The Java SASL API
Programming and Deployment Guide

  1. Introduction
  2. API Overview
  3. How SASL Mechanisms are Installed and Selected
  4. The SunSASL Provider
  5. Implementing a SASL Security Provider

Introduction

What is SASL?

Simple Authentication and Security Layer, or SASL, is an Internet standard (RFC 2222) that specifies a protocol for authentication and optional establishment of a security layer between client and server applications. SASL defines how authentication data is to be exchanged but does not itself specify the contents of that data. It is a framework into which specific authentication mechanisms that specify the contents and semantics of the authentication data can fit.

SASL is used by protocols, such as the Lightweight Directory Access Protocol, version 3 (LDAP v3), and the Internet Message Access Protocol, version 4 (IMAP v4) to enable pluggable authentication. Instead of hardwiring an authentication method into the protocol, LDAP v3 and IMAP v4 use SASL to perform authentication, thus enabling authentication via various SASL mechanisms.

There are a number of standard SASL mechanisms defined by the Internet community for various levels of security and deployment scenarios. These range from no security (e.g., anonymous authentication) to high security (e.g., Kerberos authentication) and levels in between.

The Java SASL API

The Java SASL API defines classes and interfaces for applications that use SASL mechanisms. It is defined to be mechanism-neutral: the application that uses the API need not be hardwired into using any particular SASL mechanism. The API supports both client and server applications. It allows applications to select the mechanism to use based on desired security features, such as whether they are susceptible to passive dictionary attacks or whether they accept anonymous authentication.

The Java SASL API also allows developers to use their own, custom SASL mechanisms. SASL mechanisms are installed by using the Java Cryptography Architecture (JCA).

When to Use SASL

SASL provides pluggable authentication and security layer for network applications. There are other features in the Java SE that provide similar functionality, including the Java Secure Socket Extension (JSSE), and Java Generic Security Service (Java GSS). JSSE provides a framework and an implementation for a Java language version of the SSL and TLS protocols. Java GSS is the Java language bindings for the Generic Security Service Application Programming Interface (GSS-API). The only mechanism currently supported underneath this API on Java SE is Kerberos v5.

When compared with JSSE and Java GSS, SASL is relatively lightweight and is popular among more recent protocols. It also has the advantage that several popular, lightweight (in terms infrastructure support) SASL mechanisms have been defined. Primary JSSE and Java GSS mechanisms, on the other hand, have relatively heavyweight mechanisms that require more elaborate infrastructures (Public Key Infrastructure and Kerberos, respectively).

SASL, JSSE, and Java GSS are often used together. For example, a common pattern is for an application to use JSSE for establishing a secure channel, and to use SASL for client, username/password-based authentication. There are also SASL mechanisms layered on top of GSS-API mechanisms; one popular example is a SASL GSS-API/Kerberos v5 mechanism that is used with LDAP.

Except when defining and building protocols from scratch, often the biggest factor determining which API to use is the protocol definition. For example, LDAP and IMAP are defined to use SASL, so software related to these protocols should use the Java SASL API. When building Kerberos applications and services, the API to use is Java GSS. When building applications and services that use SSL/TLS as their protocol, the API to use is JSSE. See the Java Security Documentation for further details about when to use JSSE versus Java GSS.

API Overview

SASL is a challenge-response protocol. The server issues a challenge to the client, and the client sends a response based on the challenge. This exchange continues until the server is satisfied and issues no further challenge. These challenges and responses are binary tokens of arbitrary length. The encapsulating protocol (such as LDAP or IMAP) specifies how these tokens are encoded and exchanged. For example, LDAP specifies how SASL tokens are encapsulated within LDAP bind requests and responses.

The Java SASL API is modeled according to this style of interaction and usage. It has interfaces, SaslClient and SaslServer, that represent client-side and server-side mechanisms, respectively. The application interacts with the mechanisms via byte arrays that represent the challenges and responses. The server-side mechanism iterates, issuing challenges and processing responses, until it is satisfied, while the client-side mechanism iterates, evaluating challenges and issuing responses, until the server is satisfied. The application that is using the mechanism drives each iteration. That is, it extracts the challenge or response from a protocol packet and supplies it to the mechanism, and then puts the response or challenge returned by the mechanism into a protocol packet and sends it to the peer.

