Information and Computer Security - PowerPoint Presentation

Slide1

Information and Computer Security CPIS 312 Lab 9

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MAC & HASH FUNCTION

TRIGUI Mohamed SalimSlide2

To know what hashing is forPractice how to implement MD cipher

2Lab

ObjectivesSlide3

Cryptographic hash function is another type of cryptographic algorithm.A (one-way) hash function takes variable length input and produces a fixed length output called hash value. Also known as “message digest” or digest. The hash function ensures that if the information has changed, an entirely different output value will be produced.

What is a Hash FunctionSlide4

Hash Function maps any message of any length, to an element in a different set.2 different messages could map to the same value

Uses of hash functions are with digital signatures and for data integrity.

What is a Hash FunctionSlide5

Common hash algorithmsSlide6

Message digest algorithms take a message of arbitrary size and create a digest of fixed size.The algorithm takes the message and splits it into blocks of equal length (the block size of the algorithm)The last block is padded, with a total message length attached

Each block is sent through the function in order.After all blocks are processed, the fixed digest value is retrieved

Message

Block 0

Block 1

Block 2

…

H(x)

Digest Value

<2

64

Block n

Technical Definition of MDAsSlide7

7MAC is an algorithm that requires the use of a secret key.MAC takes a variable-length message and a secret key as input and produces an authentication code.

Typically, MAC are used between two parties, say Alice and Bob, that share a secret key

K in order to validate information transmitted between these parties.When Alice has a message to send to Bob, she calculates the MAC as a function of the message and the key:

MAC = C(K, M)

where M=input message, C=MAC function, K=shared secret key.

Message authentication code MACSlide8

8Message authentication code MACSlide9

Standardized secure hash function that uses an input message and secret to compute a message authentication Code (MAC).Algorithm characteristics: nonreversible, collision resistant, avalanche effect (slight change in the input will cause a significant change in the MAC output.

Highly secure and easy to implement Technical Definition of SHA-1Slide10

10Alice sends to Bob a document as well as a MAC. Bob can authenticate who sent the document by performing the same MAC on the document and comparing his MAC to the one that Alice sent. If they match, he knows that Alice sent the document.

diamond icon represents a comparison process

Message authentication code MACSlide11

11Java Package: javax.crypto Java Class : Mac

Methods: getInstance(), init

(), update(), doFinal().Algorithms: HMAC (Hashed MAC)

MAC in JAVASlide12

12Java package: java.security

Java class: MessageDigestMethods: getInstance(), reset(), update(), digest().Algorithms:

MD5, SHA, SHA-1MD in JAVASlide13

13MessageDigest Class: A MessageDigest object starts out initialized.

The data is processed through it using the update methods. Once all the data to be updated has been updated, one of the digest

methods should be called once to complete the hash computation. After digest has been called, the MessageDigest object is reset

to its initialized state.

Ex:

MessageDigest

test =

MessageDigest.getInstance

("SHA-1");

test.update(data1);

// data1 is a byte array that holds the original massage

byte[] msgDigest = test.digest();

test.reset();

test.update(data2);

....

MD in JAVASlide14

14Alternative classes for computing a message digest on a file:

DigestInputStream and DigestOutputStream

Java pakage: java.securityDigestInputStream class:

To complete the message digest computation, call one of

read

methods. Then call one of the

digest

methods on the associated message digest .

int

read()

: Reads a byte, and updates the message digest and then return an integer value of the byte that it read.

Ex:

FileInputStream

in = new

FileInputStream

("MD.txt");

MessageDigest

md

=

MessageDigest.getInstance

("MD5");

DigestInputStream

digestIn

= new

DigestInputStream

(in,

md);

MD in JAVASlide15

15Java pakage:

java.securityDigestOutputStreamTo complete the message digest computation, call one of the digest methods on the associated message digest after that call one of the

write methods. void write(byte[] b) : Updates the message digest using the specified array, and in any case writes the array to the output stream.

Ex:

MessageDigest

md

=

MessageDigest.getInstance

("MD5");

FileOutputStream

out = new

FileOutputStream

("MDout.txt");

DigestOutputStream

dout

= new

DigestOutputStream

(out,

md

);

MD in JAVA