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1© 2009 Cisco Learning Institute.
CCNA Security
Chapter Seven
Cryptographic Systems
222© 2009 Cisco Learning Institute.
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
333© 2009 Cisco Learning Institute.
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
444© 2009 Cisco Learning Institute.
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
555© 2009 Cisco Learning Institute.
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)
666© 2009 Cisco Learning Institute.
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
777© 2009 Cisco Learning Institute.
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 BranchVPN
VPN
Iron Port
Firewall
IPS
CSA
Web Server
Email Server DNS
CSACSA CSA
CSA
CSA
CSA
CSA
888© 2009 Cisco Learning Institute.
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.
999© 2009 Cisco Learning Institute.
Integrity
• An unbroken wax seal on an envelop ensures integrity.
• The unique unbroken seal ensures no one has read the contents.
101010© 2009 Cisco Learning Institute.
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
111111© 2009 Cisco Learning Institute.
History
Scytale - (700 BC)
Jefferson encryption device
Vigenère table
German Enigma Machine
121212© 2009 Cisco Learning Institute.
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
3FKTTAW
LNESATAKTANAATCD
The clear text message would be encoded using a key of 3.
1FLANK 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
131313© 2009 Cisco Learning Institute.
Substitution CiphersCaesar Cipher
Cipherered text
3IODQN 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
The clear text message would be encoded using a key of 3.
1FLANK 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
The clear text message would be encrypted as follows using a key of 3.
Clear text
141414© 2009 Cisco Learning Institute.
Cipher Wheel
Cipherered text
3IODQN HDVW
DWWDFN DW GDZQ
The clear text message would be encoded using a key of 3.
1FLANK EAST
ATTACK AT DAWN
Shifting the inner wheel by 3, then the A becomes D, B becomes E, and so on.
2
The clear text message would appear as follows using a key of 3.
Clear text
151515© 2009 Cisco Learning Institute.
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
161616© 2009 Cisco Learning Institute.
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.
171717© 2009 Cisco Learning Institute.
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!
181818© 2009 Cisco Learning Institute.
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
191919© 2009 Cisco Learning Institute.
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 PlaintextUse 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
202020© 2009 Cisco Learning Institute.
Choosing a Cryptanalysis Method
Cipherered text
2IODQN 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.
1The graph outlines the frequency of letters in the English language.
For example, the letters E, T and A are the most popular.
212121© 2009 Cisco Learning Institute.
Defining Cryptology
Cryptography
Cryptology
+
Cryptanalysis
222222© 2009 Cisco Learning Institute.
Cryptanalysis
232323© 2009 Cisco Learning Institute.
Cryptographic Hashes, Protocols,and Algorithm Examples
IntegrityIntegrity AuthenticationAuthentication ConfidentialityConfidentiality
MD5
SHA
HMAC-MD5
HMAC-SHA-1
RSA and DSA
DES
3DES
AES
SEALRC (RC2, RC4, RC5, and RC6)
NIST Rivest
HASH HASH w/Key
Encryption
242424© 2009 Cisco Learning Institute.
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 ArbitraryLength
Fixed-LengthHash Value e883aa0b24c09f
252525© 2009 Cisco Learning Institute.
Hashing Properties
XWhy is x not inParens?
h e883aa0b24c09f
H
(H)Why is H inParens?
= (x)h
Hash Value
Hash Function
Arbitrary length text
262626© 2009 Cisco Learning Institute.
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 changesNo match = Alterations
Internet
I would like to cash this check.
272727© 2009 Cisco Learning Institute.
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
282828© 2009 Cisco Learning Institute.
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
292929© 2009 Cisco Learning Institute.
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.
303030© 2009 Cisco Learning Institute.
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 ArbitraryLength
e883aa0b24c09f
313131© 2009 Cisco Learning Institute.
HMAC Example
Data
HMAC(Authenticated
Fingerprint)
SecretKey
Pay to Terry Smith $100.00
One Hundred and xx/100 Dollars
4ehIDx67NMop9
Pay to Terry Smith $100.00
One Hundred and xx/100 Dollars
4ehIDx67NMop9
Received Data
HMAC(Authenticated
Fingerprint)
Secret Key
4ehIDx67NMop9
Pay to Terry Smith $100.00
One Hundred and xx/100 Dollars
If the generated HMAC matches the sent HMAC, then integrity and authenticity have been verified.
If they don’t match, discard the message.
323232© 2009 Cisco Learning Institute.
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
333333© 2009 Cisco Learning Institute.
Key Management
Key Management
Key Generation
Key Storage
Key Verification
Key Exchange
Key Revocation and Destruction
343434© 2009 Cisco Learning Institute.
Keyspace
DES Key Keyspace # of Possible Keys
56-bit256
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 11111111 111
576,000,000,000,000,000
60-bit
260
11111111 11111111 11111111
11111111 11111111 11111111 11111111 1111
1,152,000,000,000,000,000For 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 asmuch time
Four time asmuch time
353535© 2009 Cisco Learning Institute.
Types of Keys
2242242432112Protection up to 20 years
192192177696Protection up to 10 years
160160124880Protection up to 3 years
HashDigital Signature
Asymmetric Key
Symmetric Key
2562563248128Protection up 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.
363636© 2009 Cisco Learning Institute.
Shorter keys = faster processing, but less secure
Longer keys = slower processing, but more secure
Key Properties
373737© 2009 Cisco Learning Institute.
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)
383838© 2009 Cisco Learning Institute.
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
393939© 2009 Cisco Learning Institute.
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
404040© 2009 Cisco Learning Institute.
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
414141© 2009 Cisco Learning Institute.
Asymmetric Example : Diffie-Hellman
Get Out Your Calculators?
424242© 2009 Cisco Learning Institute.
