrypt the ciphertext with
all possible keys.
Successfully
Unencrypted
Key found

Meet-in-the-Middle Attack
With a Meet-in-the-Middle attack, the attacker has some portion of
text in both plaintext and ciphertext. The attacker attempts to
unencrypt the ciphertext with all possible keys while at the same time
encrypt the plaintext with another set of possible keys until one match
is found.
Known Ciphertext Known Plaintext
Use every possible
decryption key until a result
is found matching the
corresponding plaintext.
Use every possible
encryption key until a
result is found matching
the corresponding
ciphertext.
MATCH of
Ciphertext!
Key found

Choosing a Cryptanalysis Method
Cipherered text
2
IODQN HDVW
DWWDFN DW GDZQ
There are 6 occurrences of the cipher
letter D and 4 occurrences of the cipher
letter W.
Replace the cipher letter D first with
popular clear text letters including E, T,
and finally A.
Trying A would reveal the shift pattern of 3.
1
The graph outlines the
frequency of letters in the
English language.
For example, the letters E,
T and A are the most
popular.

Defining Cryptology
Cryptography
Cryptology
+
Cryptanalysis

Cryptanalysis

Cryptographic Hashes, Protocols,
and Algorithm Examples
IntegrityIntegrity AuthenticationAuthentication ConfidentialityConfidentiality
MD5
SHA
HMAC-MD5
HMAC-SHA-1
RSA and DSA
DES
3DES
AES
SEAL
RC (RC2, RC4, RC5, and RC6)
NIST Rivest
HASH HASH w/Key
Encryption

Hashing Basics
• Hashes are used for
integrity assurance.
• Hashes are based on
one-way functions.
• The hash function hashes
arbitrary data into a fixed-
length digest known as
the hash value, message
digest, digest, or
fingerprint.
Data of Arbitrary
Length
Fixed-Length
Hash Value
e883aa0b24c09f

Hashing Properties
XWhy is x not in
Parens?
h e883aa0b24c09f
H
(H)Why is H in
Parens?
= (x)h
Hash
Value
Hash
Function
Arbitrary
length text

Hashing in Action
• Vulnerable to man-in-the-middle attacks
- Hashing does not provide security to transmission.
• Well-known hash functions
- MD5 with 128-bit hashes
- SHA-1 with 160-bit hashes
Pay to Terry Smith
$100.00
One Hundred and
xx/100
Dollars
Pay to Alex Jones
$1000.00
One Thousand and
xx/100 Dollars
4ehIDx67NMop9 12ehqPx67NMoX
Match = No changes
No match = Alterations
Internet
I would like to
cash this
check.

MD5
• MD5 is a ubiquitous hashing
algorithm
• Hashing properties
- One-way function—easy to
compute hash and infeasible to
compute data given a hash
- Complex sequence of simple
binary operations (XORs,
rotations, etc.) which finally
produces a 128-bit hash.
MD5

SHA
• SHA is similar in design to the MD4 and
MD5 family of hash functions
- Takes an input message of no more than 264
bits
- Produces a 160-bit message digest
• The algorithm is slightly slower than MD5.
• SHA-1 is a revision that corrected an
unpublished flaw in the original SHA.
• SHA-224, SHA-256, SHA-384, and SHA-
512 are newer and more secure versions of
SHA and are collectively known as SHA-2.
SHA

Hashing Example
In this example the clear text entered is displaying hashed
results using MD5, SHA-1, and SHA256. Notice the
difference in key lengths between the various algorithm. The
longer the key, the more secure the hash function.

Features of HMAC
• Uses an additional secret
key as input to the hash
function
• The secret key is known
to the sender and receiver
- Adds authentication to
integrity assurance
- Defeats man-in-the-middle
attacks
• Based on existing hash
functions, such as MD5
and SHA-1.
The same procedure is used for
generation and verification of
secure fingerprints
Fixed Length
Authenticated
Hash Value
+
Secret
Key
Data of Arbitrary
Length
e883aa0b24c09f

HMAC Example
Data
HMAC
(Authenticated
Fingerprint)
Secret
Key
Pay to Terry Smith $100.00
One Hundred and xx/100 Dollars
4ehIDx67NMop9
4ehIDx67NMop9
Received Data
HMAC
(Authenticated
Fingerprint)
Secret Key
4ehIDx67NMop9
If the generated HMAC matches the
sent HMAC, then integrity and
authenticity have been verified.
If they don’t match, discard the
message.

