密码学课件1(英文)

– no matter how much computer power or time is available, the cipher cannot be broken since the ciphertext provides insufficient information to uniquely determine the corresponding plaintext
[Syllabus]
IA Discussion
• Tell me what you know about IA • IA tenets? • Attacks and attack trees? • Risk tradeoffs?
Some Math
• You will need to know some math • Mainly algebra and arithmetic
Cryptographic System
• A cryptographic system is characterized by:
– type of encryption operations used
• substitution / transposition / product
– number of keys used
From the Greek words Kryptos (hidden or secret) and gráphō (write) writing The modern term is concerned with developing algorithms to, e.g.,
• Conceal the context of some message from all except sender and receiver (Privacy or Secrecy) • Verify the correctness of a message to the receiver (authentication) • Form the basis of many technological solutions to computer and communication sec to share a key?
Protocol Failure
• Protocol failure or a mechanism failure occurs when a mechanism fails to meet the goals for which it was intended
– Where an adversary gains the advantage by not breaking the algorithm but by manipulating the protocol or mechanism directly
• Example: traffic analysis
Terminology Continued
• cryptography - study of encryption principles/methods • cryptanalysis (codebreaking) - study of principles/ methods of deciphering ciphertext without knowing key • cryptology - field of both cryptography and cryptanalysis
History of Ciphers
• Cryptography has been around for centuries
– It has a history of at least 4000 years, when Egyptians enciphered some hieroglyphics on monuments.
Week 11 Web Security & Email
Week 3 Hashing
Week 9 Identity
Week 12 Wireless &IPsec Week 4 Public Key Week 13 Other Systems Week 5 Digital Signatures
What is Cryptography?
Cryptanalysis
• Usually, the main objective is to recover the key, and not just the message • Cryptanalytic attack
– Uses the nature of the algorithm, plaintext, and ciphertext
– Just that the underlying security ultimately depends on just a small amount of hidden information
Introduction - Cryptographic Protocols
• A cryptographic protocol is a distributed algorithm defined by a sequence of steps precisely specifying the actions required of two or more entities to achieve a specific security objective • Protocols play a major role in cryptography and are essential in meeting IA goals
Some Basic Terminology
• plaintext - original message • ciphertext - coded message • cipher - algorithm for transforming plaintext to ciphertext • key - info used in cipher known only to sender/receiver • encipher (encrypt) - converting plaintext to ciphertext • decipher (decrypt) - converting ciphertext to plaintext
Applied Cryptography IA 5120
Week 1 Lecture Introduction
Dr. Joe J. Rushanan
Adapted from Professor Agnes Chan and Travis Mayberry
Outline for today
• Administrivia • IA discussion • Some math • Terminology • Basics of ciphers • Classic ciphers
• computational security
– given limited computing resources (e.g. time needed for calculations is greater than age of universe), the cipher cannot be broken
56
256 = 7.2 × 1016
255 µs
= 1142 years
10.01 hours
128
2128 = 3.4 × 1038
2127 µs
= 5.4 × 1024 years
5.4 × 1018 years
168
2168 = 3.7 × 1050
2167 µs
= 5.9 × 1036 years
• Brute-force attack
– Try everything
Cryptanalytic Attacks
• ciphertext only • known plaintext
– only know algorithm & ciphertext, is statistical, know or can identify plaintext – know/suspect plaintext & ciphertext – select plaintext and obtain ciphertext – select ciphertext and obtain plaintext – select plaintext or ciphertext to en/decrypt
Key Size (bits) Number of Alternative Keys Time required at 1 decryption/µs Time required at 106 decryptions/µs
32
232 = 4.3 × 109
231 µs
= 35.8 minutes
2.15 milliseconds
• chosen plaintext
• chosen ciphertext • chosen text
Brute Force Search
• Always possible to simply try every key • Effort is proportional to key size • Assume either know or can recognize plaintext
5.9 × 1030 years
26 characters (permutation)
26! = 4 × 1026
2 × 1026 µs
= 6.4 × 1012 years
6.4 × 106 years