Tài Liệu - Võ Tấn Dũng (votandung) chapter6-crypto

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Chapter 6 Cryptographic System

Objectives

Explain how cryptology consists of cryptography and cryptanalysis and how these concepts apply to modern day cryptography

Explain how securing communications by various cryptographic

methods, including encryption, hashing and digital signatures, ensures confidentiality, integrity, authentication and non-repudiation

Describe the use and purpose of hashes and digital signatures in

providing authentication and integrity

Explain how authentication is ensured

Explain how integrity is ensured

Explain how data confidentiality is ensured using symmetric encryption algorithms and pre-shared keys

Explain how data confidentiality is ensured using asymmetric

algorithms in a public key infrastructure to provide and guarantee

digital certificates

Cryptographic Services

Securing communication

Securing communication

Authentication - Guarantees that the message is not a forgery and does actually come from who it states it comes from

Integrity - Similar to a checksum function in a frame, guarantees that

no one intercepted the message and altered it

Confidentiality - Guarantees that if the message is captured, it cannot

be deciphered

Cryptography

Cryptography is both the practice and the study of hiding information

A cipher is a series of well-defined steps that can be followed as a procedure when encrypting and decrypting messages

Transposition

Substitution

Vernam

Cryptography

Transposition

Cryptography

Subtitution cipher – ceasar cipher

Cryptography

Subtitution cipher- caesar cipher wheel

Subtitution cipher – Vigenere table

Cryptography

Vernam - Teletype Cipher

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

Plaintext message is exclusively ORed with a key tape containing a random stream of data of the same length to generate the cipher

text

After a message was encrypted, the key tape was destroyed

At the receiving end, the process was reversed using an identical key tape to decode the message

Cryptanalysis

Cryptanalysis is the practice and study of determining the meaning of encrypted information (cracking the code), without access to the shared secret key

Cryptanalysis

Cryptology

Cryptology is the science of making and breaking secret codes

Basic Integrity and Authenticity

The hash function hashes

arbitrary data into a

fixed-length digest known as the

hash value, message digest,

digest, or fingerprint

Cryptographic hashes

2 well-known hash functions:

Message Digest 5 (MD5) with 128-bit digests

Secure Hash Algorithm 1 (SHA-1) with 160-bit digests

But

Vulnerable to man-in-the-middle attacks

Does not provide security to transmission

Integrity with MD5 and SHA1

MD5 Algorithm

Is used in a variety of internet applications

Uses a one-way hashing function - easy to compute hash and infeasible to compute data given a hash

Produces a 128-bit hash from a complex sequence of simple binary operations

Integrity with MD5 and SHA1

SHA1- Secure Hash Algorithm

Takes an input message of less than 2^64 bits and 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

Authenticity with HMAC

HMACs use an additional secret key

as input to the hash function

adding authentication to integrity

assurance

The secret key is known to the

sender and receiver and defeats

man-in-the-middle attacks

HMAC is based on existing hash

functions, such as MD5 and SHA-1

Cisco products use hashing for

entity authentication, data integrity,

and data authenticity purposes

Authenticity with HMAC

Key Management

Key length is the measure in bits

Shorter key : less secure, but faster procesing

Longer key: more secure, but slower processing

Keyspace is the number of possibilities that can be generated by a specific key length

Key Management

Key Management

Confidentiality

Encryption

Protect the algorithm

Protect the key

Algorithm are public

Key ensure the secrecy of data

Key are sequence of bits

Two type of encryption algorithm to protect the keys

Symmetric key

Asymmetric key

Encryption

Symmetric encryption algorithms

Shared-secret key algorithms

The usual key length is 80 - 256 bits

A sender and receiver must share a secret key

They are usually quite fast (wire speed) because these algorithms are based on simple mathematical operations

Examples of symmetric encryption algorithms are DES, 3DES, AES, IDEA, RC2/4/5/6, and Blowfish

Encryption

Asymmetric encryption algorithms

Public key algorithms

The usual key length is 512–4096 bits

A sender and receiver do not share a secret key

These algorithms are relatively slow because they are based on difficult computational algorithms

