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

2. 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
© 2009 Cisco Learning Institute. 2

3. 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
© 2009 Cisco Learning Institute. 3

4. 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
© 2009 Cisco Learning Institute. 4

5. 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)
© 2009 Cisco Learning Institute. 5

6. 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
© 2009 Cisco Learning Institute. 6

7. Secure Communications
CSA
MARS
Firewall
VPN
IPS
CSA
VPN Iron Port CSA
Remote Branch CSA
CSA CSA
CSA
CSA
Web Email
Server Server DNS
• Traffic between sites must be secure
• Measures must be taken to ensure it cannot be altered, forged, or
deciphered if intercepted
© 2009 Cisco Learning Institute. 7

8. 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.
© 2009 Cisco Learning Institute. 8

9. Integrity
• An unbroken wax seal on an envelop ensures integrity.
• The unique unbroken seal ensures no one has read the
contents.
© 2009 Cisco Learning Institute. 9

10. Confidentiality
• Julius Caesar
would send
encrypted
messages to his
I O D Q N H D V W generals in the
battlefield.
D W W D F N D W G D Z Q • Even if
intercepted, his
enemies usually
could not read, let
alone decipher,
the messages.
© 2009 Cisco Learning Institute. 10

11. History
Scytale - (700 BC)
Vigenère table
German Enigma Machine
Jefferson encryption device
© 2009 Cisco Learning Institute. 11

12. Transposition Ciphers
1
FLANK EAST The clear text message would be
ATTACK AT DAWN encoded using a key of 3.
Clear Text
2
F...K...T...T...A...W.
.L.N.E.S.A.T.A.K.T.A.N Use a rail fence cipher and a
..A...A...T...C...D... key of 3.
3
FKTTAW The clear text message would
LNESATAKTAN
AATCD appear as follows.
Ciphered Text
© 2009 Cisco Learning Institute. 12

13. Substitution Ciphers
Caesar Cipher
1
FLANK EAST The clear text message would be
ATTACK AT DAWN encoded using a key of 3.
Clear text
Shift the top
2 scroll over by
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 three characters
(key of 3), an 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 A B C
becomes D, B
becomes E, and
so on.
3
IODQN HDVW The clear text message would
DWWDFN DW GDZQ be encrypted as follows using a
key of 3.
Cipherered text
© 2009 Cisco Learning Institute. 13

14. Cipher Wheel
1
FLANK EAST The clear text message would be
ATTACK AT DAWN encoded using a key of 3.
Clear text
2
Shifting the inner wheel by 3, then
the A becomes D, B becomes E,
and so on.
3
IODQN HDVW The clear text message would
DWWDFN DW GDZQ appear as follows using a key of 3.
Cipherered text
© 2009 Cisco Learning Institute. 14

15. 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
© 2009 Cisco Learning Institute. 15

16. 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.
© 2009 Cisco Learning Institute. 16

17. Defining Cryptanalysis
Allies decipher secret
NAZI encryption code!
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.
© 2009 Cisco Learning Institute. 17

18. Cryptanalysis Methods
Brute Force Attack
Known Ciphertext
Successfully
Unencrypted
Key found
With a Brute Force attack, the attacker has some portion of
ciphertext. The attacker attempts to unencrypt the ciphertext with
all possible keys.
© 2009 Cisco Learning Institute. 18

19. Meet-in-the-Middle Attack
Known Ciphertext Known Plaintext
Use every possible Use every possible
decryption key until a result encryption key until a
is found matching the result is found matching
corresponding plaintext. the corresponding
ciphertext.
MATCH of
Ciphertext!
Key found
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.
© 2009 Cisco Learning Institute. 19

20. Choosing a Cryptanalysis Method
The graph outlines the
1
frequency of letters in the
English language.
For example, the letters E,
T and A are the most
popular.
There are 6 occurrences of the cipher
letter D and 4 occurrences of the cipher
letter W.
2 Replace the cipher letter D first with
IODQN HDVW
DWWDFN DW GDZQ popular clear text letters including E, T,
and finally A.
Cipherered text
Trying A would reveal the shift pattern of 3.
© 2009 Cisco Learning Institute. 20

