A public key infrastructure (PKI) is a hierarchy of digital certificates. This section provides an overview of public key infrastructures and the types required for electronic ID documents.

PKIs in general

A public key infrastructure is based on the principle of asymmetric encryption. With asymmetric encryption, a key pair is created for everyone who wants to communicate in encrypted form. This consists of a private (secret) key and a public key. These are generated in such a way that a file encrypted with the public key can only be decrypted with the corresponding private key (we will not go into the mathematical background here). In addition, it is possible to digitally sign a file with the same private key. The associated public key can then be used to check whether the file has been unchanged since the signature.

A digital certificate contains the public key of a key pair like this with other information, such as who issued the certificate, for whom it was issued (= the owner of the matching private key) and the validity period. If two communication partners want to send messages to each other securely, they exchange their certificates and thus have the option to encrypt messages in such a way that only the other can decrypt them. In addition, they can also verify each other's digital signature.

In order for the certificates to be exchanged, however, the communication partners would have to know each other and find a secure way for the exchange, to ensure that they actually receive the certificate of the person or organisation they wish to communicate with. One option would be to send the certificates by e-mail and then to check the respective electronic fingerprint of the two certificates (this is a letter-number combination unique to each certificate) by telephone.

Public key infrastructures are formed to simplify the exchange of certificates and make the process possible, even if the communication partners do not know each other personally in advance.

In a public key infrastructure, i.e. a hierarchy of certificates, a root certificate with an associated key pair is created at a location that is trustworthy for all participants, known as a Certificate Authority (CA). This root certificate can be used as a trust anchor. Further certificates in this PKI are signed with the private key belonging to the root certificate. A signature for a certificate is only issued if all the requirements specified by the Certificate Authority have been met. Among other things, these include proof of the identity of the person who wants to use the certificate and their secure storage of the private key.

Not every certificate of a PKI has to be signed with the private key of the root certificate. Private keys whose associated certificates were signed with the private key of the root certificate can also be used. Theoretically, a chain like this can be any length as long as it starts with the root certificate. To check the authenticity and trustworthiness of a certificate from a PKI, all certificates that lie between the certificate to be checked and the root certificate must be verified.

The certificates are only valid for a limited time.

Public key infrastructures for electronic identity documents

Two public key infrastructures are required for the electronic ID documents in use or planned. A PKI for the authentication of electronic ID documents (protocol: passive authentication), the Country Signing Certificate Authority (CSCA) and a PKI for the protection of fingerprints on the electronic ID documents (protocol: terminal authentication), the Country Verifying Certificate Authority (CVCA).

For more information, please refer to our Technical Guideline BSI TR-03110.