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**Virtual Private Networks**

Fred Baker

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What is a VPN Public networks are used to move information between trusted network segments using shared facilities like frame relay or atm A VIRTUAL Private Network replaces all of the above utilizing the public Internet Performance and availability depend on your ISP and the Internet

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Why?

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HomeNet to the office.

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VPN Types

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VPN Implementations

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VPN as your Intranet

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**What a VPN needs VPNs must be encrypted VPNs must be authenticated**

so no one can read it VPNs must be authenticated No one outside the VPN can alter the VPN All parties to the VPN must agree on the security properties

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VPN Components

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Parts of a VPN

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**VPN works via crypto/Encapsulation**

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**Encryption and Decryption**

Clear-Text Clear-Text Bob Is a Fink Bob Is a Fink 8vyaleh31&d ktu.dtrw8743 $Fie*nP093h Encryption is the masking of secret or sensitive information such that only an authorized party may view (or decrypt) it Encryption Decryption Cipher Text 43 11 43

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**Basic Crypto – Keys are key**

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2 Kinds Key Systems

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**Symmetric Key Algorithms**

DES—56-bit key Triple-DES—encrypt, decrypt, encrypt, using either two or three 56-bit keys IDEA—128-bit key Blowfish—variable-length key, up to 448 bits 15

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**Public Key Encryption Example**

Alice wants to send Bob encrypted data Alice gets Bob’s public key Alice encrypts the data with Bob’s public key Alice sends the encrypted data to Bob Bob decrypts the data with his private key Alice Bob What is encrypted with the public key can only be decrypted with the private key. Anyone can encrypt with someone’s private key but only the intended recipient can decrypt What is encrypted with the private key can only be decrypted with the public key. Only the holder of the private key can encrypt, anyone can decrypt—proof of signature. Message Encrypted Message Message Encryption Decrypt Bob’s Public Key Bob’s Private Key 13 49 49

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PKI vs Symmetric Key PKI easier as you don’t have to manage keys on a per user basis But MUCH more compute intensive (up to 1000 times faster) Many systems do a combination I.e. PGP Use PKI to send a symmetric key Then use the symmetric key to crypto the data

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**Using Crypto in real life**

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**PKI to send Private Keys**

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**PKI Certs a way to authenticate**

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**Prove the user cert Certificates of authority**

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**Digital Signature to verify data not changed in transit**

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PKI the full picture

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Where you do Crypto

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Technologies

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**Application Layer: SSL**

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**Transport Layer: IPSEC**

A standard is composed of: Diffie-Huffman key exchange PKI for the DH exchanges DES and other bulk encryption Hash to authenticate packets Digital Certificates to validate keys

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**Transport Layer: IPSEC VPNs 3 parts**

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**Tunnel vs Transport Transport Tunnel**

Implemented by the end point systems Real address to real address Cannot ‘go through’ other networks Tunnel Encapsulation of the original IP packet in another packet Can ‘go through’ other networks End systems need not support this Often PC to a box on the ‘inside’

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**Diffie-Hellman Key Exchange (1976)**

By openly exchanging non-secret numbers, two people can compute a unique shared secret number known only to them Diffie-Hellman Key Generation By exchanging numbers in the clear, two entities can determine a new unique number known only to them Result is a shared secret Neither party can alone control value Neither party knows—no needs to know—private key of other 45 16 45

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**Modular Exponentiation**

Both g and p Are Shared and Well-Known Generator, g Modulus (prime), p Y = gX mod p Diffie-Hellmann exchange is based upon the concept of modular exponentiation. The prime p denotes a galois field. The numbers are HUGE—e.g, bits 2^ mod 17

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**Diffie-Hellman Public Key Exchange**

Private Value, XA Public Value, YA Private Value, XB Public Value, YB Alice Bob YA =g mod p XA YB = g mod p XB YA D-H exchange is in fact two exponentiations One exponentiation produces a “public value”. Private value is a random (but very large) number Public numbers are exchanged and another exponentiation is performed. Each party derives the same shared secret Susceptible to man-in-the-middle attack YB XA XA XB XB YB mod p = g mod p = YA mod p (shared secret) 46 18 46

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**Security Association is the agreement on how to secure**

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**create the ISAKMP SA (Internet Security Association Key Management Protocol)**

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**IPSEC Key Exchange (IKE)**

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**IKE allows scale as I do not need to hard code passwords for each pair**

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**Link Layer: L2TP for VPDN (Vir Pvt Dial Net)**

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**PPTP: Free from Microsoft**

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PPTP: Security

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VPN Comparisons

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**So why have a private network: QOS not fully cooked**

Very dependent on your ISP Real hard to do across ISPs So no guarantee of performance

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Other Issues

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Like Nat

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**Wireless: a new big driver, WAS (Work At Starbucks)**

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**Many security protocols, depends on deployer**

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**VPN means I don’t care how you connect**

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Example

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**So what could be wrong? VPN clients hit the network stack**

May not play well with personal firewalls Or other software May not need full access to the target network just encrypted access

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**One answer: clientless VPN**

Use SSL as the transport protocol to an appliance Can add NT authentication to the appliance Clientless mode: Use web enabled applications over the Internet, the appliance SSLifies web sites Java Applet: Use an downloadable applet to send traffic over SSL, get more support for applications. Can work well if you want to have encrypted web based apps without redoing the application to use SSL you need certs and have to change EVERY link to HTTPs Also big hit on the server cpu

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**Summary: VPNs Very big in the work access space Wireless**

Exploit High speed Wireless in the office public ‘hot spots’ like Borders Replaces direct dial into the work network Replace dedicated Business partners May replace the corporate WAN

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