Encrypting Text with JavaScript

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A First Attempt: Encoding Text

There are no perfect solutions to this problem, but there are two ways you can slow down a cookie interloper: encoding and encrypting. Encoding means converting each cookie character into a code, while encrypting means scrambling the entire cookie value using some kind of key value. This section shows you an algorithm for encoding, while the next section tackles encryption.

Each character has its own numeric character code. Table 1 lists the codes that corresponds to the characters you see on your keyboard.

Table 1 Numeric Codes for Keyboard Characters

So a quick-and-dirty encoding scheme would simply convert each character in a cookie value to its equivalent numeric code. You'd also need to insert a character such as a plus sign (+) between each code for easy splitting when decoding. For example, if the cookie value is password, then the encoded version would look like this:

112+97+115+115+119+111+114+100

Listing 1 presents two functions that do the encoding and decoding.

Listing 1: Encoding and Decoding Cookie Values

<script language="JavaScript" type="text/javascript">
<!--

function encode_cookie(cookie_value) {

  // This variable holds the encoded cookie characters
  var coded_string = ""
  
  // Run through each character in the cookie value
  for (var counter = 0; counter < cookie_value.length; counter++) {
  
    // Add the character's numeric code to the string
    coded_string += cookie_value.charCodeAt(counter)
    
    // Separate each code with a plus sign (+)
    if (counter < cookie_value.length - 1) {
      coded_string += "+"
    }
  }
  return coded_string
}

function decode_cookie(coded_string) {

  // This variable holds the decoded cookie value
  var cookie_value = ""
  
  // Use + to split the coded string into an array
  var code_array = coded_string.split("+")
  
  // Loop through the array
  for (var counter = 0; counter < code_array.length; counter++) {

    // Convert the code into a character and 
    // add it to the cookie value string
    cookie_value += String.fromCharCode(code_array[counter])
  }
  return cookie_value
}


//-->
</script>

The encode_cookie() function takes the cookie value in the cookie_value argument, and the variable coded_string is used to store the encoded value. A for() loop runs through each character in cookie_value and converts it to its numeric code using the String object's charCodeAt() method:

cookie_value.charCodeAt(counter)

This value is concatenated to coded_string, and then a plus sign (+) is added. To use this function with the set_cookie() function from earlier in this article, you'd use a statement such as the following:

set_cookie("coded_cookie", encode_cookie("test"))

The decode_cookie() function takes the encoded string in the coded_string argument. The variable named cookie_value is used to store the decoded value. The function uses the split() method to split the coded string using the plus sign, and the resulting array of codes is stored in code_array. Then a for() loop runs though each array element. With each pass, the fromCharCodeAt() method converts the code into the original character, which is concatenated to cookie_value.

To use this function with the get_cookie() function from earlier in this article, you'd use a statement such as this:

decode_cookie(get_cookie("coded_cookie"))

To test these functions, use the following links:

<a href="javascript:set_cookie('coded_cookie', encode_cookie('test'))">
Set encoded cookie
</a>
<p>
<a href="javascript:alert(get_cookie('coded_cookie'))">
Display encoded cookie value
</a>
<p>
<a href="javascript:alert(decode_cookie(get_cookie('coded_cookie')))">
Display decoded cookie value
</a>

Using True Encryption

Encoding the cookie characters will thwart the casual or unsophisticated snoop, but the cookie value is really only one step removed from plain text. For a higher level of security, you need to encrypt the cookie so that its text is undecipherable to a casual inspection.

Entire books can (and certainly have been) written on the topic of encryption, so I can't hope to do the subject justice in a short section of a JavaScript programming book. What I'll try to do here is give you a taste of cryptography, and if it whets your appetite for more, then there's no shortage of material available for you to study.

The most basic form of encryption that's worthy of the name involves applying some kind of "operation" to each character in a plaintext (as unencrypted text is called) string. This operation must have the following characteristics (at a minimum):

• It must produce a different character from the original.

• It must be reversible, which means that if you apply the operation to an encrypted character, the result is the original plaintext character.

• It must include some kind of key that plays a part in how the characters are encrypted, and that must be specified in order to decrypt the characters.

Many such operations have been discovered over the years, but JavaScript has one that's built into the language. It's an operator called exclusive OR, or XOR, for short. The XOR symbol is the caret (^) and here's its syntax:

Boolean1 ^ Boolean2

The XOR result is calculated as follows:

• If both Boolean1 and Boolean2 are true, or if both Boolean1 and Boolean2 are false, then XOR returns false.

• If only one of Boolean1 and Boolean2 is true, then XOR returns true.

In other words, XOR returns true only if Boolean1 and Boolean2 are different.

XOR is known in JavaScript circles as a bitwise operator because you can apply it to a number and it actually works on the individual binary bits that number is composed of. For example, the number 115 looks like this when you convert it to binary:

01110011

Each of those 1s and 0s is a bit, and the XOR operator works on such a number bit by bit. For example, consider the following expression:

115 ^ 93

Here's what it looks like in binary:

01110011 ^ 01011101

What XOR does here is apply its rules to the first bit of the left number and the first bit of the right number, as follows:

• If the bits are the same (that is, both bits are 1 or both bits are 0) return 0.

• If the bits are different, return 1.

For the above example, the answer is 00101110, which is 46 in decimal.

However, the most important characteristic of XOR is that it's reversible. Consider the following statement:

Result = Number1 ^ Number2

Given this, then the following statement is always valid:

Number1 = Result ^ Number2

In other words, knowing the result and one of the numbers, you can calculate the other number. For example, the expression 46 ^ 93 will return 115.

