cs 356 – lecture 3 cryptographic toolsgersch/cs356/356lecture03.pdfcs 356 – lecture 3...

CS 356 – Lecture 3Cryptographic Tools

Fall 2013

Tuesday, September 3, 13

Today’s Plan• Homework # 1 is due by the time I finish presenting the next slide.

• Homework #2 is posted on web and RamCT. Submit it on RamCT by start of class next Tuesday.– note: this one’s got 7 questions and they are harder to answer, so do

them early. You may need to look some things up.

• Finish Symmetric Key Crypto

• Explain Public Key Crypto

• Explain Message Digests and Authentication

• Feedback form hand-out; return at end of class– goal: improve the class and tune it to your style of learning

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Did I mention the HOMEWORK is due NOW!?!!!

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FINISH SYMMETRIC ENCRYPTION


Data Encryption Standard (DES)

the most widely used encryption scheme

• FIPS PUB 46• referred to as the Data Encryption Algorithm (DEA)

• uses 64 bit plaintext block and 56 bit

strength concerns:• concerns about algorithm

• DES is the most studied encryption algorithm in existence

• use of 56-bit key• Electronic Frontier Foundation (EFF)

announced in July 1998 that it had broken a


•Figure 2.2 Time to Break a Code (assuming 106 decryptions/ms) The graph assumes that a symmetric encryption algorithm is attacked using•a brute-force approach of trying all possible keys


Triple DES (3DES) repeats basic DES algorithm three times using

either two or three unique keys first standardized for use in financial applications

in ANSI standard X9.17 in 1985 attractions:

168-bit key length overcomes the vulnerability to brute-force attack of DES

underlying encryption algorithm is the same as in DES drawbacks:

algorithm is sluggish in software uses a 64-bit block size


Advanced Encryption Standard (AES)

needed a replacement for 3DES

• 3DES was not reasonable for long term use

NIST called for proposals for a new AES in 1997

• should have a security strength equal to or better than 3DES• significantly improved efficiency• symmetric block cipher• 128 bit data and 128/192/256 bit keys

selected Rijndael in November 2001

• published as FIPS 197


DES, 3DES, and AES

•Comparison of Three Popular Symmetric Encryption Algorithms


Practical Security Issues


Practical Security Issues typically symmetric encryption is applied to a unit

of data larger than a single 64-bit or 128-bit block



of data larger than a single 64-bit or 128-bit block electronic codebook (ECB) mode is the simplest

approach to multiple-block encryptioneach block of plaintext is encrypted using the same key cryptanalysts may be able to exploit regularities in the

plaintext



of data larger than a single 64-bit or 128-bit block electronic codebook (ECB) mode is the simplest

approach to multiple-block encryptioneach block of plaintext is encrypted using the same key cryptanalysts may be able to exploit regularities in the

plaintext

modes of operationalternative techniques developed to increase the security of

symmetric block encryption for large sequencesovercomes the weaknesses of ECB


Block Cipher Concepts1. Divide (plaintext) Data Into Fixed Blocks

• DES divides message into 64 bit blocks2. Apply The Algorithm to Each Block

• Input is block and symmetric key• Output is a block of encrypted data

3. Transmit the Encrypted Block4. Decrypt the Block

• Input is block and symmetric key• Output is a block of decrypted data


Block Cipher Encryption

Stream Encryption


Block & Stream CiphersBlock Cipher• processes the input one block of elements at a time• produces an output block for each input block• can reuse keys• more common

Stream Cipher• processes the input elements continuously• produces output one element at a time• primary advantage is that they are almost always faster and use far less code


Public Key Encryption Pioneers Diffie and Hellman

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Public-Key Encryption Structure


Public-Key Encryption Structure

publicly proposed by

Diffie and Hellman in

1976

based on mathematical

functions

asymmetric

• uses two separate keys• public key and private key• public key is made public for others to use

some form of protocol is needed for distribution


RSA algorithm: Ron Rivest, Adi Shamir & Len Adleman

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Fun with Prime Numbers

• Can you find the prime factors of 33? – (you should have learned in the fifth grade....)

• Can you find the prime factors of 6784578994572859509348579856944875935693048176987467439593085670934573849570497394857039475039475639838574896704932487589457489784867882112834380380009792837643846856785764987698664531?

• A BIG PRIME NUMBER (e.g. 100 digits) times another BIG PRIME is really really BIG. And practically impossible to factor to get the prime numbers back again.

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Remember our Number Game last lecture?

• Using prime numbers 3 and 11– 3 x 11 = 33

• Public Key based on 2 numbers, 7 and 33: encrypt as n^7 mod 33

• Private key based on 3 and 33: decrypt as n^3 mod 33

• Not very secure, not very practical (can’t encrypt large numbers). So use really BIG prime numbers.

