23. Kea Security

This is a work in progress. Currently it contains only the TLS/HTTPS support documentation.

23.1. TLS/HTTPS support

Since version 1.9.6 Kea includes TLS support for better security. TLS is used on HTTP communication providing three increasing levels of protection:

  • no TLS. The connection is plain text, unencrypted HTTP. This is the only option available in prior versions.
  • encryption i.e. protection against passive attacks and eavesdropping, the server is still authenticated but the client is not. This is the typical mode when securing a web site, where clients and servers are not under the control of a common entity.
  • mutual authentication between the client and the server. This mode is stricter than the previous one and is the default when TLS is enabled.


The mutual authentication is for the TLS entities. When TLS and a HTTP authentication scheme are used there is no binding between the two security mechanisms so no proof that the TLS client and server are the same as the HTTP authentication client and server.

23.1.1. Kea Installation with TLS/HTTPS support

TLS/HTTPS support is available with OpenSSL and Botan cryptographic backends with some constraints including on the boost library:

  • OpenSSL versions older than 1.0.2 are obsolete so should not be considered. The TLS support was not tested and is not supported on these versions.
  • OpenSSL version 1.0.2 has extended support for OpenSSL premium customers so should not be considered at the exception of these premium customers. The TLS support was tested but is not supported on this version.
  • OpenSSL versions >= 1.1.x were tested and are supported. Note enough recent versions include TLS 1.3 support.
  • OpenSSL 3.x was not yet released and Kea does not build with it.
  • LibreSSL 3.2.4 was tested. LibreSSL shares the OpenSSL 1.0.2 API so it should work but is not supported.
  • Botan 1.x versions are obsolete so should not be considered. The TLS support was not tested and is not supported on these versions.
  • Botan 2.x versions were tested and are supported. Note the TLS support requires the four asio header files which are included in Botan packages / installation only when Botan was configured with the --with-boost option. It is still possible to take these files from the corresponding Botan distribution and to install them manually in the Botan include directory but of course this should be a last resort procedure. Note that without these header files Kea can still build but the TLS support is disabled: any attempt to use it will fail with a fatal error.
  • very old boost versions provide SSL support (based on OpenSSL) without choice of the TLS version: Kea can still use them but they are not recommended.
  • boost versions older than 1.64 provide SSL support with a rigid choice of the TLS version: Kea enforces the use of TLS 1.2 with them.
  • boost versions 1.64 or newer provide SSL support with a generic TLS version: the best (higher) version available on both peers is selected.

23.1.2. TLS/HTTPS configuration

The new TLS configuration parameters are:

  • the trust-anchor string parameter specifies the name of a file or directory where the certification authority (CA) certificate of the other peer can be found. With OpenSSL the directory must include hash symbolic links. With Botan the directory is recursively searched for certificates.
  • the cert-file string parameter specifies the name of the file containing the end-entity certificate of the Kea instance being configured.
  • the key-file string parameter specifies the private key of the end-entity certificate of Kea instance being configured. The file must not be encrypted and it is highly recommended to restrict its access.

The three string parameters must be either all not specified (TLS disabled) or all specified (TLS enabled).

TLS is asymmetric: the authentication of the server by the client is mandatory but the authentication of the client by the server is optional. In TLS terms this means the server can require the client certificate or not so there is a server specific TLS parameter.

  • the cert-required boolean parameter allows a server to not require the client certificate. Its default value is true which means to require the client certificate and to authenticate the client. This flag has no meaning on the client side: the server always provides a certificate which is validated by the client.

Objects in files must be in the PEM format. Files can contain more than one certificate but this was not tested and is not supported.

Botan requires CA certificates to have the standard CA certificate attributes, verifies that end-entity certificates are version 3 certificates (as required by the TLS standard) and supports only PKCS 8 files for the private key.


Some cryptographic libraries (e.g. Botan and recent OpenSSL) enforce minimal strength (i.e. key length), e.g. at least 2048 for RSA.

A sample set of certificates and associated objects is available at src/lib/asiolink/testutils/ca in sources with a doc.txt file explaining how they were generated using the openssl command. These files are for testing purposes only. Do not use them in production,

TLS handshake, the phase where the cryptographic parameters are exchanged and authentication is verified, can fail in a lot of ways. Error messages often do not really help to find the source of the problem. Both OpenSSL and Botan provide a command line tool with a verify command which can be used to understand and fix it.