CrytographyChapter 8

Cryptology

Cryptography Comes from Greek Kryptos meaning “hidden” Grahein meaning “to write” Process of making and using codes to

secure the transmission of information

Cryptanalysis Process of obtaining the original

message form encrypted message

Cryptology

Encryption Process of converting an original

message into a form that is unreadable to unauthorized individuals

Decryption Process of converting the encrypted

message (cipertext) into an easily read message (plain text)

Basic Definitions

Algorithm Programmatic steps to encrypt message

Cipher Encryption method or process

Ciphertext or cryptogram Encrypted message

Code Process of converting unencrypted

components into encrypted components

Basic Definitions

Decipher Convert to plaintext

Encipher To encrypt

Key or crypto-variable Information used with the algorithm to

encrypt

Key-space Entire range of values that can possibly be

used to construct an individual key

Basic Definitions Link encryption

Series of encryptions /decryptions between a number of systems

Plaintext or clear text The original message

Steganography Process of hiding messages

Work factor Amount - effort required to perform

cryptanalysis

Cipher methods

Bit stream method Each bit in the plaintext is transformed

bit by bit Most common use XOR

Block cipher method Messaged divided into blocks Each block is encoded Substitution, transposition, XOR or

combination

Substitution Cipher

Substitute one value for another 3 character substitution to the right

Original alphabet: ABCDEFGHIJKLMNOPQRSTUVWXYZ

Encrypted alphabet:

DEFGHIJKLMNOPQRSTUVWXYZABC

Simple by itself – powerful when combined with other operations

Substitution Cipher

Polyalphabetic substitution

Orig: 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

Sub1: 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

Sub2: 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

Sub3: 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

Sub4: 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

Sub5: 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

Using this technique what is? ZTPG

3 shift to the right is know as the Caesar Cipher

Vigenere Cipher

Implemented using the Vigenere Square

26 distinct cipher alphabets Header row – normal order Each additional row – shift right Start in first row and find a substitution

for one letter Move down the rows for each

subsequent letter of plaintext

Transposition Cipher

Simply rearranges the values within a block Can be done at the bit level or the byte

level

Key pattern: 1 4, 2 8, 3 1, 4 5,5 7,

7 6,8 3 Julius Caesar was associated with the early

version of this cipher also Larger blocks or keys makes cipher stronger

Exclusive OR (XOR)

Function of Boolean algebra Two bits are compared If identical, result is binary 0 If not identical, result is binary 1 Very simple encryption Not very secure

Vernam Cipher

One-time pad Uses a set of characters only one time for

each encryption process Each character of the plaintext is turned

into a number and a pad value for that position is added

Sum is then converted back to a cipher text

Decryption requires knowledge of pad values or difficult cryptanalysis.

Book or Running Key Cipher

Cipher text List of codes representing a

page number, line number, and word number of the plaintext word.

Must know which book was used

Hash functions

Not an encryption methodology Mathematical algorithm -generates a

message summary or digest Fingerprinting Used to determine if it is the same

message Not used to decypher Message always provide same hash

value if unaltered

Hash functions

Do not require keys Uses Message Authentication

Code (MAC) Key-dependent Used in password verification

systems

Secure Hash Standard (SHS)

Secure algorithm Standard issued by National Institute of

Standards and Technology (NIST) SHA-1 Produces a 160 bit digest Family of SHA SHA-256

A 256-bit cipher algorithm Creates a key - encrypting the intermediate

hash value with the message block functioning as the key

Cryptographic Algorithms

Symmetric and asymmetric and hybrid Distinguished by the types of keys they

use Symmetric Encryption

Requires the same secret key

Encryption methods use mathematical operations

Both the sender and receiver must have the secret key

Primary challenge – getting key to receiver

Symmetric Encryption Cryptosystems Data Encryption Standard (DES)

Key length of 128 bits

64-bit block size

56-bit key

Too weak

Triple DES (3DES)

Advanced Encryption Standards Used by federal agencies other than

national defense

Declassified, publicly disclosed, royalty-free

Uses block cipher, variable length block, key length of 128, 192, or 256

Asymmetric Encryption

Uses two different but related keys Either key can encrypt or decrypt Must use other to perform other function One key private One key public Also know as public key encryption Based on one-way functions One is simple to compute , the opposite

is complex

Asymmetric Encryption

Based on hash value Uses mathematical trapdoor

Secret mechanism that enable you to easily accomplish the reverse function in a one-way function.