Creating the Mechanisms

The client and server code that uses the SASL mechanisms are not hardwired to use specific mechanism(s). In many protocols that use SASL, the server advertises (either statically or dynamically) a list of SASL mechanisms that it supports. The client then selects one of these based on its security requirements.

The Sasl class is used for creating instances of SaslClient and SaslServer. Here is an example of how an application creates a SASL client mechanism using a list of possible SASL mechanisms.

String[] mechanisms = new String[]{"DIGEST-MD5", "PLAIN"}; 
SaslClient sc = Sasl.createSaslClient(mechanisms, authzid, protocol, 
                serverName, props, callbackHandler);

Based on the availability of the mechanisms supported by the platform and other configuration information provided via the parameters, the Java SASL framework selects one of the listed mechanisms and return an instance of SaslClient.

The name of the selected mechanism is usually transmitted to the server via the application protocol. Upon receiving the mechanism name, the server creates a corresponding SaslServer object to process client-sent responses. Here is an example of how the server would create an instance of SaslServer.

SaslServer ss = Sasl.createSaslServer(mechanism, protocol, 
                  myName, props, callbackHandler);

Passing Input to the Mechanisms

Because the Java SASL API is a general framework, it must be able to accommodate many different types of mechanisms. Each mechanism needs to be initialized with input and may need input to make progress. The API provides three means by which an application gives input to a mechanism.

  1. Common input parameters. The application uses predefined parameters to supply information that are defined by the SASL specification and commonly required by mechanisms. For SASL client mechanisms, the input parameters are authorization id, protocol id, and server name. For SASL server mechanisms, the common input parameters are prototol id and (its own fully qualified) server name.
  2. Properties parameter. The application uses the properties parameter, a mapping of property names to (possibly non-string) property values, to supply configuration information. The Java SASL API defines some standard properties, such as quality-of-protection, cipher strength, and maximum buffer size. The parameter can also be used to pass in non-standard properties that are specific to particular mechanisms.
  3. Callbacks. The application uses the callback handler parameter to supply input that cannot be predetermined or might not be common across mechanisms. When a mechanism requires input data, it uses the callback handler supplied by the application to collect the data, possibly from the end-user of the application. For example, a mechanism might require the end-user of the application to supply a name and password.

    Mechanisms can use the callbacks defined in the javax.security.auth.callback package; these are generic callbacks useful for building applications that perform authentication. Mechanisms might also need SASL-specific callbacks, such as those for collecting realm and authorization information, or even (non-standardized) mechanism-specific callbacks. The application should be able to accommodate a variety of mechanisms. Consequently, its callback handler must be able to service all of the callbacks that the mechanisms might request. This is not possible in general for arbitrary mechanisms, but is usually feasible due to the limited number of mechanisms that are typically deployed and used.

Using the Mechanisms

Once the application has created a mechanism, it uses the mechanism to obtain SASL tokens to exchange with the peer. The client typically indicates to the server via the application protocol which mechanism to use. Some protocols allows the client to accompany the request with an optional initial response for mechanisms that have an initial response. This feature can be used to lower the number of message exchanges required for authentication. Here is an example of how a client might use SaslClient for authentication. 
// Get optional initial response
byte[] response = 
    (sc.hasInitialResponse() ? sc.evaluateChallenge(new byte[]) : null);

String mechanism = sc.getMechanismName();

// Send selected mechanism name and optional initial response to server
send(mechanism, response);

// Read response
msg = receive();
while (!sc.isComplete() && (msg.status == CONTINUE || msg.status == SUCCESS)) {
    // Evaluate server challenge
    response = sc.evaluateChallenge(msg.contents);

    if (msg.status == SUCCESS) {
        // done; server doesn't expect any more SASL data
        if (response != null) {
           throw new IOException(
               "Protocol error: attempting to send response after completion");
        }
        break;
    } else {
        send(mechanism, response);
        msg = receive();
    }

The client application iterates through each step of the authentication by using the mechanism (sc) to evaluate the challenge gotten from the server and to get a response to send back to the server. It continues this cycle until either the mechanism or application-level protocol indicates that the authentication has completed, or if the mechanism cannot evaluate a challenge. If the mechanism cannot evaluate the challenge, it throws an exception to indicate the error and terminates the authentication. Disagreement between the mechanism and protocol about the completion state must be treated as an error because it might indicate a compromise of the authentication exchange.

