SPKAC is an acronym that stands for Signed Public Key and Challenge, also known as Netscape SPKI.
Contents
It is a format for sending a Certification Signing Request: it encodes a public key, that can be manipulated using openssl. It is created using the little documented HTML keygen element inside a number of Netscape compatible browsers.
Implementations
HTML5 originally specificed the <keygen> element to support SPKAC in the browser to make it easier to create client side certificates through a web service for protocols such as WebID; however, subsequent work for HTML 5.1 placed the keygen element "at-risk", and the first public working draft of HTML 5.2 removes the keygen element entirely. The removal of the keygen element is due to non-interoperability and non-conformity from a standards perspective in addition to security concerns. The W3C Web Authentication Working Group is working on the Web Authentication API to replace the keygen element.
Bouncy Castle provides a Java class.
An implementation for Erlang/OTP exists too.
An implementation for Python is named pyspkac.
PHP OpenSSL extension as of version 5.6.0.
node.js implementation.
Deficiencies
The user interface needs to be improved in browsers, to make it more obvious to users when a server is asking for the client certificate.