[owasp-modsecurity compatibility] hexDecode method

[tty2]

Summary

coraza doesn't implement method hexDecode
owasp-modsecurity has this method

Basic example

https://github.com/owasp-modsecurity/ModSecurity/wiki/Reference-Manual-(v3.x)#hexdecode

Motivation

OWASP modsecurity compatibility

[fzipi]

Thanks for reporting! Are you up for a PR?

[tty2]

Hi
I'm gonna take it to implement soon.

[tty2]

Could you help me to understand the approach used in the tests for hexDecode?
https://github.com/corazawaf/coraza/blob/main/internal/transformations/testdata/hexDecode.json

@jcchavezs I see you are an author of these tests, you are probably more in the context. And thanks you've done it in advance!

My current assumption that coraza tends to "best-effort" approach (opposite to "fail fast" approach). In that case it explains everything.
As example this test case:

 {
    "type": "tfn",
    "name": "hexDecode",
    "input": "01234567890a0",
    "output": "\\x01#Eg\\x89\\x0a",
    "ret": 1
 }

From RFC 4648:
"The encoding process represents 8-bit groups (octets) of input bits as output strings of 2 encoded characters." => the valid lenght of the input should be even. In this case it is odd, 13 characters.
As I understand the offered logic is pretty simple: from the expected output I can guess we just trim the input and process the rest. Right?

The other case is more tricky:

  {
    "name": "hexDecode",
    "type": "tfn",
    "ret": 1,
    "input": "01234567890a0z01234567890a",
    "output": "\\x01#Eg\\x89\\x0a#\\x01#Eg\\x89\\x0a"
  }

We have even number of characters at the begging but with the invalid symbol inside ("z" in this case). With removing invalid symbol only we'll get to the situation from the previous case (odd number of symbols). Which logic we should follow here?
I guess find the next valid pair and follow, right?
The most interesting part is how do we get "#" from "0z". Is that some specific behavior? I couldn't find any mentioning about it in RFC. Could you clarify this part, please?

Thanks in advance