Java安全基础 | Baeldung中文网

1. Overview

In this tutorial, we’ll go through the basics of security on the Java platform. We’ll also focus on what’s available to us for writing secure applications.

Security is a vast topic that encompasses many areas. Some of these are part of the language itself, like access modifiers and class loaders. Furthermore, others are available as services, which include data encryption, secure communication, authentication, and authorization, to a name a few.

Therefore, it’s not practical to gain meaningful insight into all of these in this tutorial. However, we’ll try to gain at least a meaningful vocabulary.

2. Language Features

Above all, security in Java begins right at the level of language features. This allows us to write secure code, as well as benefit from many implicit security features:

Static Data Typing: Java is a statically typed language, which reduces the possibilities of run-time detection of type-related errors
Access Modifiers: Java allows us to use different access modifiers like public and private to control access to fields, methods, and classes
Automatic Memory Management: Java has garbage-collection based memory management, which frees developers from managing this manually
Bytecode Verification: Java is a compiled language, which means it converts code into platform-agnostic bytecode, and runtime verifies every bytecode it loads for execution

This is not a complete list of security features that Java provides, but it’s good enough to give us some assurance!

3. Security Architecture in Java

Before we begin to explore specific areas, let’s spend some time understanding the core architecture of security in Java.

The core principles of security in Java are driven by interoperable and extensible Provider implementations. A particular implementation of Provider may implement some or all of the security services.

For example, some of the typical services a Provider may implement are:

Cryptographic Algorithms (such as DSA, RSA, or SHA-256)
Key generation, conversion, and management facilities (such as for algorithm-specific keys)

Java ships with many built-in providers. Also, it’s possible for an application to configure multiple providers with an order of preference.

Consequently, the provider framework in Java searches for a specific implementation of a service in all providers in the order of preference set on them.

Moreover, it’s always possible to implement custom providers with pluggable security functions in this architecture.

4. Cryptography

Cryptography is the cornerstone of security features in general and in Java. This refers to tools and techniques for secure communication in the presence of adversaries.

4.1. Java Cryptography

The Java Cryptographic Architecture (JCA) provides a framework to access and implement cryptographic functionalities in Java, including:

Digital signatures
Message digests
Symmetric and asymmetric ciphers
Message authentication codes
Key generators and key factories

Most importantly, Java makes use of Provider-based implementations for cryptographic functions.

Moreover, Java includes built-in providers for commonly used cryptographic algorithms like RSA, DSA, and AES, to name a few. We can use these algorithms to add security to data in rest, in use, or in motion.

4.2. Cryptography in Practice

A very common use case in applications is to store user passwords. We use this for authentication at a later point in time. Now, it’s obvious that storing plain text passwords compromises security.

So, one solution is to scramble the passwords in such a way that the process is repeatable, yet only one-way. This process is known as the cryptographic hash function, and SHA1 is one such popular algorithm.

So, let’s see how we can do this in Java:

MessageDigest md = MessageDigest.getInstance("SHA-1");
byte[] hashedPassword = md.digest("password".getBytes());

Here, MessageDigest is a cryptographic service that we are interested in. We’re using the method getInstance() to request this service from any of the available security providers.

5. Public Key Infrastructure

Public Key Infrastructure (PKI) refers to the setup that enables the secure exchange of information over the network using public-key encryption. This setup relies on trust that is built between the parties involved in the communication. This trust is based on digital certificates issued by a neutral and trusted authority known as a Certificate Authority (CA).

5.1. PKI Support in Java

Java platform has APIs to facilitate the creation, storage, and validation of digital certificates:

KeyStore: Java provides the KeyStore class for persistent storage of cryptographic keys and trusted certificates. Here, KeyStore can represent both key-store and trust-store files. These files have similar content but vary in their usage.
CertStore: Additionally, Java has the CertStore class, which represents a public repository of potentially untrusted certificates and revocation lists. We need to retrieve certificates and revocation lists for certificate path building amongst other usages.

Java has a built-in trust-store called “cacerts” that contains certificates for well known CAs.

