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Digital Signatures

As we said earlier, applets were what started the craze over the Java platform. In practice, people discovered that although they could write animated applets like the famous "nervous text" applet, applets could not do a whole lot of useful stuff in the JDK 1.0 security model. For example, because applets under JDK 1.0 were so closely supervised, they couldn't do much good on a corporate intranet, even though relatively little risk attaches to executing an applet from your company's secure intranet. It quickly became clear to Sun that for applets to become truly useful, it was important for users to be able to assign different levels of security, depending on where the applet originated. If an applet comes from a trusted supplier and it has not been tampered with, the user of that applet can then decide whether to give the applet more privileges.

To give more trust to an applet, we need to know two things:

  • Where did the applet come from?
  • Was the code corrupted in transit?

In the past 50 years, mathematicians and computer scientists have developed sophisticated algorithms for ensuring the integrity of data and for electronic signatures. The java.security package contains implementations of many of these algorithms. Fortunately, you don't need to understand the underlying mathematics to use the algorithms in the java.security package. In the next sections, we show you how message digests can detect changes in data files and how digital signatures can prove the identity of the signer.

Message Digests

A message digest is a digital fingerprint of a block of data. For example, the so-called SHA1 (secure hash algorithm #1) condenses any data block, no matter how long, into a sequence of 160 bits (20 bytes). As with real fingerprints, one hopes that no two messages have the same SHA1 fingerprint. Of course, that cannot be true—there are only 2160 SHA1 fingerprints, so there must be some messages with the same fingerprint. But 2160 is so large that the probability of duplication occurring is negligible. How negligible? According to James Walsh in True Odds: How Risks Affect Your Everyday Life (Merritt Publishing 1996), the chance that you will die from being struck by lightning is about one in 30,000. Now, think of nine other people, for example, your nine least favorite managers or professors. The chance that you and all of them will die from lightning strikes is higher than that of a forged message having the same SHA1 fingerprint as the original. (Of course, more than ten people, none of whom you are likely to know, will die from lightning strikes. However, we are talking about the far slimmer chance that your particular choice of people will be wiped out.)

A message digest has two essential properties:

  • If one bit or several bits of the data are changed, then the message digest also changes.
  • A forger who is in possession of a given message cannot construct a fake message that has the same message digest as the original.

The second property is again a matter of probabilities, of course. Consider the following message by the billionaire father:

  • "Upon my death, my property shall be divided equally among my children; however, my son George shall receive nothing."

That message has an SHA1 fingerprint of

2D 8B 35 F3 BF 49 CD B1 94 04 E0 66 21 2B 5E 57 70 49 E1 7E

The distrustful father has deposited the message with one attorney and the fingerprint with another. Now, suppose George can bribe the lawyer holding the message. He wants to change the message so that Bill gets nothing. Of course, that changes the fingerprint to a completely different bit pattern:

2A 33 0B 4B B3 FE CC 1C 9D 5C 01 A7 09 51 0B 49 AC 8F 98 92

Can George find some other wording that matches the fingerprint? If he had been the proud owner of a billion computers from the time the Earth was formed, each computing a million messages a second, he would not yet have found a message he could substitute.

A number of algorithms have been designed to compute these message digests. The two best-known are SHA1, the secure hash algorithm developed by the National Institute of Standards and Technology, and MD5, an algorithm invented by Ronald Rivest of MIT. Both algorithms scramble the bits of a message in ingenious ways. For details about these algorithms, see, for example, Cryptography and Network Security, 4th ed., by William Stallings (Prentice Hall 2005). Note that recently, subtle regularities have been discovered in both algorithms. At this point, most cryptographers recommend avoiding MD5 and using SHA1 until a stronger alternative becomes available. (See http://www.rsa.com/rsalabs/node.asp?id=2834 for more information.)

The Java programming language implements both SHA1 and MD5. The MessageDigest class is a factory for creating objects that encapsulate the fingerprinting algorithms. It has a static method, called getInstance, that returns an object of a class that extends the MessageDigest class. This means the MessageDigest class serves double duty:

  • As a factory class
  • As the superclass for all message digest algorithms

For example, here is how you obtain an object that can compute SHA fingerprints:

MessageDigest alg = MessageDigest.getInstance("SHA-1");

(To get an object that can compute MD5, use the string "MD5" as the argument to getInstance.)

After you have obtained a MessageDigest object, you feed it all the bytes in the message by repeatedly calling the update method. For example, the following code passes all bytes in a file to the alg object just created to do the fingerprinting:

InputStream in = . . .
int ch;
while ((ch = in.read()) != -1)
   alg.update((byte) ch);

Alternatively, if you have the bytes in an array, you can update the entire array at once:

byte[] bytes = . . .;
alg.update(bytes);

When you are done, call the digest method. This method pads the input—as required by the fingerprinting algorithm—does the computation, and returns the digest as an array of bytes.

byte[] hash = alg.digest();

The program in Listing 9-15 computes a message digest, using either SHA or MD5. You can load the data to be digested from a file, or you can type a message in the text area. Figure 9-11 shows the application.

