DEPARTMENT OF COMPUTER TECHNOLOGY,

YCCE, NAGPUR

VIII-SEMESTERNETWORK SECURITY

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UNIT NO.4

B. L. PARNEAsst. Prof.

CT Department

UNIT 3:- SYLLABUS

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Message Authentication and Hash Function1

Authentication Requirement2

Authentication Function Code3

Hash Functions and it’s Security4

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SHA-1, RIPEMD-160, HMAC5

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Digital Signature Standard, Authentication Protocol 6

Background :

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The cryptography systems that we have studied so far provide secrecy, or confidentiality, but not integrity.

However, there are occasions where we may not even need secrecy but instead must have integrity.

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Background :

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• Message authentication is a mechanism or

service used to verify the integrity of message.

• Message authentication assures that data

received are exactly as sent by (no

modification, insertion, deletion or reply)

sender.

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Document and Fingerprint :

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• One way to preserve the integrity of a document is through the use of a fingerprint. If Alice needs to be sure that the contents of her document will not be changed, she can put her fingerprint at the bottom of the document.

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Message and Message Digest :

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• The electronic equivalent of the document and fingerprint pair is the message and digest pair.

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Difference :

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• The two pairs (document / fingerprint) and (message

/ message digest) are similar, with some differences.

• The document and fingerprint are physically linked

together.

• The message and message digest can be unlinked

separately, and, most importantly, the message

digest needs to be safe from change.

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Checking Integrity :

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• Run the cryptographic hash function again and compare the new message digest with previous one.

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Cryptographic Hash Function Criteria :

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• Pre-image resistance: Means that it must be extremely hard for intruder to create any message from the digest.

• Second Pre-image resistance: Ensures that if intruder has a message and the corresponding digest, he should not be able to create second message whose digest is same as the first.

• Collision Resistance: Ensures that intruder cannot find two messages that hash to same digest.

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Authentication Requirements :

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• Disclosure

• Traffic analysis

• Masquerade

• Content modification

• Sequence modification

• Timing modification

• Source repudiation

• Destination repudiation

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Authentication Function:

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• Message Encryption : The ciphertext of the entire

message serves as its authenticator.

• Message Authentication Code : A function of the

message and a secret key that produces a fixed

length value that serves as authenticator.

• Hash Function : A function that maps a message of

any length into a fixed length hash value, which

serves as the authenticator.

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Message Encryption:

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• Symmetric (Private key) Encryption

• Asymmetric (Public key) Encryption

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Message Authentication :

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• A message digest does not authenticate the sender of the message.

• To provide message authentication, Alice needs to provide proof that it is Alice sending the message and not an impostor.

• The digest created by a cryptographic hash function is normally called a modification detection code (MDC).

• What we need for message authentication is a message authentication code (MAC).

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Modification Detection Code :

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Modification Authentication Code :

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The security of a MAC depends on the security of the underlying hash algorithm.

Security of MAC :

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• Suppose Eve has intercepted the message M and the

digest h(K/M). There are three possible way to forge

a message without knowing the secret key :

– If the size of key allow exhaustive search.

– By applying preimage attack until Eve will find X such

that h(X) is equal to MAC she has intercepted.

– Given some pairs of messages and their MAC’s, Eve can

manipulate them to come up with new message and its

MAC.

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Nested MAC :

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HMAC Design Objective :

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• To allow easy replace-ability of the embedded hash

function in case faster or more secure hash function

are found or required.

• To use and handle keys in a simple way.

• To preserve the original performance of the hash

function without incurring a significant degradation.

• To use without modifications, available hash

functions.

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HMAC :

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CMAC :

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References Books:

Cryptography and Network Security Principles and Practices , 4/e, by William Stallings (Pearson Edu Asia).

Cryptography and Network Security, 2/e by Behrouz A. Forouzan, The McGraw-Hill Publication.

Networks Security Essentials, Applications and Standards,4/e by William Stalling (Pearson Edu).

https://www.dropbox.com/s/467ms83kfdy9f90/Cryptography%20and%20Network%20Security%20Principles%20and%20Practices%2C%204th%20Ed%20-%20William%20Stallings.pdf

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