Secure Hash Algorithm

Secure Hash Algorithm ( SHA )
 Secure Hash Algorithm (SHA) was developed
by NIST along with NSA.
 In 1993, SHA was published as a Federal
Information Processing Standard.
 It has following versions-
 SHA-0
 SHA-1
 SHA-2
 SHA-3

SHA-1
 It works for any input message that is less than 264 bits.
 The output of SHA is a message digest of 160 bits in length.
 This is designed to be computationally infeasible to:
a) Obtain the original message , given its message digest.
b) Find two messages producing the same message digest.

How SHA-1 works?
 Step 1: Padding of Bits
 Step 2: Append Length
 Step 3: Divide the input into 512-bit blocks
1 0 1 1 0 1 0 0 0 0 0 0 0 0 …………………………… 0 0 0 0 1 1 0
Original
string
Padding of 0’s to 64 bits short of X 512
Length of
message
Total Message length multiple of 512

How SHA-1 works cont…
 Step 4: Initialize chaining variables
 Step 5: Process Blocks- Now the actual algorithm
begins….
Chaining Variables Hex values
A 01 23 45 67
B 89 AB CD EF
C FE DC BA 98
D 76 54 32 10
E C3 D2 E1 F0

 Step 5.1 : Copy chaining variables A-E into variables a-e.
 Step 5.2 : Divide current 512-bit block into 16 sub-blocks of
32-bits.
 Step 5.3 : SHA has 4 rounds, each consisting of 20 steps.
Each round takes 3 inputs-
• 512-bit block,
• The register abcde
• A constant K[t] (where t= 0 to 79)
Round Value of t between
1 1 and 19
2 20 and 39
3 40 and 59
4 60 and 79

 Step 5.4 : SHA has a total of 80 iterations (4 rounds X 20
-iterations). Each iteration consists of following
operations:-
abcde = ( e +Process P + S5(a) + W[t] + K[t] ), a, S30(b) , c , d
Where,
abcde = The register made up of 5 variables a, b, c, d, e.
Process P = The logic operation.
St = Circular-left shift of 32-bit sub-block by t bits.
W[t[ = A 32-bit derived from the current 32-bit sub-block.
K[t] = One of the five additive constants.

 Process P in each SHA round
Round Process P
1 (b AND c) OR (( NOT b) AND (d))
2 b XOR c XOR d
3 (b AND c ) OR (b AND d) OR (c AND d)
4 b XOR c XOR d

Single SHA-1 iteration

 The values of W[t] are calculated as follows :
 For the first 16 words of W (i.e. t=0 to 15) , the contents of the input
message sub-block M[t] become the contents of W[t].
 For the remaining 64 values of W are derived using the equation
W[t] = s1 ( W[t-16] XOR W[t-14] XOR W[t-8] XOR W[t-3])

Comparison between MD5 and SHA-1
Point of discussion MD5 SHA-1
Message digest length in
bits
128 160
Attack to try and find the
original message given a
message digest
Requires 2128 operations to
break in.
break in, therefore more
secure.
Attack to try and find two
messages producing same
message digest
break in.
break in.
Speed Faster Slower
Successful attempts so far There have been reported
attempts to some extent.
No such claims so far.

Parameters for various versions of SHA
Parameter SHA-1 SHA-256 SHA-384 SHA-512
Message digest size(in
bits)
160 256 384 512
Message size(in bits) <264 264 2128 2128
Block size (in bits) 512 512 1024 1024
Word size (in bits) 32 32 64 64
Steps in algorithm 80 64 80 80

SHA-512
 SHA-512 algorithm takes a message of length
2128 bits and produces a message digest of size
512 bits.
 SHA-512 was closely modeled after SHA-1 ,
which itself is modeled on MD5.

How SHA-512 works?
 Step 1 : Padding of bits – 128 bits short of a multiple of
1024.
 Step 2 : Append length.
 Step 3 : Divide the input into 1024-bit blocks.
 Step 4: Initialize chaining variables.
A= 6A09E667F3BCC908 B= BB67AE8584CAA73B
C= 3C6EF372FE94F82B D= A54FF53A5F1D36F1
E= 510E527FADE6821D F= 9B05688C2B3E6C1F
G= 1F83D9ABFB41BD6B H= 5BE0CD19137E2179

 Step 5 : Process blocks.
 Step 5.1 : Copy the variables A-H to a-h.
 Step 5.2 : Divide the 1024-bit block into 16 sub-blocks of
64-bit each.
 Step 5.3 : It takes 80 rounds with 1024-bit block, abcdefgh
register and K[t] constant as three inputs.
Each round consists of following operations:
Temp1 = h + Ch(e,f,g) + Sum (e[i] for i=1 to 512) + W[t] + K[t]
Temp2 = Sum (a[i] for i=0 to 512) + Maj(a,b,c)
a = Temp1 + Temp2
b = a
c = b

d = c
e = d + Temp1
f = e
g = f
h = g
Where,
t = round number
Ch (e,f,g) = (e AND f) XOR (NOT e AND g)
Maj (a,b,c) = (a AND b) XOR (a AND c) XOR (b AND c)
Sum (a[i]) = ROTR (a[i] by 28 bits) XOR ROTR (a[i] by 34 bits)
XOR ROTR (a[i] by 39 bits)
Sum (e[i]) = ROTR (e[i] by 14 bits) XOR ROTR (e[i] by
18 bits) XOR ROTR (e[i] by 41 bits)

ROTR(x) = Circular right shift, i.e. rotation , of the 64-bit
array x by the specified no. of bits.
W[t] = 64-bit word derived from the current 512-bit input
block.
K[t] = 64-bit additive constant.
+ (or Add) = Addition mod 264.
 In case of values of W[t] , for the first 16 rounds (0 to 15) ,
the value of W[t] is equal to the corresponding word in the
message block.
 For the remaining 64 steps, the value W[t] is equal to the
circular left shift by one bit of the XOR of the four preceding
values of W[t] with two of them subjected to shift and rotate
operations

Single SHA-512 iteration

Conclusion
 Developing Secure Hash Algorithm was initially major
concern for defense authorities.
 SHA produces message digest which has an application
in digital signature.
 In this way, this technique took a contributed in secure
and robust encryption.

Thank you for your attention!
Encrypt Safe…

Secure Hash Algorithm

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