public string DecryptText(string cipherText)
{
int revA = SimpleMaths.ReverseElement(a, M);
StringBuilder decryptedText = new StringBuilder();
foreach (char c in cipherText)
{
if (c == ' ')
{
decryptedText.Append(' ');
continue;
}
int
code = c - 'A',
x = (revA * (code - b)) % M;
if (x < 0)
{
x += M;
}
char next = (x == 26) ? ' ' : (char)(x + 'A');
decryptedText.Append(next);
}
return(decryptedText.ToString());
}
public string Analyze()
{
for (int a = 1; a < M; a++)
{
if (SimpleMaths.GCD(a, M) != 1)
{
continue;
}
for (int b = 0; b < M; b++)
{
AffineCipher affine = new AffineCipher(a, b);
string text = affine.DecryptText(cipherText);
double score = NgramStatistics.CountTextScore(text);
if (score > rate)
{
rate = score;
this.decryptedText = text;
}
}
}
return(decryptedText);
}
public AffineCipher(int a, int b)
{
if (SimpleMaths.GCD(a, M) != 1)
{
throw new ArgumentException("Error:"
+ " \"a\" parameter and modulus M should be coprime.");
}
this.a = a;
this.b = b;
}
