Restrict Characters used in Encryption

Question

I have a very simple encryption class using tripleDES to encrypt the query string for a particular page on my site. I do this to prevent people page scraping sequentially based on our database IDs.

Anyhow, I've used this encryption method

However, it includes 3d% and some other special characters that shouldn't be in a query string and are rejected by Url Scan for security purposes. There is a = in the actual encrypted string that is generated. I don't want to change URL scan, but I was wondering if there is a way to limit the encryption characters for the tripleDES crypto provider or something. I know next to nothing about encyrption and I'm really just obfuscating the query string, so I'm open to other options regarding my encryption of the query string.

Answer 1

The methods you have linked use Base64 encoding to convert the encrypted byte array - which could have all kinds of "non-printable" bytes in it - into a form that will only contain A-Z, a-z, 0-9, +, / and =.

However, these last 3 are not suitable for URLs.

You could do a simple String.Replace on the Base64 string, replacing these characters with URL-safe characters, e.g. + => -, / => _ and = => .. You can even drop the = off the end completely, as they are only padding characters. (Making the first two substitutions and dropping the = is suggested by RFC3548.)

Then simply reverse this replacement when you want to decrypt your string. If you dropped the = completely, add = until the length of the string is a multiple of 4.

Answer 2

You shouldn't mess with crypto if you don't know what you're doing (and even if you do). Instead, use the crypto as is, and UrlEncode the result.