21. Defining Cryptology
Cryptology
+
Cryptography Cryptanalysis
© 2009 Cisco Learning Institute. 21

22. Cryptanalysis
© 2009 Cisco Learning Institute. 22

23. Cryptographic Hashes, Protocols,
and Algorithm Examples
Integrity Authentication Confidentiality
DES
HMAC-MD5 3DES
MD5
HMAC-SHA-1 AES
SHA
RSA and DSA SEAL
RC (RC2, RC4, RC5, and RC6)
HASH HASH w/Key
NIST Rivest Encryption
© 2009 Cisco Learning Institute. 23

24. Hashing Basics
• Hashes are used for
integrity assurance. Data of Arbitrary
Length
• 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.
Fixed-Length
Hash Value
e883aa0b24c09f
© 2009 Cisco Learning Institute. 24

25. Hashing Properties
Arbitrary X
length text Why is x not in
Parens?
h = H (x)
Hash
Function
(H)
Why is H in
Parens?
Hash h e883aa0b24c09f
Value
© 2009 Cisco Learning Institute. 25

26. Hashing in Action
• Vulnerable to man-in-the-middle attacks
- Hashing does not provide security to transmission.
• Well-known hash functions
I would like to
- MD5 with 128-bit hashes cash this
- SHA-1 with 160-bit hashes check.
Internet
Pay to Terry Smith Pay to Alex Jones
$100.00 $1000.00
One Hundred and One Thousand and
xx/100
xx/100 Dollars
Dollars
4ehIDx67NMop9 12ehqPx67NMoX
Match = No changes
No match = Alterations
© 2009 Cisco Learning Institute. 26

27. MD5
• MD5 is a ubiquitous hashing
algorithm
• Hashing properties
- One-way function—easy to
compute hash and infeasible to MD5
compute data given a hash
- Complex sequence of simple
binary operations (XORs,
rotations, etc.) which finally
produces a 128-bit hash.
© 2009 Cisco Learning Institute. 27

28. 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
SHA
• 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.
© 2009 Cisco Learning Institute. 28

29. 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.
© 2009 Cisco Learning Institute. 29

30. Features of HMAC
• Uses an additional secret
key as input to the hash Data of Arbitrary Secret
function Length + Key
• The secret key is known
to the sender and receiver
- Adds authentication to
integrity assurance
- Defeats man-in-the-middle
Fixed Length
attacks Authenticated e883aa0b24c09f
Hash Value
• Based on existing hash
functions, such as MD5 The same procedure is used for
generation and verification of
and SHA-1. secure fingerprints
© 2009 Cisco Learning Institute. 30

31. HMAC Example
Data Received Data Secret Key
Pay to Terry Smith $100.00 Secret
Pay to Terry Smith $100.00
One Hundred and xx/100 Dollars Key One Hundred and xx/100 Dollars
HMAC HMAC
(Authenticated 4ehIDx67NMop9 (Authenticated 4ehIDx67NMop9
Fingerprint) Fingerprint)
Pay to Terry Smith $100.00 If the generated HMAC matches the
One Hundred and xx/100 Dollars sent HMAC, then integrity and
authenticity have been verified.
4ehIDx67NMop9 If they don’t match, discard the
message.
© 2009 Cisco Learning Institute. 31

32. Using Hashing
Data Integrity Data Authenticity
e883aa0b24c09f
Fixed-Length Hash
Value
Entity Authentication
• 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
© 2009 Cisco Learning Institute. 32

33. Key Management
Key Generation Key Verification
Key
Management Key Storage
Key Exchange
Key Revocation and Destruction
© 2009 Cisco Learning Institute. 33

