Message Authentication and Hash Functions in Network Security, Slides of Cryptography and System Security

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Network

Security

(Message

Authentication

Hash

Functions)

Message

Authentication

Message authentication is a mechanism used to verify the integrity of the message.  Message authentication assures that data received are exactly as sent(contains no modification, insertion, deletion, or replay) and that the purported identity of the sender is valid.  Symmetric encryption provides authentication among those who share the secret key.  Encryption of the message by a sender’s private key also provides a form of authentication.

MAC

A

MAC

is an algorithm that requires the use of secret key. 

A

MAC

takes a variable

length message and a secret key as input and produces an authentication code. 

A

recipient in possession of the secret key can generate an authentication code to verify the integrity of the message.

Hash

functions

A

hash function maps a variable

length message into a fixed length hash value, or message digest.  For message authentication, a secure hash function must be combined in some fashion with a secret key.

Authentication

Requirements

 Content modification: Changing the content of the message, including insertion, deletion etc.  Sequence modification: Any modification to sequence of messages between parties including insertion, deletion.  Timing modification: Delayed or replay of messages.  Source repudiation: Denial of transmission of message by source.  Destination repudiation: Denial of receipt of message by destination.

Authentication

Requirements

The first two attacks deal with message confidentiality. 

to

deal with message authentication.  And last two deal with digital signatures.

Message

Encryption

Message encryption can itself provide measure of authentication. The analysis differ for

 Symmetric  public ‐ key encryption schemes

Symmetric

Encryption

Symmetric

Encryption

We can say that symmetric encryption provide both confidentiality and authentication.  Consider what is happening at

B

Given decryption function

D

B

will accept any cipher text

X

and produce

Y.

If

X

is the cipher text of legitimate message then Y is some cipher text

otherwise meaning less sequence of bits.  So if

Y

is legitimate plaintext therefore must have come from

A.

Symmetric

Encryption

 For example we are transmitting English language message using caeser cipher with a shift of one (K=1) .A sends the following legitimate cipher text.  Decrypts to produce the following plaintext  A Simple Frequency analysis confirms that message has the profile of ordinary English.  On the other hand if opponent generates the following random sequence of letters.

Symmetric

Encryption

 Append an error correcting code or a frame check sequence(FCS) to each message before encryption.  A prepares plaintext message and provide this input to the function F that produce an FCS.  FCS is appended to M and entire block is encrypted.  At destination B decrypts the incoming block and treat the result as a message with appended FCS.  B applies the same function F to attempt to reproduce the FCS.  If the calculated FCS is equal to the incoming FCS, then message is considered authentic.  It is unlikely that any random sequence of bits would exhibit the desired relationship.

Symmetric

Encryption

Public

Key

Encryption

Message

Authentication

Code

 Authentication technique involves the use of secret key to generate a small fixed ‐ size block of data, known as cryptographic checksum or MAC that is appended to message.  Assume the two parties say A and B, share a common secret key K. when A has a message to send to B , it 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  MAC = message authentication code