While digitally signing a PDF document in Acrobat or Acrobat Reader, you may see the following warning message:
Background: SHA256 has been the default hashing algorithm in Acrobat since version 9.1. However, in some cases, for example if the signature device (like a smart card or USB token) or its driver doesn’t support SHA256 hashing, to prevent failure while creating the signature, Acrobat or Reader will fall back to use SHA1.
Recently, researchers have succeeded in generating collisions with the SHA1 hash algorithm when applied to digital signatures. This means that, under certain conditions, it might be possible to generate a digital signature based on SHA1 hash which would not be unique to a document, but it might be valid also when applied to different documents.
What has changed in Acrobat and Acrobat Reader (2017.009.20044): With Acrobat and Acrobat Reader release 2017.009.20044, Adobe is warning users against using the deprecated SHA1 hash algorithm for digital signatures. The user can continue to sign using SHA1 although this is not recommended as SHA1 is considered deprecated industry wide.
There are multiple solutions to prevent this warning dialog, based on specific situations: