Error during signature verification. Signature contains incorrect, unrecogniz.. (Byte range invalid)

Hi @just_0161,

Hope you are doing well. Thanks for writing in!

All the comments below are based strictly on the file you attached here: 

You are on the right track, and your suspicion is very close to the actual root cause. What you are seeing is a well-known Acrobat-specific validation behavior that is stricter than DSS and most ETSI validators.

Acrobat requires that the ByteRange:

• Covers everything except the signature contents

• Excludes:

  • The < and > of /Contents

  • Exactly the correct number of hex characters

• Does not include:

  • Trailing whitespace added by incremental updates

  • Cross-reference fixes

  • Repaired offsets

Many custom signers calculate ByteRange on the raw file bytes, but Acrobat recalculates using its parsed object model. More info here: https://adobe.ly/44Alb2Y

For PAdES signatures, Acrobat requires:

• Signature added as a true incremental update

• No modification of:

  • Previous xref table

  • Existing objects

• A new xref section only

If your Flutter signer:

• Rewrites the xref

• Normalizes line endings

• Reorders objects

• Rewrites streams

Then Acrobat detects the file as modified after signing, which explains the "Error 26 on save."

The ETSI PAdES standard (EN 319 142) that Acrobat implements references the correct creation of the signature, including byte range and how signatures must be embedded and validated in a PDF document: https://adobe.ly/44ErbHQ

This standard describes how PAdES signatures must be embedded and validated — including the requirement that the cryptographic hash covers the correct byte ranges and that the signature container meets precise structural expectations.

Hope I was able to clarify your question.