MD5



 MD5 also known as message digest algorithm was  a widely used hash function producing 128 bit hash value.

I has been found to suffer from extensive vulnerabilities.

specified as internet standard RFC1321.

WORKING OF MD5:
  1. Pad message so it's length is 448 mod 512.
  2. append a 64 bit length value to the message.
  3. Initialize four word MD5 buffer (A,B,C,D).
  4. Process message in 16 word blocks
    1. Compression function that consists of four rounds of processing labelled as HMD5.
    2. Using 4 rounds of 16 step operation message block and buffer.
    3. Each round takes the current 512 bit block being processed(Yq) and 128 bit buffer value ABCD and updates the content of the buffer.
  5. Output of the fourth round is added to the input (Cq) to produce Cq+1 Output.









MD5 compression function:

Each round has 16 steps of the form:
    
          a=b+((a+g(b,c,d)+X[k]+T[i])<<<S)

a,b,c,d are the four words of the buffer
g() is one of the four functions F,G,H,I
<<<S is the circuit shift of 32 bit argument by s bit
T[i] ith 32 bit in matrix T

Each round the buffer input with the next word of the  message in a complex, non linear manner.
A different non-linear function is used in each of the 4 rounds.
The four buffer words are rotated from step to step so all are used and updated.
g is the primitive functions F,G,H,I of four rounds respectively.
X[i] is the 32 bit word in the current message block.
T[i] is the entry in the matrix of constants.


Four 32 bit register:
  •  A=67452301
  • B=EFCDAB89
  • C=98BADCFE
  • D=10325476
F(B,C,D)=(B AND C)  OR (NOT B AND D)
G(B,C,D)=(B AND D) OR (C OR NOT D)
H(B,C,D)=B XOR C XOR D
I(B,C,D)=C XOR(B OR NOT D)





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