Codesign an executable and allow the modification of some bytes

Question

I use signtool.exe to codesign a Windows executable.

Which options of this tool can be used to define that a few bytes should be omitted from the hash calculation, and thus allowing that a digitally signed executable can have 8 or 16 bytes modified later?

This technique has been used by Mozilla (their .exe installer is different for each download, but has the same digital signature), see How can a .exe be modified and still keep a valid digital signature?.

The WinAPI function ImageGetDigestStream has an option DigestLevel to exclude resource information from the hash computation, but how to use this option when actually with signtool.exe or a similar tool?

Answer 1

This is explained by Didier Stevens in an article here: It's in the signature and he also provides a tool "disitool" to manipulate the signature.

Here are the steps to sign a file using Authenticode, and to append some data to it without breaking the signature:

A) Create a code siging certificate (you'll be required to enter passwords):

MakeCert /n "CN=MyOrg" /r /h 0 /eku "1.3.6.1.5.5.7.3.3,1.3.6.1.4.1.311.10.3.13" /sv MyOrg.pvk MyOrg.cer

note: 1.3.6.1.5.5.7.3.3 is szOID_PKIX_KP_CODE_SIGNING and 1.3.6.1.4.1.311.10.3.13 is szOID_KP_LIFETIME_SIGNING. This will create a private key file and a certificate file.

B) Add certificate to store (needs admin rights, could be a different store):

Certutil -addStore TrustedPeople MyOrg.cer

C) Create a Pfx file to sign:

Pvk2Pfx /pvk MyOrg.pvk /pi [Password goes here] /spc MyOrg.cer /pfx MyOrg.pfx

D) Sign your file:

SignTool.exe sign /fd SHA256 /v /a /f MyOrg.pfx /p [Password goes here] MyFile.exe

At this point MyFile.exe is signed using Authenticode:

E) Create some data.txt file. I've created one that just contains the "[Kilroy was here!]" text.

F) Now run disitool:

python.exe disitool.py inject --paddata MyFile.exe data.txt MyFile2.exe

And here is the result, MyFile2.exe is still valid without resigning the file:

While you can see data.txt's content added to its end:

From the original file's end: