Federation servers require the use of token-signing certificates to prevent attackers from altering or counterfeiting security tokens in an attempt to gain unauthorized access to federated resources. Every token-signing certificate contains cryptographic private keys and public keys that are used to digitally sign (by means of the private key) a security token. Later, after they are received by a partner federation server, these keys validate the authenticity (by means of the public key) of the encrypted security token.
When you deploy the first federation server in a new Active Directory Federation Services (AD FS) installation, you must obtain a token-signing certificate and install it in the local computer personal certificate store on that federation server. You can obtain a token-signing certificate by requesting one from an enterprise certification authority (CA) or a public CA or by creating a self-signed certificate.