Here is an example of how a server might use SaslServer.

// Read request that contains mechanism name and optional initial response
msg.receive();

// Obtain a SaslServer to perform authentication
SaslServer ss = Sasl.createSaslServer(msg.mechanism, 
    protocol, myName, props, callbackHandler);

// Perform authentication steps until done
while (!ss.isComplete()) {
    try {
        // Process response
        byte[] challenge = sc.evaluateResponse(msg.contents);

        if (ss.isComplete()) {
            send(mechanism, challenge, SUCCESS);
        } else {
            send(mechanism, challenge, CONTINUE);
            msg.receive();
        }
    } catch (SaslException e) {
        send(ERROR);
        sc.dispose();
        break;
    }
}
The server application iterates through each step of the authentication by giving the client's response to the mechanism (ss) to process. If the response is incorrect, the mechanism indicates the error by throwing a SaslException so that the server can report the error and terminate the authentication. If the response is correct, the mechanism returns challenge data to be sent to the client and indicates whether the authentication is complete. Note that challenge data can accompany a "success" indication. This might be used, for example, to tell the client to finalize some negotiated state.

Using the Negotiated Security Layer

Some SASL mechanisms support only authentication while others support use of a negotiated security layer after authentication. The security layer feature is often not used when the application uses some other means, such as SSL/TLS, to communicate securely with the peer.

When a security layer has been negotiated, all subsequent communication with the peer must take place using the security layer. To determine whether a security layer has been negotiated, get the negotiated quality-of-protection (QOP) from the mechanism. Here is an example of how to determine whether a security layer has been negotiated.

String qop = (String) sc.getNegotiatedProperty(Sasl.QOP);
boolean hasSecurityLayer = (qop != null && 
    (qop.equals("auth-int") || qop.equals("auth-conf")));

A security layer has been negotiated if the Sasl.QOP property indicates that either integrity and/or confidentiality has been negotiated.

To communicate with the peer using the negotiated layer, the application first uses the wrap method to encode the data to be sent to the peer to produce a "wrapped" buffer. It then transfers a length field representing the number of octets in the wrapped buffer followed by the contents of the wrapped buffer to the peer. The peer receiving the stream of octets passes the buffer (without the length field) to unwrap to obtain the decoded bytes sent by the peer. Details of this protocol are described in RFC 2222. Here is an example of how a client application sends and receives application data using a security layer.

// Send outgoing application data to peer
byte[] outgoing = ...;
byte[] netOut = sc.wrap(outgoing, 0, outgoing.length);

send(netOut.length, netOut);   // send to peer

// Receive incoming application data from peer
byte[] netIn = receive();      // read length and ensuing bytes from peer

byte[] incoming = sc.unwrap(netIn, 0, netIn.length);

How SASL Mechanisms are Installed and Selected

SASL mechanism implementations are provided by SASL security providers. Each provider may support one or more SASL mechanisms and is registered with the Java Cryptography Architecture (JCA). By default, in J2SE 5, the SunSASL provider is automatically registered as a JCA provider. To remove it or reorder its priority as a JCA provider, change the line 
security.provider.7=com.sun.security.sasl.Provider
in the Java Security Properties file ($JAVA_HOME/lib/security/java.security).

To add or remove a SASL provider, you add or remove the corresponding line in the Security Properties file. For example, if you want to add a SASL provider and have its mechanisms be chosen over the same ones implemented by the SunSASL provider, then you would add a line to the Security Properties file with a lower number.

security.provider.7=com.example.MyProvider
security.provider.8=com.sun.security.sasl.Provider

Alternatively, you can programmatically add your own provider using the java.security.Security class. For example, the following sample code registers the com.example.MyProvider to the list of available SASL security providers.

