Affine Cipher written in C#

The following code written in C# encrypts and decrypts using the Affine Cipher. For more information, check out Making, Breaking Codes by Paul Garrett. All questions/comments are always appreciated.



///

/// This function takes plain text and encrypts it using the Affine Cipher
/// e(x) = (ax + b)(mod m). Note: a & m should be coprime.
///
public static string AffineEncrypt(string plainText, int a, int b)
{
string cipherText = "";

// Put Plain Text (all capitals) into Character Array
char[] chars = plainText.ToUpper().ToCharArray();

// Compute e(x) = (ax + b)(mod m) for every character in the Plain Text
foreach (char c in chars)
{
int x = Convert.ToInt32(c - 65);
cipherText += Convert.ToChar((( a * x + b ) % 26) + 65);
}

return cipherText;
}

///

/// This function takes cipher text and decrypts it using the Affine Cipher
/// d(x) = aInverse * (e(x) − b)(mod m).
///
public static string AffineDecrypt(string cipherText, int a, int b)
{
string plainText = "";

// Get Multiplicative Inverse of a
int aInverse = MultiplicativeInverse(a);

// Put Cipher Text (all capitals) into Character Array
char[] chars = cipherText.ToUpper().ToCharArray();

// Computer d(x) = aInverse * (e(x) − b)(mod m)
foreach (char c in chars)
{
int x = Convert.ToInt32(c - 65);
if (x - b < 0) x = Convert.ToInt32(x) + 26;
plainText += Convert.ToChar(((aInverse * (x - b)) % 26) + 65);
}

return plainText;
}

///

/// This functions returns the multiplicative inverse of integer a mod 26.
///
public static int MultiplicativeInverse(int a)
{
for (int x = 1; x < 27; x++)
{
if ((a * x) % 26 == 1)
return x;
}

throw new Exception("No multiplicative inverse found!");
}