The Vigenere algorithm in C# explanation

Question

I came across this code:

Byte Vigenere Cipher, error with decryption

But trying to follow the rules I made a new question about it.

The following algorithm is used and I'm trying to get a better understanding into it:

Byte[] result= new Byte[plaintext.Length];

key = key.Trim().ToUpper();

int keyIndex = 0;
int keylength = key.Length;

for (int i = 0; i < plaintext.Length; i++)
{
    keyIndex = keyIndex % keylength;
    int shift = (int)key[keyIndex] - 65;
    result[i] = (byte)(((int)plaintext[i] + shift) % 256);
    keyIndex++;
}

Am I right in thinking the key needs to be trimmed as it is in Unicode? therefore subtracting 65 from a capital produces a common character/symbol?

Answer 1

The ASCII value for the capital A is 65. All characters in key are converted to uppercase, this would simply return the alphabetical index of each letter in key.

Each letter in key is converted to a number that way, and each letter in the original string is "shifted up the alphabet" that number of positions.

If your key was BAD, this would turn into the numbers 1, 0 and 3, then applied to "hello world" as follows:

Hello world
10310310310 <-- added to each character
Ieomo#xoumd

You can demonstrate this by adding this code below yours:

StringBuilder demonstration = new StringBuilder();
foreach (byte b in result)
{
    demonstration.Append((char)b);
}
Console.WriteLine(demonstration.ToString());