Using Hashing
• Routers use hashing with secret keys
• Ipsec gateways and clients use hashing algorithms
• Software images downloaded from the website have checksums
• Sessions can be encrypted
Fixed-Length Hash
Value
e883aa0b24c09f
Data Integrity
Entity Authentication
Data Authenticity

Key Management
Key
Management
Key Generation
Key Storage
Key Verification
Key Exchange
Key Revocation and Destruction

Keyspace
DES Key Keyspace # of Possible Keys
56-bit
256
11111111 11111111 11111111
11111111 11111111 11111111 11111111
72,000,000,000,000,000
57-bit
257
11111111 11111111 11111111
11111111 11111111 11111111 11111111 1
144,000,000,000,000,000
58-bit
258
11111111 11111111 11111111
11111111 11111111 11111111 11111111 11
288,000,000,000,000,000
59-bit
259
11111111 11111111 11111111
11111111 11111111 11111111 11111111111
576,000,000,000,000,000
60-bit
260
11111111 11111111 11111111
11111111 11111111 11111111 111111111111
1,152,000,000,000,000,000For each bit added to the DES key, the attacker would require twice the amount of time to
search the keyspace.
Longer keys are more secure but are also more resource intensive and can affect throughput.
With 60-bit DES
an attacker would
require sixteen
more time than
56-bit DES
Twice as
much time
Four time as
much time

Types of Keys
2242242432112Protection up
to 20 years
to 10 years
to 3 years
Hash
Digital
Signature
Asymmetric
Key
Symmetric
Key
to 30 years
51251215424256Protection against
quantum computers
 Calculations are based on the fact that computing power will continue to
grow at its present rate and the ability to perform brute-force attacks will
grow at the same rate.
 Note the comparatively short symmetric key lengths illustrating that
symmetric algorithms are the strongest type of algorithm.

Shorter keys = faster
processing, but less secure
Longer keys = slower
processing, but more
secure
Key Properties

Confidentiality and the OSI Model
• For Data Link Layer confidentiality, use proprietary link-
encrypting devices
• For Network Layer confidentiality, use secure Network
Layer protocols such as the IPsec protocol suite
• For Session Layer confidentiality, use protocols such as
Secure Sockets Layer (SSL) or Transport Layer Security
(TLS)
• For Application Layer confidentiality, use secure e-mail,
secure database sessions (Oracle SQL*net), and secure
messaging (Lotus Notes sessions)

Symmetric Encryption
• Best known as shared-secret key algorithms
• The usual key length is 80 - 256 bits
• A sender and receiver must share a secret key
• Faster processing because they use simple mathematical operations.
• Examples include DES, 3DES, AES, IDEA, RC2/4/5/6, and Blowfish.
Key Key
Encrypt Decrypt
$1000 $1000$!@#IQ
Pre-shared
key

Symmetric Encryption and XOR
Plain Text 1 1 0 1 0 0 1 1
Key (Apply) 0 1 0 1 0 1 0 1
XOR (Cipher Text) 1 0 0 0 0 1 1 0
Key (Re-Apply) 0 1 0 1 0 1 0 1
XOR (Plain Text) 1 1 0 1 0 0 1 1
The XOR operator results in a 1 when the value of
either the first bit or the second bit is a 1
The XOR operator results in a 0 when neither or both
of the bits is 1

Asymmetric Encryption
• Also known as public key algorithms
• The usual key length is 512–4096 bits
• A sender and receiver do not share a secret key
• Relatively slow because they are based on difficult computational
algorithms
• Examples include RSA, ElGamal, elliptic curves, and DH.
Encryption Key Decryption Key
Encrypt Decrypt
$1000 $1000%3f7&4
Two separate
keys which are
not shared

Asymmetric Example : Diffie-Hellman
Get Out Your Calculators?

