Cryptography

Introduction to Cryptography

Objectives

• A conceptual understanding of secret-key, public-key, and hashing cryptographic algorithms and how they fit into the notion of certificates and how these certificates form the basis of Public Key Infrastructure (PKI).

Why Cryptography

• Scramble data to keep it private

• Detect whether data has been changed

• Provide proof of origin, receipt, transaction..

• Provide digital identities

Cryptographic Functions

• Three types - each with strengths and weaknesses depending upon their intended use.

• Secret-Key (Symmetric) Cryptography - Same shared key

• Public-Key (Asymmetric) Cryptography - Different but related keys

• Hash Function

Secret-Key Cryptography

• Characteristics

Relatively fast

Poses key delivery challenges when faced with large numbers

of senders/receivers

• Popular Implementations

Data Encryption Standard (DES) 56 bit key (plus 8 parity bits)

Triple DES

RSA’s RC2, RC4, RC5

Secret-Key Cryptography

• Examples of usage

Encryption of bulk data and protection of ATM PINs

Public-Key Cryptography

• Characteristics

Slower than secret-key cryptography

Designed to accommodate key delivery and scalability

Key Strengths 512 to 2048 bits

• Popular Implementations

RSA - encryption and digital signature

El Gamal and DSS - digital signatures

Diffie-Hellman -secret-key establishment

Public-Key Cryptography

• Examples of Usage

Authentication - encrypting challenge

Key Distribution - encrypting session key

Hash Algorithms

• Message of length n - Hash Function ------> h (Fixed length, short number

• CharacteristicsEasy to compute

There is no way to get from the hash to the original message, that is any simpler than going though all possible values of the original message and computing the hash for each one

It should be computationally infeasible to find two messages that hash to the same thing.

• Popular implementations - SHA-1, MD2, MD4, MD5

Hash Algorithms

• Examples of UsageSystem can store hashes of passwords instead of the password

itself

Message integrity - send messages and the hash of (message/secret-password)

Digital signature

Digital Signatures

• Alice Signs with her Private Key - emails / file transfer/ floppy ----> Recipient Verifies with Alice’s Public Key

• Definition/Characteristics A digital signature is a number associated with a message, generated

using the private key of the sender

Anyone with Alice’s public key can verify that it is Alice’s signature

Proves the message has not been altered in any way since the signature was applied

Provides non-reputable source of origin

Digital Signatures

• Popular implementations - RSA+MD5, RSA+SHA-1, DSS+SHA-1

• Examples of Usage

Signed mail

Signed code (Java applets, Active X controls)

Hashes and Digital Signatures

Alice’s Plain textSHA-1

hAlice signs the hash (encryptsthe hash with her private key)

Bob h

h

Bob decrypts the signed hash with Alice’s Public key

Bob hashes theplain text to derive the hash

If hashed matchOnly Alice could havesignedPlain text did not change

Signed

Signed

Certificates (x.509 V3)

• Question - Can I trust the Public Key?

• Answer - Yes, if it backed up by some trustworthy authority.

• User CertificatesTrusted Method to Store Public Key

Binds Public Key to User

• CA CertificatesWell Known Public Key for Signature Verification

Delivered with Browser, Server, Mutually installed on Browser / Server

Encryption Can be employed at Different Levels

• Field Level - Client Browser ---> s-HTTP Web Server Status- not supported - being dropped from products

• Message Level - Mail Sender ---> S-MIME /Open PGP - Mail Server - Advantage - Mail is often intended to be kept secret as an entire unit

• Transport Level - Client Browser ---> SSL/TLS - Web Server - Advantage - Useful when a session’s interactions needs to be kept secret

• Datagram Level - Firewall ---> IPSec Firewall - Advantage Encrypted pipe enables multiple sessions to be encrypted with only single IPSec setup overhead

Public-Key Infrastructure (PKI)

Alice BobAlice & BobRequest signing certificateGenerate key pairSend public key to CARequest encryption certificate

CA (Encryption Certificate) CA (Signing Certificate)Generate Key Pair Receive signing public keyBuild certificate Build & return signing certificateReturn private key and certificateSave private key for recoveryPublish certificate with LDAP server

