Slide an toàn và hệ thống bảo mật thông tin chapter 7 cryptographic hash funtions

.c
om

cu

u

du
o

ng

th

an

co

ng

CRYPTOGRAPHIC HASH FUNTIONS

CuuDuongThanCong.com

/>

.c
om

Contents
Hash function

2)

Applications of Cryptographic Hash Functions

3)

Simple Hash Functions

4)

Requirements and Security

5)

Secure Hash Algorithm (SHA)

cu

u

du
o

ng

th

an

co

ng

1)

CuuDuongThanCong.com

/>

.c
om

1. Hash function

cu

u

du
o

ng

th

an

co

value, or message digest: h= H(M)

ng

 A hash function maps a variable-length message into a fixed-length hash

CuuDuongThanCong.com

/>

.c
om

1. Cryptographic hash function.
 A cryptographic hash function is an algorithm for which it is

co

ng

computationally infeasible to find either a data object that maps to a pre-

cu

u

du
o

ng

th

an

specified hash result or two data objects that map to the same hash result

CuuDuongThanCong.com

/>

.c
om

2. Applications Of Cryptographic Hash Functions

ng

 Message Authentication

cu

u

du
o

ng

th

an

co

 Digital signature

CuuDuongThanCong.com

/>

.c
om

Message Authentication
 Message authentication is a mechanism or service used to verify the

co

ng

integrity of a message. Message authentication assures that data received

cu

u

du
o

ng

th

an

are exactly as sent

CuuDuongThanCong.com

/>

cu

u

du
o

ng

th

an

co

ng

.c

om

Message Authentication

CuuDuongThanCong.com

/>

.c
om

Digital signature
 The hash value of a message is encrypted with a user’s private key.

co

ng

 Anyone who knows the user’s public key can verify the integrity of the

cu

u

du
o

ng

th

an

message that is associated with the digital signature

CuuDuongThanCong.com

/>

.c
om
ng
co
an
th
ng
du
o
u
cu
CuuDuongThanCong.com

/>

.c
om

Other Applications

ng

 Hash functions are commonly used to create a one-way password file

cu

u

du
o

ng

th

an

co

 Hash functions can be used for intrusion detection and virus detection

CuuDuongThanCong.com

/>

.c
om

3. Simple Hash Functions

ng

 Input (message, file,etc.) is viewed as a sequence of n-bit blocks

co

 One of the simplest hash functions is the bit-by-bit exclusive-OR (XOR)

cu

u

du
o

ng

th

an

of every block. This can be expressed as:

CuuDuongThanCong.com

/>

cu

u

du
o

ng

th

an

co

ng

.c
om

4. Requirements And Security

CuuDuongThanCong.com

/>

cu

u

du
o

ng

th

an

co

ng

.c
om

5. Secure Hash Algorithm (SHA)

CuuDuongThanCong.com

/>

cu

u

du
o

ng

th

an

co

ng

.c
om

SHA-512

CuuDuongThanCong.com

/>

.c
om

SHA-512
 Step 1 Append padding bits: The padding consists of a single 1 bit

co

ng

followed by the necessary number of 0 bits

th

an

 Step 2 Append length: A block of 128 bits is appended to the message.

ng

This block is treated as an unsigned 128-bit integer and contains the length

cu

u

du
o

of the original message (before the padding).

CuuDuongThanCong.com

/>

.c
om

SHA-512
 Step 3 Initialize hash buffer: A 512-bit buffer is used to hold intermediate

co

ng

and final results of the hash function. The buffer can be represented as

an

eight 64-bit registers (a, b, c, d, e, f, g, h). These registers are initialized to

cu

u

du
o

ng

th

the following 64-bit integers (hexadecimal values):

CuuDuongThanCong.com

/>

.c
om

SHA-512

cu

u

du
o

ng

th

an

co

ng

 Step 4 Process message in 1024-bit (128-word) blocks:

CuuDuongThanCong.com

/>

.c
om

SHA-512
 Each round also makes use of an additive constant, where indicates one of

cu

u

du
o

ng

th

an

co

ng

the 80 rounds.

CuuDuongThanCong.com

/>

cu

u

du
o

ng

th

an

co

ng

.c
om

SHA-512

CuuDuongThanCong.com

/>

cu

u

du
o

ng

th

an

co

ng

.c
om

SHA-512: Word Expansion

CuuDuongThanCong.com

/>

.c
om

SHA-512
 Step 5 Output: After all 1024-bit blocks have been processed, the output

cu

u

du
o

ng

th

an

co

ng

from the Nth stage is the 512-bit message digest.

CuuDuongThanCong.com

/>

.c
om
ng
co
an
th
ng
du
o
u
cu
CuuDuongThanCong.com

/>