Bài giảng Cryptography and Network Security - Chapter 4, Part B: Message authetication codes - Nguyễn Đức Thái
Cryptography and Network Security
Chapter 4 – Part B
Message Authetication Codes
Lectured by
Nguyễn Đức Thái
Outline
. Message Authentication Requirements
. Message Authentication Functions
. Basic Use of MACs
. MACs based on Hash Functions: HMAC
2
Message Authentication
. Message authentication is a mechanism or service
used to verify the integrity of a message.
. Message authentication assures that data received
are exactly as sent by (i.e., contain 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.
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Message Authentication
. A message authentication code (MAC) is an
algorithm that requires the use of a 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
. One way: a MAC is to combine a cryptographic hash
function in some fashion with a secret key
. Another way: to use a symmetric block cipher in
such a way that it produces a fixed-length output for
a variablelength input
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Message Authentication Requirements
. Disclosure
. Sequence modification
• Release of message contents
• Any modification to a
to any person or process not
sequence of messages
possessing the appropriate
between parties, including
cryptographic key
insertion, deletion, and
reordering
. Traffic analysis
. Timing modification
• Discovery of the pattern of
traffic between parties
• Delay or replay of
messages
. Masquerade
. Source repudiation
• Insertion of messages into
the network from a
• Denial of transmission of
fraudulent source
message by source
. Content modification
. Destination repudiation
• Changes to the contents of a
message, including insertion,
deletion, transposition, and
modification
• Denial of receipt of
message by destination
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Message Authentication
. Message authentication is a procedure to verify that
received messages come from the alleged source
and have not been altered.
. Message authentication may also verify sequencing
and timeliness
. A digital signature is an authentication technique
that also includes measures to counter repudiation
by the source.
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Message Authentication Functions
• Hash function
Two levels of
• A function that maps a
message of any length into a
fixed-length hash value
which serves as the
Lower level
authenticator
• There must be some sort of function
that produces an authenticator
• Message encryption
• The ciphertext of the entire
message serves as its
authenticator
• Message authentication code
(MAC)
• A function of the message
and a secret key that
produces a fixed-length
value that serves as the
authenticator
Higher-level
•Uses the lower-level function as a primitive in an
authentication protocol that enables a receiver to
verify the authenticity of a message
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Message Encryption
8
Internal and External Error Control
TCP Segment
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Public Key Encryption
. The straightforward use of public-key encryption
provides confidentiality but not authentication
. To provide both confidentiality and authentication, A
can encrypt M first using its private key which
provides the digital signature, and then using B’s
public key, which provides confidentiality
. Disadvantage is that the public-key algorithm must
be exercised four times rather than two in each
communication
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Basic Uses of MAC
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Requirements for MAC
The final
requirement
dictates that the
authentication
algorithm should
not be weaker
with respect to
certain parts or
bits of the
The first requirement
deals with message
replacement attacks,
in which an opponent
is able to construct a
new message to
match a given MAC,
even though the
opponent does not
know and does not
learn the key
The second
requirement
deals with the
need to thwart a
brute-force
Taking into account
the types of attacks,
the MAC needs to
satisfy the following:
attack based on
chosen plaintext
message than
others
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Brute-Force Attacks
. Requires known message-tag pairs
• A brute-force method of finding a collision is to pick
a random bit string y and check if H(y) = H(x)
Two lines of attack:
•Attack the key space
•If an attacker can determine the MAC key then it is possible to
generate a valid MAC value for any input x
•Attack the MAC value
•Objective is to generate a valid tag for a given message or to find a
message that matches a given tag
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Cryptanalysis
. Cryptanalytic attacks seek to exploit some property
of the algorithm to perform some attack other than
an exhaustive search
. An ideal MAC algorithm will require a cryptanalytic
effort greater than or equal to the brute-force effort
. There is much more variety in the structure of MACs
than in hash functions, so it is difficult to generalize
about the cryptanalysis of MACs
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MACs based on Hash Functions: HMAC
. There has been increased interest in developing a
MAC derived from a cryptographic hash function
. Motivations:
• Cryptographic hash functions such as MD5 and SHA
generally execute faster in software than symmetric block
ciphers such as DES
• Library code for cryptographic hash functions is widely
available
. HMAC has been chosen as the mandatory-to-
implement MAC for IP security
. Has also been issued as a NIST standard (FIPS 198)
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Summary
. Message Authentication Requirements
. Message Authentication Functions
. Basic Use of MACs
. MACs based on Hash Functions: HMAC
17
References
1. Cryptography and Network Security, Principles
and Practice, William Stallings, Prentice Hall,
Sixth Edition, 2013
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