Security.addProvider(new com.example.MyProvider());
When an application requests a SASL mechanism by supplying one or more mechanism names, the SASL framework looks for registered SASL providers that support that mechanism by going through, in order, the list of registered providers. The providers must then determine whether the requested mechanism matches the selection policy properties and if so, return an implementation for the mechanism.

The selection policy properties specify the security aspects of a mechanism, such as its susceptibility to certain attacks. These are characteristics of the mechanism (definition), rather than its implementation so all providers should come to the same conclusion about a particular mechanism. For example, the PLAIN mechanism is susceptible to plaintext attacks regardless of how it is implemented. If no selection policy properties are supplied, there are no restrictions on the selected mechanism. Using these properties, an application can ensure that it does not use unsuitable mechanisms that might be deployed in the execution environment. For example, an application might use the following sample code if it does not want to allow the use of mechanisms susceptible to plaintext attacks.

Map props = new HashMap();
props.add(Sasl.POLICY_NOPLAINTEXT, "true");
SaslClient sc = Sasl.createSaslClient(mechanisms,
    authzid, protocol, serverName, props, callbackHandler);
See the Sasl class for descriptions of the selection policy properties.

The SunSASL Provider

The SunSASL provider supports the following client and server mechanisms.

Client Mechanisms

The SunSASL provider supports several SASL mechanisms used in popular protocols such as LDAP, IMAP, and SMTP. The following table summarizes the client mechanisms and their required input.
Client Mechanism Name Parameters/Input Callbacks Configuration Properties Selection Policy
CRAM-MD5 authorization id (as default username) NameCallback
PasswordCallback		   Sasl.POLICY_NOANONYMOUS
Sasl.POLICY_NOPLAINTEXT
DIGEST-MD5 authorization id
protocol id
server name		 NameCallback
PasswordCallback
RealmCallback
RealmChoiceCallback		 Sasl.QOP
Sasl.STRENGTH
Sasl.MAX_BUFFER
Sasl.SERVER_AUTH
"javax.security.sasl.sendmaxbuffer"
"com.sun.security.sasl.digest.cipher"
 Sasl.POLICY_NOANONYMOUS
Sasl.POLICY_NOPLAINTEXT
EXTERNAL authorization id
external channel		     Sasl.POLICY_NOPLAINTEXT
Sasl.POLICY_NOACTIVE
Sasl.POLICY_NODICTIONARY
GSSAPI JAAS Subject
authorization id
protocol id
server name		   Sasl.QOP
Sasl.MAX_BUFFER
Sasl.SERVER_AUTH
"javax.security.sasl.sendmaxbuffer"
 Sasl.POLICY_NOACTIVE
Sasl.POLICY_NOANONYMOUS
Sasl.POLICY_NOPLAINTEXT
PLAIN authorization id NameCallback
PasswordCallback		   Sasl.POLICY_NOANONYMOUS

An application that uses these mechanisms from the SunSASL provider must supply the required parameters, callbacks and properties. The properties have reasonable defaults and only need to be set if the application wants to override the defaults. Most of the parameters, callbacks, and properties are described in the API documentation. The following sections describe mechanism-specific behaviors and parameters not already covered by the API documentation.

Cram-MD5

The Cram-MD5 client mechanism uses the authorization id parameter, if supplied, as the default username in the NameCallback to solicit the application/end-user for the authentication id. The authorization id is otherwise not used by the Cram-MD5 mechanism; only the authentication id is exchanged with the server.

Digest-MD5

The Digest-MD5 mechanism is used for digest authentication and optional establishment of a security layer. It specifies the following ciphers for use with the security layer: Triple DES, DES and RC4 (128, 56, and 40 bits). The Digest-MD5 mechanism can support only ciphers that are available on the platform. For example, if the platform does not support the RC4 ciphers, then the Digest-MD5 mechanism will not use those ciphers.

The Sasl.STRENGTH property supports "high", "medium", and "low" settings; its default is "high,medium,low". The ciphers are mapped to the strength settings as follows.

