How a PKI works

Public keys, private keys, and certificates

To encrypt and sign data, you need 2 pairs of keys:

  • public and private encryption keys
  • public and private signing keys

The keys consist of very long numbers linked in a particular way so the public key can be derived from the private key but not the other way around. Entities within a PKI generate their own keys. They always retain their private keys. However, they share their public keys with other entities to allow secure communication through encryption and signing.

Public keys are shared using certificates. A certificate is a file that contains:

  • a copy of the certificate owner’s public key
  • information about the identity of the certificate owner
  • an indication of the purpose of the certificate, for example, encryption or signing

Certificates are issued and signed by a certificate authority. The certificate authority acts as the trust anchor in the PKI. An entity that receives data from a third party can request confirmation from the certificate authority that the third party’s certificate is valid.

Keys and certificates in the GOV.UK Verify federation

The IDAP PKI and GOV.UK Verify federation use the X509 standard for digital certificates.

In the GOV.UK Verify federation, the IDAP certificate authority issues and signs certificates. As a government service using GOV.UK Verify, you need to request 4 certificates:

  • encryption and signing certificates for your service
  • encryption and signing certificates for your Matching Service Adapter

To obtain a certificate:

  1. Generate pair of keys (private and public). You must generate a pair of encryption keys for each encryption certificate request and a pair of signing keys for each signing certificate request.
  2. Submit a certificate signing request to the IDAP certificate authority.
  3. The certificate authority generates your certificate and sends it to you.

The certificate authority also issues certificates for the GOV.UK Verify hub and identity providers.

Data encryption and signing

Encryption certificates provide the sender with assurance that only the intended receiver can decrypt the message.

The sender extracts the public encryption key from the receiver’s encryption certificate and uses it to encrypt a message. The receiver decrypts the message using their corresponding private encryption key.

Diagram with an overview of how data encryption works. The GOV.UK Verify hub extracts the Matching Service Adapter public encryption key from the Matching Service Adapter encryption certificate. The GOV.UK Verify hub encrypts data and sends it to the Matching Service Adapter. The Matching Service Adapter decrypts the data using the Matching Service Adapter private encryption key.

Example of data encryption

Signing certificates provide the receiver with assurance of who authored the message. They also guarantee that the message hasn’t been tampered with since the author signed it.

When signing data, you use your private signing key to create a digital signature, and then send the signed message. The receivers check the digital signature using your public signing key from your signing certificate.

Diagram with an overview of how data signing works. The government service signs data using their private signing key. The government service sends the signed data to the GOV.UK Verify hub. The hub checks the signature using the government service public signing key.

Example of data signing

You can use encryption and signing together or alone.

Signing alone

When a user indicates that they would like to prove their identity, the government service sends a message to the GOV.UK Verify hub. The message is unencrypted because it contains no personal user information. However, the message is signed because the hub needs assurance that the message originated from the government service.

Encryption and signing

When a user has verified their identity, the identity provider sends a signed message to the hub containing the user’s encrypted personal information. This ensures the confidentiality of the message and that the message originated from the identity provider.

A message can contain embedded messages which may be encrypted and/or signed by different entities within the GOV.UK Verify federation. For more information, see the diagram showing the SAML message flow within the GOV.UK Verify federation.