Crytography
Chapter 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:
ABCDEFGHIJKLMNOPQRSTUVWXYZ
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