Figure 9-11

Figure 9-11 Computing a message digest

Listing 9-15. MessageDigestTest.java

  
  1. import java.io.*;
  2. import java.security.*;
  3. import java.awt.*;
  4. import java.awt.event.*;
  5. import javax.swing.*;
  6.
  7. /**
  8.  * This program computes the message digest of a file or the contents of a text area.
  9.  * @version 1.13 2007-10-06
 10.  * @author Cay Horstmann
 11.  */
 12. public class MessageDigestTest
 13. {
 14.    public static void main(String[] args)
 15.    {
 16.       EventQueue.invokeLater(new Runnable()
 17.          {
 18.             public void run()
 19.             {
 20.                JFrame frame = new MessageDigestFrame();
 21.                frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
 22.                frame.setVisible(true);
 23.             }
 24.          });
 25.    }
 26. }
 27.
 28. /**
 29.  * This frame contains a menu for computing the message digest of a file or text area, radio
 30.  * buttons to toggle between SHA-1 and MD5, a text area, and a text field to show the
 31.  * messge digest.
 32.  */
 33. class MessageDigestFrame extends JFrame
 34. {
 35.    public MessageDigestFrame()
 36.    {
 37.       setTitle("MessageDigestTest");
 38.       setSize(DEFAULT_WIDTH, DEFAULT_HEIGHT);
 39.
 40.       JPanel panel = new JPanel();
 41.       ButtonGroup group = new ButtonGroup();
 42.       addRadioButton(panel, "SHA-1", group);
 43.       addRadioButton(panel, "MD5", group);
 44.
 45.       add(panel, BorderLayout.NORTH);
 46.       add(new JScrollPane(message), BorderLayout.CENTER);
 47.       add(digest, BorderLayout.SOUTH);
 48.       digest.setFont(new Font("Monospaced", Font.PLAIN, 12));
 49.
 50.       setAlgorithm("SHA-1");
 51.
 52.       JMenuBar menuBar = new JMenuBar();
 53.       JMenu menu = new JMenu("File");
 54.       JMenuItem fileDigestItem = new JMenuItem("File digest");
 55.       fileDigestItem.addActionListener(new ActionListener()
 56.          {
 57.             public void actionPerformed(ActionEvent event)
 58.             {
 59.                loadFile();
 60.             }
 61.          });
 62.       menu.add(fileDigestItem);
 63.       JMenuItem textDigestItem = new JMenuItem("Text area digest");
 64.       textDigestItem.addActionListener(new ActionListener()
 65.          {
 66.             public void actionPerformed(ActionEvent event)
 67.             {
 68.                String m = message.getText();
 69.                computeDigest(m.getBytes());
 70.             }
 71.          });
 72.       menu.add(textDigestItem);
 73.       menuBar.add(menu);
 74.       setJMenuBar(menuBar);
 75.    }
 76.
 77.    /**
 78.     * Adds a radio button to select an algorithm.
 79.     * @param c the container into which to place the button
 80.     * @param name the algorithm name
 81.     * @param g the button group
 82.     */
 83.    public void addRadioButton(Container c, final String name, ButtonGroup g)
 84.    {
 85.       ActionListener listener = new ActionListener()
 86.          {
 87.             public void actionPerformed(ActionEvent event)
 88.             {
 89.                setAlgorithm(name);
 90.             }
 91.          };
 92.       JRadioButton b = new JRadioButton(name, g.getButtonCount() == 0);
 93.       c.add(b);
 94.       g.add(b);
 95.       b.addActionListener(listener);
 96.    }
 97.
 98.    /**
 99.     * Sets the algorithm used for computing the digest.
100.     * @param alg the algorithm name
101.     */
102.    public void setAlgorithm(String alg)
103.    {
104.       try
105.       {
106.          currentAlgorithm = MessageDigest.getInstance(alg);
107.          digest.setText("");
108.       }
109.       catch (NoSuchAlgorithmException e)
110.       {
111.          digest.setText("" + e);
112.       }
113.    }
114.
115.    /**
116.     * Loads a file and computes its message digest.
117.     */
118.    public void loadFile()
119.    {
120.       JFileChooser chooser = new JFileChooser();
121.       chooser.setCurrentDirectory(new File("."));
122.
123.       int r = chooser.showOpenDialog(this);
124.       if (r == JFileChooser.APPROVE_OPTION)
125.       {
126.          try
127.          {
128.             String name = chooser.getSelectedFile().getAbsolutePath();
129.             computeDigest(loadBytes(name));
130.          }
131.          catch (IOException e)
132.          {
133.             JOptionPane.showMessageDialog(null, e);
134.          }
135.       }
136.    }
137.
138.    /**
139.     * Loads the bytes in a file.
140.     * @param name the file name
141.     * @return an array with the bytes in the file
142.     */
143.    public byte[] loadBytes(String name) throws IOException
144.    {
145.       FileInputStream in = null;
146.
147.       in = new FileInputStream(name);
148.       try
149.       {
150.          ByteArrayOutputStream buffer = new ByteArrayOutputStream();
151.          int ch;
152.          while ((ch = in.read()) != -1)
153.             buffer.write(ch);
154.          return buffer.toByteArray();
155.       }
156.       finally
157.       {
158.          in.close();
159.       }
160.    }
161.
162.    /**
163.     * Computes the message digest of an array of bytes and displays it in the text field.
164.     * @param b the bytes for which the message digest should be computed.
165.     */
166.    public void computeDigest(byte[] b)
167.    {
168.       currentAlgorithm.reset();
169.       currentAlgorithm.update(b);
170.       byte[] hash = currentAlgorithm.digest();
171.       String d = "";
172.       for (int i = 0; i < hash.length; i++)
173.       {
174.          int v = hash[i] & 0xFF;
175.          if (v < 16) d += "0";
176.          d += Integer.toString(v, 16).toUpperCase() + " ";
177.       }
178.       digest.setText(d);
179.    }
180.
181.    private JTextArea message = new JTextArea();
182.    private JTextField digest = new JTextField();
183.    private MessageDigest currentAlgorithm;
184.    private static final int DEFAULT_WIDTH = 400;
185.    private static final int DEFAULT_HEIGHT = 300;
186. }
  • static MessageDigest getInstance(String algorithmName)

