C functions to Javascript

I'm trying to convert 2 functions from C to Javascript, but I'm failing on that.

The C functions im trying to convert is:

void encrypt(char password,int key)

unsigned int i;
for(i=0;i<strlen(password);++i)

password[i] = password[i] - key;



void decrypt(char password,int key)

unsigned int i;
for(i=0;i<strlen(password);++i)

password[i] = password[i] + key;




The C functions is from: http://c-program-example.com/2012/04/c-program-to-encrypt-and-decrypt-a-password.html

What i did in Javascript is this:

var password = "hello"

var key = 322424;

for (var i=0; i<password.length; i++)

var char = password[i].charCodeAt();


var s = String.fromCharCode(char-key);


alert(s);




I'm putting the alert to see if it working correctly before making them as functions. Can someone please show me how it is done correctly in Js?

charCodeAt(i)

encrypt

decrypt

-key

char

(char - key) & 0xff

2 Answers
2

Try this, as said by Clifford in the comments, each char code has to be masked to modulo-256 because the size of char. Also, since decrypt is the same but substracting key, I reused encrypt but with a negative key.

char

decrypt

key

encrypt

key

Here you have a working snippet, let me know if it gives the desired output.

function encrypt (password, key)
var i,
output = '';

for (i = 0; i < password.length; i++)
var charCode = password.charCodeAt(i),
keyedCharCode = (charCode - key) & 0xff;

output += String.fromCharCode(keyedCharCode);


return output;


function decrypt (password, key)
return encrypt(password, -key);


var password = 'hello',
key = 322424,
encrypted = encrypt(password, key),
decrypted = decrypt(encrypted, key);

console.log('encrypt', encrypted);
console.log('decrypt', decrypted);

A char in C is actually a tiny integer, which value in hexa goes from 00 to FF. char is signed in many implementations (unsigned char is not), so its values go from -128 to 127 (256 values total, or 0 to 255 if unsigned). An array of chars, char is an array of tiny integers, conventionally the last char of a 'string' in C is the byte 0x00 that marks the end (for instance, printf knows the end is there when the byte 0 is met).

char

00

FF

char

unsigned char

-128

127

0

255

char

printf

0

In Javascript a string is a set of Unicode characters.

In C,

char s = "hello";
int k = 322424;
char *t = s;
while(*t) *t++ -= k; // 't' is the encoded version of 's'
// print the integer values of the new encoded 'string'
for(t=s ; *t ; t++) printf("%d ", *t); // -16 -19 -12 -12 -9
// (unsigned: 240 237 244 244 247, or add 256 to negative values above)
printf("n");


A char cannot contain more than a byte.

char

In Javascript, you have to keep the resulting number in a byte-range, otherwise, a multi-byte character might be stored instead. For instance

String.fromCharCode(0x3042) gives "あ" (Unicode [character 0x3042](https://www.key-shortcut.com/en/writing-systems/%E3%81%B2%E3%82%89%E3%81%8C%E3%81%AA-japanese/))


In C,

char c = 0x3042; // will only keep one byte, the LSB, 0x42


Thus in Javascript, try something like

var s = "hello";
var k = 322424;
var i,l = s.length;
var t = '';
// Note the `& 0xff` that keep the result within a byte range
for(i=0 ; i<l ; i++) t += String.fromCharCode((s.charCodeAt(i) - k) & 0xff);
// 't' is the result, let see its char codes
var u = '';
for(i=0 ; i<l ; i++) u += t.charCodeAt(i) + ' ';

console.log(u); // 240 237 244 244 247 (same as in C, 240-256 = -16 ...)


The JS char codes are unsigned, thus the positive results. They're the same as in C.

char

char

unsigned char

unsigned char

char

char

char

char c = 0x3042; // will only keep one byte, the LSB, 0x42

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