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MT311 (Oct 2004) Java Application Development. Java Security, Cryptography, Authentication, PKI, Digital Signature & SSL. Tutorial 4. Tutor Information. Edmund Chiu (Group 4) Email: OR Please begin your email subject with [MT311]

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Mt311 oct 2004 java application development

MT311 (Oct 2004)Java Application Development

Java Security, Cryptography, Authentication, PKI, Digital Signature & SSL

Tutorial 4

Tutor information
Tutor Information

  • Edmund Chiu (Group 4)

  • Email:

  • Please begin your email subject with [MT311]

  • Webpage:

Security issues in java
Security Issues in Java

  • When a computer is connected to a network, it is not enough to have physical security alone.

    • Communication security is a concern when computers communicate across a network

  • In Java applications, there are two main areas of security issues:

    • System security – safety and stability of the computing environment against malicious applications (e.g. virus)

    • Information security – the secrecy and integrity of data

Sandbox model
Sandbox Model

  • In early days, Java use a basic security model called the sandbox model

    • Local applications are considered as trustable and are granted full access to the system

    • Download applets are not trusted and could not use any local resources

  • It is a quick solution related to downloaded applications at that time, but

    • has lots of shortcomings because you can either trust or not trust the entire application/applet

    • Cannot prevent a local application downloaded from the Internet from accessing a sensitive file

    • Even printing from an applet is impossible

Security policy model
Security Policy Model

  • In a security policy model, resource can be granted or denied different access independently

    • For example, if the resource is a file, you can grant a “read”, “write”, “execute” and “delete” permission

    • We can grant different types of permission to the following different resources also

      • Audio, AWT, Network, Property, Runtime and etc

      • Details are given in the Java Documentation on Security

  • To provide an even more flexible implementation, Java allows different policies to apply to different application, or even to different invocations of the same application.

Applet security
Applet Security

  • Consider an Applet writing a file to your PC

    • Click Here to view the Applet source code ( on course homepage)

  • By default, Sandbox Model is used

    • Access denied. A security exception is shown in the applet

Applying security policy in applet
Applying Security Policy in Applet

  • You local PC needs to release the restriction by specifying a different policy

    grant { permission "AppletSecurityTest.txt", "write";};

    • Save the above text as a policy file named “policy.AllowWrite” in you current directory

    • View your applet using applet viewer applying the policy

      appletviewer AppletSecurity.html

Security policy
Security Policy

  • As in the previous example, you can see a policy is identified in the following format:

    permission <permission> <target> <action>;

    • Permission type (

    • The target source (

    • The action (write)

  • You can also specifying the permission for applets loaded from a URL specified by the code base

    grant codeBase {

    permission “<<ALL FILES>>”, “write”;


    grant codeBase {

    permission “<<ALL FILES>>”, “read”;


Using the policy tool
Using the Policy Tool

  • To save the time and avoid from making mistakes that will lead to security vulnerabilities, Java provides a GUI tool policytool to manage the policies

Code signing
Code Signing

  • We can grant a permission to applets from a particular URL to access our computer

    • URL can be hijacked or forged, how can we really know the applet file is from the exact party

  • Java provides another layer of security called code signing

  • To apply code signing, you need:

    • Create a JAR file (Java Archive, which is Java’s mechanism for bundling multiple files) using jar tool (Details)

    • Sign it using a Java tool jarsigner. You need a key pair generated by Java keytool to sign the JAR file (Details)

      jarsigner <jar_file> <key_file>

Code signing ii
Code Signing II

  • The HTML code to read a jarred applet

    <applet code = AppletClassName.class archive=“JarFileNaem.jar” width=XXX height=YYY>

  • When the runtime loads a JAR file, it verifies if the JAR file has been signed and determine if it can be trusted

    • A signed JAR file authenticates the file owner and integrity of the signed files in the JAR archive

    • Web browser can apply a standard policy to trust an applet if the applet is signed by a trusted authority

Java archive jar
Java Archive (JAR)

  • What is in a JAR file?

