1. Cryptography
Ch-1
prepared by:
Diwan

2. Essential Terms Secret writing Cryptography Encryption Decryption
Plain text  Cipher text
Decryption
Cipher text  Plain text
Cryptanalysis
Cryptology
Secret writing
Source:

3. 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

4. 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

5. Basic Terminologies Examples: Symmetric key methods
DES 56-bit
Triple DES bit
AES bit and higher
Blowfish bit and higher
Asymmetric key methods
RSA
PGP

6. Basic Terminologies Plaintext is text that is in readable form
Ciphertext results from plaintext by applying the encryption key
Notations:
M message, C ciphertext, E encryption, D decryption, k key
E(M) = C
E(M, k) = C
Fact: D(C) = M, D(C, k) = M

7. Cryptographic Methods
Symmetric
Same key for encryption and decryption
Key distribution problem
Asymmetric
Mathematically related key pairs for encryption and decryption
Public and private keys
Cryptographic Algorithms generally fall into one of two different categories, or are a combination of both.
Symmetric
Fast
Only provide confidentiality
Need secure channel for key distribution
Key management headaches from large number of key pairs to maintain N(N-1)/2
That’s over 6.3 million key pairs to let all 3556 Purdue A/P staff members exchange encrypted messages
To do the same for all students would require over half a billion key pairs!
Examples: DES, AES, Blowfish, RC4, RC5
Asymmetric
Large mathematical operations make it slower than symmetric algorithms
No need for out of band key distribution (public keys are public!)
Scales better since only a single key pair needed per individual
Can provide authentication and nonrepudiation
Examples: RSA, El Gamal, ECC, Diffie-Hellman

8. Cryptographic Methods
Hybrid
Combines strengths of both methods
Asymmetric distributes symmetric key
Also known as a session key
Symmetric provides bulk encryption
Example:
SSL negotiates a hybrid method
A hybrid cryptosystem is the best of both worlds. In this case, an asymmetric encryption scheme is used to transmit a generated symmetric key to the other party, then that key is used for all further communications.
This combines the scalability and key management features of the asymmetric algorithms with the speed of symmetric ones. The Secure Sockets Layer (SSL) protocol negotiates which asymmetric and symmetric algorithms to use in a hybrid system to protect TCP connections, such as an HTTP connection between a web browser and web server.

9. Types of Secret Writing
Steganography
Cryptography

10. 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

11. Steganography Hiding a message within another medium, such as an image
No key is required
Example
Modify color map of JPEG image
Invisible ink, hidden tattoos, and microdots are all examples of steganography.
By taking a color digital image and slightly altering the color of each pixel, you can hide a message in the image without noticeably altering the appearance. The receiver can then extract the message if they have the original, unaltered image.

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

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

14. Types of Cryptography Stream-based Ciphers Block Ciphers
One at a time, please
Mixes plaintext with key stream
Good for real-time services
Block Ciphers
Amusement Park Ride
Substitution and transposition
Stream Ciphers are fast and easy to implement in hardware.
Block ciphers are stronger, but slower and often implemented in hardware.

15. Types of Secret Writing
Steganography
Cryptography
Substitution
Transposition
Mon alphabetic
Polyalphabetic

16. Encryption Systems Substitution Cipher Transposition Cipher
Convert one letter to another
Cryptoquip
Transposition Cipher
Change position of letter in text
Word Jumble
Monoalphabetic Cipher
Caesar
Polyalphabetic Cipher
Vigenère
Substitution Cipher: Replacing one letter with another
Transposition Cipher: World Jumble. Rearranging or reordering the letters within a message
Monoalphabetic Cipher: Algorithm that substitutes one letter in the ciphertext alphabet for one in the plaintext alphabet

17. 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

18. Currently Available Crypto Algorithms (private key)
DES (Data Encryption Standard) and derivatives: double DES and triple DES
IDEA (International Data Encryption Standard)
Blowfish
RC5 (Rivest Cipher #5)
AES (Advance Encryption Standard)

19. Currently Available Crypto Algorithms (public key)
RSA (Rivest, Shamir, Adleman)
DH (Diffie-Hellman Key Agreement Algorithm)

20. Hashing Algorithms MD5 SHA-1 Computes 128-bit hash value
Widely used for file integrity checking
SHA-1
Computes 160-bit hash value
MD-5 is based on MD-4 and was created to address vulnerabilities found in MD-4. MD5 generates 128-bit hash values over 512-bit blocks in 4 rounds of 16 steps each.
SHA-1 also operates on 512-bit blocks, but produces a 160-bit hash value in 4 rounds of 20 steps each.