Public key becomes the true key Private is derived form public key

using trapdoor

Public Key

RSA (Rivest-Shamir-Adlemann) First public key encryption algorithm Published for commercial use E-commerce browsers Has become the de facto standard

Encryption Key Size

Cryptovariable or key size must be decided Length of key increase the number of

random selections to be guessed Length of key influences strength The security of encrypted data is NOT

dependent on keeping the algorithm secret Depends on keeping some or all of the

elements of the keys secret See table on page 367

Public Key Infrastructure (PKI)

Integrated system of software, encryption methodologies, protocols, legal agreements, and 3rd part services

Based on public key Include digital certificates and

certificate authorities

Digital Certificates

Public key container files that allow computer program to validate the key and identify to whom it belongs.

Allows integration of key characteristics to be integrated into business practices Authentication Integrity Privacy Authorization Non-repudiation

Digital Certificate

Used by third party Certifies the authenticity of the Digital signature is attached

Certify that file is from the entity that it claims to be Has not been modified

Certificate authority Software agent Manages the issuance of certificates Serves as the electronic notary pubic Verify the certificates worth and integrity

PKI

Common implementation Systems to issue digital certificates

to users and servers Directory enrollment Key issuing systems Tools for managing the key issuance Verification and return cetificates

Digital Signatures

Created to verify information transferred using electronic systems

Currently asymmetric encryption processes are used to create digital signatures

Encrypted messages that can be mathematically authentic

Used when using DSS (digital Signature Standard)

Digital Signatures

Process Create a message digest using the hash

Input into the digital signature algorithm along with a random number to be used for generating the digital signature

Depends upon the sender’s private key and other info provided by the CA

Verified by the recipient through use of the sender’s public key

Hybrid Cryptography Systems

Pure asymmetric keys encryption is not widely used except in digital certificates

More widely used as part of hybrid system

Diffie-Hellman Key Exchange method Exchanging private keys using public key

encryption Asymmetric encryption is used to exchange

session keys Limited use keys Temporary communications

Steganography

Process of hiding information Not technically a form of cryptography Most popular version

Hiding information within files that appear to contain digital picture or other images

Use one bit per color or 3 bits per pixel to store information.

Compute files that don’t use all available bits

Protocols for Secure Communication

Secure Socket Layer (SSL) Used public key encryption to secure channel Support by most internet browsers Client and server establish HTTP session Client requests access part of web site - requires

secure communications Server sends message to client Client respond - sending its public key & security

parameters Server finds a public key match Sends a digital certificate to the client Client must verify - digital certificate –received,

valid & trustworthy Lasts for duration of session

Protocols for Secure Communication

SSL Two layers of protocol

SSL Record Protocol Compression, encryption and attachment of

SSL header

Received encrypted messages are decrypted and reassembled

Basic security at top level of SSL protocol stack

Standard HTTP Internet communication services

Protocols for Secure Communication

S-HTTP (Secure Hypertext Transfer Protocol) Extended version of hypertext

transfer protocol Provides for encryption of individual

messages between client and server No session Designed for sending individual

messages

Securing E-mail

Secure Multipurpose Internet mail Extensions (S/MIME) Adds encryption of MIME (Multipurpose

Internet Mail Extensions)

PEM Uses 3DES symmetric key encryption and

RSA for key exchanges and digital signature

PGP Pretty Good Privacy Used IDEA Cipher