Policy
Configure invalid RSA public key size
This policy setting allows you to configure whether Office displays a digital signature as invalid because of the number of RSA 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 RSA public key size to be the next largest option.
eea755459641 L_SelectDigitalSignatureInvalidRSABits Registry
Copy registry mappings
HKCU\software\policies\microsoft\office\16.0\common\signatures\invalidrsabitsPolicy notes
This policy setting allows you to configure whether Office displays a digital signature as invalid because of the number of RSA 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 RSA public key size to be the next largest option.