    • Create by: jar cf <jar_file> <all_files>

    • Execute by: java –jar myJAR.jar

    • Can also be viewed, extracted or even executed (Main-Class header is required in the Manifest file)

    • After code signing, the JAR files includes the class files, a fingerprint of the jarred files, code signer’s credentials, expiry date, the manifest and other related information

    • Fingerprint is an unique authentication token generated using a one-way hash algorithm – any alteration in code will generate a totally different fingerprint

    • Manifest is the directory of the archive – listing the files, the signatures, and other information (like Main-Class)


  • The main objectives of cryptography are:

    • Confidentiality – the information cannot be understood by others whom were not intended

    • Integrity – the information cannot be altered in the transition between sender and intended receiver without being detected

    • Non-repudiation – sender cannot deny his/her intention of sending/creating a message

    • Authentication – the sender and receiver can confirm other’s identity and the origin/destination of the information

Secure communication
Secure Communication

  • The simplest form of secure communication

    • Sender and recipient agree on a shared key

    • Communication is encrypted using the shared key

    • Only the objective of confidentiality is achieved

Sending a message securely
Sending a Message Securely

  • Steps of sending messages achieving all four cryptography objectives

    • Encrypt the original message with sender’s secret key

      • Confidentiality and authentication – encryption done using sender’s secret key

    • Create a message digest of the message

      • Integrity – digest is unique to the message

    • Encrypt the encrypted message and the message digest using the sender’s secret key

      • Non-repudiation – digest and message were encrypted together

    • Encrypt the encrypted message, message digest and the sender’s signature with the recipient's secret key

      • Authentication and confidentiality – only recipient can decrypt the message

Receiving a message securely
Receiving a Message Securely

  • Steps of receiving messages in a secured way

    • Decrypt the package with the recipient’s key

    • Decrypt the pieces with the sender’s key

    • Compute the message digest to verify the integrity of the message

    • Decrypt the encrypted message using the sender’s key

Problem in a secret key algorithm
Problem in a Secret-Key Algorithm

  • We require a number of secret keys, one for each step

  • The sender and recipient need to agree on the secret keys used in steps 1 and 2.

  • The sender and recipient need to give their secret keys to each other

    • If sender uses the same key for all recipients, the key in not secret then.

    • If sender uses different keys for each recipient, key management is complex

    • Key distribution is difficult, especially when the two parties are previously unknown

Public key algorithm
Public-Key Algorithm

  • Each party has a pair of keys

    • public key –made public to all parties you want to communicate with

    • private key –kept confidential

    • The key pair is generated using mathematical properties of prime number.

    • Each key pair is unique

    • It is difficult to compute the private key from the public key

  • The key pair has an interesting property

    • Decrypt (Encrypt(message, privateKey), publicKey) = message

    • Decrypt (Encrypt(message, publicKey), privateKey) = message

Public key algorithm cont d
Public-Key Algorithm (cont’d)

  • How public-key algorithm achieve the objectives

    • Confidentiality and integrity can be achieved when used with signatures

    • Non-refutable – the private key is secret and there is only one right combination of public/private keys

    • Authentication – a message that A can read will be encrypted using A’s public key, only A, who have the respective private key can read the message

  • Messages in public-key algorithm

    • Sender: Encrypt( Encrypt(msg, SendPrivKey), RecPubKey)

    • Recipient: Decrypt( Decrypt(msg, RecPriKey), SendPubKey)

  • Key distribution problem is solved because public key can be made available publicly.

Symmetric algorithm shared key
Symmetric Algorithm (Shared Key)

  • Public key algorithm uses two different keys for encryption and decryption – we call it an asymmetric algorithm

    • Public key algorithm requires a lot of computations

    • In practical, public key algorithm is used to establish the identities of the two parties and exchange a session key

  • Symmetric algorithm (shared key algorithm) use the same key for encryption and decryption

    • Decrypt (Encrypt (message,key), key) = message

    • The session key agreed in the above step is used to encrypt and decrypt the communication after the initial handshake

Java cryptography extension jce
Java Cryptography Extension (JCE)

