Clipboard Content Encryption and Decryption Tool Based on C# 100010896

Cryptography Big Homework

Customize an encryption algorithm and its programming implementation

1. Encryption principle

plaintext space	plaintext length	Ciphertext Space	Ciphertext Length
a-z	Greater than 1	a-z	Hill cipher encryption matrix Integer multiples of order

Encryption order	Decryption order
plaintext	Ciphertext
Virginia Cipher	Hill Cipher
Affine Cipher	Affine Cipher
Hill Cipher	Virginia Cipher
ciphertext	plaintext

1.1. Virginia code

Encryption function:

(

)

(

)

e_K(x_1, x_2, … x_m) = (x_1 + k_1, x_2 + k_2, … x_m + k_m)

eK?(x1?,x2?,…xm?)=(x1? + k1?,x2? + k2?,…xm? + km?)

Decryption function:

(

the y

)

(

the y

)

d_K(y_1, y_2, … y_m) = (y_1 – k_1, y_2 – k_2, … y_m – k_m)

dK?(y1?,y2?,…ym?)=(y1k1?,y2k2?,…ymkm?)

When encrypting, data is grouped. The length of each group is

m. If the last grouping is insufficient
$$

bit only

bit

, then only encrypt the former

n$$ bits.

1.2. Affine cipher

Encryption function:

(

)

(

)

e(x) = (ax + b) \mod m

e(x)=(ax + b)modm

Decryption function:

(

the y

)

(

the y

)

d(y) = a^{-1} (y – b) \mod m

d(y)=a?1(y?b)modm

a and
$$

reciprocal, if

but

is relatively prime, if m = 26, then

Reciprocal prime, if m=26, then

a \in {1,3,5,7,9,11,15,17,19,21,23,25}$$

1.3. Hill password

Encryption function:

(

)

e_K(x) = xK

eK?(x)=xK

Decryption function:

(

the y

)

the y

dk(y) = yK^{-1}

dk(y)=yK?1

where the key K is a reversible square matrix

1.3.1. Encrypted matrix inversion in finite fields

set matrix

A is reversible,
$$

A^-

is its inverse matrix,

is its determinant,

A^*$$ is its adjoint matrix, then we have

A^- = \frac{A*}{D} = A^* D^-

A?=DA=A?D?

D^-

for

D in die
$$

The inverse element below, the above operations are all in the modulo

Calculated under m$$.

1.3.2. Encryption

If there is the following plaintext:

Plain text serial number	1	2	3	4	5	6	7	8	9	10
corresponds The value of	12	0	1	6	18	25	6	20	25	25

1.3.2.1. Process the plaintext first, then add the terminator

According to statistics, the word with the lowest frequency is z = 0.08, and z corresponds to 25, that is, the frequency of z in plaintext is relatively low, and 25 will be used as the terminator of plaintext. Therefore, if you encounter 25 during encryption, you need to escape it and replace it with two 25s.

Analyze the above plaintext, replace all 25 with two 25, and get the following plaintext:

Number	1	2	3	4	5	6	7	8	9	10	11	12	13
value	12	0	1	6	18	25	25	6	20	25	25	25	25< /strong>

Add a 25 at the end of the plaintext as a terminator to get the following ciphertext:

Number	1	2	3	4	5	6	7	8	9	10	11	12	13	14
value	12	0	1	6	18	25	25	6	20	25	25	25	25	25

1.3.2.2. Padding groups

The plaintext is grouped, and the last group is insufficient to supplement 0 (it can also be filled with random values, but it is not easy to analyze, so here we simply fill in the same characters). If the block length is 8, the following ciphertext is obtained:

Number	1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	16
value	12	0	1	6	18	25	25	6	20	25	25	25	25	25	0	0

1.3.2.3. Encryption

Number	1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	16
value	12	0	1	6	18	25	25	6	20	25	25	25	25	25	0	0

Matrix encryption for each packet

1.3.2.4. At this point, the encryption is over, just convert the numbers into corresponding letters

1.3.3. Decrypt

If the last group is not a complete group after the ciphertext is grouped, it means that the ciphertext and the key do not match and cannot be decrypted. If the packet is normal, the reverse process of encryption is used.

1.3.3.1. Decrypt

Decrypt each packet using the matrix. If the following plaintext is obtained after decryption:

Number	1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	16
value	12	0	1	6	18	25	25	6	20	25	25	25	25	25	0	0

1.3.3.2. Remove padding data

From the front to the back, find the subscript of the terminal 25, and then delete itself and all subsequent data to get the following plaintext:

Number	1	2	3	4	5	6	7	8	9	10	11	12	13
value	12	0	1	6	18	25	25	6	20	25	25	25	25

1.3.3.3. Combine two consecutive 25s into one to get the following plaintext:

Number	1	2	3	4	5	6	7	8	9	10
value	12	0	1	6	18	25	6	20	25	25

1.3.3.4. At this point, the decryption is over, just put the serial number

2. Programmatic implementation

A winform program developed with C#.

2.1. Interface effects

2.1.1 Key Configuration

After the app is opened, the user is required to configure the key. If you don’t want to enter it yourself, you can click the “Generate Random Key” button.

generate random key

After generating a random key, you can click the “Export Key” button, and the program will save the key as a “JSON” file, and then you can send the key to others.