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Public Key Encryption


Public Key Authentication


Requirements for Public-Key Cryptosystems

computationally easy to create

key pairs

computationally easy for sender knowing public key to encrypt

messages

computationally easy for receiver

knowing private key to decrypt

ciphertextcomputationally

infeasible for opponent to determine

private key from public key

computationally infeasible for opponent to otherwise

recover original message

useful if either key can be used

for each role


Public Key Algorithms• RSA (Rivest, Shamir, Adleman)

– developed in 1977– only widely accepted public-key encryption algorithm– given tech advances need 1024+ bit keys. 2048 bits now

common.

• Diffie-Hellman key exchange algorithm– only allows exchange of a secret key

• Digital Signature Standard (DSS)– provides only a digital signature function with SHA-1

• Elliptic curve cryptography (ECC)– new, security like RSA, but with much smaller keys


Table 2.3

• Applications for Public-Key Cryptosystems


Practical Application: Encryption of Stored Data

• common to encrypt transmitted data• much less common for stored data

– which can be copied, backed up, recovered• approaches to encrypt stored data:

– back-end appliance– library based tape encryption– background laptop/PC data encryption


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Cryptography is like magic fairy dust,we just sprinkle it on our protocols and its makes everything secure


Recent Headline in MIT Technology Review

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There is no magic fairy dust


Digital Signatures


Digital Signaturesused for authenticating both source and

data integrity



data integritycreated by encrypting hash code with

private key



data integritycreated by encrypting hash code with

private keydoes not provide confidentiality

even in the case of complete encryptionmessage is safe from alteration but not

eavesdropping


Message Authentication

protects against active attacks

verifies received message is authentic• contents have not been altered• from authentic source• timely and in correct sequence

can use conventional encryption• only sender & receiver share a key


Message Authentication Codes


Secure Hash Functions


Hash Function Requirements


Hash Function Requirements• can be applied to a block of data of any size• produces a fixed-length output• H(x) is relatively easy to compute for any given x• one-way or pre-image resistant

– computationally infeasible to find x such that H(x) = h

• second pre-image resistant or weak collision resistant– computationally infeasible to find y ≠ x such

that H(y) = H(x)• collision resistant or strong collision resistance

– computationally infeasible to find any pair (x, y) such that H(x) = H(y)


Security of Hash Functions


Security of Hash Functions there are two approaches to attacking a

secure hash function:cryptanalysis

exploit logical weaknesses in the algorithm

brute-force attack strength of hash function depends solely on the length of the hash

code produced by the algorithm

SHA most widely used hash algorithm

additional secure hash function applications:passwords

hash of a password is stored by an operating system intrusion detection

store H(F) for each file on a system and secure the hash values


Message Authentication

Using a One-Way

Hash Function


Public Key Certificates


Digital Envelopes

protects a message without needing to first arrange for sender and receiver to have the same secret key

• ***equates to the same thing as a sealed envelope containing an unsigned letter


Random Numbers

keys for public-key algorithms

stream key for symmetric stream cipher

symmetric key for use as a temporary session key or in creating a digital envelope

handshaking to prevent replay attacks

session key• Uses include generation of:


Random Numbers

• random numbers have a range of uses• requirements:• randomness

– based on statistical tests for uniform distribution and independence

• unpredictability– successive values not related to previous– clearly true for truly random numbers– but more commonly use generator


Pseudorandom verses Random Numbers

• often use algorithmic technique to create pseudorandom numbers– which satisfy statistical randomness tests– but likely to be predictable

• true random number generators use a nondeterministic source– e.g. radiation, gas discharge, leaky

capacitors– increasingly provided on modern

processors


Summary• introduced cryptographic algorithms• symmetric encryption algorithms for

confidentiality• message authentication & hash

functions• public-key encryption• digital signatures and key management• random numbers


Thursday• You will be receiving LOGIN credentials from the DETER

project in your student email sometime this week. For now, just login and change your password.– If you don’t get it by Thursday, inform Pinalkumar

• Project 1 will be posted by Thursday. It is due on September 26th. – 3 weeks looks like a lot of time. It isn’t. No, really!– Don’t count on doing it at the last minute. There are only 20

systems at DETER reserved for 50 students. That means that only 20 of you can login at one time to run the experiment. If you wait, you are likely going to be shut out!!!

• We’ll be starting chapter 3 in the text.

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cs 356 – lecture 3 cryptographic toolsgersch/cs356/356lecture03.pdfcs 356 – lecture 3...

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