  • JCE provides cryptographic functions, include

    • Key generation

    • Encryption/decryption

    • Message digest

    • Certificates and more…

  • The standard APIs are defined, but the implementation can be substituted

    • JDK 1.4.2 includes some service implementation provided by Suns

    • You can still easily install other cryptography providers implementation (like IBM’s) to the Java Security API and the implementation should still work

    • Advantages: Different algorithm can be used – some may only trust certain cryptographic implementation

Some commonly used cryptographic services
Some commonly used Cryptographic Services

  • SecureRandom – SHA1PRNG

  • KeyGenerator – DES, TripleDES, Blowfish

  • KeyPairGenerator – DSA, RSA, DH

  • MessageDigest – SHA1, MD5

  • MAC – HmacMD5, HmacSHA1

  • Signature – SHA1withDSA, SHA1WithRSA

  • CertificateFactory – X509

  • Cipher – DES, TripleDES, Blowfish

  • KeyAgreement – DH

  • List can be retrieved through Provider class in package (see for details)

Encryption and decryption
Encryption and Decryption

Plain text

Cipher text



Encryption strength
Encryption Strength

  • Encryption algorithms are measured by their strength

    • Example: 64-bit SSL and 128 bit-SSL

    • Key lengths provide a basic measure of strength when the same protocol is used

      • 128-bit SSL is 264 stronger than 64-bit SSL

      • Actually, the strength also depends on other factor, e.g., randomness of keys

    • Key lengths are not comparable when different algorithm is used

      • In RSA, when not all possible keys are valid, typical RSA keys are 1024-bit or even 2048-bit.

Secret key encryption and decryption
Secret-Key Encryption and Decryption

  • Most encryption and encryption are based on secret key algorithm

  • DES (Data Encryption Standard) was the de facto standard until recently

    • 56-bit key length

    • Possible to break a DES encryption by searching all possible keys – in less than 23 hours using 100,000 computers in 1999

    • Now, Triple-DES is used by running DES three times using three different key

  • Newer standard used in US government is AES (Advanced Encryption Standard)

    • More extensible algorithm whose strength can be extended by using different key lengths

Using jce to encrypt decrypt a message
Using JCE to Encrypt/Decrypt a Message

  • Java cryptographic APIs are defined in and javax.crypto pacakges

  • For a basic encryption, we need a secret key and a cryptographic algorithm

    • javax.crypto.SecretKey

    • javax.crypto.Cipher

  • To create a SecretKey,

    • we first need a KeySpec (e.g, javax.crypto.spec.DESKeySpec), which defines the attributes of the cryptographic key

    • KeySpec is then passed to a SecretKeyFactory in which the actual SecretKey object is created

Creating a secret key des key
Creating a Secret Key (DES Key)

// the key itself as a byte array

byte[] key = {'a','b','c','d','e','f','g','h'};

// create a DESKeySpec for out key

DESKeySpec spec = new DESKeySpec(key);

// retrieve a DES SecretKeyFactory

SecretKeyFactory factory =


// generate the actual SecretKey object

SecretKey secret = factory.generateSecret(spec);

  • DES key = 56-bit  7 bytesHowever, it is customary to initialize a 8-byte array and use the lower 7 bytes only (InvalidKeyException if only 7 bytes are input)

  • The secret key can be printed out by using secret.getEncoded().bytesToHexString()

Creating a cipher
Creating a Cipher

  • To create a cipher, you need to specify a right provider and call the getInstance static method from the Cipher class

    • Cipher c = Cipher.getInstance("DES");

  • Then, you can initialize the Cipher object to either encrypt or decrypt a message (reinitialize will clear all previous data)

    • c.init(Cipher.ENCRYPT_MODE, secret);

    • secret refers to the key you obtained in the secret key factory

  • Now, you can call the update method to encrypt/decrypt the message (repeatedly call to process multi-parts)

    • c.update(buffer); // buffer is a byte array

  • To retrieve the encrypted/decrypted result, you invoke the doFinal method

    • Byte[] isEncrypted = c.doFinal();

