1. Cryptography – Test Review
CSPrinciples
February 2, 2017

2. History 50 B.C. Julius Caesar uses cryptographic technique
400 A.D. Kama Sutra in India mentions
cryptographic techniques
1250 British monk Roger Bacon
describes simple ciphers
1466 Leon Alberti develops a cipher
disk
1861 Union forces use a cipher during Civil War

3. History 1914 World War I – British, French, and
German forces use encryption
technology
1917 William Friedman, Father of U.S.
encryption efforts starts a school
for teaching cryptanalysis in
Illinois
1917 AT&T employee Gilbert Vernam
invents polyalphabetic cipher
1919 Germans develop the Engima machine
for encryption

4. History 1937 Japanese design the Purple machine for encryption
1942 Navajo windtalkers help with secure
communication during World War II
1948 Claude Shannon develops statistical
methods for encryption/decryption
1976 IBM develops DES
1976 Diffie – Hellman develop public key /
private key cryptography
1977 Rivest – Shamir – Adleman develop the
RSA algorithm for public key / private key

5. Basic Terminologies
Cryptography deals with creating documents that can be shared secretly over public communication channels
Cryptographic documents are decrypted with the key associated with encryption, with the knowledge of the encryptor
The word cryptography comes from the Greek words: Krypto (secret) and graphein (write)
Cryptanalysis deals with finding the encryption key without the knowledge of the encryptor
Cryptology deals with cryptography and cryptanalysis
Cryptosystems are computer systems used to encrypt data for secure transmission and storage

6. Types of Secret Writing
Steganography
Cryptography
Substitution
Transposition
Code
Cipher

7. Basic Terminologies
Steganography is the method of hiding secret messages in an ordinary document
Steganography does not use encryption
Steganography does not increase file size for hidden messages
Example: select the bit patterns in pixel colors to hide the message

8. Steganography
Steganography – covered writing – is an art of hiding information
Popular contemporary steganographic technologies hide information in images
New York Times, August 3rd, 2001

9. Hiding information in pictures
Image in which to hide another image
Image to hide within the other image

10. Basic Terminologies
Keys are rules used in algorithms to convert a document into a secret document
Keys are of two types:
Symmetric
Asymmetric
A key is symmetric if the same key is used both for encryption and decryption
A key is asymmetric if different keys are used for encryption and decryption

11. Caesar Cipher
A substitution cipher where each plaintext letters is replaced by some letter a fixed number spaces down in the alphabet

12. ROT13 Network data encryption / decryption using ROT13 algorithm
Rotates characters by 13 places
‘A’  ‘N’, ‘M’  ‘Z’, ‘a’  ‘n’, ‘m’  ‘z’
Encryption Example:
‘Hello World’ encrypts to ‘Uryyb Jbeyq’
Decryption Example:
‘Uryyb Jbeyq’ decrypts to ‘Hello World’

13. Public Key Cryptography
Private (symmetric, secret) key – the same key used for encryption/decryption
Problem of key distribution
Public (asymmetric) key cryptography – a public key used for encryption and private key for decryption
Key distribution problem solved
Very popular technique:
Large Prime Numbers

14. Transmitting over an insecure channel
Alice wants to send Bob a private message.
Apublic is Alice’s public key.
Aprivate is Alice’s private key.
Bpublic is Bob’s public key.
Bprivate is Bob’s private key.
Netprog: Cryptgraphy

15. Hello Bob, Wanna get together?
Alice
Bob
encrypt using Bpublic
decrypt using Bprivate
Netprog: Cryptgraphy

16. OK Alice, where do we meet?
Bob
decrypt using Aprivate
encrypt using Apublic
Netprog: Cryptgraphy

17. Bob’s Dilemma
Nobody can read the message from Alice, but anyone could produce it.
How does Bob know that the message was really sent from Alice?
Bob may be comforted to know that only Alice can read his reply.
Netprog: Cryptgraphy

18. Alice can sign her message!
Alice can create a digital signature and prove she sent the message (or someone with knowledge of her private key).
The signature can be a message digest encrypted with Aprivate.
Netprog: Cryptgraphy

19. Digital Certificates
Issued by trusted third parties known as Certificate Authorities (CAs)
Verisign is a trusted third party
Used to authenticate an individual or an organization
Digital Certificates are usually given for a period of one year
They can be revoked
It is given at various security levels. Higher the security level, the CA verifies the authenticity of the certificate seeker more.

20. Digital Certificates
Digital Certificates are part of the authentication mechanism. The other part is Digital Signature.
When a user uses the digital signature, the user starts with their private key and encrypts the message and sends it. The receiver uses the sender’s public key and decrypts the message
In traditional encryption, the sender uses the public key of the receiver and encrypts the message and sends it and the receiver decrypts the message with their private key

21. Without Authentication
With Authentication

23. Authentication and Digital Signatures
Preventing impostor attacks
Preventing content tampering
Preventing timing modification
Preventing repudiation
By:
Encryption itself
Cryptographic checksum and hash functions

24. Potential Problems with Cryptographic Technologies?
False sense of security if badly implemented
Government regulation of cryptographic technologies/export restrictions
Encryption prohibited in some countries

25. Sample Encryption Scheme

26. Key