DATA ENCRYPTION STANDARD ALGORITHM
Data Encryption Standard (DES) encrypts blocks of size 64 bit.It was developed by IBM based on the cipher Lucifer under influence of the National Security Agency (NSA).It was a most popular block cipher for most of the last 30 years. • By far best studied symmetric algorithm. • Nowadays considered insecure due to the small key length of 56 bit.It mainly includes confusion and diffusion.
Confusion: An encryption operation where the relationship between key and ciphertext is obscured.
Diffusion: An encryption operation where the influence of one plaintext symbol is spread over many ciphertext symbols with the goal of hiding statistical properties of the plaintext. It is a fiestal cipher .
It consist of 16 rounds.I have not used the inital and the final permutation step because that is not required.
Algorithm for DES Encryption:
Step1: The plain text is entered which is of 64 bit in binary. In my program you have to enter 16 bit of heaxadecimal number. Then the key bit is entered which is of same 16 bit hexadecimal.
Step2: The whole 64 bit string is divided into two parts each of 32 bits let's say L&R.
Step 3: Then the non invertible function is calculated using R and the key stream.
This is the most important step It involves following steps:-
Sub1(3):The round key are generated which is of 48 bit.This involves following steps:
Sub1(3)(1): Using the parity bit table 8 bits of the key are removed leaving us with 56 bits.
Parity bit configuration:
Sub2(3)(1): The 56 bit key is divided into two 28 bit key say keyL and keyR.
Sub3(3)(1):Both the key are shifted by '1' for 1,2,9 & 16 rounds otherwise shifted by '2';
Sub4(3)(1): Using the Dbox each 28 is converted to 24 bit key which are then combined to get a 48 bit key which is used in each round.
Dbox Configuration:
The first 3 rows are used by keyL and last 3 rows by keyR.
Sub2(3):After the round key generated we have R which is of 32 bit. It is then converted to 48 bit by using expansion P-box.The 32 bit is divided into 8 parts each of 4 bit which is converted to 6 bit by pbox.
P-box configuration:
As the diagram shows the extra two bit in each blocks comes from its neighbor.It is consider as circular so that the first bit comes from the last bit and viceversa.
Sub3(3):So we get a 48 bit output from the P-box which is "bitwise xor" with the 48 bit key of that round.
Sub4(3):the xor output is of 48 bit which is then fitted in a compression S-box. The S-box consist of 8 small Sbox which takes 6 bit and results in 4 bit.Each S-box has a different configuration which is fixed for each round.
S-box 1 configuration:
Consider the first 6 bit, Find the decimal value of 1st and last bit which will give the row number and the remaining 4 bits are converted to decimal to get the column.The corresponding value is fetched from the above table and is again converted to binary which will be of 4 bit since every number in the matrix is less than 15. Similarly you can do it for the rest of the bit.The configuration of the other S -box as follows:
S2
15 1 8 14 6 11 3 4 9 7 2 13 12 0 5 10
3 13 4 7 15 2 8 14 12 0 1 10 6 9 11 5
0 14 7 11 10 4 13 1 5 8 12 6 9 3 2 15
13 8 10 1 3 15 4 2 11 6 7 12 0 5 14 9
S3
10 0 9 14 6 3 15 5 1 13 12 7 11 4 2 8
13 7 0 9 3 4 6 10 2 8 5 14 12 11 15 1
13 6 4 9 8 15 3 0 11 1 2 12 5 10 14 7
1 10 13 0 6 9 8 7 4 15 14 3 11 5 2 12
S4
7 13 14 3 0 6 9 10 1 2 8 5 11 12 4 15
13 8 11 5 6 15 0 3 4 7 2 12 1 10 14 9
10 6 9 0 12 11 7 13 15 1 3 14 5 2 8 4
3 15 0 6 10 1 13 8 9 4 5 11 12 7 2 14
S5
2 12 4 1 7 10 11 6 8 5 3 15 13 0 14 9
14 11 2 12 4 7 13 1 5 0 15 10 3 9 8 6
4 2 1 11 10 13 7 8 15 9 12 5 6 3 0 14
11 8 12 7 1 14 2 13 6 15 0 9 10 4 5 3
S6
12 1 10 15 9 2 6 8 0 13 3 4 14 7 5 11
10 15 4 2 7 12 9 5 6 1 13 14 0 11 3 8
9 14 15 5 2 8 12 3 7 0 4 10 1 13 11 6
4 3 2 12 9 5 15 10 11 14 1 7 6 0 8 13
S7
4 11 2 14 15 0 8 13 3 12 9 7 5 10 6 1
13 0 11 7 4 9 1 10 14 3 5 12 2 15 8 6
1 4 11 13 12 3 7 14 10 15 6 8 0 5 9 2
6 11 13 8 1 4 10 7 9 5 0 15 14 2 3 12
S8
13 2 8 4 6 15 11 1 10 9 3 14 5 0 12 7
1 15 13 8 10 3 7 4 12 5 6 11 0 14 9 2
7 11 4 1 9 12 14 2 0 6 10 13 15 3 5 8
2 1 14 7 4 10 8 13 15 12 9 0 3 5 6 11
Sub5(3): From the above step we will get 32 bit since each 6 bit is converted into 4 bit i.e. 4*8=32.