  • Please refer to API specification for other forms of update/doFinal

Encryption and decryption example
Encryption and Decryption Example

  • Main method

    • Open a file, encrypt the contents and write the encrypted data in another file

  • encrypt method

    • Create a cipher and initialize it using the secret key in the input parameter

    • Encrypt the input byte array and return the encrypted result

  • makeDESKey method

    • Create and return a DES cryptographic key according to the given byte array

  • Exactly the same as except the Cipher is initialized to decrypt instead of encrypt

Encrypted echo server client
Encrypted Echo Server/Client

  • The Echo server/client can have a more secured communication by adding DES encryption/decryption

    • Messages cannot be processed if the key was wrongly typed

  • See:



Aes encryption and decryption
AES Encryption and Decryption

  • In AES, apart from creating the KeySpec, we need to also create a IvParameterSpec for the initialization vector

    • Example: IvParameterSpec params = new IvParameterSpec(iVBytes);

  • We call Cipher's init method with the mode (encrypt/decrypt), keySpec and the parameterSpec:

    • Example:cipher.init(Cipher.ENCRYPT_MODE, key, params);

Message digest
Message Digest

  • Message digests is used to guarantee the integrity of a message, is also known as a fingerprint or a hash.

  • A message digest is generated using a mathematical transformation called a hash function

    • The resultant message digest is typically fixed length

  • The hash function used to produce message digest should have the following properties:

    • Extremely difficult to produce a same message digest from two different messages – the hash must be one-to-one

    • Extremely difficult to produce the original message from a given message digest – the hash must be irreversible

  • The most popular algorithms are SHA1 and MD5.

Md5 algorithm
MD5 Algorithm

  • MD5 algorithm takes in a message of arbitrary length and produces a 128-bit or 16-byte message digest

  • MD5 is popular because it is one of the faster message digest algorithm

  • MD5 ensure a download file has not tampered by providing a MD5 checksum

    • After the file is downloaded, MD5 checksum is generated and checked against the original one

  • In this way, two parties can know the secret without transferring the secret itself

    • Example: a password's hash is transferred to a homepage and the hash is checked with the hash stored in the web server

Message digest in java
Message Digest in Java

  • Message digests are encapsulated in the class

  • Like Cipher, we create a MessageDigest using getInstance method

    MessageDigest md = MessageDigest.getInstance("MD5");

  • Data can then feed in to the MessageDigest object using the update method

    md.update(buffer); // buffer is a byte array

  • The hash is retrieved using the digest method


  • See the sample code in

Sha1 and keyed md5
SHA1 and Keyed MD5

  • SHA1 is similar to MD5 but it produces a 160-bit message digest.

  • Going back to the previous password handling technique

    • Man-in-the-middle can still impersonate the user without knowing the actual password

  • To prevent the man-in-the-middle attack, we modify the protocol to a challenge-and-response authentication

    • Server sends the client a random string

    • The client computes the MD5 hash of the random string using the password as a secret key

    • Previously computed hash cannot be reused because the string is randomly generated each time

Message authentication code mac
Message Authentication Code (MAC)

  • MAC (message authentication code) is similar to a message digest, but with the addition of a secret key.

    • In addition to integrity, MAC also ensures the authenticity of the message digest

    • MAC still has the same key distribution problem

  • To generate a MD5 MAC, we first create the necessary key from javax.crypto.KeyGenerator

    KeyGenerator kg = KeyGenerator.getInstance("HmacMD5");

    SecretKey sk = kg.generateKey();

  • MAC object is then created and initialized using the key

    Mac mac = Mac.getInstance("HmacMD5"); mac.init(sk);

  • Data is then feed into the MAC object and the resultant code can be retrieved using doFinal

    mac.update(buffer); // buffer is a byte array

    byte[] result = mac.doFinal();

Public key infrastructure pki
Public-Key Infrastructure (PKI)

  • Public-key algorithm solve the key distribution issue

    • Public key is distributed widely

    • Anyone can send you a message that only you can read

    • If message was also encrypted with the sender's private key, you can also verify the identity of the sender

    • But, how do we know a particular public key really belongs to a particular person?