Strength Cipher Cipher Id
high Triple DES
RC4 128 bits		 3des rc4
medium DES
RC4 56 bits		 des rc4-56
low RC4 40 bits rc4-40

When there is more than one choice for a particular strength, the cipher selected depends on the availability of the ciphers in the underlying platform. To explicitly name the cipher to use, set the "com.sun.security.sasl.digest.cipher" property to the corresponding cipher id. Note that this property setting must be compatible with Sasl.STRENGTH and the ciphers available in the underlying platform. For example, Sasl.STRENGTH being set to "low" and "com.sun.security.sasl.digest.cipher" being set to "3des" are incompatible. The "com.sun.security.sasl.digest.cipher" property has no default.

The "javax.security.sasl.sendmaxbuffer" property specifies (the string representation of) the maximum send buffer size in bytes. The default is 65536. The actual maximum number of bytes will be the minimum of this number and the peer's maximum receive buffer size.

GSSAPI

The GSSAPI mechanism is used for Kerberos v5 authentication and optional establishment of a security layer. The mechanism expects the calling thread's Subject to contain the client's Kerberos credentials or that the credentials could be obtained by implicitly logging in to Kerberos. To obtain the client's Kerberos credentials, use the Java Authentication and Authorization Service (JAAS) to log in using the Kerberos login module. See the Java GSS-API and JAAS Tutorials for Use with Kerberos for details and examples. After using JAAS authentication to obtain the Kerberos credentials, you put the code that uses the SASL GSSAPI mechanism within doAs or doAsPrivileged. 
LoginContext lc = new LoginContext("JaasSample", new TextCallbackHandler());
lc.login();
lc.getSubject().doAs(new SaslAction());

class SaslAction implements java.security.PrivilegedAction {
   public class run() {
       ...
       String[] mechanisms = new String[]{"GSSAPI"};
       SaslClient sc = Sasl.createSaslClient(mechanisms,
           authzid, protocol, serverName, props, callbackHandler);
       ...
   }
}
To obtain Kerberos credentials without doing explicit JAAS programming, see the Java GSS-API and JAAS Tutorials for Use with Kerberos. When using this approach, there is no need to wrap the code within doAs or doAsPrivileged.

The "javax.security.sasl.sendmaxbuffer" property specifies (the string representation of) the maximum send buffer size in bytes. The default is 65536. The actual maximum number of bytes will be the minimum of this number and the peer's maximum receive buffer size.

Server Mechanisms

Here is a table that summarizes the server mechanisms and their required input.
Server Mechanism Name Parameters/Input Callbacks Configuration Properties Selection Policy
CRAM-MD5 server name AuthorizeCallback
NameCallback
PasswordCallback		   Sasl.POLICY_NOANONYMOUS
Sasl.POLICY_NOPLAINTEXT
DIGEST-MD5 protocol id
server name		 AuthorizeCallback
NameCallback
PasswordCallback
RealmCallback		 Sasl.QOP
Sasl.STRENGTH
Sasl.MAX_BUFFER
"javax.security.sasl.sendmaxbuffer"
"com.sun.security.sasl.digest.realm"
"com.sun.security.sasl.digest.utf8"		 Sasl.POLICY_NOANONYMOUS
Sasl.POLICY_NOPLAINTEXT
GSSAPI JAAS Subject
protocol id
server name		 AuthorizeCallback Sasl.QOP
Sasl.MAX_BUFFER
"javax.security.sasl.sendmaxbuffer"
 Sasl.POLICY_NOACTIVE
Sasl.POLICY_NOANONYMOUS
Sasl.POLICY_NOPLAINTEXT

An application that uses these mechanisms from the SunSASL provider must supply the required parameters, callbacks and properties. The properties have reasonable defaults and only need to be set if the application wants to override the defaults.

All users of server mechanisms must have a callback handler that deals with the AuthorizeCallback. This is used by the mechanisms to determine whether the authenticated user is allowed to act on behalf of the requested authorization id, and also to obtain the canonicalized name of the authorized user (if canonicalization is applicable).

Most of the parameters, callbacks, and properties are described in the API documentation. The following sections describe mechanism-specific behaviors and parameters not already covered by the API documentation.

Cram-MD5

The Cram-MD5 server mechanism uses the NameCallback and PasswordCallback to obtain the password required to verify the SASL client's response. The callback handler should use the NameCallback.getDefaultName() as the key to fetch the password.