This 32 bit is then given to a straight P-box which is a permutation box which takes 32 bit as input and gives 32 bit as output.
Straight P-box configuration:
The first bit of the 32 bit output from the S-box will be the 16 bit of it and so on.
Step 4:The 32 bit output from the above step is then "bitwise xor" with L and the result is stored in R, the L will contain the copy of R which was in the Step2.
Step 5:Continue the step 3 and Step 4 16 times and then the L and R is combined to get 64 bit encrypted cipher which is converted back to hexadecimal.
Algorithm for DES Decryption:
Same as DES encryption only the steps are reversed. The L & R we got from the last step is used we used the key in reversed order Same Encryption is done with reversing the L & R.
Program of DES Encryption and Decryption in C++
#include<bits/stdc++.h>
using namespace std;
string hex2bin(string p)//hexadecimal to binary
{
string ap="";
int l=p.length();
for(int i=0;i<l;i++)
{
string st="";
if(p[i]>='0'&&p[i]<='9')
{
int te=int(p[i])-48;
while(te>0)
{
st+=char(te%2+48);
te/=2;
}
while(st.length()!=4)
st+='0';
for(int j=3;j>=0;j--)
ap+=st[j];
}
else
{
int te=p[i]-'A'+10;
while(te>0)
{
st+=char(te%2+48);
te/=2;
}
for(int j=3;j>=0;j--)
ap+=st[j];
}
}
return ap;
}
int main()
{
string p,l,r,ap="",ke,kp,rtem;
pre:;
//Input of the plain text and the key
cout<<"Enter the plain text in hexadecimal form (16bit) \n";
cin>>p;
if(p.length()!=16)
{
cout<<"enter all the bits\n";
goto pre;
}
for(int i=0;i<16;i++)
{
if((p[i]>='0'&&p[i]<='9')||(p[i]>='A'&&p[i]<='F'))
;
else
{
cout<<"Not a valid hexadecimal string\n";
goto pre;
}
}
int key1[64];
pr:;
cout<<"Enter the key in hexadecimal form (16bit) \n";
cin>>ke;
if(ke.length()!=16)
{
cout<<"enter all the key bits\n";
goto pr;
}
for(int i=0;i<16;i++)
{
if((ke[i]>='0'&&ke[i]<='9')||(ke[i]>='A'&&ke[i]<='F'))
;
else
{
cout<<"Not a valid hexadecimal key string\n";
goto pr;
}
}
//Input is completed
p=hex2bin(p);
kp=hex2bin(ke);
for(int i=0;i<64;i++)
key1[i]=kp[i]-'0';
//getting 56 bit key from 64 bit using the parity bits
int keyp[56]={57 , 49 , 41 ,33 , 25 , 17 , 9,
1 , 58 ,50 , 42 , 34 , 26 , 18,
10 , 2 , 59 , 51 nbsp;, 43 , 35 , 27,
19 , 11 , 3 , 60 , 52 , 44 , 36,
63 , 55 , 47 , 39 , 31 , 23 , 15,
7 , 62 , 54 , 46 , 38 , 30 , 22,
14 , 6 , 61 , 53 , 45 , 37 , 29,
21 , 13 , 5 , 28 , 20 , 12 , 4};
//D box configuration each 28 to 24 bit
int key2[48]={14, 17, 11, 24, 1, 5,
3 , 28 , 15 , 6 , 21 , 10 ,
23 ,19 , 12 , 4 , 26 , 8,
16 ,7 , 27 , 20 , 13 , 2,
41 , 52 , 31 , 37 , 47 , 55,
30 , 40 , 51 , 45 , 33 , 48,
