Chapter 11: Message Authentication & Hash Functions

Study Notes

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.

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.

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 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.

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.

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.

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.

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.

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.

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 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).

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).