34. Keyspace
DES Key Keyspace # of Possible Keys
256
56-bit 11111111 11111111 11111111
72,000,000,000,000,000
11111111 11111111 11111111 11111111
Twice as
much time
2 57
11111111 11111111 11111111
57-bit 144,000,000,000,000,000 Four time as
11111111 11111111 11111111 11111111 1
much time
258
58-bit 11111111 11111111 11111111
288,000,000,000,000,000
11111111 11111111 11111111 11111111 11 With 60-bit DES
an attacker would
require sixteen
259 more time than
56-bit DES
11111111 11111111 11111111
59-bit 11111111 11111111 11111111 11111111 111
576,000,000,000,000,000
260
For each bit added to the DES key, the attacker 1,152,000,000,000,000,000amount of time to
60-bit 11111111 11111111 11111111
would require twice the
search the keyspace. 11111111 11111111 11111111 1111
11111111
Longer keys are more secure but are also more resource intensive and can affect throughput.
© 2009 Cisco Learning Institute. 34

35. Types of Keys
Symmetric Asymmetric Digital
Hash
Key Key Signature
Protection up
to 3 years 80 1248 160 160
Protection up
to 10 years 96 1776 192 192
Protection up
to 20 years 112 2432 224 224
Protection up
to 30 years 128 3248 256 256
Protection against
quantum computers 256 15424 512 512
 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.
© 2009 Cisco Learning Institute. 35

36. Key Properties
Shorter keys = faster
processing, but less secure
Longer keys = slower
processing, but more
secure
© 2009 Cisco Learning Institute. 36

37. 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)
© 2009 Cisco Learning Institute. 37

38. Symmetric Encryption
Pre-shared
Key key Key
Encrypt Decrypt
$1000 $!@#IQ $1000
• 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.
© 2009 Cisco Learning Institute. 38

39. Symmetric Encryption and XOR
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
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
© 2009 Cisco Learning Institute. 39

40. Asymmetric Encryption
Two separate
keys which are
Encryption Key not shared Decryption Key
Encrypt Decrypt
$1000 %3f7&4 $1000
• 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.
© 2009 Cisco Learning Institute. 40

41. Asymmetric Example : Diffie-Hellman
Get Out Your Calculators?
© 2009 Cisco Learning Institute. 41

42. Symmetric Algorithms
Symmetric Key length
Encryption Description
Algorithm (in bits)
Designed at IBM during the 1970s and was the NIST standard until 1997.
Although considered outdated, DES remains widely in use.
DES 56
Designed to be implemented only in hardware, and is therefore extremely
slow in software.
Based on using DES three times which means that the input data is
encrypted three times and therefore considered much stronger than DES.
3DES 112 and 168
However, it is rather slow compared to some new block ciphers such as
AES.
Fast in both software and hardware, is relatively easy to implement, and
AES 128, 192, and 256 requires little memory.
As a new encryption standard, it is currently being deployed on a large scale.
Software SEAL is an alternative algorithm to DES, 3DES, and AES.
Encryption 160 It uses a 160-bit encryption key and has a lower impact to the CPU when
Algorithm (SEAL) compared to other software-based algorithms.
RC2 (40 and 64) A set of symmetric-key encryption algorithms invented by Ron Rivest.
RC4 (1 to 256) RC1 was never published and RC3 was broken before ever being used.
The RC series RC5 (0 to 2040) RC4 is the world's most widely used stream cipher.
RC6 (128, 192, RC6, a 128-bit block cipher based heavily on RC5, was an AES finalist
and 256) developed in 1997.
© 2009 Cisco Learning Institute. 42

43. Symmetric Encryption Techniques
Enc
Mes rypted
blank blank 1100101 01010010110010101 sag
e
01010010110010101
64 bits 64bits 64bits
Block Cipher – encryption is completed
in 64 bit blocks
Enc
Mes rypted
sag
e
0101010010101010100001001001001 0101010010101010100001001001001
Stream Cipher – encryption is one bit
at a time
© 2009 Cisco Learning Institute. 43

