Chapter 13. Digest Authentication

Table of Contents

Overview
Configuration

Overview

Spring Security provides a DigestProcessingFilter which is capable of processing digest authentication credentials presented in HTTP headers. Digest Authentication attempts to solve many of the weaknesses of Basic authentication, specifically by ensuring credentials are never sent in clear text across the wire. Many user agents support Digest Authentication, including FireFox and Internet Explorer. The standard governing HTTP Digest Authentication is defined by RFC 2617, which updates an earlier version of the Digest Authentication standard prescribed by RFC 2069. Most user agents implement RFC 2617. Spring Security DigestProcessingFilter is compatible with the "auth" quality of protection (qop) prescribed by RFC 2617, which also provides backward compatibility with RFC 2069. Digest Authentication is a highly attractive option if you need to use unencrypted HTTP (ie no TLS/HTTPS) and wish to maximise security of the authentication process. Indeed Digest Authentication is a mandatory requirement for the WebDAV protocol, as noted by RFC 2518 Section 17.1, so we should expect to see it increasingly deployed and replacing Basic Authentication.

Digest Authentication is definitely the most secure choice between Form Authentication, Basic Authentication and Digest Authentication, although extra security also means more complex user agent implementations. Central to Digest Authentication is a "nonce". This is a value the server generates. Spring Security's nonce adopts the following format:

    base64(expirationTime + ":" + md5Hex(expirationTime + ":" + key))
            
    expirationTime:   The date and time when the nonce expires, expressed in milliseconds
    key:              A private key to prevent modification of the nonce token
        

The DigestProcessingFilterEntryPoint has a property specifying the key used for generating the nonce tokens, along with a nonceValiditySeconds property for determining the expiration time (default 300, which equals five minutes). Whist ever the nonce is valid, the digest is computed by concatenating various strings including the username, password, nonce, URI being requested, a client-generated nonce (merely a random value which the user agent generates each request), the realm name etc, then performing an MD5 hash. Both the server and user agent perform this digest computation, resulting in different hash codes if they disagree on an included value (eg password). In Spring Security implementation, if the server-generated nonce has merely expired (but the digest was otherwise valid), the DigestProcessingFilterEntryPoint will send a "stale=true" header. This tells the user agent there is no need to disturb the user (as the password and username etc is correct), but simply to try again using a new nonce.

An appropriate value for DigestProcessingFilterEntryPoint's nonceValiditySeconds parameter will depend on your application. Extremely secure applications should note that an intercepted authentication header can be used to impersonate the principal until the expirationTime contained in the nonce is reached. This is the key principle when selecting an appropriate setting, but it would be unusual for immensely secure applications to not be running over TLS/HTTPS in the first instance.

Because of the more complex implementation of Digest Authentication, there are often user agent issues. For example, Internet Explorer fails to present an "opaque" token on subsequent requests in the same session. Spring Security filters therefore encapsulate all state information into the "nonce" token instead. In our testing, Spring Security implementation works reliably with FireFox and Internet Explorer, correctly handling nonce timeouts etc.