Basic Security Cryptography, Concepts and IPsec. Security What do we require from a data security system? Confidentiality –Only the intended recipient

T825 ... Duncan Maidens 1

BasicSecurity

Cryptography, Concepts

and IPsec


Security• What do we require from a data security system?

• Confidentiality– Only the intended recipient can read the data

• Integrity– No one can change data in transit

• Authentication– I am who I say I am

• Non Repudiation– Proof of message sent or received


Encryption

• Solution Encryption

–Take the message (plain text)

–Apply some algorithm to each of the letters (the cipher)

–Generate the encrypted message (cipher text)


Simple Encryption


Method and Key

• Encryption has two major components

–Method

•This is publicly known

–Key

•This is a secret known only to the two parties who are communicating


Simple Encryption• Substitution

– Caesar Cipher – Replace each letter in the plain text by another letter from ‘n’

places up in the alphabet

• Encryption method– Substitution

• Encryption key– N=3

A B C D E Z

D E F G H C

I C A M E I S A W I C O N Q U E R E DL F D P H L V D Z L F R Q T X H U H G

Veni, vidi, vici


Caesar’s 26 keys

0 A B C D E F G H I J K L M N O P

1 B C D E F G H I J K L M N O P Q

2 C D E F G H I J K L M N O P Q R

3 D E F G H I J K L M N O P Q R S

4 E F G H I J K L M N O P Q R S T

5 F G H I J K L M N O P Q R S T U

6 G H I J K L M N O P Q R S T U V

7 H I J K L M N O P Q R S T U V W


Simple Encryption Problems

• Simple encryption systems can be broken by statistical analysis

– The letter ‘e’ occurs 13% in English texts– Distribution statistics exist for all the letters in an

alphabet/language – With the power of computing, easy to try several

alternatives– With Ceasar’s there were only 26 possible keys.


Making it harder to crack

• Polyalphabetic substitution ciphers

–Rotate the key in use

–Means each time a specific letter occurs it may be substituted by a different letter

–Hence it is more complex to analysis


Polyalphabetic substitution









Keyword BED Use 1, 4, 3, 1, 4, 3……



1 4 3 1 4 3 1 4 3 1 4 3 1 4 3 1 4 3 1I C A M E I S A W I C O N Q U E R E DJ G D N I L T E Z J G R O U X F V H E

• Keyword is BED

• Encryption method– Substitution

• Encryption key– 1, 4 , 3 and then repeat











• How to crack this?• Statistical analysis

– Guess the codeword length ..say 4– Analyse every 4th letter for a distribution– If no pattern emerges try every 3rd…. Etc– With the power of computing, easy to try several

alternatives– Again with modern computing we can analyse the

cipher text and crack the code


Other Encryption Systems

Transposition ciphers– Re-order the letters in the message– Use a key to specify the re-ordering sequence


Transposition ciphers

1 2 3 4 5

I C A M E

I S A W I

C O N Q U

E R E D Z

Message :-I CAME I SAW I CONQUERED

Write the message into the box ignoring the spaces.Insert random characters at the end of message to fill grid

Chose the KEY 3, 2, 4, 1, 5Read out the letters in column order

AANE CSOR MWQD IICE EIUZ


Breaking Transposition ciphers

A A N E C S O R M W Q D I I C E E I U Z

2 1 3 RED

2 1 3

Look for common letter groups “THE” “ING” “RED” “CH” at regular spacing e.g. 4

When finding a pattern, look with same spacing for other groups

SAW


Confusion and Diffusion

• Cracking substitution relies on spotting letter frequency and knowing the letters are in the correct position

• Cracking transposition relies on knowing we have the correct letters and moving the letters into the correct place

• When used together to produce a cipher that is ‘virtually impossible’ to crypt analysis

• We break these by brute force– Try all possible keys– Bigger keys (more bits) takes longer to break.


Cipher Machines

• Down the ages cipher machines were developed such as the famous German Enigma machine


Security• Data Encryption Standard (DES)

– Uses substitution and transposition– Initial proposal used 128 bit key– US NSA requested key to be reduced to 56 bits– With enough computing power this can now be broken in less

than 3 hours by brute force attack

• Replaced with Triple DES– Uses cascaded DES encryption

• 3 * 56 bit key = 168 bit key– 2 key version 1013 times stronger then DES


Symmetric encryption

• In a shared key encryption, a single key is used for both encryption and decryption of the data

• Only the trusted parties must know the shared secret key• DES, 3DES, IDEA, and Blowfish are examples of shared key

encryption


Does this encryption system solve all our requirements?• Confidentiality

– Yes

• Integrity– No

• Message may be incorrect • We can use a hash algorithm

• Authentication– Yes

• Assuming no one has stolen the key


Problems with symmetric encryption• Two big problems exist:

–Scalability

•For each secure channel we need a unique pair.

–Key distribution

•How do we distribute the key to the two ends of the link?

•What if I want 1 million secure links?