44. Selecting an Algorithm
DES 3DES AES
The algorithm is trusted by Been
Verdict is
the cryptographic replaced by Yes
still out
community 3DES
The algorithm adequately
protects against brute-force No Yes Yes
attacks
© 2009 Cisco Learning Institute. 44

45. DES Scorecard
Description Data Encryption Standard
Timeline Standardized 1976
Type of Algorithm Symmetric
Key size (in bits) 56 bits
Speed Medium
Time to crack Days (6.4 days by the COPACABANA machine, a specialized
(Assuming a computer could try cracking device)
255 keys per second)
Resource
Medium
Consumption
© 2009 Cisco Learning Institute. 45

46. Block Cipher Modes
ECB CBC
Message of Five 64-Bit Blocks Message of Five 64-Bit Blocks
Initialization
Vector
DES
DES
DES
DES
DES
DES
DES
DES
DES
DES
© 2009 Cisco Learning Institute. 46

47. Considerations
• Change keys frequently to help
prevent brute-force attacks. DES
• 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.
© 2009 Cisco Learning Institute. 47

48. 3DES Scorecard
Description Triple Data Encryption Standard
Timeline Standardized 1977
Type of Algorithm Symmetric
Key size (in bits) 112 and 168 bits
Speed Low
Time to crack
(Assuming a computer could try 4.6 Billion years with current technology
255 keys per second)
Resource
Medium
Consumption
© 2009 Cisco Learning Institute. 48

49. Encryption Steps
The clear text from Alice is
encrypted using Key 1. That
ciphertext is decrypted
using a different key, Key 2.
1 Finally that ciphertext is
encrypted using another
key, Key 3.
When the 3DES ciphered text
2 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.
© 2009 Cisco Learning Institute. 49

50. AES Scorecard
Description Advanced Encryption Standard
Timeline Official Standard since 2001
Type of Algorithm Symmetric
Key size (in bits) 128, 192, and 256
Speed High
Time to crack
(Assuming a computer could try 149 Trillion years
255 keys per second)
Resource
Low
Consumption
© 2009 Cisco Learning Institute. 50

51. 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
© 2009 Cisco Learning Institute. 51

52. SEAL Scorecard
Description Software-Optimized Encryption Algorithm
Timeline First published in 1994. Current version is 3.0 (1997)
Type of Algorithm Symmetric
Key size (in bits) 160
Speed High
Time to crack
(Assuming a computer could try Unknown but considered very safe
255 keys per second)
Resource
Low
Consumption
© 2009 Cisco Learning Institute. 52

53. Rivest Codes Scorecard
Description RC2 RC4 RC5 RC6
Timeline 1987 1987 1994 1998
Stream
Type of Algorithm Block cipher Block cipher Block cipher
cipher
0 to 2040
128, 192, or
Key size (in bits) 40 and 64 1 - 256 bits (128
256
suggested)
© 2009 Cisco Learning Institute. 53

54. 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 Unknown but considered very safe
try 255 keys per second)
Resource
Medium
Consumption
© 2009 Cisco Learning Institute. 54

55. Using Diffie-Hellman
Alice Bob
Shared Secret Calc Shared Secret Calc
1 5, 23 1 5, 23
3
2 6 56mod 23 = 8 8
1. Alice and Bob agree to use the same two numbers. For example, the base number
g= 5 and prime number p=23
2. Alice now chooses a secret number x= 6.
3. Alice performs the DH algorithm: gx modulo p = ( 56 modulo 23) = 8 (Y) and
sends the new number 8 (Y) to Bob.
© 2009 Cisco Learning Institute. 55

56. Using Diffie-Hellman
Alice Bob
Shared Secret Calc Shared Secret Calc
5, 23 5, 23
6 56mod 23 = 8 8 15 4
19 515mod 23 = 19
19 mod 23 = 2 2
5
6
6 815mod 23 =
15, performed the DH algorithm:
4. Meanwhile Bob has also chosen a secret number x=
g modulo p = (515 modulo 23) = 19 (Y) and sent the new number 19 (Y) to
x 23
Alice. The result (2) is the same
2
for both Alice and Bob.
196 modulo 23) = 2.
5. Alice now computes Yx modulo p = (
This number can now be
used as a shared secret
key by the encryption
6. Bob now computes Y modulo p = (86 modulo 23) = 2.
x algorithm.
© 2009 Cisco Learning Institute. 56

