# (Solved Homework): Implement the Tiny Encryption Algorithm (TEA). a. Use your TEA algorithm to encrypt the 64-bit plaintext blo…

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(Solved Homework): Implement the Tiny Encryption Algorithm (TEA). a. Use your TEA algorithm to encrypt the 64-bit plaintext blo…
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Implement the Tiny Encryption Algorithm (TEA). a. Use your TEA algorithm to encrypt the 64-bit plaintext block 0x0123456789ABCDEF using the 128-bit key OxA56BABCD00000000FFFFFFFFABCDEF01. Decrypt the resulting ciphertext and verify that you obtain the original plaintext. b. Using the key in part a, encrypt and decrypt the following message using each of the three block cipher modes discussed in the text (ECB mode, CBC mode, and CTR mode). Four score and seven years ago our fathers brought forth on this continent, a new nation, conceived in Liberty, and dedicated to the proposition that all men are created equal.

```import java.math.BigInteger;

public class TEA {

BigInteger [] K ; //128 bits key
private String plainText;
public static final BigInteger delta = new BigInteger("9e3779b9",16);

//constructor receives a string of plaintext and 128 bit key in hexadecimal
public TEA(String plainText, String key)
{
parseKey(key);

}

public TEA(String key)
{

parseKey(key);

}

//parses a 128 bit key, given in hexadecimal form, and store its value in 4 integers (total of 128 bits),
private void parseKey(String key)
{
if(key.substring(0,2).equals("0x"))
key= key.substring(2);

//validating input
if(key.length() != 32)
{
System.out.println("Invalid key size!");
return;
}

//dividing the key into 4 strings
String[] kStr = new String[4];
int index=-1;
for(int i=0; i<key.length(); i++)
{
if(i%8 == 0)
{
index++;
kStr[index]="";

}
kStr[index] = kStr[index] + key.charAt(i);
}

//converting the 4 hex strings into 4 integers
K= new BigInteger[4];
for(int i=0; i<4; i++)
K[i] = new BigInteger(kStr[i], 16);

}

//receives a plaintext block of 64 bits in hexadecimal to be encrypted
//returns the cipher block
String encryptBlock(String plainTextBlock)
{
if(plainTextBlock.substring(0,2).equals("0x"))
plainTextBlock= plainTextBlock.substring(2);

//validating input
if(plainTextBlock.length()!=16)
{
System.out.println("Invalid block size!");
return null;

}

//separating the string block into left and right blocks
String LStr = plainTextBlock.substring(0, 8); //left block (32 bit)
String RStr = plainTextBlock.substring(8); //right block (32 bit)

//converting left and right blocks to integers
BigInteger L = new BigInteger(LStr, 16);
BigInteger R = new BigInteger(RStr, 16);

BigInteger sum= new BigInteger("0");
//32 rounds
for(int i=0; i<32; i++)
{
L= sum(L,  (sum(shiftLeft(R,4),K[0]))   .xor(sum(R,sum))    .xor(sum(shiftRight(R,5),K[1]))) ;
R= sum(R,  (sum(shiftLeft(L,4),K[2]))   .xor(sum(L,sum))    .xor(sum(shiftRight(L,5),K[3]))) ;

}

//joining back the blocks as hex
String cipherBlock = "0x"+L.toString(16)+R.toString(16)+"";

return cipherBlock;
}

//receives a ciphertext block of 64 bits in hexadecimal to be decrypted
//returns the plaintext block
String decryptBlock(String cipherBlock)
{
if(cipherBlock.substring(0,2).equals("0x"))
cipherBlock= cipherBlock.substring(2);

//validating input
if(cipherBlock.length()!=16)
{
System.out.println("Invalid block size!");
return null;

}

//separating the string block into left and right blocks
String LStr = cipherBlock.substring(0, 8); //left block (32 bit)
String RStr = cipherBlock.substring(8); //right block (32 bit)

//converting left and right blocks to integers
BigInteger L = new BigInteger(LStr, 16);
BigInteger R = new BigInteger(RStr, 16);

BigInteger sum= shiftLeft(delta,5);
//32 rounds
for(int i=0; i<32; i++)
{

R= subtract(R,  (sum(shiftLeft(L,4),K[2]))   .xor(sum(L,sum))    .xor(sum(shiftRight(L,5),K[3]))) ;
L= subtract(L,  (sum(shiftLeft(R,4),K[0]))   .xor(sum(R,sum))    .xor(sum(shiftRight(R,5),K[1]))) ;

sum = sum.subtract(delta);
}

//joining back the blocks as hex
String plainTextBlock = "0x"+L.toString(16)+R.toString(16)+"";

return plainTextBlock;
}

private BigInteger shiftLeft(BigInteger x, int steps)
{

BigInteger shifted=null;
boolean negative =false;

String xStr = x.toString(2);

//removing negative sign while shifting (currently)
if(xStr.charAt(0)=='-')
{
negative= true;
xStr = xStr.substring(1);
}

xStr= "0"+xStr;

for(int i=0; i<steps; i++)
{
xStr = xStr.substring(1);
xStr = xStr+"0";
}

//one last addition of negative sign if the number is negative
if(negative==true)
xStr= "-"+xStr;

//System.out.println(xStr);
shifted = new BigInteger(xStr,2);

return shifted;
}

private BigInteger shiftRight(BigInteger x, int steps)
{
BigInteger shifted=null;
boolean negative = false;

String xStr = x.toString(2);

//removing negative sign while shifting (currently)
if(xStr.charAt(0)=='-')
{
negative= true;
xStr = xStr.substring(1);
}

xStr= "0"+xStr;

for(int i=0; i<steps; i++)
{
xStr = xStr.substring(0,xStr.length()-1);
xStr = "0"+xStr;
}

//one last addition of negative sign if the number is negative
if(negative==true)
xStr= "-"+xStr;

shifted = new BigInteger(xStr,2);

return shifted;
}

private BigInteger sum(BigInteger a, BigInteger b)
{

String sumStr = sum.toString(2);
if(sumStr.length()>32)
{
int diff = sumStr.length()- 32;
sumStr = sumStr.substring(diff);
}

BigInteger newSum = new BigInteger(sumStr,2);

return newSum;
}

private BigInteger subtract(BigInteger a, BigInteger b)
{

BigInteger sub = a.subtract(b);

String subStr = sub.toString(2);
if(subStr.length()>32)
{
int diff = subStr.length()- 32;
subStr = subStr.substring(diff);
}

BigInteger newSub = new BigInteger(subStr,2);

return newSub;
}

public static void main(String[] args)
{

String plainText="0x0123456789ABCDEF";
String key= "0xA56BABCD00000000FFFFFFFFABCDEF01";
TEA tea = new TEA(key);
String cipherText = tea.encryptBlock(plainText);
System.out.println("Original Plain Text:"+plainText);
System.out.println("CipherText:"+cipherText);
System.out.println("Decrypted CipherText is:"+tea.decryptBlock(cipherText));

}

}```

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