Reversing PRONOM syntax

Last updated on 2025-10-29 | Edit this page

Estimated time: 15 minutes

Overview

As you write more signatures you begin to develop a need to debug yours, and existing signatures.
It can be useful to work backwards from a PRONOM signature to create an outline digital file that triggers a signature’s patterns.
These outline digital files are called skeleton files.

Challenge

Given the sequence:

AABB??CC{1-10}DD*010203

Can you write a byte sequence that will match in DROID?

AABB00CCFFFFFFDD00000000010203

AABB00CCFFFFFFDD00000000010203

You can jot down byte-sequences in any notepad application you have available to you (even google docs!).
Byte sequences must have an even number of characters, i.e. one byte is always two characters.
You can copy and paste the bytes into a hex editor. As we’ve seen, these are usually split into two panes, one for bytes and one for a representation of bytes in ASCII or another encoding.

Callout

Try it on https://hexed.it/ if you have a moment.

Take one of the solutions above and copy it into your clipboard.
Click anywhere in the editor pane and press ctrl-v.
Select “create new file” and specify how the data should be interpreted as “hexadecimal values”.
You will see the bytes from the solution in the left hand side of the window and its ASCII interpretation on the right.
You can then elect to download and name the file via ‘Save As’.

Challenge

Create a skeleton file using the bytes 5A5854617065211A01 and run it against DROID, FIDO, or Siegfried, what PUID do you get?

fmt/1000 ZX Tape Format

There are no concrete rules for how to convert a PRONOM signature into a skeleton file.
A good rule of thumb is to always convert wildcards and unknown values ( *, ??, {n}, {n-m} ) to zero bytes as they will help make it easier to see how the file is spaced out.
For sequences described by PRONOM as optional or belonging to one out of a set of values ( (aa|bb), (aa|bb|cccc), (aaaa|ffff) ) you only need to select one byte or set of bytes..
It’s a good idea to create some space between sets of sequences, for example between BOF and EOF sequences. Pick a number between 1 and 20 bytes (or as many bytes as you like) and enter that many null bytes between sequences.
Observe the way the skeleton file (bytes) below is spaced where zeros are used to replace wildcards in the signature.

Skeleton file	`41 75 74 6F 43 41 44 20 42 69 6E 61 72 79 20 44 58 46 0D 0A 1A 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 53 45 43 54 49 4F 4E 00 02 48 45 41 44 45 52 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 09 24 41 43 41 44 56 45 52 00 01 41 43 31 30 31 32 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 45 4E 44 53 45 43 00 00 45 4F 46 00`
BOF	4175746F4341442042696E617279204458460D0A1A000053454354494F4E0002{0-1}4845414445520009{0-1}24414341445645520001{0-1}41433130313200*00454E4453454300
EOF	00454F4600
Format	fmt/82: Drawing Interchange File Format (Binary): R13

You can always measure your success in creating a skeleton file against the signature itself. Once you’ve created a good skeleton file it will match the signature that you are investigating.

Callout

Skeleton files have different uses. They enable us to:

Callout

Siegfried’s build process uses a set of skeleton files and stores them in a repository called builder. You can find those at the link below.

Key Points