57. Asymmetric Key Characteristics
Encryption Decryption
Key Key
Plain Encryption Encrypted Decryption Plain
text text text
• 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
© 2009 Cisco Learning Institute. 57

58. Public Key (Encrypt) + Private Key
(Decrypt) = Confidentiality
Computer A acquires
Computer B’s public key
Can I get your Public Key please? Bob’s Public
1 Key
Here is my Public Key.
Bob’s Public
Computer A transmits Bob’s Private
2 4
Key The encrypted message Key
Computer
Computer to Computer B Encrypted
Text B
A
Encryption Encryption
Algorithm Algorithm
Encrypted 3 Computer B uses
Text its private key to
decrypt and reveal
Computer A uses Computer B’s
the message
public key to encrypt a message
using an agreed-upon algorithm
© 2009 Cisco Learning Institute. 58

59. Private Key (Encrypt) + Public Key
(Decrypt) = Authentication
Bob uses the public key to
Alice encrypts a message successfully decrypt the message
with her private key and authenticate that the message
did, indeed, come from Alice.
Alice’s Private
1 Key
Encrypted
Text
Encryption
Alice transmits the 4
Alice’s Public
Key
Algorithm encrypted message Encrypted
2 to Bob Text
Encrypted
Computer Text
3 Computer Encryption
A B
Algorithm
Alice’s Public Can I get your Public Key please?
Key
Here is my Public Key
Bob needs to verify that the message
actually came from Alice. He requests
and acquires Alice’s public key
© 2009 Cisco Learning Institute. 59

60. Asymmetric Key Algorithms
Key
length Description
(in bits)
Invented in 1976 by Whitfield Diffie and Martin Hellman.
512, 1024, Two parties to agree on a key that they can use to encrypt messages
DH
2048 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 Created by NIST and specifies DSA as the algorithm for digital signatures.
Standard (DSS) and
Digital Signature
512 - 1024 A public key algorithm based on the ElGamal signature scheme.
Algorithm (DSA) Signature creation speed is similar with RSA, but is slower for verification.
Developed by Ron Rivest, Adi Shamir, and Leonard Adleman at MIT in 1977
RSA encryption Based on the current difficulty of factoring very large numbers
512 to 2048
algorithms Suitable for signing as well as encryption
Widely used in electronic commerce protocols
Based on the Diffie-Hellman key agreement.
Described by Taher Elgamal in 1984and is used in GNU Privacy Guard software,
EIGamal 512 - 1024 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
Invented by Neil Koblitz in 1987 and by Victor Miller in 1986.
Elliptical curve
160 Can be used to adapt many cryptographic algorithms
techniques
Keys can be much smaller
© 2009 Cisco Learning Institute. 60

61. 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
© 2009 Cisco Learning Institute. 61

62. 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.
© 2009 Cisco Learning Institute. 62

63. The Digital Signature Process
The sending device creates
a hash of the document
The receiving device Validity of the digital
Data accepts the document signature is verified
Confirm with digital signature
and obtains the public key Signature Verified
Order
0a77b3440…
1 hash Signed Data 6
Signature Confirm
Key Order 4
____________
Encrypted 0a77b3440…
hash Signature is
2 Signature
Algorithm verified with
The sending device 3 the verification
encrypts only the hash key
0a77b3440…
with the private key
of the signer The signature algorithm Verification
5
generates a digital signature Key
and obtains the public key
© 2009 Cisco Learning Institute. 63

64. 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.
© 2009 Cisco Learning Institute. 64

65. 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
© 2009 Cisco Learning Institute. 65

