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Published on January 12, 2008

Author: Doride

Source: authorstream.com

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Slide1:  INTRODUCTION to CRYPTOGRAPHY Fred Piper Codes & Ciphers Ltd 12 Duncan Road, Richmond Surrey, TW9 2JD ENGLAND Royal Holloway, University of London Egham Hill, Egham Surrey TW20 0EX ENGLAND Some Security Issues:  Sender Am I happy that the whole world sees this ? What am I prepared to do to stop them ? What am I allowed to do to stop them ? Recipient Do I have confidence in : the originator the message contents and message stream no future repudiation. Network Manager Do I allow this user on to the network ? How do I control their privileges ? Some Security Issues Cipher System:  Cipher System cryptogram c Enciphering Algorithm Deciphering Algorithm Key k(E) Key k(D) message m message m Interceptor The Attacker’s Perspective:  The Attacker’s Perspective Deciphering Algorithm Unknown Key k(D) Known c Wants m Note: k(E) is not needed unless it helps determine k(D) Two Types of Cipher System:  Two Types of Cipher System Conventional or Symmetric k(D) easily obtained from k(E) Public or Asymmetric Computationally infeasible to determine k(D) from k(E) Slide6:  Mortice Lock. If you can lock it, then you can unlock it. Bevelled Sprung Lock. Anyone can lock it, only keyholder can unlock it. Types of Attack:  Types of Attack Ciphertext only Known plaintext Chosen ciphertext Assumptions About Attacker (1):  Assumptions About Attacker (1) Military/Government: Try to keep details of system (including algorithm) secret Worst Case Conditions: Commercial: Assume he knows: System (including algorithm) All ciphertext Some corresponding plaintext/ ciphertext Warning:  Warning THE FACT THAT AN ALGORITHM HAS BEEN PUBLISHED SAYS NOTHING ABOUT ITS STRENGTH. Breaking Algorithm:  Breaking Algorithm Finding a method of determining message from cryptogram without being given deciphering key. Exhaustive Key Search Attacks:  Exhaustive Key Search Attacks The security depends on the secrecy of the deciphering key. One potential attack, if the algorithm is known, is to try all possible deciphering keys and to eliminate all incorrect ones. To withstand this type of attack a large key space is required. Exhaustive Key Searches:  Exhaustive Key Searches Estimating time required for key search requires assumptions about the attacker’ resources Saints or Sinners ?:  Saints or Sinners ? Receiver Interceptor Sender Who are the ‘good’ guys ? If Someone Wants Your Plaintext:  If Someone Wants Your Plaintext Give it to them Give them the decryption key They may break algorithm They may ‘find’ plaintext in system They may ‘find’ key in system Practical Considerations when Implementing Encryption:  Practical Considerations when Implementing Encryption Key management is the difficult part Keys need to be generated, distributed, stored, changed securely History shows that most cryptanalytic attacks exploit poor implementation and/or key management Example: Enigma in World War 2 Cryptography is used to provide::  Cryptography is used to provide: 1. Confidentiality 2. Data Integrity 3. Entity/Origin Verification 4. Non-Repudiation 5. Access Control Choosing an Algorithm:  Choosing an Algorithm The choice of algorithm depends upon the application. Applications of encryption include : Data confidentiality Data integrity Digital Signatures. Misuse of Encryption:  Misuse of Encryption Grade Good student xxxxx Bad student xxxxx Grades can be changed Classification of Techniques:  Classification of Techniques Bit / Block operation Positional dependence/independence Message dependence/independence Vernam Cipher:  Vernam Cipher Random sequence k1,k2,…,kn Message m1,m2,…,mn + Ciphertext k1m1,k2 m2,…,kn mn The message and key are bit strings Stream Cipher:  Stream Cipher Plaintext data Keystream sequence Ciphertext Key Sequence Generator XOR Stream Cipher:  Stream Cipher Enciphers bit by bit Positional dependence Security depends on properties of the keystream Stream Ciphers:  Stream Ciphers Applications Widely used for military and paramilitary applications for both data and digitised speech The main reason for their wide use is that military communications are often over poor channels and error propagation is unacceptable Symmetric Block Cipher System:  Symmetric Block Cipher System Key dependent permutation on s-bit blocks s-bit plaintext block s-bit ciphertext block Key Block Ciphers : Key Sizes:  Block Ciphers : Key Sizes Depends on security requirement Key searches on size 290 are currently considered infeasible DES: Key Search on Internet (1997):  DES: Key Search on Internet (1997) DES has 256 keys DES key found Search took 140 days Search used over 10,000 computers Peak rate: 7.109 keys/sec ‘Might’ have taken 32 days DES Breaker (1998):  DES Breaker (1998) Electronic Frontier Foundation Design cost $ 80,000 Manufacturing cost $130,000 Test key found in 56 hours Complete search in 220 hours 90 Billion keys per second Design details published DES : Double Length Key:  DES : Double Length Key k = (k1,k2) k1,k2 DES keys Ek(m) = Ek1(Dk2(Ek1(m))) key is 112 bits key search with 2112 trials is infeasible. Advanced Encryption Standard (AES):  Advanced Encryption Standard (AES) Block ciphers Block size 128 bits Key lengths 128, 192, 256 bits Must be faster than triple DES AES (Continued):  AES (Continued) June 1998: 15 candidates August 1998: 11 April 1999: 5 Decision October 2000 Rijndael Slide31:  The following slides will not be discussed but are included for completeness Applications:  Applications Access Control Authentication The Challenge / Response Principle:  The Challenge / Response Principle Key Key Random number Challenge PIN-Controlled A A Response A - Encipher or OWF Y/N = ? Digital Signatures:  Digital Signatures According to ISO, the term Digital Signature is used: ‘to indicate a particular authentication technique used to establish the origin of a message in order to settle disputes of what message (if any) was sent’. Digital Signatures:  Digital Signatures A signature on a message is some data that validates a message and verifies its origin a receiver can keep as evidence a third party can use to resolve disputes. It depends on the message a secret parameter only available to the sender It should be easy to compute (by one person only) easy to verify difficult to forge Principle of Digital Signatures:  Principle of Digital Signatures There is a (secret) number which: Only one person can use Is used to identify that person ‘Anyone’ can verify that it has been used NB: Anyone who knows the value of a number can use that number. Certification Authority:  Certification Authority Aim : To guarantee the authenticity of public keys. Method : The Certification Authority guarantees the authenticity by signing a certificate containing user’s identity and public key with its secret key. Requirement : All users must have an authentic copy of the Certification Authority’s public key. Certification Process:  Certification Process Verifies credentials Creates Certificate Receives (and checks) Certificate Presents Public Key and credentials Generates Key Set Distribution Centre Owner How Does it Work?:  How Does it Work? The Certificate can accompany all Fred’s messages The recipient must directly or indirectly: Trust the CA Validate the certificate The CA certifies that Fred Piper’s public key is……….. Electronically signed by the CA Fundamental Requirement:  Fundamental Requirement Internal infrastructure to support secure technological implementation

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