5.2. Java Tools for PKI

Java has some really handy tools to facilitate trusted communication:

There is a built-in tool called “keytool” to create and manage key-store and trust-store
There is also another tool “jarsigner” that we can use to sign and verify JAR files

5.3. Working with Certificates in Java

Let’s see how we can work with certificates in Java to establish a secure connection using SSL. A mutually authenticated SSL connection requires us to do two things:

Present Certificate — We need to present a valid certificate to another party in the communication. For that, we need to load the key-store file, where we must have our public keys:

KeyStore keyStore = KeyStore.getInstance(KeyStore.getDefaultType());
char[] keyStorePassword = "changeit".toCharArray();
try(InputStream keyStoreData = new FileInputStream("keystore.jks")){
    keyStore.load(keyStoreData, keyStorePassword);
}

Verify Certificate — We also need to verify the certificate presented by another party in the communication. For this we need to load the trust-store, where we must have previously trusted certificates from other parties:

KeyStore trustStore = KeyStore.getInstance(KeyStore.getDefaultType());
// Load the trust-store from filesystem as before

We rarely have to do this programmatically and normally pass system parameters to Java at runtime:

-Djavax.net.ssl.trustStore=truststore.jks 
-Djavax.net.ssl.keyStore=keystore.jks

6. Authentication

Authentication is the process of verifying the presented identity of a user or machine based on additional data like password, token, or a variety of other credentials available today.

6.1. Authentication in Java

Java APIs makes use of pluggable login modules to provide different and often multiple authentication mechanisms to applications. LoginContext provides this abstraction, which in turn refers to configuration and loads an appropriate LoginModule.

While multiple providers make available their login modules, Java has some default ones available for use:

Krb5LoginModule, for Kerberos-based authentication
JndiLoginModule, for username and password-based authentication backed by an LDAP store
KeyStoreLoginModule, for cryptographic key-based authentication

One of the most common mechanisms of authentication is the username and password. Let’s see how we can achieve this through JndiLoginModule.

This module is responsible for getting the username and password from a user and verifying it against a directory service configured in JNDI:

LoginContext loginContext = new LoginContext("Sample", new SampleCallbackHandler());
loginContext.login();

Here, we are using an instance of LoginContext to perform the login. LoginContext takes the name of an entry in the login configuration — in this case, it’s “Sample”. Also, we have to provide an instance of CallbackHandler, using the LoginModule that interacts with the user for details like username and password.

Let’s take a look at our login configuration:

Sample {
  com.sun.security.auth.module.JndiLoginModule required;
};

Simple enough, it suggests that we’re using JndiLoginModule as a mandatory LoginModule.

7. Secure Communication

Communication over the network is vulnerable to many attack vectors. For instance, someone may tap into the network and read our data packets as they’re being transferred. Over the years, the industry has established many protocols to secure this communication.

7.1. Java Support for Secure Communication

Java provides APIs to secure network communication with encryption, message integrity, and both client and server authentication:

SSL/TLS: SSL and its successor, TLS, provide security over untrusted network communication through data encryption and public-key infrastructure. Java provides support of SSL/TLS through SSLSocket defined in the package “java.security.ssl“.
SASL: Simple Authentication and Security Layer (SASL) is a standard for authentication between client and server. Java supports SASL as part of the package “java.security.sasl“.
GGS-API/Kerberos: Generic Security Service API (GSS-API) offers uniform access to security services over a variety of security mechanisms like Kerberos v5. Java supports GSS-API as part of the package “java.security.jgss“.

7.2. SSL Communication in Action

Let’s now see how we can open a secure connection with other parties in Java using SSLSocket:

SocketFactory factory = SSLSocketFactory.getDefault();
try (Socket connection = factory.createSocket(host, port)) {
    BufferedReader input = new BufferedReader(
      new InputStreamReader(connection.getInputStream()));
    return input.readLine();
}

Here, we are using SSLSocketFactory to create SSLSocket. As part of this, we can set optional parameters like cipher suites and which protocol to use.

For this to work properly, we must have created and set our key-store and trust-store as we saw earlier.

8. Access Control

Access Control refers to protecting sensitive resources like a filesystem or codebase from unwarranted access. This is typically achieved by restricting access to such resources.

8.1. Access Control in Java

We can achieve access control in Java using classes Policy and Permission mediated through the SecurityManager class. SecurityManager is part of the “java.lang” package and is responsible for enforcing access control checks in Java.

When the class loader loads a class in the runtime, it automatically grants some default permissions to the class encapsulated in the Permission object. Beyond these default permissions, we can grant more leverage to a class through security policies. These are represented by the class Policy.

During the sequence of code execution, if the runtime encounters a request for a protected resource, SecurityManager* verifies the requested Permission against the installed *Policy through the call stack. Consequently, it either grants permission or throws SecurityException.