Symmetric Algorithms
Symmetric
Encryption
Algorithm
Key length
(in bits)
Description
DES 56
Designed at IBM during the 1970s and was the NIST standard until 1997.
Although considered outdated, DES remains widely in use.
Designed to be implemented only in hardware, and is therefore extremely
slow in software.
3DES 112 and 168
Based on using DES three times which means that the input data is
encrypted three times and therefore considered much stronger than DES.
However, it is rather slow compared to some new block ciphers such as
AES.
AES 128, 192, and 256
Fast in both software and hardware, is relatively easy to implement, and
requires little memory.
As a new encryption standard, it is currently being deployed on a large scale.
Software
Encryption
Algorithm (SEAL)
160
SEAL is an alternative algorithm to DES, 3DES, and AES.
It uses a 160-bit encryption key and has a lower impact to the CPU when
compared to other software-based algorithms.
The RC series
RC2 (40 and 64)
RC4 (1 to 256)
RC5 (0 to 2040)
RC6 (128, 192,
and 256)
A set of symmetric-key encryption algorithms invented by Ron Rivest.
RC1 was never published and RC3 was broken before ever being used.
RC4 is the world's most widely used stream cipher.
RC6, a 128-bit block cipher based heavily on RC5, was an AES finalist
developed in 1997.

Symmetric Encryption Techniques
64 bits 64bits 64bits
0101001011001010101010010110010101
1100101blank blank
0101010010101010100001001001001 0101010010101010100001001001001
Block Cipher – encryption is completed
in 64 bit blocks
Stream Cipher – encryption is one bit
at a time
EncryptedMessage
EncryptedMessage

Selecting an Algorithm
DES 3DES AES
The algorithm is trusted by
the cryptographic
community
Been
replaced by
3DES
Yes
Verdict is
still out
The algorithm adequately
protects against brute-force
attacks
No Yes Yes

DES Scorecard
Description Data Encryption Standard
Timeline Standardized 1976
Type of Algorithm Symmetric
Key size (in bits) 56 bits
Speed Medium
Time to crack
(Assuming a computer could try
255 keys per second)
Days (6.4 days by the COPACABANA machine, a specialized
cracking device)
Resource
Consumption
Medium

Block Cipher Modes
DES
DES
DES
DES
DES
DES
DES
DES
DES
DES
Initialization
Vector
ECB CBC
Message of Five 64-Bit BlocksMessage of Five 64-Bit Blocks

Considerations
• Change keys frequently to help
prevent brute-force attacks.
• Use a secure channel to
communicate the DES key from
the sender to the receiver.
• Consider using DES in CBC
mode. With CBC, the
encryption of each 64-bit block
depends on previous blocks.
• Test a key to see if it is a weak
key before using it.
DES

3DES Scorecard
Description Triple Data Encryption Standard
Timeline Standardized 1977
Key size (in bits) 112 and 168 bits
Speed Low
Time to crack
4.6 Billion years with current technology
Resource
Consumption
Medium

Encryption Steps
When the 3DES ciphered text
is received, the process is
reversed. That is, the
ciphered text must first be
decrypted using Key 3,
encrypted using Key 2, and
finally decrypted using Key 1.
1
2
The clear text from Alice is
encrypted using Key 1. That
ciphertext is decrypted
using a different key, Key 2.
Finally that ciphertext is
encrypted using another
key, Key 3.

AES Scorecard
Description Advanced Encryption Standard
Timeline Official Standard since 2001
Key size (in bits) 128, 192, and 256
Speed High
Time to crack
149 Trillion years
Resource
Consumption
Low

Advantages of AES
• The key is much stronger due to the key length
• AES runs faster than 3DES on comparable hardware
• AES is more efficient than DES and 3DES on
comparable hardware
The plain text is now
encrypted using 128
AES
An attempt at
deciphering the text
using a lowercase,
and incorrect key

SEAL Scorecard
Description Software-Optimized Encryption Algorithm
Timeline First published in 1994. Current version is 3.0 (1997)
Key size (in bits) 160
Speed High
Time to crack
Unknown but considered very safe
Resource
Consumption
Low

Rivest Codes Scorecard
Description RC2 RC4 RC5 RC6
Timeline 1987 1987 1994 1998
Type of Algorithm Block cipher
Stream
cipher
Block cipher Block cipher
Key size (in bits) 40 and 64 1 - 256
0 to 2040
bits (128
suggested)
128, 192, or
256

