User Configuration > Administrative Templates > Microsoft Office 2016 > Security Settings > Digital Signatures
User
HKCU\software\policies\microsoft\office\16.0\common\signatures
This policy setting allows you to configure whether Office displays a digital signature as invalid because of the number of DSA public key bits used in the digital signature. If you enable this policy setting, you can specify the number of bits that Office treats as invalid in a digital signature. For example: 512, 768, etc. If you don’t configure this policy setting, Office won’t treat any digital signatures as invalid because of the number of bits in the public key. Enabling this policy causes the minimum DSA public key size to be the next largest option.