decoding algorithm wanted

Question

I receive encoded PDF files regularly. The encoding works like this:

• the PDFs can be displayed correctly in Acrobat Reader
• select all and copy the test via Acrobat Reader
• and paste in a text editor
• will show that the content are encoded

so, examples are:

13579 -> 3579;
hello -> jgnnq

it's basically an offset (maybe swap) of ASCII characters.

The question is how can I find the offset automatically when I have access to only a few samples. I cannot be sure whether the encoding offset is changed. All I know is some text will usually (if not always) show up, e.g. "Name:", "Summary:", "Total:", inside the PDF.

Thank you!

edit: thanks for the feedback. I'd try to break the question into smaller questions:

Part 1: How to detect identical part(s) inside string?

Answer 1

You need to brute-force it.

If those patterns are simple like +2 character code like in your examples (which is +2 char codes)

h i j
e f g
l m n
l m n
o p q

1 2 3
3 4 5
5 6 7
7 8 9
9 : ;

You could easily implement like this to check against knowns words

>>> text='jgnnq'
>>> knowns=['hello', '13579']
>>>
>>> for i in range(-5,+5): #check -5 to +5 char code range
...     rot=''.join(chr(ord(j)+i) for j in text)
...     for x in knowns:
...         if x in rot:
...             print rot
...
hello