Shift Cipher Gives Funny Characters. Related to ASCII shifting? - c++

Question

#include <iostream>
using namespace std;
const int MAXLEN = 50; // max length for input sequence of characters
const int MAXKEY = 10; // max length for key
// Shift Cipher Function. Use of ASCII Table for algorithm. (PROBLEMATIC)
char Shift(char chr, int k)
{
char c = chr;
if (c >= 'a' && c <= 'z')
{
c = c + k - 32; // For Uppercase
if (c < 65)
{
c = c + 26; // A-1 must be Z
}
else if (c > 90)
{
c = c - 26; // Z+1 must be A
}
return c;
}
else if (c >= 'A' && c <= 'Z')
{
c = c + k + 32; // + 32 to convert it to Lower case
if (c < 97)
{
c = c + 26; // a-1 must be z
}
else if (c > 122)
{
c = c - 26; // z+1 must be a
}
return c;
}
else
return c;
}
int main()
{
// to store user inputs
char s; // 'e' for encryption, 'd' for decryption
char text[MAXLEN]; // the sequence of characters to encrypt/decrypt
char key[MAXKEY]; // the key
char c, v, ch;
int n, cn = 0;
cin >> s;
// For the first input
for (int i = 0; i < 51; ++i)
{
cin >> c;
if (c != '!')
{
text[i] = c;
cn++;
}
else
{
break;
}
}
// For the second input
cin >> n;
for (int i = 0; i < n; ++i)
{
cin >> v;
key[i] = v;
}
// Execution
for (int i = 0; i < 51; ++i)
{
if (s == 'd')
{
ch = Shift(text[i], -key[i]); // 2->1 Logic
cout << ch;
}
else if (s == 'e')
{
ch = Shift(text[i], key[i]);
cout << ch;
}
if (ch != ' ')
{
cout << ch;
}
}
return 0;
}
Hi everyone. I am trying to solve a shift cipher problem but I do not know how to deal with the funny character output.
We first consider the algorithm to encrypt/decrypt a single character:
To encrypt (decrypt) a letter c (within the alphabet A-Z or a-z) with a shift of k positions:
Let x be c's position in the alphabet (0 based), e.g., the position of B is 1 and position of g is 6.
For encryption, calculate y = x + k modulo 26;
for decryption, calculate y = x − k modulo 26.
Let w be the letter corresponding to position y in the alphabet. If c is in
uppercase, the encrypted (decrypted) letter is w in lowercase; otherwise, the
encrypted (decrypted) letter is w in uppercase.
A character which is not within the alphabet A-Z or a-z will remain unchanged under
encryption or decryption.
Example. Given letter B and k = 3, we have x = 1, y = 1 + 3 mod 26 = 4, and w = E . As B
is in uppercase, the encrypted letter is e .
Now, to encrypt/decrypt a sequence of characters:
The number of positions, k, used to shift a character is determined by a key V of n
characters. For example, if V is a 4-character key 'C', 'O', 'M', 'P', and their positions in the
alphabet is 2, 14, 12 and 15, respectively. To encrypt a sequence of characters, we shift the
first character by +2 positions, the second by +14, the third by +12, the fourth by +15 and
repeat the key, i.e., we shift the fifth character by +2, the sixth by +14, until we encrypt all the
characters in the input sequence.
This is the intended I/O:
Input:
e !
3 A B C
Output:
!
But the output is problematic:
≡≡↨↨╧╧??÷÷00αα‼‼▓▓╘╘zz☻☻ §§ll??÷÷☺☺
So I suspect that the problem is in the Shift function but not the int main() ones. Yet I am not sure how.