New Solutions

• Public-Private Keys• Diffie-Hellman Key Exchange

• Developed by Mathematicians who focussed on prime numbers, modular arithmetic and one way functions


Public Key Encryption

• Ron Rivest, Adi Shamir and Lenard Adleman produced a popular encryption method called RSA, using modular mathematics they generated two inter-related keys: – Public key– Private key



• Keys are generated as a pair– Public Key– Private Key

• Given one key, the other key cannot be derived• Data encrypted with one key can only be decrypted with

the other key


Pay to Terry Smith £100.00

One Hundred and xx/100 Pounds

A priA pub

BobAlice

EncryptPay to Terry Smith £100.00

One Hundred and xx/100 Pounds


Decrypt

A pub

KJklzeAidJfdlwiej47DlItfd578MNSbXoE


Does this encryption system solve all our requirements?

• Confidentiality– Yes

• Integrity– No

• Message may be incorrect• Unless we use a hash algorithm

• Authentication– No

• Our public key is publicly available


How to make Authentication with

Public and Private Keys• Use your own private key to encrypt your

‘signature’

–Digital signature

–You’re the only one with your private key

•Hence the message can be authenticated

–The recipient can check your signature by ‘decrypting’ it with your public key


Fhlkdhfakjs asjfa1KJklzeAidJfdlwiej47DlItfd578MNSbXoE

‘signature’From Bob (12)

B pri

A pub

Bob

Encrypt

Pay to Terry Smith £100.00( 12 )

Public Key Encryption with Signatures A

pub

Encrypt

B pub

+


Fhlkdhfakjs asjfa1

KJklzeAidJfdlwiej47DlItfd578MNSbXoE

‘signature’From Bob (12)

A pri

A pub

Alice

Decrypt

Pay to Terry Smith £100.00( 12 )

Public Key Decryption with Signatures

Decrypt

B pub

+

B pub


Message in transit to Alice

Fhlkdhfakjs asjfa1KJklzeAidJfdlwiej47DlItfd578MNSbXoE

A pub

B pri

MESSAGE SIGNATURE

B pubA pri

Message encrypted with Signature encrypted (signed) with

Message decrypted with Apri. Only Alice can do this. Anyone could have encrypted it

Signature decrypted with BpubAnyone with Bpub can do this, but only the one with Bpri key could have signed it


Is this secure?

• Relies on knowing you have Bpub and it really is Bob’s public Key

• Need to be sure so get Bob’s public key from a trusted source

• Key must be signed by someone you already trust• You must have the signer’s public key to check their

signature– A trusted key authority (Certificate Authority)

• This is called a digital certificate


Digital Certificate

Bob’s public Key

AHGAH AJAHJAGJAG AKKUHAKKGK AKAKGGKAKK KGKJG

Signed CA private key


RSA Algorithm and Prime numbers

• Relies on not being able to derive one key from the other.

• Based on Prime numbers and modular arithmetic.


Prime number basics.

• Prime numbers – A whole number that can only be divided by itself and

1.– 2,3,5,7,11,13,17,19,23,…….– Do primes go on to infinity?– Can we use a formulae to calculate the list of prime

numbers?– Is there any pattern to their position in the set of

natural numbers.


Primes in cryptography• Prime numbers and Cryptography

– RSA relies on equations that are easy to compute in one direction, but very difficult in the other.

– For example • 34 * 24 = 816.• What two numbers when multiplied give 816?• Multiple solutions – factorise answer to solve.

– What happens when numbers are primes?• 19*23 = 437.• What two numbers when multiplied give 437?• There is only one solution. How can we find it?• What about the product 1506181? Can you solve it?


Cryptanalysis of RSA • One method of cracking RSA encryption tries to factorise these

very large products of two primes• Understanding the distribution of primes may be the ‘key’ to

solving this problem.• Many number theorists have tried to understand how primes are

distributed.• One such theory that may help is the Riemann Hypothesis

– The hypothesis concerns the distribution of zeros in Riemann’s zeta function.

– The distribution of zeros has been proved to be the same as the distribution of primes.

– Proving this hypothesis and thus understanding the distribution of primes, could lead to being able to crack RSA encryption.


With RSA why do we still use a Symmetric System

• RSA with certificates provides Authentication, Confidentiality, and Integrity– However the algorithm is computationally intensive– Not suitable for bulk or real time encryption

• Need Symmetrical encryption for this.– DES, 3DES and AES can be done in real time– Still need the Shared Secret KEY.


Diffie-Hellman key Exchange

• The Diffie-Hellman algorithm provides a way for two users, Bob and Alice, to establish a shared secret key that only they know– They then use the key for symmetrical encryption (e.g.