44 ,49 , 39 , 56 , 34 , 53,
46 ,42 , 50 , 36 , 29 , 32};
int key[16][48],keyl[28],keyr[28],nshift,temp1,temp2,pkey[56];
l=p.substr(0,32);
r=p.substr(32,32);
//key generation
int i,t=1,j,row,col,temp,round=16;
j=0;
for(i=0;i<56;i++)
pkey[i]=key1[keyp[i]-1];
for(i=0;i<28;i++)
keyl[i]=pkey[i];
for(i=0;i<28;i++)
keyr[i]=pkey[i+28];
//round key generation
for(i=0;i<16;i++)
{
if(i==0||i==1||i==8||i==15)
nshift=1;
else
nshift=2;
while(nshift--)
{
temp1=keyl[0];
temp2=keyr[0];
for(j=0;j<27;j++)
{
keyl[j]=keyl[j+1];
keyr[j]=keyr[j+1];
}
keyl[27]=temp1;
keyr[27]=temp2;
}
for(j=0;j<24;j++)
key[i][j]=keyl[key2[j]-1];
for(j=24;j<48;j++)
key[i][j]=keyr[key2[j]-1-28];
}
//straight pbox
int per[32]={16 , 7 , 20 ,21,
29 , 12 , 28 , 17,
1 , 15, 23, 26,
5 , 18 , 31 ,10,
2 , 8 , 24 , 14,
32 ,27, 3 , 9,
19 ,13, 30, 6,
22 , 11 , 4 , 25};
//sbox configuration 6 bit to 4 bit
int s[8][4][16]=
{{
14,4,13,1,2,15,11,8,3,10,6,12,5,9,0,7,
0,15,7,4,14,2,13,1,10,6,12,11,9,5,3,8,
4,1,14,8,13,6,2,11,15,12,9,7,3,10,5,0,
15,12,8,2,4,9,1,7,5,11,3,14,10,0,6,13
},
{
15,1,8,14,6,11,3,4,9,7,2,13,12,0,5,10,
3,13,4,7,15,2,8,14,12,0,1,10,6,9,11,5,
0,14,7,11,10,4,13,1,5,8,12,6,9,3,2,15,
13,8,10,1,3,15,4,2,11,6,7,12,0,5,14,9
},
{
10,0,9,14,6,3,15,5,1,13,12,7,11,4,2,8,
13,7,0,9,3,4,6,10,2,8,5,14,12,11,15,1,
13,6,4,9,8,15,3,0,11,1,2,12,5,10,14,7,
1,10,13,0,6,9,8,7,4,15,14,3,11,5,2,12
},
{
7,13,14,3,0,6,9,10,1,2,8,5,11,12,4,15,
13,8,11,5,6,15,0,3,4,7,2,12,1,10,14,9,
10,6,9,0,12,11,7,13,15,1,3,14,5,2,8,4,
3,15,0,6,10,1,13,8,9,4,5,11,12,7,2,14
},
{
2,12,4,1,7,10,11,6,8,5,3,15,13,0,14,9,
14,11,2,12,4,7,13,1,5,0,15,10,3,9,8,6,
4,2,1,11,10,13,7,8,15,9,12,5,6,3,0,14,
11,8,12,7,1,14,2,13,6,15,0,9,10,4,5,3
},
{
12,1,10,15,9,2,6,8,0,13,3,4,14,7,5,11,
10,15,4,2,7,12,9,5,6,1,13,14,0,11,3,8,
9,14,15,5,2,8,12,3,7,0,4,10,1,13,11,6,
4,3,2,12,9,5,15,10,11,14,1,7,6,0,8,13
},
{
4,11,2,14,15,0,8,13,3,12,9,7,5,10,6,1,
13,0,11,7,4,9,1,10,14,3,5,12,2,15,8,6,
1,4,11,13,12,3,7,14,10,15,6,8,0,5,9,2,
6,11,13,8,1,4,10,7,9,5,0,15,14,2,3,12
},
{
13,2,8,4,6,15,11,1,10,9,3,14,5,0,12,7,
1,15,13,8,10,3,7,4,12,5,6,11,0,14,9,2,
7,11,4,1,9,12,14,2,0,6,10,13,15,3,5,8,
2,1,14,7,4,10,8,13,15,12,9,0,3,5,6,11
}};
//DES Encryption
while(round--)
{
rtem=r;
t=1;
string ep="",xorout="",sout="",soutt;
//the expansion P box
ep+=r[31];
for(i=0;i<32;i++)
{
if((t+1)%6==0)
{
ep+=r[4*((t+1)/6)];
t++;
}
if(t%6==0&&i!=0)
{
ep+=r[4*(t/6)-1];
t++;
}
ep=ep+r[i];
t++;
}
ep+=r[0];
//Key xor with output of expansion p box
for(i=0;i<48;i++)
xorout+=char(((int(ep[i])-48)^key[16-round-1][i])+48);
//sbox compression 48bit to 32 bit
