Bài giảng Cryptography and Network Security - Chapter 4, Part A: Cryptographic hash functions - Nguyễn Đức Thái
Cryptography and Network Security
Chapter 4 – Part A
Cryptographic Hash Functions
Lectured by
Nguyễn Đức Thái
Outline
. Cryptographic Hash Functions
. Message Authentication
. Attacks on Hash Functions
• Brute-Force Attacks
• Cryptanalysis Attacks
. Secure Hash Algorithm (SHA)
2
Hash functions
. A hash function maps a variable-length message into
a fixed-length hash value, or message digest
. A hash function H accepts a variable-length block of
data as input and produces a fixed-size hash value
h = H(M)
. The principal object of a hash function is data
integrity
3
Cryptographic Hash functions
. The kind of hash function needed for security
applications is referred to as a cryptographic hash
function.
. A cryptographic hash function is an algorithm for
which it is computationally infeasible
. Because of these characteristics, hash functions are
often used to determine whether or not data has
changed
4
Cryptographic Hash functions
5
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 (i.e., contain no modification,
insertion, deletion, or replay).
. When a hash function is used to provide message
authentication, the hash function value is often
referred to as a message digest.
6
Hash Functions & Msg Authentication
7
Message Authentication – Picture a)
. The message plus concatenated hash code is
encrypted using symmetric encryption.
. Because only A and B share the secret key, the
message must have come from A and has not been
altered.
. The hash code provides the structure or redundancy
required to achieve authentication.
. Because encryption is applied to the entire message
plus hash code, confidentiality is also provided
8
Message Authentication – Picture b)
. Only the hash code is encrypted, using symmetric
encryption.
. This reduces the processing burden for those
applications that do not require confidentiality
9
Message Authentication – Picture c)
. It is possible to use a hash function but no
encryption for message authentication.
. The technique assumes that the two communicating
parties share a common secret value S.
. A computes the hash value over the concatenation
of M and S and appends the resulting hash value to.
. Because B possesses, it can recompute the hash
value to verify.
. Because the secret value itself is not sent, an
opponent cannot modify an intercepted message
and cannot generate a false message.
10
Message Authentication – Picture d)
. Confidentiality can be added to the approach of
method (c) by encrypting the entire message plus
the hash code
11
Hash Functions & Digital Signatures
12
Hash Functions & Dig. Signatures (1/2)
. The hash code is encrypted, using public-key
encryption with the sender’s private key.
. It also provides a digital signature, because only the
sender could have produced the encrypted hash
code.
. In fact, this is the essence of the digital signature
technique.
13
Hash Functions & Dig. Signatures (2/2)
. If confidentiality as well as a digital signature is
desired, then the message plus the private-key-
encrypted hash code can be encrypted using a
symmetric secret key.
14
Other Hash Functions Uses
. Hash functions are commonly used to create a one-way
password file.
• Thus, the actual password is not retrievable by a hacker who gains
access to the password file.
• This approach to password protection is used by most operating
systems.
. Hash functions can be used for intrusion detection and virus
detection.
• Store H(F) for each file on a system and secure the hash values (e.g.,
on a CD-R that is kept secure).
• One can later determine if a file has been modified by recomputing
H(F).
• An intruder would need to change F without changing H(F).
. Can be used to construct a pseudorandom function (PRF) or
a pseudorandom number generator (PRNG).
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Hash Functions Requirements
16
Attacks on Hash Functions
. Brute-Force attacks
• Preimage and second preimage attacks
• Collision resistant attacks
. Cryptanalysis attacks
17
Brute-Force Attacks
. A brute-force attack does not depend on the specific
algorithm but depends only on bit length.
. In the case of a hash function, a brute-force attack
depends only on the bit length of the hash value.
. A cryptanalysis, in contrast, is an attack based on
weaknesses in a particular cryptographic algorithm.
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Preimage & Second Preimage Attacks
. For a preimage or second preimage attack, an
adversary wishes to find a value such that H(y) is
equal to a given hash value.
. The brute-force method is to pick values of y at
random and try each value until a collision occurs.
. For an m-bit hash value, the level of effort is
proportional to 2m
. Specifically, the adversary would have to try, on
average, 2m-1 values of y to find one that generates a
given hash value h.
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Collision Resistant Attacks
. For a collision resistant attack, an adversary wishes
to find two messages or data blocks, x and y, that
yield the same hash function: H(x) = H(y).
. In essence, if we choose random variables from a
uniform distribution in the range 0 through N – 1,
then the probability that a repeated element is
encountered exceeds 0.5 after N1/2 choices have
been made
. Thus, for an m-bit hash value, if we pick data blocks
at random, we can expect to find two data blocks
with the same hash value within 2m/2 attempts
20
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