Process and convert array of bytes to string

I'm trying to generate the structure of a FamousWriter file (fwd). I'm halfway through the file. I'm using the WEB IDE, and I'd like to see the translation to string of the byte arrays I've found.

This should be doable with process and encoding but I'm struck by a unkown key found error.

As an example, the string:

10 00 3F 79 7D 77 3F 67 79 7E 74 7F 67 24 3E 7A 60 77

is shown in it's ASCII form:

?y}w?gy~tg$>z`w

But I know the first byte (in this case '10') it's the encryption key.

So, after processing it:

00 10 2F 69 6D 67 2F 77 69 6E 64 6F 77 34 2E 6A 70 67

And it would be shown as:

/img/window4.jpg

My code at the moment is:

  - id: firstfile
    type: file_name
    repeat: expr
    repeat-expr: 1

  file_name:
    seq:
    - id: sizedecoder
      type: u2
    - id: data
      size: sizedecoder
      process: xor(sizedecoder)
      #type: str
      #encoding: SJIS

Affiliate disclaimer: I'm a Kaitai Struct maintainer (see my GitHub profile).

As of Kaitai Struct 0.10, using type: str and process on the same attribute hasn't been implemented yet. See https://github.com/kaitai-io/kaitai_struct/issues/503 and https://github.com/kaitai-io/kaitai_struct/issues/706#issuecomment-590125139:

2. using process is not implemented on string type at all. I don't think it's intentional, it's just another bug.

But it's easy to work around both issues, you can also wrap it into a new user type.

So in your case, you should add a wrapper type for the string - let's call it str_wrapper:

  file_name:
    seq:
      - id: sizedecoder
        type: u2
      - id: data
        size: sizedecoder
        process: xor(sizedecoder)
        type: str_wrapper

  str_wrapper:
    seq:
      - id: value
        size-eos: true
        type: str
        encoding: SJIS