3DES)– The shared secret key can be established even

though Bob and Alice are communicating over an insecure channel


Diffie-Hellman Key Exchange

1. Generate large integer p Send p to Peer B Receive q Generate g2. Generate private key XA

5. Generate shared secret number ZZ = YB^ XA mod p

2. Generate private key XB

3. Generate public key YA = g ^ XA mod p

3. Generate public key YB = g ^ XB mod p

4. Send public key YA 4. Send public key YB 5. Generate shared secret number ZZ = YA^ XB mod p

6. Generate shared secret key from ZZ (56-bit for DES, 168-bit for 3DES)

6. Generate shared secret key from ZZ (56-bit for DES, 168-bit for 3DES)

Peer BPeer A

1. Generate large integer q Send q to Peer A Receive p Generate g


Very Simple Guide to Diffie Hellman

(without modular arithmetic)Pub = g Pri Let g = 2Alice BobApri = 3 Bpri = 4

Apub = 2 3 = 8 Bpub = 2 4 = 16Apub = 8 Bpub = 16

Remember it is really Pub = g Pri MOD p

We can’t go back from public key

to work out the private key.

SS= BpubApri

= 163= 4096

SS= ApubBpri

= 84= 4096

SS= ( 2Bpri)Apri

= ( 24)3

SS= 2(Bpri * Apri)

= 212 =4096

SS= ( 2Apri)Bpri

= ( 23)4

Choose Private Keys

Generate public keys

Swap public keys

Compute shared secret key

How does it work?

Both ends have same shared secret key


More Realistic Guide to Diffie Hellman

Pub = g Pri MOD p

g = 5; p = 23

Alice Bob

Apri = 6 Bpri = 15

Apub = 5 6 MOD 23 Bpub = 5 15 MOD 23

Apub = 8Bpub = 19

Remember Pub = g Pri MOD pWe can’t go back !

SS= BpubApri MOD 23 SS= ApubBpr MOD 23 i

SS= 2 SS= 5(6*15) MOD 23 SS= 2SS= 196 MOD 23

SS= 815 MOD 23


Problem

• How can Bob be sure that he is speaking to Alice when running Diffie-Hellman?

• Some sort of authentication is required– Pre-share keys

• Not scalable– Digital certificates

• Needs a certificate authority• Scalable


Virtual Private Networks

• A connection over a network (such as the internet) between two other networks, that makes the two networks operate as if they were connected.

• They need not be encrypted, but often are.– Not encrypted GRE - VPN– Encrypted IPSEC - VPN


Why have VPN’s


IPSEC - VPN

• IPSEC – VPN

–Encrypt the whole IP datagram (possibly header and all)

–Build a tunnel (Embed the packet in a new IP header)

–Send over insure network to other end of tunnel


IPSEC Virtual Private Networks

• Use authentication to establish who is at the other end

• Use Diffie-Hellman to generate shared secret keys

• Use symmetrical system (DES) to encrypt data


VPN usage scenarios


Pulling it all together

IKE – Internet Key Exchange

IKE – Phase 1 – Uses ISAKMP

IKE – Phase 2 – Uses Transform sets

Access list specifies interesting traffic

Crypto map – pulls it all together


IPSEC VPN

• ISAKMP POLICY

–Authenticate

–Agree encryption

–Generate keys (group 1,2 or 5)

• IPSEC transform set

–Bulk encryption and authentication method

–Tunnel / transport mode


IPSEC VPN

• Interesting Traffic for VPN

–Specified by an access list

• Crypto map

–Pulls it all together

–Applied on an interface


See it in action !

fa 0/0

int fa 0/0crypto map MYMAP

crypto isakmp policy 1 authentication pre-share encryption des hash md5 group 1

crypto map MYMAP 10 ipsec-isakmp set peer x.x.x.x set transform-set MYTRANS match address 100set pfs group 2

access-list 100 permit ip SA DA

crypto isakmp key MYKEY address x.x.x.x

crypto ipsec transform-set MYTRANS esp-des

crypto map MYMAP 10 ipsec-isakmp set peer x.x.x.x set transform-set MYTRANS match address 100set pfs group 2

access-list 100 permit ip SA DA

crypto isakmp policy 1 authentication pre-share encryption des hash md5 group 1

crypto isakmp key MYKEY address x.x.x.xcrypto ipsec transform-set MYTRANS esp-desint fa 0/0crypto map MYMAP


Perfect Forward Secrecy

• With no pfs set the keys for IKE phase 2 are derived from the keys in phase 1.

• With pfs set we re-run Diffie Hellman for each set of keys we need in phase 2.


GRE Generic Route Encapsulation

• Why– To allow routing traffic over a VPN by making it unicast

• Could we run unicast with neighbour statements?• Multicast has been supported for IPSEC since 12.24 T• We need it to make the interfaces we wish to neighbour

be the same Subnet . That way they appear directly connected.


R1 and R3 can neighbour.


The data flow with Secure GRE

Tunnel 0

Serial 0

Routed

GRE traffic

IP traffic

IPSEC traffic•CRYPTO MAP HERE•GRE is interesting traffic.


The Encapsulation with Secure GRE

IP GRE IPSEC

Tunnel Interface

Serial Interface


And …

• Multiple VPNs from a single interface.• IPSEC – MANUAL• SSL VPN in CCNA-SEC


The End

Documents

Basic Security Cryptography, Concepts and IPsec. Security What do we require from a data security system? Confidentiality –Only the intended recipient