Information and
Computer Security
CPIS 312 Lab 9
MAC & HASH FUNCTION
TRIGUI Mohamed Salim
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Lab Objectives
1.
2.
To know what hashing is for
Practice how to implement MD cipher
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What is a Hash Function
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 Function



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.
Common hash algorithms
Technical Definition of MDAs





Message digest algorithms take
Message
<264
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 Block 0 Block 1 Block 2 … Block n
of the algorithm)
The last block is padded, with a
total message length attached
H(x)
Each block is sent through the
function in order.
After all blocks are processed,
the fixed digest value is
retrieved
Digest Value
Message authentication code MAC
MAC 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.
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Message authentication code MAC
K
Message
M
Sender A
Message
M
MAC
algorithm
Transmit
MAC’
MAC
MAC
algorithm
MAC
Compare
MAC
Cryptographic
Checksum
Receiver B
K
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Technical Definition of SHA-1

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

Message authentication code MAC

Alice 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
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MAC in JAVA
Java Package: javax.crypto
 Java Class : Mac
 Methods: getInstance(), init(),
update(), doFinal().
 Algorithms: HMAC (Hashed MAC)

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MD in JAVA


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Java package: java.security
Java class: MessageDigest
Methods: getInstance(), reset(),
update(), digest().
Algorithms: MD5, SHA, SHA-1
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