Then M is broken up into words of length 32 bits as follows:

M = M [0] M [1] … M [ N -1]

Where each M [ i ] is 32 bit long and N º 0 mod 16. The overall algorithm proceeds as follows:

1. 1.  A = 67452301 _hex

2.  B = efcdab89 _hex

3.  C = 98badcfe _hex

4.  D = 10325476 _hex

5. for i = 0 to N/16 -1 do

6.  for j = 0 to 15 do

7.  X [ j] = M [ 16i + j ]

8.  AA = A

9.  BB = B

10.  CC = C

11.  DD = D

12.  Round1

13.  Round2

14.  Round3

15.  A = A + AA

16.  B = B + BB

17. C = C + CC

18.  D = D + DD

 

We maintain 4 registers A , B , C , D each of length 32 bits. In each iteration of the outer for loop we process a message block X [0] X [1] … X [15] of length 512 bits to produce a message digest of lenghth 128 bits formed by concatenating the contents of those 4 register A , B , C , D .

The above algorithm of MD4 was subsequently extended to MD5 that works in 4 rounds instead of 3 rounds.