Following information is from a blog post
IOS IPS uses a crypto key to verify the digital signature for the master signature file, which is signed using a private key. To verify the signature, we need a corresponding public key. This key is available as a text file on Cisco’s site.
Which means we need to inform the router about realm-cisco.pub.key.txt file . This is mentioned in http://blog.ine.com/2010/01/14/ios-ips-ccie-rs-ccie-sc/ but I dont see this file name or configuration in Cisco's DOC CD link http://www.cisco.com/en/US/docs/ios/sec_data_plane/configuration/guide/sec_cfg_ips_external_docbase_0900e4b180de56d7_4container_external_docbase_0900e4b180e076b5.html
The file is needed for verification, as already mentioned. The public key is located at Cisco's main Web site. See this link. http://www.cisco.com/en/US/prod/collateral/iosswrel/ps6537/ps6586/ps6634/prod_white_paper0900aecd805c4ea8.html