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MD5
Message Digest
Algorithm
KHAWAJA ROKHAM TARIQ
C O M P U T E R S C I E N C E D E P A R T M E N T
U N V E R S I T Y O F K O T L I A Z A D A N D J A M M U K A S H M I R

Content
Introduction
Hashing knowledge
MD5 Algorithm Structure
Implementation Steps
Performance
MD5 vs. SHA

Hashing knowledge
It is a mathematical function that convert a numerical values into another compressed
numerical values.
Output will always of fixed length.
Same message have same particular
same output.

Introduction
MD5 algorithm was developed by Professor Ronald L. Rivest in 1991. MD5 message-digest
algorithm takes as input a message of arbitrary length and produces as output a 128-bit
"fingerprint" or "message digest" of the input.
The MD5 algorithm is intended for digital signature applications, where a large file must be
"compressed" in a secure manner before being encrypted with a private (secret) key under a
public-key cryptosystem such as RSA.

Original message Add padding
Original message Add padding Add length
512 bits
blocks
512 bits
blocks

Step1 Append padding bits
The input message is "padded" (extended) so that its length (in bits) equals to 448 mod 512.
Padding is always performed, even if the length of the message is already 448 mod 512.
Padding is performed as follows: a single "1" bit is appended to the message, and then "0" bits
are appended so that the length in bits of the padded message becomes congruent to 448 mod
512. At least one bit and at most 512 bits are appended.

Padding is done such that the total length is 64bit < exact multiple of 512.
Suppose original message is 1000bit.
Now check exact multiple of 512
Total 1000bits
512*2=1024 which is not good.
512*3=1536
Now sub 64 from 1536
1536-64=1472
We need to add 472 bits more for calculation.
Also 1472 is 64bit less than exact multiple of 512.

Append the original length before padding.
We use modulus 64 operation.
Get exact multiple of 512 after 64 bits adding.
Now create blocks according to bit length.
Before padding 1000bits.
Now apply mod64.
1000] length 264

Step2. Append length
A 64-bit representation of the length of the message is appended to the result of step1. If the
length of the message is greater than 2^64, only the low-order 64 bits will be used.
The resulting message (after padding with bits and with b) has a length that is an exact multiple
of 512 bits. The input message will have a length that is an exact multiple of 16 (32-bit) words.

Processing blocks
Copy 4-chaining blocks into small corresponding variables.
Dividing 512 bit block into 16(32bits blocks)
Apply four rounds.
16 sub blocks Constant (t)
One round
a b c d

Step3. Initialize MD buffer
A four-word buffer (A, B, C, D) is used to compute the message digest. Each of
A, B, C, D is a 32-bit register. These registers are initialized to the following values
in hexadecimal, low-order bytes first):
word A: 01 23 45 67
word B: 89 ab cd ef
word C: fe dc ba 98
word D: 76 54 32 10

Intinliaze 4-Chaining varbles
32 bits
Name A,B,C,D.
Values in hexadecimal.

Step4. Process message in 16-word blocks
Four functions will be defined such that each function takes an input of three
32-bit words and produces a 32-bit word output.
F (X, Y, Z) = XY or not (X) Z
G (X, Y, Z) = XZ or Y not (Z)
H (X, Y, Z) = X xor Y xor Z
I (X, Y, Z) = Y xor (X or not (Z))

Round 1.
[abcd k s i] denote the operation a = b + ((a + F (b, c, d) + X [k] + T [i]) <<< s).
Do the following 16 operations.
[ABCD 0 7 1] [DABC 1 12 2] [CDAB 2 17 3] [BCDA 3 22 4]

Single MD5 operation mathmeticaly defined like this:
a=b+((a+process,p(b,c,d)+M[i]+T[k]))<<shift
M[i]=message is i.
T[k]=constant is k.
<<shift=circular shift

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MD5

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