DH Scorecard
Description Diffie-Hellman Algorithm
Timeline 1976
Type of Algorithm Asymmetric
Key size (in bits) 512, 1024, 2048
Speed Slow
Time to crack
(Assuming a computer could
try 255 keys per second)
Unknown but considered very safe
Resource
Consumption
Medium

Using Diffie-Hellman
AliceAlice BobBob
Calc Calc
5566
mod 2323 = 88
1. Alice and Bob agree to use the same two numbers. For example, the base numberbase number
gg=55and prime numberprime number pp=2323
2. Alice now chooses a secret numbersecret number xx=66.
3. Alice performs the DH algorithm: ggxx modulo pp = (5566 modulo 2323))= 8 (Y)8 (Y)and
sends the new number 8 (Y)8 (Y) to Bob.
55,, 2323 55,, 2323
66
Secret SharedShared Secret
1 1
2
3
88

Using Diffie-Hellman
Alice Bob
66
Secret Calc Shared Calc
15155566
mod 2323 = 88
4. Meanwhile Bob has also chosen a secret numbersecret number xx=1515, performed the DH algorithm:
ggxx modulo pp = (551515 modulo 2323) = 19 (Y)19 (Y) and sent the new number 19 (Y)19 (Y)to
Alice.
5. Alice now computes YYxx modulo pp = (191966 modulo 23)23)= 22.
6. Bob now computes YYxx modulo pp = (8866 modulo 23)23)= 22.
551515
mod 2323 = 1919
191966
mod 2323 = 22 881515
mod 2323 = 22
The result (22) is the same
for both Alice and Bob.
This number can now be
used as a shared secret
key by the encryption
algorithm.
The result (22) is the same
for both Alice and Bob.
This number can now be
used as a shared secret
key by the encryption
algorithm.
Shared Secret
88
1919
44
5
6
55,, 2323 55,, 2323

Asymmetric Key Characteristics
• Key length ranges from 512–4096 bits
• Key lengths greater than or equal to 1024 bits can be
trusted
• Key lengths that are shorter than 1024 bits are
considered unreliable for most algorithms
Plain
text
Encrypted
text
Plain
text
Encryption Decryption
Encryption
Key
Decryption
Key

Public Key (Encrypt) + Private Key
(Decrypt) = Confidentiality
Computer
A
Bob’s Public
Key
Can I get your Public Key please?
Here is my Public Key.
1
Bob’s Public
Key
3
2
Encrypted
Text
Bob’s Private
Key4
Encryption
Algorithm
Encryption
Algorithm
Encrypted
Text
Computer
B
Computer A acquires
Computer B’s public key
Computer A uses Computer B’s
public key to encrypt a message
using an agreed-upon algorithm
Computer A transmits
The encrypted message
to Computer B
Computer B uses
its private key to
decrypt and reveal
the message

Private Key (Encrypt) + Public Key
(Decrypt) = Authentication
Bob uses the public key to
successfully decrypt the message
and authenticate that the message
did, indeed, come from Alice.
Alice’s Private
Key
1 Encrypted
Text
Encryption
Algorithm
Encrypted
Text
2
Alice’s Public
Key
Can I get your Public Key please?
Here is my Public Key
3
4
Encryption
Algorithm
Encrypted
Text
Alice’s Public
Key
Computer
A
Computer
B
Alice encrypts a message
with her private key
Alice transmits the
encrypted message
to Bob
Bob needs to verify that the message
actually came from Alice. He requests
and acquires Alice’s public key

Asymmetric Key Algorithms
Key
length
(in bits)
Description
DH
512, 1024,
2048
Invented in 1976 by Whitfield Diffie and Martin Hellman.
Two parties to agree on a key that they can use to encrypt messages
The assumption is that it is easy to raise a number to a certain power, but
difficult to compute which power was used given the number and the outcome.
Digital Signature
Standard (DSS) and
Digital Signature
Algorithm (DSA)
512 - 1024
Created by NIST and specifies DSA as the algorithm for digital signatures.
A public key algorithm based on the ElGamal signature scheme.
Signature creation speed is similar with RSA, but is slower for verification.
RSA encryption
algorithms
512 to 2048
Developed by Ron Rivest, Adi Shamir, and Leonard Adleman at MIT in 1977
Based on the current difficulty of factoring very large numbers
Suitable for signing as well as encryption
Widely used in electronic commerce protocols
EIGamal 512 - 1024
Based on the Diffie-Hellman key agreement.
Described by Taher Elgamal in 1984and is used in GNU Privacy Guard software,
PGP, and other cryptosystems.
The encrypted message becomes about twice the size of the original message
and for this reason it is only used for small messages such as secret keys
Elliptical curve
techniques
160
Invented by Neil Koblitz in 1987 and by Victor Miller in 1986.
Can be used to adapt many cryptographic algorithms
Keys can be much smaller

