Cookbook for Assignment 4

1
Cookbook for Assignment 4

• william_at_cs.pitt.edu

2
Outline

• Overview of Assign 4
• Concepts and Algorithm
• Implementation Details

3
Overview

• To implement a primitive digital signature system
  using the RSA algorithm
• 3 pieces of program
• RSA Envelop
• Sending and Receiving emulator
• Byte editor to tamper the file content

4
Concepts and Algorithm

• Digital signature system
• To allow the receiver to verify the authenticity
  of the message
• Instead of decrypting the entire message, only
  the signature of the message is being decrypted
• Procedure

"DECRYPTED" Signature
Signature
Signature
yes
DECRYPT
ENCRYPT
Digested

?
No
MESSAGE
MESSAGE
Signature
Digested
5
Concepts and Algorithm

• RSA Algorithm
• The well known algorithm in public key
  cryptography
• E,D,NP,Q?N,(P,Q)??,E, (?,E)?D
• Encryption and Decryption of M
• Encryption C ME mod N
• Decryption M CD mod N

6
Impl. Details

• BigInteger
• probablePrime() //to generate a prime
• subtract(), multiply() //arithmetic ops
• modInverse() //compute D from E and ?
• modPow() //to encrypt and decrypt

7
Impl. Details

• Objects and Bytes I/O
• ObjectOutputStream.writeObject()
• ObjectInputStream.readObject()
• FileInputStream.read()
• FileOutputStream.write()

8
Impl. Details

• MessageDigest

byte contentsnew byte(int)orig_text.length()
MessageDigest m1 MessageDigest.getInstance("M
D5") in.read(contents) m1.update(contents) byte
digest m1.digest()
9
(No Transcript)