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Message Digest - MD5 Algorithm

Part:   1  2  3  4  5 

(Continued from previous part...)

Step 3. Initializing MD Buffer. MD5 algorithm requires a 128-bit buffer with a specific initial value. The rules of initializing buffer are:

• The buffer is divided into 4 words (32 bits each), named as A, B, C, and D.
• Word A is initialized to: 0x67452301.
• Word B is initialized to: 0xEFCDAB89.
• Word C is initialized to: 0x98BADCFE.
• Word D is initialized to: 0x10325476.

Step 4. Processing Message in 512-bit Blocks. This is the main step of MD 5 algorithm, which loops through the padded and appended message in blocks of 512 bits each. For each input block, 4 rounds of operations are performed with 16 operations in each round. This step can be described in the following pseudo code slightly modified from the RFC 1321's version:

Input and predefined functions: 
   A, B, C, D: initialized buffer words

   F(X,Y,Z) = (X AND Y ) OR (NOT X AND Z)
   G(X,Y,Z) = (X AND Z ) OR (Y AND NOT Z)
   H(X,Y,Z) = X XOR Y XOR Z
   I(X,Y,Z) = Y XOR (X OR NOT Z)

   T[1, 2, ..., 64]: Array of special constants (32-bit integers) as:
      T[i] = int(abs(sin(i)) * 2**32)
   
   M[1, 2, ..., N]: Blocks of the padded and appended message

   R1(a,b,c,d,X,s,i): Round 1 operation defined as: 
      a = b + ((a + F(b,c,d) + X + T[i]) <<< s)

   R2(a,b,c,d,X,s,i): Round 1 operation defined as: 
      a = b + ((a + G(b,c,d) + X + T[i]) <<< s)

   R3(a,b,c,d,X,s,i): Round 1 operation defined as: 
      a = b + ((a + H(b,c,d) + X + T[i]) <<< s)

   R4(a,b,c,d,X,s,i): Round 1 operation defined as: 
      a = b + ((a + I(b,c,d) + X + T[i]) <<< s)

Algorithm:
   For k = 1 to N do the following

     AA = A
     BB = B
     CC = C
     DD = D
     (X[0], X[1], ..., X[15]) = M[k] /* Divide M[k] into 16 words */
     
     /* Round 1. Do 16 operations. */
     R1(A,B,C,D,X[ 0], 7, 1)
     R1(D,A,B,C,X[ 1],12, 2)
     R1(C,D,A,B,X[ 2],17, 3)
     R1(B,C,D,A,X[ 3],22, 4)
     R1(A,B,C,D,X[ 4], 7, 5)
     R1(D,A,B,C,X[ 5],12, 6)
     R1(C,D,A,B,X[ 6],17, 7)
     R1(B,C,D,A,X[ 7],22, 8)
     R1(A,B,C,D,X[ 8], 7, 9)
     R1(D,A,B,C,X[ 9],12,10)
     R1(C,D,A,B,X[10],17,11)
     R1(B,C,D,A,X[11],22,12)
     R1(A,B,C,D,X[12], 7,13)
     R1(D,A,B,C,X[13],12,14)
     R1(C,D,A,B,X[14],17,15)
     R1(B,C,D,A,X[15],22,16)

     /* Round 2. Do 16 operations. */
     R2(A,B,C,D,X[ 1], 5,17)
     R2(D,A,B,C,X[ 6], 9,18)
     R2(C,D,A,B,X[11],14,19)
     R2(B,C,D,A,X[ 0],20,20)
     R2(A,B,C,D,X[ 5], 5,21)
     R2(D,A,B,C,X[10], 9,22)
     R2(C,D,A,B,X[15],14,23)
     R2(B,C,D,A,X[ 4],20,24)
     R2(A,B,C,D,X[ 9], 5,25)
     R2(D,A,B,C,X[14], 9,26)
     R2(C,D,A,B,X[ 3],14,27)
     R2(B,C,D,A,X[ 8],20,28)
     R2(A,B,C,D,X[13], 5,29)
     R2(D,A,B,C,X[ 2], 9,30)
     R2(C,D,A,B,X[ 7],14,31)
     R2(B,C,D,A,X[12],20,32)
     
     /* Round 3. Do 16 operations. */
     R3(A,B,C,D,X[ 5], 4,33)
     R3(D,A,B,C,X[ 8],11,34)
     R3(C,D,A,B,X[11],16,35)
     R3(B,C,D,A,X[14],23,36)
     R3(A,B,C,D,X[ 1], 4,37)
     R3(D,A,B,C,X[ 4],11,38)
     R3(C,D,A,B,X[ 7],16,39)
     R3(B,C,D,A,X[10],23,40)
     R3(A,B,C,D,X[13], 4,41)
     R3(D,A,B,C,X[ 0],11,42)
     R3(C,D,A,B,X[ 3],16,43)
     R3(B,C,D,A,X[ 6],23,44)
     R3(A,B,C,D,X[ 9], 4,45)
     R3(D,A,B,C,X[12],11,46)
     R3(C,D,A,B,X[15],16,47)
     R3(B,C,D,A,X[ 2],23,48)

     /* Round 4. Do 16 operations. */
     R4(A,B,C,D,X[ 0], 6,49)
     R4(D,A,B,C,X[ 7],10,50)
     R4(C,D,A,B,X[14],15,51)
     R4(B,C,D,A,X[ 5],21,52)
     R4(A,B,C,D,X[12], 6,53)
     R4(D,A,B,C,X[ 3],10,54)
     R4(C,D,A,B,X[10],15,55)
     R4(B,C,D,A,X[ 1],21,56)
     R4(A,B,C,D,X[ 8], 6,57)
     R4(D,A,B,C,X[15],10,58)
     R4(C,D,A,B,X[ 6],15,59)
     R4(B,C,D,A,X[13],21,60)
     R4(A,B,C,D,X[ 4], 6,61)
     R4(D,A,B,C,X[11],10,62)
     R4(C,D,A,B,X[ 2],15,63)
     R4(B,C,D,A,X[ 9],21,64)
     
     A = A + AA
     B = B + BB
     C = C + CC
     D = D + DD
   End of for loop
   
Output: 
   A, B, C, D: Message digest

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Part:   1  2  3  4  5