To apply this to encryption, consider this revised statement:

Encrypted_Value = Plaintext_Value ^ Key

In other words, given a plaintext string and key value, XORing these gives you some encrypted value. But XOR's reversibility also means that the following is valid:

Plaintext_Value = Encrypted_Value ^ Key

That is, you can derive the plaintext from the encrypted value if you XOR the latter with the same key. Therefore, XOR meets all our criteria for an encryption operation.

Listing 2 puts XOR to work in encrypting a cookie value.

Listing 2: Encrypting and Decrypting Cookie Values

<script language="JavaScript" type="text/javascript">
<!--

var legal_characters = "!\"#$%&'()*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQ
[ic:ccc]RSTUVWXYZ[\]^_´abcdefghijklmnopqrstuvwxyz{|}~€_‚ƒ„...†‡ˆ‰_‹Œ____''""–—˜™_›
[ic:ccc]œ__Ÿ ¡¢£€x_§¨©ª«¬_®¯°±__´μ¶·¸_º»___¿ÀÁÃÄÅÆÇÈÉÊËÌÍÎÏ_ÑÒÓÔÕÖ_ØÙÚÛÜ__ßàáãäå
[ic:ccc]æçèéêëìíîï_ñòóôõö÷øùúûü__ÿ"

function encrypt_cookie(cookie_value, encrypt_key) {

  var cookie_character
  var character_location
  var encrypted_location
  var encrypted_character

  // This variable holds the encrypted cookie characters
  var encrypted_string = ""
  
  // Run through each character in the cookie value
  for (var counter = 0; counter < cookie_value.length; counter++) {
  
    // Get the current cookie character
    cookie_character = cookie_value.substring(counter, counter + 1)
    
    // Get the character's location in the string of legal characters
    character_location = legal_characters.indexOf(cookie_character)
    
    // XOR the character location with the encrypt_key
    encrypted_location = character_location ^ encrypt_key
    
    // Use the encrypted location to specify the encrypted 
    // character within the string of legal characters
    encrypted_character = legal_characters.substring(encrypted_location, 
               encrypted_location + 1)
    
    // Add the encrypted character to the string
    encrypted_string += encrypted_character 
    
  }
  return encrypted_string
}

function decrypt_cookie(encrypted_string, encrypt_key) {

  var cookie_character
  var character_location
  var encrypted_location
  var encrypted_character

  // This variable holds the decrypted cookie value
  var cookie_value = ""
  
  // Run through each character in the encrypted string
  for (var counter = 0; counter < encrypted_string.length; counter++) {
  
    // Get the current encrypted character
    encrypted_character = encrypted_string.substring(counter, counter + 1)
    
    // Get the character's location in the string of legal characters
    encrypted_location = legal_characters.indexOf(encrypted_character)
    
    // XOR the character location with the encrypt_key
    character_location = encrypted_location ^ encrypt_key
    
    // Use the character location to specify the plain text 
    // character within the string of legal characters
    cookie_character = legal_characters.substring(character_location, 
              character_location + 1)
    
    // Add the plain text character to the string
    cookie_value += cookie_character 
  }
  return cookie_value
}

//-->
</script>

The legal_characters variable stores a string that contains every possible legal character, except the space. As you'll see, this string will be used to determine the encrypted characters after XOR has been used.

The encrypt_cookie() function takes two arguments: cookie_value is the string to be encrypted, and encrypt_key is the encryption key (a number). The function begins by declaring and initializing encrypted_string, which will hold the encrypted cookie value.

Then a for() loop is used to run through every character in cookie_value. Here's what happens:

1. The substring() method extracts the current character from cookie_value.

2. The current character's location in the legal_characters string is determined using the indexOf() method. The purpose of this is to turn the character into a numeric value. This value is stored in the character_location variable. For example, if the current character is t, it's the 83rd character in legal_characters, so its index is 82, which is what would be stored in character_location.

3. The character_location value is XORed with the encrypt_key value. The resulting number is stored in the encrypted_location variable. For example, 82 XORed with 25 equals 75.

4. The encrypted_location value specifies the location of a character in the legal_characters string. This character is the encrypted version of the plaintext character, and it's extracted using the substring() method. It's stored in the encrypted_character variable. For example, index number 75 in the legal_characters string is the letter m. Therefore, the encrypted value of t is m.

CAUTION

The legal_characters string contains 222 characters. Therefore, when extracting a character from this string, the index value must be less than or equal to 221. However, there are certain values of encrypt_key that will produce values greater than 221 during the XOR operation. These values are 0, 1, and anything larger than 31. Therefore, make sure your encrypt_key value is greater than or equal to 2 and less than or equal to 31.

5. The encrypted_character value is concatenated to encrypted_string.

The decrypt_cookie() function is almost identical to the encrypt_cookie() function. In this case, the arguments are the encrypted cookie value (encrypted_string) and the key (encrypt_key). The cookie_value variable is used to store the decrypted text, and then a for() loop uses the same procedure as before to reverse the encryption.

To test these functions, use the following links.

<a href="javascript:set_cookie('encrypted_cookie', encrypt_cookie('test', 25))">
Set encrypted cookie
</a>
<p>
<a href="javascript:alert(get_cookie('encrypted_cookie'))">
Display encrypted cookie value
</a>
<p>
<a href="javascript:alert(decrypt_cookie(get_cookie('encrypted_cookie'), 25))">
Display decrypted cookie value
</a>