  • Instead of distributing the public keys in PKI, the public keys are distributed as certificates

    • Certificates should contain enough info to authenticate the owner and associate the owner of a particular public key

    • Most popular standards for certificates include X.509 and PGP

Certificate authorities
Certificate Authorities

  • How do you know the certificate is authentic

    • It needs to be signed by a trusted party called the Certificate Authority (CA) using CA's private key – it validate the integrity and authenticity of the cert.

    • To verify a certificate, we check the signature using CA's public key, obtainable from CA's root certificate (self-signed)

      • CA must guarantee its own private key has not been compromised

  • All certificate related classes in Java are in package



Keys management
Keys Management

  • Keys and certificates can be stored in binary or text files, and in Java, a special format called key store.

  • To create a key store and generate a certificate, you can invoke keytool as:

    keytool –keystore myKeyStore –genKey –alias myKey

    • Password is required for the key store

    • You will also be prompted for the information necessary for the X.509 certificate

Certificates management
Certificates Management

  • The certificate created by the recent step can be exported by calling:

    • For binary file export:

      keytool -keystore myKeyStore -storepass <passwd> -alias <myKey> -export –file <file>

    • For text format export:

      keytool -keystore myKeyStore -storepass <passwd> -alias <myKey> -export –rfc –file <file>

  • The certificates generated by keytool are self-signed

Using certificate in java
Using Certificate in Java

  • Import a certificate from a file

    FileInputStream is = new FileInputStream(file);

    CertificateFactory cf = CertificateFactory.getInstance("X.509");

    Certificate cert = cf.generateCertificate(is);

Information within the certificate
Information within the Certificate

  • Information within a cert is abstracted as Principle

    • Subject is retrieved by: Principal p = cert.getSubjectDN(); // p.getName() will return the subject

    • Issuer is retrieved by:Principal p = cert.getIssuerDN();// p.getName() will return the issuer name

  • See

Verifying the certificate
Verifying the Certificate

  • To verify the certificate

    • Get the public key from the cert. (normally from CA's cert):PublicKey key = cert.getPublicKey();

    • Verify the certificate using the public keycert.verify(key); // give out exceptions if verification fails

  • Possible exceptions in a certificate

    • NoSuchAlgorithmException

    • InvalidKeyException

    • NoSuchProviderException

    • SignatureException

    • CertificateException

  • See

Other certificate issues
Other Certificate Issues

  • Trust is transitive in most PKI

    • Certificate can be chained

    • Once you validate a certificate, you need to validate all intermediate certificates

  • Revocation

    • Certificates that are no longer valid is to be put in the certificate revocation lists

Comparing secret key and public key algorithm
Comparing Secret-Key and Public-Key Algorithm

  • Secret-key are much faster than public-key

  • Public key requires certificate authorities as a trusted third party

  • Secret-key requires pre-agreement on keys and key distribution using public key is easier (using certificates)

Digital signature
Digital Signature

  • Secret-key MAC key distribution problem revisit

    • Can be solved by using private key to generate MAC and use public key to verify it – too slow

    • Hash the message and sign using our private key, the process is called digital signing

    • Digital signature verifies the identity and integrity of the message

Digital signature in java
Digital Signature in Java

  • In Java, contains the function used in digital signing

    • The most popular algorithm used is Digital Signature Algorithm (DSA)

  • Key pair is first generated

    • KeyPairGenerator kpg = new KeyPairGenerator.getInstance("DSA");

  • Then, initialize the key generator with the desired length

    • kpg.intialize(1024, new SecureRandom());

  • The key pair can be generated according to followings

    • KeyPair keypair = kpg.generateKeyPair();

    • DSAPublicKey public = (DSAPublicKey) keypair.getPublic());

    • DSAPrivateKey private = (DSAPrivateKey) keypair.getPrivate());

Digital signature in java cont d
Digital Signature in Java (cont'd)

  • The signing process

    Signature sign = Signature.getInstance("DSA");



    return sign.sign(); // signature is created

  • The verifying process

    Signature sign = Signature.getInstance("DSA");



    return sign.verify();

Other issues on digital signature
Other Issues on Digital Signature

  • A DSA algorithm requires a key of at least 1024 bits and the resultant signature is 46-bytes long

  • Though we can create a DSA for the entire message, it works very slowly.

