KB - Public Key Infrastructure and Certificates

Security: Public Key Infrastructure and Certificates

Use of digital certificates for encryption and digital signing relies on a combination of supporting elements known as a public key infrastructure (PKI). These elements include software applications such as PK Protect that work with certificates and keys as well as underlying technologies and services.

The heart of PKI is a mechanism by which two cryptographic keys associated with a piece of data called a certificate are used for encryption/decryption and for digital signing and authentication. One of the keys is private and must be kept secure so that only its owner can use it. The other is a public key that may be freely distributed for anyone to use to encrypt data intended for the owner of the certificate or to authenticate signatures. The keys look like long character strings but represent very large numbers. 

End entity certificates and their related keys are used for signing and authentication. They are created at the end of the trust hierarchy of certificate authorities. Each certificate is signed by its CA issuer and is identified in the “Issued By” field in the end certificate. In turn, a CA certificate can also be issued by a higher level CA. Such certificates are known as intermediate CA certificates. At the top of the issuing chain is a self-signed certificate known as the root.

How the Keys Are Used

With encryption/decryption, a copy of the public key is used to encrypt data such that only the possessor of the private key can decrypt it. Thus anyone with the public key can encrypt for a recipient, and only the targeted recipient has the key with which to decrypt.

With digital signing and authentication, the owner of the certificate uses the private key to sign data, and anyone with access to a copy of the certificate containing the public key can authenticate the signature and be assured that the signed data really proceeds unchanged from the signer.

Authentication for an X.509 key has one additional step. As an assurance that the signer is who he says he is—that the certificate with Bob’s name on it is not fraudulent—the signer’s certificate itself is signed by an issuing certificate authority (CA). The CA in effect vouches that Bob is who he says he is. The CA signature is authenticated using the public key of the CA certificate used. This CA certificate too may be signed, but at some point the trust chain stops with a self-signed root CA certificate that is simply trusted. The PKI provides for these several layers of end-user public key certificates, intermediate CA certificates, and root certificates, as well as for users’ private keys.