Digest-MD5

The Digest-MD5 server mechanism uses the RealmCallback, NameCallback, and PasswordCallback to obtain the password required to verify the SASL client's response. The callback handler should use
RealmCallback.getDefaultText() and NameCallback.getDefaultName() as keys to fetch the password.

The "javax.security.sasl.sendmaxbuffer" property is described in the Digest-MD5 client section.

The "com.sun.security.sasl.digest.realm" property is used to specify a list of space-separated realm names that the server supports. The list is sent to the client as part of the challenge. If this property has not been set, the default realm is the server's name (supplied as a parameter).

The "com.sun.security.sasl.digest.utf8" property is used to specify the character encoding to use. "true" means to use UTF-8 encoding; "false" means to use ISO Latin 1 (ISO-8859-1). The default is "true".

GSSAPI

The GSSAPI server mechanism has the same requirements as the GSSAPI client mechanism in terms of Kerberos credentials and the "javax.security.sasl.sendmaxbuffer" property.

Debugging and Monitoring

The SunSASL provider uses the Logging APIs to provide implementation logging output. This output can be controlled by using the logging configuration file and programmatic API (java.util.logging). The logger name used by the SunSASL provider is "javax.security.sasl". Here is a sample logging configuration file that enables the FINEST logging level for the SunSASL provider. 
javax.security.sasl.level=FINEST
handlers=java.util.logging.ConsoleHandler
java.util.logging.ConsoleHandler.level=FINEST

The table below shows the mechanisms and the logging output that they generate.

Mechanism Logging Level Information Logged
CRAM-MD5 FINE Configuration properties; challenge/response messages
DIGEST-MD5 INFO Message discarded due to encoding problem
(e.g., unmatched MACs, incorrect padding)
DIGEST-MD5 FINE Configuration properties; challenge/response messages
DIGEST-MD5 FINER More detailed information about challenge/response messages
DIGEST-MD5 FINEST Buffers exchanged at the security layer
GSSAPI FINE Configuration properties; challenge/response messages
GSSAPI FINER More detailed information about challenge/response messages
GSSAPI FINEST Buffers exchanged at the security layer

Implementing a SASL Security Provider

There are three basic steps in implementing a SASL security provider.
  1. Write a class that implements the SaslClient or SaslServer interface.
  2. Write a factory class (that implements SaslClientFactory or SaslServerFactory) that creates instances of the class.
  3. Write a JCA provider that registers the factory.

The first step involves providing an implementation for the SASL mechanism. To implement a client mechanism, you need to implement the methods declared in the SaslClient interface. Similarly, for a server mechanism, you need to implement the methods declared in the SaslServer interface. For the purposes of this discussion, suppose you are developing an implementation for the client mechanism "SAMPLE-MECH", implemented by the class, com.example.SampleMechClient. You must decide what input are needed by the mechanism and how the implementation is going to collect them. For example, if the mechanism is username/password-based, then the implementation would likely need to collect that information via the callback handler parameter.

The next step involves providing a factory class that will create instances of com.example.SampleMechClient. The factory needs to determine the characteristics of the mechanism that it supports (as described by the Sasl.POLICY_* properties) so that it can return an instance of the mechanism when the API user requests it using compatible policy properties. The factory may also check for validity of the parameters before creating the mechanism. For the purposes of this discussion, suppose the factory class is named com.example.MySampleClientFactory. Although our sample factory is responsible for only one mechanism, a single factory can be responsible for any number of mechanisms.

The final step involves creating a JCA provider. The steps for creating a JCA provider is described in detail in the document, How to Implement a Provider for the Java Cryptography Architecture. SASL client factories are registered using property names of the form

SaslClientFactory.mechName

while SASL server factories are registered using property names of the form

SaslServerFactory.mechName

mechName is the SASL mechanism's name. This is what's returned by SaslClient.getMechanismName() and SaslServer.getMechanismName(). Continuing with our example, here is how the provider would register the "SAMPLE-MECH" mechanism.

put("SaslClientFactory.SAMPLE-MECH", "com.example.MySampleClientFactory");

A single SASL provider might be responsible for many mechanisms. Therefore, it might have many invocations of put to register the relevant factories. The completed SASL provider can then be made available to applications using the instructions given previously.

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