LDAPServer

PKI - ContinuedALice Bob

Signature

LDAPServer

CertificateAuthority

AliceSigns the messageEncrypts the message for Bob usinga generated secret keyEncrypts the secret key with BOB’spublic KeySends mail that includes:The signatureThe encrypted messageThe encrypted secret keyAlice’s signing certificate

PKI - ContinuedALice Bob

Signature

LDAPServer

CertificateAuthority

BOBDecrypts the secret key with hisprivate encryption keyDecrypts the message with the secretkeyVerifies the CA signature on Alice’scertificateChecks with the LDAP server toensure Alice’s signing certificate notrevokedValidates Alice’s signatureEnjoys private communication,assured it’s from Alice

PKI Aspects - Establishing Trust

• Need to establish trust with other CA’s

divisions of a corporation

trading partners

suppliers

competitors

• Trust Models

Flat certificate model

Hierarchical certificate model

Cross-certification

- allows one CA to recognize the authority of another

- CAs certify each other

Link X.500 directories

PKI Aspects - CAs & RAs

• Certificate AuthorityPolicy

Profile

CA key management

Handle exceptions

Support users

• Registration AuthorityAuthenticate applicants

Select attributer

Approve and forward

Request revocation

Help manage policy

Network Security -SSL

• Secure Sockets Layer (SSL)

Cryptographic protocol for protecting digital communication between client & server

Can also be used server to server

Standard, efficient and widely deployed

(Web Servers - Apache, Netscape, Microsoft -IIS …etc)

SSL- How it works

• Negotiate OptionsOne or Two way Authentication

Which Crypto Suite

• AuthenticateExchange Certificate(s)

Validate Certificate(s)

Random Number Challenge(s)

• Distribute KeysGenerate Secret Keys

Exchange Secret Keys

• Flow Secure DataEncryption

Modification Detection

Compression

SSL- How it works

• Client Server (Alice) (Bob)

<-------------------------------->

Negotiate Options

Authenticate

Distribute Keys

Flow Secure Data

<---------------------------------->

History of SSL/TLS

• SSL Version 1 - was quickly replaced by SSL version 2 . Not in use today

• SSL Version 2 - some security problems; 1-way (server) authentication

• Private Communication Technology (PCT) - Microsoft’s response to SSL 2.0. Fixes some problems, but supplanted by SSL V3.

• SSL version 3 - Complete redesign of SSL. Fixed the problems in previous versions and added many features

• Transport Layer Security (TLS) - IETF standard, based on SSL V3, under development

IPSec Overview

• A framework that provides network layer security via protection for IP datagram transit

Origin authentication, integrity, confidentiality, and key management

Basis for virtual private networks (VPNs)

• IPSec support in TCP/IP

Firewall - All connections among IP-Addressable entities

• Involves 3 protocols:

Authentication Header (AH) protocol

Encapsulating security payload (ESP)

ISAKMP/Oakley

IPSec Enables Virtual Private Networks

• Via the internet, the corporate network can be securely extended to include remote nodes/networks

• IPSec enables a secure pipe or tunnel to be established

ISAKMP/Oakley• Internet Security Association and Key Management

Protocol

• IETF Standardized Framework

Negotiate security associations

Crypto key generation/refresh

• Oakley is the mandatory key management protocol within ISAKMP

• Essential for success/growth of VPNs

Secure key exchange is most important element

Automated key distribution, without manual intervention facilitates scalability

IPSec Modes - Tunnel and Transport• IPSec tunnel and transport work together to allow tailoring of

protection: end-to-end and tunnel specific

• Transport modes:

end-to-end

original IP datagram header is used for routing intranet addresses

typical example - exchanges between a server in one intranet and client in another

• Tunnel modes:

tunnel-specific protection

outer IP header created and used for routing internet addresses

typical example - firewall to firewall VPN

IP Authentication Header (AH)

• Two Modes: Transport and Tunnel• Datagram content is clear text• AH provides data integrity and data origin

authentication • Protection for entire datagram• Data integrity is assured by the checksum

generated by a message authentication code (MD5)

Encapsulating Security Payload (ESP)

• Two Modes: Transport and Tunnel

• IP datagram can be encrypted entirely (Tunnel Mode), or payload only (transport mode)

• Also provides data integrity and origin authentication

• ESP’s encryption uses a symmetric shared key