autokey vigenere decryption in c#

Question

This is my code to decrypt autokey vigenere cipher algorithm

string cipherText = "zicvtwqngkzeiigasxstslvvwla";
string key = "deceptive";

key = key.ToLower();
cipherText = cipherText.ToLower();

int klength = key.Length;
int kslength = (int)(cipherText.Length - key.Length);

string pl = string.Empty;
char[] newpl = new char[cipherText.Length];
int a = Convert.ToInt32('a');
for (int i = 0; i < cipherText.Length - kslength; i++)
{
    int c = Convert.ToInt32(cipherText[i]) - a;
    if (c< 0) c += 26;
    int k = Convert.ToInt32(key[i]) - a;
    if (k < 0) k += 26;
    int p = (c - k);
    p %= 26;
    if (p < 0) p += 26;
    p += a;
    char temp = Convert.ToChar(p);
    newpl[i] = temp;

}

char[] NewKey = new char[cipherText.Length];
char[] ciphertext = new char[cipherText.Length];
char[] chars = new char[cipherText.Length];
int count =0;
for (int i = 0; i < key.Length; i++)
{
    NewKey[i] = key[i];
    count++;
}
int j = 0;
for (int i = count; i < cipherText.Length; i++)
{
    NewKey[i] = newpl[j];
    j++;
}

Console.WriteLine(NewKey);

for (int i = klength; i < cipherText.Length; i++)
{
    int c = Convert.ToInt32(cipherText[i]) - a;
    int k = Convert.ToInt32(NewKey[i]) - a;
    int p = (c - k);
    p %= 26;
    if (p < 0) p += 26;
    p += a;

    char temp = Convert.ToChar(p);
    newpl[i] = temp;
}
pl = new string(newpl);
Console.WriteLine(pl);

it gives me output:

deceptivewearedisc

wearediscoveredsavlmleoopet

while the right output should be:

deceptivewearediscoveredsav

wearediscoveredsaveyourself

first line of the output refers to the auto generated key

second line refers to the decrypted text.

Answer 1

There are few errors in your code:

1) Look at this line:

for (int i = 0; i < cipherText.Length - kslength; i++)

kslength = cipherText.Length - key.Length so your code is

for (int i = 0; i < key.Length; i++)

Key length is < than text length so you finish decryption too early.

2)

char temp = Convert.ToChar(p);
newpl[i] = temp;

You decrypted symbol but with autokey decryption you should add decrypted symbol to your key.

3)

for (int i = 0; i < key.Length; i++)

Should be NewKey.Length instead, because key is longer that we really need after fixing #2.

Fixed code:

string cipherText = "zicvtwqngkzeiigasxstslvvwla";
string key = "deceptive";

key = key.ToLower();
cipherText = cipherText.ToLower();

int klength = key.Length;

string pl = string.Empty;
char[] newpl = new char[cipherText.Length];
int a = Convert.ToInt32('a');
for (int i = 0; i < cipherText.Length; i++)
{
    int c = Convert.ToInt32(cipherText[i]) - a;
    if (c < 0) c += 26;
    int k = Convert.ToInt32(key[i]) - a;
    if (k < 0) k += 26;
    int p = (c - k);
    p %= 26;
    if (p < 0) p += 26;
    p += a;
    char temp = Convert.ToChar(p);
    key += temp;
    newpl[i] = temp;

}

char[] NewKey = new char[cipherText.Length];
int count = 0;
for (int i = 0; i < NewKey.Length; i++)
{
    NewKey[i] = key[i];
    count++;
}
int j = 0;
for (int i = count; i < cipherText.Length; i++)
{
    NewKey[i] = newpl[j];
    j++;
}

Console.WriteLine(NewKey);

for (int i = klength; i < cipherText.Length; i++)
{
    int c = Convert.ToInt32(cipherText[i]) - a;
    int k = Convert.ToInt32(NewKey[i]) - a;
    int p = (c - k);
    p %= 26;
    if (p < 0) p += 26;
    p += a;

    char temp = Convert.ToChar(p);
    newpl[i] = temp;
}
pl = new string(newpl);
Console.WriteLine(pl);

Output:

deceptivewearediscoveredsav

wearediscoveredsaveyourself