for(i=0;i<48;i+=6)
{
row=(int(xorout[i+5])-48)+(int(xorout[i])-48)*2;
col= (int(xorout[i+1])-48)*8+(int(xorout[i+2])-48)*4+(int(xorout[i+3])-48)*2+(int(xorout[i+4])-48);
temp=s[i/6][row][col];
soutt="";
while(temp>0)
{
soutt+=char(temp%2+48);
temp/=2;
}
while(soutt.length()!=4)
soutt+='0';
for(j=soutt.length()-1;j>=0;j--)
sout+=soutt[j];
}
//straight pbox that is permutation of the sbox output
char pc[32];
for(i=0;i<32;i++)
pc[i]=sout[per[i]-1];
r="";
for(i=0;i<32;i++)
r+=char(((int(pc[i])-48)^(int(l[i])-48))+48);
l=rtem;
cout<<"Output after Round"<<16-round<<endl;
string cip="";
for(i=0;i<32;i+=4)
{
int te;
te=(int(l[i])-48)*8+(int(l[i+1])-48)*4+(int(l[i+2])-48)*2+(int(l[i+3])-48);
if(te<10)
cip+=char(te+48);
else
cip+=char(te+55);
}
for(i=0;i<32;i+=4)
{
int te;
te=(int(r[i])-48)*8+(int(r[i+1])-48)*4+(int(r[i+2])-48)*2+(int(r[i+3])-48);
if(te<10)
cip+=char(te+48);
else
cip+=char(te+55);
}
cout<<cip<<endl;
}
//test case
/*
PLAIN=
14A7D67818CA18AD
KEY=
AABB09182736CCDD
*/
//DES decryption
round=16;
string ltem;
while(round--)
{
ltem=l;
t=1;
string ep="",xorout="",sout="",soutt;
//the expansion P box
ep+=l[31];
for(i=0;i<32;i++)
{
if((t+1)%6==0)
{
ep+=l[4*((t+1)/6)];
t++;
}
if(t%6==0&&i!=0)
{
ep+=l[4*(t/6)-1];
t++;
}
ep=ep+l[i];
t++;
}
ep+=l[0];
//Key xor with output of expansion p box
for(i=0;i<48;i++)
xorout+=char(((int(ep[i])-48)^key[round][i])+48);
//sbox compression 48bit to 32 bit
for(i=0;i<48;i+=6)
{
row=(int(xorout[i+5])-48)+(int(xorout[i])-48)*2;
col= (int(xorout[i+1])-48)*8+(int(xorout[i+2])-48)*4+(int(xorout[i+3])-48)*2+(int(xorout[i+4])-48);
temp=s[i/6][row][col];
soutt="";
while(temp>0)
{
soutt+=char(temp%2+48);
temp/=2;
}
while(soutt.length()!=4)
soutt+='0';
for(j=soutt.length()-1;j>=0;j--)
sout+=soutt[j];
}
//straight pbox that is permutation of the sbox output
char pc[32];
for(i=0;i<32;i++)
pc[i]=sout[per[i]-1];
l="";
for(i=0;i<32;i++)
l+=char(((int(pc[i])-48)^(int(r[i])-48))+48);
r=ltem;
cout<<"Decrypted Output after Round"<<16-round<<endl;
string cip="";
for(i=0;i<32;i+=4)
{
int te;
te=(int(l[i])-48)*8+(int(l[i+1])-48)*4+(int(l[i+2])-48)*2+(int(l[i+3])-48);
if(te<10)
cip+=char(te+48);
else
cip+=char(te+55);
}
for(i=0;i<32;i+=4)
{
int te;
te=(int(r[i])-48)*8+(int(r[i+1])-48)*4+(int(r[i+2])-48)*2+(int(r[i+3])-48);
if(te<10)
cip+=char(te+48);
else
cip+=char(te+55);
}
cout<<cip<<endl;
}
system("pause");
}
- Each string should contain values from [0-9] or [A-F] which is in hexadecimal.
Output:It will give output after every round the intermediate cipher text.After that th reverse of it.