Examples of asymmetric encryption algorithms are RSA, ElGamal, elliptic curves, and DH

Encryption

Encryption

Two criteria when choosing encryption algorithm

trusted by the cryptographic community

adequately protects against brute-force attacks

supports variable and long key lengths and scalability not have export or import restrictions

Data Encryption Standard (DES)

Data Encryption Standard (DES)

Data Encryption Standard (DES)

Data Encryption Standard (DES)

Data Encryption Standard (DES)

Data Encryption Standard (DES)

Data Encryption Standard (DES)

Securing DES

Change key

Use a secure channel to communicate the DES key from the sender to the receiver

Using DES in CBC mode

Testing key to avoid weak key

Data Encryption Standard (DES)

3DES

Data Encryption Standard (DES)

3DES

Data Encryption Standard (DES)

AES is available in the following Cisco VPN devices as an encryption transform:

IPsec-protected traffic using Cisco IOS Release 12.2(13)T and later Cisco PIX Firewall software version 6.3 and later

Cisco ASA software version 7.0 and later

Cisco VPN 3000 software version 3.6 and later

Alternate Encryption Algorithm

SEAL has several restrictions:

The Cisco router and the peer must support IPsec

The Cisco router and the other peer must run an IOS image with k9 long keys (the k9 subsystem)

The router and the peer must not have hardware IPsec encryption

Alternate Encryption Algorithm

Designed all or in part by Ronald Rivest, who also invented MD5 The RC algorithms are widely deployed in many networking

applications because of their favorable speed and variable key-length capabilities

Diffie-Hellman Key Exchange

A method to securely exchange the keys that encrypt data

Encrypt the data using symmetric algorithm and use DH to create key

DH is a mathematical algorithm that allows two computers to

generate an identical shared secret on both systems, without having communicated before

Public Key Cryptography

Asymmetric Encryption

IKE, a fundamental component of IPsec VPNs

SSL, now implemented as IETF standard TLS

SSH

PGP, a computer program that provides cryptographic privacy and authentication and often used to increase the security of email

communications

Asymmetric Encryption

Asymmetric Encryption

Asymmetric Encryption

Asymmetric Encryption

The typical key length is 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

Some well-know asymmetric algorithm

Diffie-Hellman

Digital Signature Standard (DSS)

RSA encryption algorithms

ElGamal

Elliptical curve techniques

Slower than symmetric key

Key exchage or digital signatures

Key management is simpler than

Digital signature

Digital signature can provide the same funtion as handwritten signature and much more

Digital signature

Digital signature

Digital signature

Digital signature

RSA

The RSA algorithm is based on a public key and a private key

used widely in digital signature, e-commerce systems and Internet protocols

The performance problem is the main reason that RSA is typically used only to protect small amounts of data

Public Key Infrastructure (PKI)

PKI is a service framework (hardware, software, people, policies and procedures) needed to support large-scale public key-based

Public Key Infrastructure (PKI)

CA vendors

Public Key Infrastructure (PKI)

PKI usage key or special key : two key pair per entity

One public and private key pair for encryption operations The second pair for digital signature

 Two certificates

PKI Standard

Standardization and interoperability of different PKI vendors is still

an issue when interconnecting PKIs

IETF – X509

Secure web servers: SSL and TLS

Web browsers: SSL and TLS

Email programs: S/MIME

IPsec VPNs: IKE

PKI Standard

The Public-Key Cryptography Standards (PKCS) by RSA laboratory

Certificate Authorities (CA)

CA topologies

Single-root PKI Topology

Hierarchical CA Topology

Cross-certified CA Topology

Certificate Authorities (CA)

CA topologies

Certificate Authorities (CA)

CA topologies

Certificate Authorities (CA)

RA

Authentication of users when they enroll with the PKI

Key generation for users that cannot generate their own keys Distribution of certificates after enrollment

Digital Signature and CA

PKI as the authentication mechanism

Authentication

Nonrepudiation

Easier key management

Long lifetime for the certificates

Disadvantages

A user certificate is compromised (stolen private key)

The certificate of the CA is compromised (stolen private key) The CA administrator makes an error (the human factor)

Digital Signature and CA