|Subject||Computer and Network Security|
|NU Year||Set: 5.(b) Marks: 4+3 Year: 2017|
• Step 1 – append padded bits: – The message is padded so that its length is congruent to 448, modulo 512.
• Means extended to just 64 bits shy of being of 512 bits long. – A single “1” bit is appended to the message, and then “0” bits are appended so that the length in bits equals 448 modu
Step 2 – append length: – A 64 bit representation of b is appended to the result of the previous step. – The resulting message has a length that is an exact multiple of 512 bits.
Step 3 – Initialize MD Buffer • A four-word buffer (A,B,C,D) is used to compute the message digest. – Here each of A,B,C,D, is a 32 bit register.
Step 3 cont. • These registers are initialized to the following values in hexadecimal: word A: 01 23 45 67 word B: 89 ab cd ef word C: fe dc ba 98 word D: 76 54 32 10
Step 4 – Process message in 16-word blocks. – Four auxiliary functions that take as input three 32-bit words and produce as output one 32-bit word. F(X,Y,Z) = XY v not(X) Z G(X,Y,Z) = XZ v Y not(Z) H(X,Y,Z) = X xor Y xor Z I(X,Y,Z) = Y xor (X v not(Z))
Step 4 – Process message in 16-word blocks cont. – if the bits of X, Y, and Z are independent and unbiased, the each bit of F(X,Y,Z), G(X,Y,Z), H(X,Y,Z), and I(X,Y,Z) will be independent and unbiased.
Step 5 – output – The message digest produced as output is A, B, C, D. – That is, output begins with the low-order byte of A, and end with the high-order byte of D.