Security Services- Digital Signatures
• Authenticates a source,
proving a certain party
has seen, and has signed,
the data in question
• Signing party cannot
repudiate that it signed
the data
• Guarantees that the data
has not changed from the
time it was signed Authenticity
Integrity
Nonrepudiation

Digital Signatures
• The signature is authentic and
not forgeable: The signature is
proof that the signer, and no one
else, signed the document.
• The signature is not reusable:
The signature is a part of the document and cannot be moved to a
different document.
• The signature is unalterable: After a document is signed, it cannot
be altered.
• The signature cannot be repudiated: For legal purposes, the
signature and the document are considered to be physical things.
The signer cannot claim later that they did not sign it.

The Digital Signature Process
Confirm
Order
Encrypted
hash
Confirm
Order
____________
0a77b3440…
Signature
Algorithm
Signature
Key
Data
Signature Verified
0a77b3440…
Verification
Key
0a77b3440…
Signed Data1
2
3
4
6
Validity of the digital
signature is verified
hash
5
The sending device creates
a hash of the document
The sending device
encrypts only the hash
with the private key
of the signer The signature algorithm
generates a digital signature
and obtains the public key
The receiving device
accepts the document
with digital signature
and obtains the public key
Signature is
verified with
the verification
key

Code Signing with Digital Signatures
• The publisher of the software attaches a digital signature to the
executable, signed with the signature key of the publisher.
• The user of the software needs to obtain the public key of the
publisher or the CA certificate of the publisher if PKI is used.

DSA Scorecard
Description Digital Signature Algorithm (DSA)
Timeline 1994
Type of Algorithm Provides digital signatures
Advantages: Signature generation is fast
Disadvantages: Signature verification is slow

RSA Scorecard
Description Ron Rivest, Adi Shamir, and Len Adleman
Timeline 1977
Type of Algorithm Asymmetric algorithm
Key size (in bits) 512 - 2048
Advantages: Signature verification is fast
Disadvantages: Signature generation is slow

Properties of RSA
• One hundred times slower than
DES in hardware
• One thousand times slower
than DES in software
• Used to protect small amounts
of data
• Ensures confidentiality of data
thru encryption
• Generates digital signatures for
authentication and
nonrepudiation of data

Public Key Infrastructure
Alice applies for a driver’s license.
She receives her driver’s license
after her identity is proven.
Alice attempts to cash a check.
Her identity is accepted after her
driver’s license is checked.

PKI:
A service framework (hardware, software, people,
policies and procedures) needed to support large-
scale public key-based technologies.
Certificate:
A document, which binds together the name of the
entity and its public key and has been signed by the
CA
Certificate authority (CA):
The trusted third party that signs the public keys
of entities in a PKI-based system
Public Key Infrastructure
PKI terminology to remember:

CA Vendors and Sample Certificates
http://www.verizonbusiness.com/
http://www.verisign.com
http://www.rsa.com/
http://www.entrust.com
http://www.novell.com
http://www.microsoft.com

Usage Keys
• When an encryption certificate is used much more frequently than a
signing certificate, the public and private key pair is more exposed
due to its frequent usage. In this case, it might be a good idea to
shorten the lifetime of the key pair and change it more often, while
having a separate signing private and public key pair with a longer
lifetime.
• When different levels of encryption and digital signing are required
because of legal, export, or performance issues, usage keys allow
an administrator to assign different key lengths to the two pairs.
• When key recovery is desired, such as when a copy of a user’s
private key is kept in a central repository for various backup reasons,
usage keys allow the user to back up only the private key of the
encrypting pair. The signing private key remains with the user,
enabling true nonrepudiation.