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.
434343© 2009 Cisco Learning Institute.
Symmetric Encryption Techniques
64 bits 64bits 64bits01010010110010101010100101100101011100101blank blank
0101010010101010100001001001001 0101010010101010100001001001001
Block Cipher – encryption is completed in 64 bit blocks
Stream Cipher – encryption is one bit at a time
Encrypted Message
Encrypted Message
444444© 2009 Cisco Learning Institute.
Selecting an Algorithm
DES 3DES AES
The algorithm is trusted by the cryptographic community
Been replaced by
3DESYes
Verdict is still out
The algorithm adequately protects against brute-force attacks
No Yes Yes
454545© 2009 Cisco Learning Institute.
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
464646© 2009 Cisco Learning Institute.
Block Cipher Modes
DE
S
DE
S
DE
S
DE
S
DE
S
DE
S
DE
S
DE
S
DE
S
DE
S
Initialization Vector
ECB CBC
Message of Five 64-Bit BlocksMessage of Five 64-Bit Blocks
474747© 2009 Cisco Learning Institute.
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
484848© 2009 Cisco Learning Institute.
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
255 keys per second)
4.6 Billion years with current technology
Resource Consumption
Medium
494949© 2009 Cisco Learning Institute.
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.
505050© 2009 Cisco Learning Institute.
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
255 keys per second)
149 Trillion years
Resource Consumption
Low
515151© 2009 Cisco Learning Institute.
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
525252© 2009 Cisco Learning Institute.
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
255 keys per second)
Unknown but considered very safe
Resource Consumption
Low
535353© 2009 Cisco Learning Institute.
Rivest Codes Scorecard
Description RC2 RC4 RC5 RC6
Timeline 1987 1987 1994 1998
Type of Algorithm Block cipherStream cipher
Block cipher Block cipher
Key size (in bits) 40 and 64 1 - 2560 to 2040 bits (128
suggested)
128, 192, or 256
545454© 2009 Cisco Learning Institute.
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
555555© 2009 Cisco Learning Institute.
Using Diffie-Hellman
AliceAlice BobBob
Calc Calc
5566mod 2323 = 88
1. Alice and Bob agree to use the same two numbers. For example, the base numberbase number
gg=55 and 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
565656© 2009 Cisco Learning Institute.
Using Diffie-Hellman
Alice Bob
66
Secret Calc Shared Calc
15155566mod 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.
551515mod 2323 = 1919
191966mod 2323 = 22 881515mod 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
881919
44
56
55,, 2323 55,, 2323
575757© 2009 Cisco Learning Institute.
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
Plaintext
Encryptedtext
Plaintext
Encryption Decryption
EncryptionKey
DecryptionKey
585858© 2009 Cisco Learning Institute.
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’spublic key to encrypt a messageusing an agreed-upon algorithm
Computer A transmits The encrypted messageto Computer B
Computer B usesits private key todecrypt and revealthe message
595959© 2009 Cisco Learning Institute.
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 messagewith her private key
Alice transmits theencrypted messageto Bob
Bob needs to verify that the messageactually came from Alice. He requestsand acquires Alice’s public key
606060© 2009 Cisco Learning Institute.
Asymmetric Key Algorithms
Key length
(in bits)Description
DH512, 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
616161© 2009 Cisco Learning Institute.
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
626262© 2009 Cisco Learning Institute.
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.
636363© 2009 Cisco Learning Institute.
The Digital Signature Process
Confirm Order
Encrypted hash
ConfirmOrder
____________0a77b3440…
SignatureAlgorithm
SignatureKey
Data
Signature Verified
0a77b3440…
VerificationKey
0a77b3440…
Signed Data1
2
3
4
6
Validity of the digital signature is verified
hash
5
The sending device createsa 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 signatureand obtains the public key
Signature is verified with the verificationkey
646464© 2009 Cisco Learning Institute.
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.
656565© 2009 Cisco Learning Institute.
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
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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
676767© 2009 Cisco Learning Institute.
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
686868© 2009 Cisco Learning Institute.
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.
696969© 2009 Cisco Learning Institute.
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:
707070© 2009 Cisco Learning Institute.
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
717171© 2009 Cisco Learning Institute.
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.
727272© 2009 Cisco Learning Institute.
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
737373© 2009 Cisco Learning Institute.
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
747474© 2009 Cisco Learning Institute.
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 EnterpriseNetwork
ExternalWeb Server
InternetMailServer
CiscoSecureACS
CAServer
SSL S/MIME
EAP-TLS
IPsecVPNConcentrator
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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
767676© 2009 Cisco Learning Institute.
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
SignedCertificate
PKCS#7
CA
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Single-Root PKI Topology
• Certificates issued by one CA
• Centralized trust decisions
• Single point of failure
Root CA
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Hierarchical CA Topology
• Delegation and distribution of trust
• Certification paths
Root CA
SubordinateCA
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Cross-Certified CAs
• Mutual cross-signing of CA certificates
CA2CA1
CA3
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Registration Authorities
The CA will sign the certificate request and send it back to the host
1Enrollment request
2
Completed Enrollment Request Forwarded to CA
3
Certificate Issued
RA
CA
Hosts will submitcertificate requeststo the RA
After the RegistrationAuthority adds specificinformation to thecertificate request andthe request is approvedunder the organization’spolicy, it is forwardedon to the CertificationAuthority
818181© 2009 Cisco Learning Institute.
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
33
Alice and Bob request the CA certificatethat contains the CA public key
Each system verifies the validity of the certificate
828282© 2009 Cisco Learning Institute.
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
838383© 2009 Cisco Learning Institute.
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
848484© 2009 Cisco Learning Institute.
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.
858585© 2009 Cisco Learning Institute.