我觉得我的数学和逻辑是正确的，我不确定我做错了什么。我尝试了几个不同的b值，没有任何作用。出于某种原因，无论使用常数，“f”总是变为“v”而“k”总是变为“q”。我的输出是：glqvafkpuzejotydinsxchmrwb，abcdwvghijqlmnolqrstgvwxyb。当b = 0时，解密方法有效。
public class Asgn2No2CodeDecode {
public static void main(String[] args) {
System.out.println(affineCipher("abcdefghijklmnopqrstuvwxyz"));
System.out.println(affineDecipher(affineCipher("abcdefghijklmnopqrstuvwxyz")));
}
public static String affineCipher(String plainTxt)
{
StringBuilder cipherTxt = new StringBuilder();
String alphabet = "abcdefghijklmnopqrstuvwxyz";
int a = 5;
int b = 6;
int charNum = 0;
char alphChar;
char ciphChar;
for (int i = 0; i < plainTxt.length(); i++)
{
ciphChar = plainTxt.charAt(i);
if (Character.isLetter(ciphChar))
{
for (int j = 0; j<alphabet.length(); j++)
{
alphChar = alphabet.charAt(j);
if (ciphChar == alphChar)
{
charNum = ((a*j + b)%26);
}//end if
}//end for alph
cipherTxt.append(alphabet.charAt(charNum));
}else cipherTxt.append(plainTxt.charAt(i));
}//end for plain
return cipherTxt.toString();
}//end affineCipher(String)
public static String affineDecipher(String cipherTxt)
{
StringBuilder plainTxt = new StringBuilder();
String alphabet = "abcdefghijklmnopqrstuvwxyz";
int aInv = 21;
int b = 6;
int charNum = 0;
char alphChar;
char ciphChar;
for (int i = 0; i < cipherTxt.length(); i++)
{
ciphChar = cipherTxt.charAt(i);
if (Character.isLetter(ciphChar))
{
for (int x = 0; x<alphabet.length(); x++)
{
alphChar = alphabet.charAt(x);
if (ciphChar == alphChar)
{
if (x>=b)
{
charNum = (((aInv*(x-b))%26));
}
else charNum = (-((aInv*(x-b))%26));
}//end if
}//end for alph
plainTxt.append(alphabet.charAt(charNum));
}else plainTxt.append(cipherTxt.charAt(i));
}//end for plain
return plainTxt.toString();
}//end affineCipher(String)
}