The Current State
• Many vendors have proposed and implemented
proprietary solutions
• Progression towards publishing a common set of
standards for PKI protocols and data formats
X.509

X.509v3
• X.509v3 is a standard that
describes the certificate
structure.
• X.509v3 is used with:
- Secure web servers: SSL
and TLS
- Web browsers: SSL and
TLS
- Email programs: S/MIME
- IPsec VPNs: IKE

X.509v3 Applications
• Certificates can be used for various purposes.
• One CA server can be used for all types of authentication
as long as they support the same PKI procedures.
Internet Enterprise
Network
External
Web Server
Internet
Mail
Server
Cisco
Secure
ACS
CA
Server
SSL S/MIME
EAP-TLS
IPsec
VPN
Concentrator

RSA PKCS Standards
• PKCS #1: RSA Cryptography Standard
• PKCS #3: DH Key Agreement Standard
• PKCS #5: Password-Based Cryptography Standard
• PKCS #6: Extended-Certificate Syntax Standard
• PKCS #7: Cryptographic Message Syntax Standard
• PKCS #8: Private-Key Information Syntax Standard
• PKCS #10: Certification Request Syntax Standard
• PKCS #12: Personal Information Exchange Syntax Standard
• PKCS #13: Elliptic Curve Cryptography Standard
• PKCS #15: Cryptographic Token Information Format Standard

Public Key Technology
• A PKI communication protocol used for VPN PKI
enrollment
• Uses the PKCS #7 and PKCS #10 standards
PKCS#7
PKCS#10
Certificate
Signed
Certificate
PKCS#7
CA

Single-Root PKI Topology
• Certificates issued by one CA
• Centralized trust decisions
• Single point of failure
Root CA

Hierarchical CA Topology
• Delegation and distribution of trust
• Certification paths
Root CA
Subordinate
CA

Cross-Certified CAs
• Mutual cross-signing of CA certificates
CA2
CA1
CA3

Registration Authorities
The CA will sign the certificate
request and send it back to
the host
1
Enrollment
request
2
Completed Enrollment
Request Forwarded to
CA
3
Certificate Issued
RA
CA
Hosts will submit
certificate requests
to the RA
After the Registration
Authority adds specific
information to the
certificate request and
the request is approved
under the organization’s
policy, it is forwarded
on to the Certification
Authority

Retrieving the CA Certificates
Alice and Bob telephone the CA
administrator and verify the public key
and serial number of the certificate
CA
Admin
CA
CA
Certificate
CA
Certificate
Enterprise Network
POTS
Out-of-Band
Authentication of
the CA Certificate
POTS
Out-of-Band
Authentication of
the CA Certificate
1
1
2
2
3
3
Alice and Bob request the CA certificate
that contains the CA public key
Each system verifies the
validity of the certificate

Submitting Certificate Requests
CA
Admin
CA
Enterprise Network
POTS
Out-of-Band
Authentication of
the CA Certificate
POTS
Out-of-Band
Authentication of
the CA Certificate
1
1
2
3 Certificate
Request
Certificate
Request 3
Both systems forward a certificate request which
includes their public key. All of this information is
encrypted using the public key of the CA
The certificate is
retrieved and the
certificate is installed
onto the system
The CA administrator telephones to
confirm their submittal and the public
key and issues the certificate by
adding some additional data to the
request, and digitally signing it all

Authenticating
Private Key (Alice)
Certificate (Alice)
CA Certificate
Private Key (Bob)
Certificate (Bob)
CA Certificate
Certificate (Bob)
Certificate (Alice)
Each party verifies the digital signature on the certificate by hashing the
plaintext portion of the certificate, decrypting the digital signature using the
CA public key, and comparing the results.
1
2
2
Bob and Alice exchange certificates. The CA is no longer involved

PKI Authentication Characteristics
• To authenticate each other, users have to obtain
the certificate of the CA and their own certificate.
These steps require the out-of-band verification
of the processes.
• Public-key systems use asymmetric keys where
one is public and the other one is private.
• Key management is simplified because two
users can freely exchange the certificates. The
validity of the received certificates is verified
using the public key of the CA, which the users
have in their possession.
• Because of the strength of the algorithms,
administrators can set a very long lifetime for the
certificates.

Chapter 7 overview

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