Why does str_rot13 fail when processing numbers? Common causes and solutions

echo str_rot13("abc123"); // Output: nop123

From the result, we can see that only "abc" is replaced with "nop", while "123" remains unchanged.

II. Why was this design chosen?

This design is intentional. ROT13 originated from the ancient Roman Caesar cipher, but its modern implementation, particularly in the computer field,

typically only replaces ASCII letter characters. This is because it is mainly used for simple text obfuscation rather than strong encryption.

There is no direct "letter correspondence" for digits in ROT13, so str_rot13 naturally does not handle numbers.

III. The problem in common scenarios

When we want to "pseudo-encrypt" an entire string (including numbers), the behavior of str_rot13 may cause "information leakage".

For example, when hiding some sensitive content, although the letters are changed, the exposed numbers may still reveal too much.

For example:

$input = "Email: user123@example.com";

echo str_rot13($input);

// Output: Rznvy: hfre123@rknzcyr.pbz

In the result, 123 and the entire structure of the email can still be easily recognized.

IV. Solutions

If you need a "more comprehensive" encryption method (including numbers), you can use the following approaches:

1. Using a custom ROT13 + ROT5 method

You can write your own function combining ROT13 and ROT5 (5-position shift for digits):

function rot13_5($str) {

$str = str_rot13($str);  

return preg_replace_callback('/\d/', function($matches) {  

    return (string)((($matches[0] + 5) % 10));  

}, $str);  

}

echo rot13_5("abc123"); // Output: nop678

This way, both letters and digits are obfuscated.

2. Using base64_encode or other lightweight encryption methods

If you just want to obfuscate data and do not consider encryption strength, you can use base64_encode:

echo base64_encode("abc123"); // Output: YWJjMTIz

3. Using openssl or sodium extensions for real encryption

For scenarios with high security requirements, you should use actual encryption methods, such as:

$ciphertext = openssl_encrypt("abc123", "AES-128-CTR", "secretkey", 0, "1234567891011121");

echo $ciphertext;

This way, not only letters and digits, but the entire structure is hidden.

V. Conclusion

str_rot13 is a quick method for letter obfuscation, but it does not handle numbers. If you want to "encrypt" numbers as well,

additional logic, such as ROT5, Base64, or more advanced encryption methods, is needed. Understanding the behavior of str_rot13

will help you choose the right tool for different application scenarios.
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