Chapter 11: Message Authentication & Hash Functions

Chapter 11: Message Authentication and Hash Functions

• Message authentication is a process to verify messages originate from the claimed source and haven't been altered. It can also verify message sequencing and timeliness.

Authentication Requirements

• Types of attacks in network communications include disclosure, traffic analysis, masquerade, content modification, sequence modification, timing modification, source repudiation, and destination repudiation.
• Confidentiality protects against disclosure and traffic analysis.
• Message authentication defends against content modification, sequence modification, timing modification, source repudiation, and destination repudiation.

Authentication Requirements

• Digital signatures are authentication techniques designed to prevent repudiation by a source.
• The goal of digital signatures is authenticating and verifying documents to safeguard against tampering, modification, or forgery.

Authentication Functions

• Authentication mechanisms can be categorized as two levels:
  • Lower level functions produce an authenticator (a value used to authenticate a message).
  • Higher level authentication protocols use lower-level functions as primitives.
• Three types of functions that can produce an authenticator are:
  • Message encryption - ciphertext itself serves as the authenticator.
  • Message authentication codes (MACs) - these are short pieces of information that authenticate messages by confirming their origin and ensuring they haven't been altered. A MAC function takes a message and a secret key as input to produce a fixed-length value.
  • Hash functions- a function mapping a message of any length into a fixed-length hash value to serve as the authenticator.

Message Encryption

• Symmetric encryption can be used for authentication and confidentiality.
• It needs a recognizable plaintext to distinguish it from random bits(like ASCII text).
• Public-key cryptography is different; it uses mathematically linked key pairs.

Basic Uses of Message Encryption

• Symmetric encryption guarantees confidentiality by sharing a secret key between sender and receiver, thereby only A and B can decrypt it. It doesn't prevent the possibility of receiver forgeries.
• Public-key encryption provides confidentiality (uses recipients public key to encrypt, and private decryption key for the receiver.). It doesn't offer authentication in the same way.

Symmetric Encryption

• A message transmitted from A to B using a shared secret key ensures only A and B can decrypt it.
• If no one else has the key, then confidentiality is guaranteed.
• B is certain the message originated from A because only A holds the key. This also authenticates the message.

Internal vs External Error Control

• Append error detecting codes(FCSs or checksums) to the message before encryption.
• Data frames may get corrupted in transit.
• FCS bits are added to the frame before transmission, and then compared at the receiving end. If matches, the frame is valid; otherwise, it's discarded.

Message Authentication Code (MAC)

• A shared secret key is used to generate a fixed-size block of data (called a MAC) appended to the message.
• This ensures message integrity and that it originated from the claimed sender, as both parties perform the same computation.
• This technique works well when only authenticity is required.

Theory of Operation MAC

• When sender A sends a message to B, it calculates the MAC using a shared secret key and the message.
• The receiver B performs the same calculation and compares the results to the received MAC. A match confirms the message's integrity and origin.

Hash Functions

• Hash functions convert variable-sized messages into fixed-sized hash codes (message digests). Hashing does not require a key. It's primarily useful for verifying message integrity.
• It can be used for authentication (e.g., with encryption or a shared key).
• There are methods for authentication including hashing with encryption, a MAC, and using a signed hash (digital signatures):
  • M || E(H(M)): Equivalent to internal error control.
  • M || signed H: Typical digital signature.
  • M || H(M || K): Keyed hash (no encryption).

Why Use MACs?

• Cleartext can remain clear if encrypting as a whole.
• MACs may be cheaper or sufficient for some use cases.
• Authentication may be needed in cases where executable code persists longer than the encryption.
• A MAC does not provide a digital signature (because the same key is used by both sender and receiver).