  • Thus, it is typical we create a hash for the message first, and use DSA to sign the hash only

Before starting ssl
Before Starting SSL

  • Special Notes: Please ignore the Installation Notes in your Course Materials if you use JDK1.4.x. It is for previous versions only.

Secure socket layer ssl
Secure Socket Layer (SSL)

  • SSL is the de facto standard for securing communication on the Internet

  • SSL uses a combination of public key algorithm and symmetric key algorithms to authenticate the parties and secure the communication

    • Public key is nice but slow and symmetric key is fast but key distribution is a headache

    • When SSL session is started, the tow parties use a public key mechanism to authenticate each other

    • Then, they exchange a session key and is used in a symmetric algorithm to encrypt and decrypt the communication

Https connection to a website
HTTPS Connection to a Website

  • You can connect a URL with https (in fact invoking SSL communication)

    • URLConnection urlConn = url.openConnection();

  • You can test if you can make a HTTPS connection using the given example (

    • java SSLTest <host>

    • Java – SSLTest <host>

Ssl in socket programming
SSL in Socket Programming

  • SSL can write socket programs similar to normal socket programs

    • Instead of Socket and ServerSocket, SSL uses SSLSocket and SSLServerSocket

    • However, SSL Sockets are constructed using the getDefault method in SSLSocketFactory (unless you have other SSL provider)

    • After obtaining a SSLSocketFactory (casting the SocketFactory obtained above), you can use the createSocket method to create an SSLSocket

    • You need to cast the obtained Socket back to SSLSocket

      SSLSocketFactory sslSocketFactory = (SSLSocketFactory) SSLSocketFactory.getDefault();

      SSLSocket sslSocket = (SSLSocket) sslSocketFactory.createSocket(server, port);

Ssl in socket programming cont d
SSL in Socket Programming (cont'd)

  • SSLServerSocket can be created in a similar method as in SSLSocket

    • First, we retrieve a SSLServerSocketFactory using getDefault method in the class

    • Casting back the obtained ServerSocketFactory to SSLServerSocketFactory, we create an SSLServerSocket using the createServerSocket method

      SSLServerSocketFactory sslSSFactory = (SSLServerSocketFactory) SSLServerSocketFactory.getFactory();

      SSLServerSocket sslSSocket = (SSLServerSocket) sslSSFactory.createServerSocket(port);

      SSLSocket sslSocket = (SSLSocket) sslSSocket.accept();

Ssl implementation
SSL Implementation

  • SSL protocol only defines the basic mechanisms for handshaking and data exchange

    • The public-key and/or secret-key algorithms are not defined

  • The secret keys, key length and the encryption algorithm are collectively called cipher suites

    • You can retrieve the default and supported cipher suites from SSLSocketFactory and SSLServerSocketFactory using the following methods:

      String[] getDefaultCipherSuites();

      String[] getSupportedCipherSuites();

Ssl communication
SSL Communication

  • Step 1 is to authenticate the two parties

    • Involves certificate checking

    • However, SSLSocket and SSLServerSocket has done all checking for you

  • Then, the two parties agree on the cipher to use

    • Cipher suites can be retrieved using methods in SSLSocket and SSLServerSocket

  • Other than the above, SSLSocket and SSLServerSocket are the same as Socket and ServerSocket

Ssl echo server client
SSL Echo Server/Client

  • In order to run the given SSL echo server and client, we need to provide the necessary certificates

  • In this example, we create a keystore using keytool

    • keytool –keystore <file> -genkey –alias <keyName>

  • In a production environment, CA will issue the server certificate and the client will have the lists of root certs and other certs it trusts.

Ssl echo server client1
SSL Echo Server/Client

  • In the server side, we specify the server certificate to use by

    • java –<cert file><passwd> SSLEchoServer

  • In the client side, we specify a key store for certificates that the clients trust

    • java –<trusted cert><passwd> SSLEchoClient