A pop-up window prompts that the save is successful

You can open it and have a look, the format is as follows

{<!-- -->
  "InvalidIntValue": 2147483647,
  "VigenereKey": [ 1, 15, 22, 24, 22, 9 ],
  "AffineKeyA": 23,
  "AffineKeyB": 6,
  "HillMatrix": [
    [ 11, 15, 7, 10, 9, 20, 24, 10 ],
    [ 15, 19, 16, 1, 16, 7, 16, 21 ],
    [ 22, 6, 10, 24, 1, 18, 6, 22 ],
    [ 6, 13, 1, 22, 13, 12, 5, 16 ],
    [ 22, 20, 18, 24, 14, 5, 20, 17 ],
    [ 2, 15, 2, 17, 2, 17, 18, 7 ],
    [ 18, 14, 13, 8, 21, 1, 22, 17 ],
    [ 10, 9, 5, 20, 20, 2, 21, 3 ]
  ],
  "AllKeysReady": true
}

If you need to decrypt other people’s ciphertext, you can click the “Import Key” button to import the other party’s key.

2.1.2 Encrypted and decrypted text

If you check “Copy result to clipboard”, when the encryption operation is performed, the ciphertext will be copied directly to the clipboard, so that it can be pasted directly to others.
If “Get text from clipboard” is checked, when the decryption operation is performed, the text decryption will be read from the clipboard
Start using the encryption and decryption function, enter the letters to be encrypted, and click the “Encrypt” button

Copy the ciphertext to “Data Source”, click the “Decrypt” button to get the plaintext. (If “Get Text from Clipboard” is checked, you can directly click the decrypt button, and the program will automatically fill the ciphertext in the clipboard into “Data Source” and perform the decryption operation)

2.2. Some source code

2.2.1 Virginia code

// Encryption
public List<char> Decrypt(List<char> cipher)
{
    var result = new List<char>();
    int groupLength = vigenereKey. Length;

    for (int i = 0; i < cipher. Count; i ++ )
    {
        var _ = cipher[i] - vigenereKey[i % groupLength];
        result.Add((char)((_ % m + m) % m));
    }

    return result;
}

// decrypt
public List<char> Encrypt(List<char> plain)
{
    var result = new List<char>();
    int groupLength = vigenereKey. Length;

    for (int i = 0; i < plain. Count; i ++ )
    {
        var _ = plain[i] + vigenereKey[i % groupLength];
        result.Add((char)(_ % m));
    }

    return result;
}

2.2.2 Affine cipher

// decryption
public List<char> Decrypt(List<char> cipher)
{
    int ap = AAP[a]; // the inverse of a
    for (int i = 0; i < cipher. Count; i ++ )
    {
        cipher[i] = (char)(((ap * (cipher[i] - b)) % 26 + 26) % 26);
    }
    return cipher;
}

// encryption
public List<char> Encrypt(List<char> plain)
{
    for (int i = 0; i < plain. Count; i ++ )
    {
        plain[i] = (char)(((a * plain[i] + b) % 26 + 26) % 26);
    }
    return plain;
}

2.2.3 Hill password

// decryption
public List<char> Decrypt(List<char> cipher)
{
    // TODO Delete
    //PrintList(cipher, "[before decryption]");

    if (cipher. Count % groupLength != 0)
    {
        throw new FormatException("The format of the ciphertext is wrong, please check if there is missing data");
    }

    // decrypt
    int groupNums = cipher.Count / groupLength; // total number of groups
    List<char> result = new List<char>(); // result

    for (int i = 0; i < groupNums; i ++ )
    {
        // Decrypt the block using the encryption matrix and add to the result
        result. AddRange(
            MatrixIntGF26.MultiplyMod26(
                cipher.GetRange(i * groupLength, groupLength).ToArray(),
                DKEY
            )
        );
    }

    // Remove redundancy, remove the data after the terminator
    var index = result.LastIndexOf(Transfer_Char);
    if (index == -1)
    {
        throw new Exception("The data is wrong, or the data and the key of Hill Cipher do not match");
    }
    result.RemoveRange(index, result.Count - index);

    // Merge two consecutive terminal symbols
    for (int i = 1; i < result. Count; i ++ )
    {
        if (result[i] == Transfer_Char)
        {
            if (result[i - 1] == Transfer_Char)
            {
                result. RemoveAt(i);
            }
        }
    }

    return result;
}

// Encryption
public List<char> Encrypt(List<char> plain)
{
    // copy plain
    var copy = new List<char>();
    for (int i = 0; i < plain. Count; i ++ )
    {
        copy. Add(plain[i]);
    }

    // Escape the terminal, that is, replace one terminal with two consecutive terminals
    for (int i = 0; i < copy.Count; i ++ )
    {
        if (copy[i] == Transfer_Char)
        {
            copy. Insert(i, Transfer_Char);
            i + + ;
        }
    }

    // Add a terminator at the end of the data
    copy.Add(Transfer_Char);

    // Check the total length of the data, if the last group is not enough, you need to fill the data
    int appendLength = copy.Count % groupLength;//filling number
    if (appendLength != 0)
    {
        appendLength = groupLength - appendLength;
    }

    for (int i = 0; i < appendLength; i ++ )
    {
        copy.Add(Append_Char);
    }
    
// total number of groups
    int groupNums = copy.Count / groupLength + ((copy.Count % groupLength) == 0 ? 0 : 1);

    List<char> result = new List<char>(); // result
    for (int i = 0; i < groupNums; i ++ )
    {
        // Encrypt the block using the encryption matrix and add it to the result
        result. AddRange(
            MatrixIntGF26.MultiplyMod26(
                copy. GetRange(
                    i * groupLength, groupLength).ToArray(),
                EKEY
            )
        );
    }

    return result;
}

2.2.4 Hill password auxiliary class

It is used to provide matrix multiplication and matrix inversion operations under mod26. There are more codes, so I won’t show them

Resources

Size: 221KB
Resource download: https://download.csdn.net/download/s1t16/87484796
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