On Cisco ISR routers, for what purpose is the realm-cisco.pub public encryption key used?

A.
used for SSH server/client authentication and encryption

B.
used to verify the digital signature of the IPS signature file

C.
used to generate a persistent self-signed identity certificate for the ISR so administrators can
authenticate the ISR when accessing it using Cisco Configuration Professional

D.
used to enable asymmetric encryption on IPsec and SSL VPNs

E.
used during the DH exchanges on IPsec VPNs

Explanation:
http://www.cisco.com/en/US/prod/collateral/iosswrel/ps6537/ps6586/ps6634/prod_white_paper0
900aecd805c4ea8.html
Step 1: Downloading IOS IPS files
The first step is to download IOS IPS signature package files and public crypto key from Cisco.com.
Step 1.1: Download the required signature files from Cisco.com to your PC
• Location:
http://tools.cisco.com/support/downloads/go/Model.x?mdfid=281442967&mdfLevel=Software%20
Family&treeName=Security&modelName=Cisco%20IOS%20Intrusion%20Prevention%20System%20F
eature%20Software&treeMdfId=268438162
• Files to download:
IOS-Sxxx-CLI.pkg: Signature package – download the latest signature package.
realm-cisco.pub.key.txt: Public Crypto key – this is the crypto key used by IOS IPS