8.2. Java Tools for Policy

Java has a default implementation of Policy that reads authorization data from the properties file. However, the policy entries in these policy files have to be in a specific format.

Java ships with “policytool”, a graphical utility to compose policy files.

8.3. Access Control Through Example

Let’s see how we can restrict access to a resource like a file in Java:

SecurityManager securityManager = System.getSecurityManager();
if (securityManager != null) {
    securityManager.checkPermission(
      new FilePermission("/var/logs", "read"));
}

Here, we’re using SecurityManager to validate our read request for a file, wrapped in FilePermission.

But, SecurityManager delegates this request to AccessController. AccessController internally makes use of the installed Policy to arrive at a decision.

Let’s see an example of the policy file:

grant {
  permission 
    java.security.FilePermission
      <<ALL FILES>>, "read";
};

We are essentially granting read permission to all files for everyone. But, we can provide much more fine-grained control through security policies.

It’s worth noting that a SecurityManager might not be installed by default in Java. We can ensure this by always starting Java with the parameter:

-Djava.security.manager -Djava.security.policy=/path/to/sample.policy

9. XML Signature

XML signatures are useful in securing data and provide data integrity. W3C provides recommendations for governance of XML Signature. We can use XML signature to secure data of any type, like binary data.

9.1. XML Signature in Java

Java API supports generating and validating XML signatures as per the recommended guidelines. Java XML Digital Signature API is encapsulated in the package “java.xml.crypto“.

The signature itself is just an XML document. XML signatures can be of three types:

Detached: This type of signature is over the data that is external to the Signature element
Enveloping: This type of signature is over the data that is internal to the Signature element
Enveloped: This type of signature is over the data that contains the Signature element itself

Certainly, Java supports creating and verifying all the above types of XML signatures.

9.2. Creating an XML Signature

Now, we’ll roll up our sleeves and generate an XML signature for our data. For instance, we may be about to send an XML document over the network. Hence, we would want our recipient to be able to verify its integrity.

So, let’s see how we can achieve this in Java:

XMLSignatureFactory xmlSignatureFactory = XMLSignatureFactory.getInstance("DOM");
DocumentBuilderFactory documentBuilderFactory = DocumentBuilderFactory.newInstance();
documentBuilderFactory.setNamespaceAware(true);
 
Document document = documentBuilderFactory
  .newDocumentBuilder().parse(new FileInputStream("data.xml"));
 
DOMSignContext domSignContext = new DOMSignContext(
  keyEntry.getPrivateKey(), document.getDocumentElement());
 
XMLSignature xmlSignature = xmlSignatureFactory.newXMLSignature(signedInfo, keyInfo);
xmlSignature.sign(domSignContext);

To clarify, we’re generating an XML signature for our data present in the file “data.xml”. Meanwhile, there are a few things to note about this piece of code:

Firstly, XMLSignatureFactory is the factory class for generating XML signatures
XMLSigntaure requires a SignedInfo object over which it calculates the signature
XMLSigntaure also needs KeyInfo, which encapsulates the signing key and certificate
Finally, XMLSignature signs the document using the private key encapsulated as DOMSignContext

As a result, the XML document will now contain the Signature element, which can be used to verify its integrity.

10. Security Beyond Core Java

As we have seen by now, the Java platform provides a lot of the necessary functionality to write secure applications. However, sometimes, these are quite low-level and not directly applicable to, for example, the standard security mechanism on the web.

For example, when working on our system, we generally don’t want to have to read the full OAuth RFC and implement that ourselves. We often need quicker, higher-level ways to achieve security. This is where application frameworks come into the picture – these help us achieve our objective with much less boilerplate code.

And, on the Java platform – generally that means Spring Security. The framework is part of the Spring ecosystem, but it can actually be used outside of pure Spring application.

In simple terms, it helps is achieve authentication, authorization and other security features in a simple, declarative, high-level manner.

Of course, Spring Security is extensively covered in a series of tutorials, as well as in a guided way, in the Learn Spring Security course.

11. Conclusion

In short, in this tutorial, we went through the high-level architecture of security in Java. Also, we understood how Java provides us with implementations of some of the standard cryptographic services.

We also saw some of the common patterns that we can apply to achieve extensible and pluggable security in areas like authentication and access control.

To sum up, this just provides us with a sneak peek into the security features of Java. Consequently, each of the areas discussed in this tutorial merits further exploration. But hopefully, we should have enough insight to get started in this direction!

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