66. 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
© 2009 Cisco Learning Institute. 66

67. 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
© 2009 Cisco Learning Institute. 67

68. 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.
© 2009 Cisco Learning Institute. 68

69. Public Key Infrastructure
PKI terminology to remember:
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
© 2009 Cisco Learning Institute. 69

70. CA Vendors and Sample Certificates
http://www.verisign.com http://www.entrust.com
http://www.verizonbusiness.com/
http://www.novell.com
http://www.rsa.com/
http://www.microsoft.com
© 2009 Cisco Learning Institute. 70

71. 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.
© 2009 Cisco Learning Institute. 71

72. The Current State
X.509
• Many vendors have proposed and implemented
proprietary solutions
• Progression towards publishing a common set of
standards for PKI protocols and data formats
© 2009 Cisco Learning Institute. 72

73. 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
© 2009 Cisco Learning Institute. 73

74. X.509v3 Applications
SSL S/MIME
Internet
Mail
External Server
Web Server EAP-TLS
Cisco
Secure
Internet Enterprise ACS
Network
CA
Server
VPN
IPsec Concentrator
• 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.
© 2009 Cisco Learning Institute. 74

75. 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
© 2009 Cisco Learning Institute. 75

76. Public Key Technology
PKCS#7
PKCS#10
CA
Certificate
Signed
Certificate
PKCS#7
• A PKI communication protocol used for VPN PKI
enrollment
• Uses the PKCS #7 and PKCS #10 standards
© 2009 Cisco Learning Institute. 76

77. Single-Root PKI Topology
• Certificates issued by one CA
• Centralized trust decisions
• Single point of failure
Root CA
© 2009 Cisco Learning Institute. 77

78. Hierarchical CA Topology
Root CA
Subordinate
CA
• Delegation and distribution of trust
• Certification paths
© 2009 Cisco Learning Institute. 78

79. Cross-Certified CAs
CA2
CA1
CA3
• Mutual cross-signing of CA certificates
© 2009 Cisco Learning Institute. 79

80. Registration Authorities
After the Registration
Authority adds specific
information to the
2 CA certificate request and
Completed Enrollment
Request Forwarded to
the request is approved
CA under the organization’s
policy, it is forwarded
Hosts will submit on to the Certification
certificate requests RA Authority
to the RA 3
1
Certificate Issued
Enrollment
request
The CA will sign the certificate
request and send it back to
the host
© 2009 Cisco Learning Institute. 80

81. Retrieving the CA Certificates
Alice and Bob telephone the CA
administrator and verify the public key
and serial number of the certificate
Out-of-Band
Out-of-Band Authentication of
Authentication of the CA Certificate
the CA Certificate CA
Admin POTS
3
POTS 3
CA
1 CA
1
Certificate
CA
Certificate
Enterprise Network
2
2
Alice and Bob request the CA certificate Each system verifies the
that contains the CA public key validity of the certificate
© 2009 Cisco Learning Institute. 81

82. Submitting Certificate Requests
The CA administrator telephones to
The certificate is confirm their submittal and the public
retrieved and the key and issues the certificate by
certificate is installed 2 adding some additional data to the
onto the system request, and digitally signing it all
Out-of-Band Out-of-Band
Authentication of Authentication of
the CA Certificate CA the CA Certificate
Admin
POTS POTS
CA
1 Certificate
3 1 Certificate Request 3
Request
Enterprise Network
Both systems forward a certificate request which
includes their public key. All of this information is
encrypted using the public key of the CA
© 2009 Cisco Learning Institute. 82

83. Authenticating
Bob and Alice exchange certificates. The CA is no longer involved
2 2
Private Key (Alice) Private Key (Bob)
Certificate (Alice)
1
Certificate (Alice) Certificate (Bob)
Certificate (Bob)
CA Certificate CA Certificate
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.
© 2009 Cisco Learning Institute. 83

84. 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.
© 2009 Cisco Learning Institute. 84

85. © 2009 Cisco Learning Institute. 85