You can import only 512-bit key (low grade) digital certificates into Internet Explorer.
Additional InformationWARNING: If you use Registry Editor incorrectly, you may cause serious problems that may require you to reinstall your operating system. Microsoft cannot guarantee that you can solve problems that result from using Registry Editor incorrectly. Use Registry Editor at your own risk.
Note that this behavior can also occur if a previously exported 1024-bit key (high-grade) is improperly imported into a different system.
When this occurs, the 128-bit Cryptographic Service Provider (CSP) is not registered as the default provider. When you try to import the 1024-bit certificate onto a 1024-bit system, it does not work because it is trying to make a call to the default 512-bit base (exportable) provider, which does not accept strong cryptographic key sizes (>512 bit).
To correct this problem, change the following registry key:
Name = "Microsoft Base Cryptographic Provider v1.0"
Name = "Microsoft Enhanced Cryptographic Provider v1.0"
This behavior can also occur if the user key for the certificate you are trying to import already exists in the following registry key:
Article ID: 182054 - Last Review: Jun 22, 2014 - Revision: 1