Practical Cryptography by Pathan Al-Sakib Khan Azad Saiful

Author:Pathan, Al-Sakib Khan, Azad, Saiful [Pathan, Al-Sakib Khan, Azad, Saiful]
Language: eng
Format: epub
Published: 0101-01-01T00:00:00+00:00

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bay zid ashik hossain

P

Q

R

S

Mi

F

Ki

<<<s

P

Q

R

S

Figure 10.1 Operational model of MD5.

and left rotation. Hence, there are three kinds of operations in MD5:

bit-wise Boolean operation, modular addition, and cyclic shift opera-

tion. All three operations are very fast on 32-bit machines, which

make MD5 quite fast.

MD5 consists of 64 of these operations, stated in Figure 10.1,

grouped in four rounds of 16 actions. F is a function that is nonlinear

in nature; in each round one function is used. Mi denotes a 32-bit

block of the message input, and Ki denotes a 32-bit constant.

The algorithm of MD5 can be described in five steps:

1. Add padding bits behind the input message.

2. Add a 64-bit binary string that is the representation of the

message’s length.

3. Initialize four 32-bit values.

4. Compress every 512-bit block.

5. Generate the 128-bit output.

10.3.1 Add Padding Bits behind the Input Message

This step is to elongate the initial message and make its length

congruent to 448 mod 512. First, a single bit 1 is appended to the

message. Then, a series of 0 bits are appended so that length (the

padded message) ≡ 448 mod 512. For example, suppose the initial

message digest algorithm 5

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message has 1000 bits. Then, this step will add 1 bit 1 and 471

bits 0. As another example, consider a message with just 448 bits.

As the algorithm does not check whether the primary length is

congruent to 448 mod 512, 1 bit 1 and 511 bits 0 will be appended

to the message. As a result, the padding bits’ length is at least 1 and

at most 512.

new_len = initial_len+1;

while((new_len% 64) ! = 56){

new_len++;

}

msg = new uint8_t[new_len+8];

10.3.2 Add a 64-Bit Binary String That Is the

Representation of the Message’s Length

Consideration should be paid to the meaning of the 64-bit binary

string. One should not regard it as the first 64 bits of the initial mes-

sage. It is the binary representation of the length of the preliminary

message. For example, assume the message length is 1000 bits. Its

64-bit binary representation would be 0x00000000000003E8. If the

message is very lengthy, larger than 264, only the lower 64 bits of its

binary representation are used.

msg[initial_len] =

0x80;//append the "1" bit; most

significant bit is "first"

for (offset =

initial_len + 1; offset <new_len;

offset++)

msg[offset] = 0;

10.3.3 Initialize Four 32-Bit Values

These four 32-bit variables stated below would be used to compute the

message digest. In the Implementation 1 section, these variables are

mentioned as h0, h1, h2, and h3 and their initial values are

h0 = 0x67452301;

h1 = 0xefcdab89;

h2 = 0x98badcfe;

h3 = 0x10325476;

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10.3.4 Compress Every 512-Bit Block

Four supplementary functions will be defined such that each func-

tion takes an input of three 32-bit words and produces a 32-bit word

output [2].

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))

In each bit position, F acts as a condition such that if X, then

Y; otherwise, Z. The function F might have been defined using addition instead of or since XY and not ( X) Z will never have 1’s in the same bit position.

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