    returns a MessageDigest object that implements the specified algorithm. Throws NoSuchAlgorithmException if the algorithm is not provided.

  • void update(byte input)
  • void update(byte[] input)
  • void update(byte[] input, int offset, int len)

    updates the digest, using the specified bytes.

  • byte[] digest()

    completes the hash computation, returns the computed digest, and resets the algorithm object.

  • void reset()

    resets the digest.

Message Signing

In the last section, you saw how to compute a message digest, a fingerprint for the original message. If the message is altered, then the fingerprint of the altered message will not match the fingerprint of the original. If the message and its fingerprint are delivered separately, then the recipient can check whether the message has been tampered with. However, if both the message and the fingerprint were intercepted, it is an easy matter to modify the message and then recompute the fingerprint. After all, the message digest algorithms are publicly known, and they don't require secret keys. In that case, the recipient of the forged message and the recomputed fingerprint would never know that the message has been altered. Digital signatures solve this problem.

To help you understand how digital signatures work, we explain a few concepts from the field called public key cryptography. Public key cryptography is based on the notion of a public key and private key. The idea is that you tell everyone in the world your public key. However, only you hold the private key, and it is important that you safeguard it and don't release it to anyone else. The keys are matched by mathematical relationships, but the exact nature of these relationships is not important for us. (If you are interested, you can look it up in The Handbook of Applied Cryptography at http://www.cacr.math.uwaterloo.ca/hac/.)

The keys are quite long and complex. For example, here is a matching pair of public and private Digital Signature Algorithm (DSA) keys.

Public key:

p:
fca682ce8e12caba26efccf7110e526db078b05edecbcd1eb4a208f3ae1617ae01f35b91a47e6df63413c5e12ed0899
bcd132acd50d99151bdc43ee737592e17

q: 962eddcc369cba8ebb260ee6b6a126d9346e38c5
g:678471b27a9cf44ee91a49c5147db1a9aaf244f05a434d6486931d2d14271b9e35030b71fd73da179069b32e29356
30e
1c2062354d0da20a6c416e50be794ca4

y:
c0b6e67b4ac098eb1a32c5f8c4c1f0e7e6fb9d832532e27d0bdab9ca2d2a8123ce5a8018b8161a760480fadd040b927
281ddb22cb9bc4df596d7de4d1b977d50

Private key:

p:
fca682ce8e12caba26efccf7110e526db078b05edecbcd1eb4a208f3ae1617ae01f35b91a47e6df63413c5e12ed0899
bcd132acd50d99151bdc43ee737592e17

q: 962eddcc369cba8ebb260ee6b6a126d9346e38c5

g:
678471b27a9cf44ee91a49c5147db1a9aaf244f05a434d6486931d2d14271b9e35030b71fd73da179069b32e2935630
e1c2062354d0da20a6c416e50be794ca4

x: 146c09f881656cc6c51f27ea6c3a91b85ed1d70a

It is believed to be practically impossible to compute one key from the other. That is, even though everyone knows your public key, they can't compute your private key in your lifetime, no matter how many computing resources they have available.

It might seem difficult to believe that nobody can compute the private key from the public keys, but nobody has ever found an algorithm to do this for the encryption algorithms that are in common use today. If the keys are sufficiently long, brute force—simply trying all possible keys—would require more computers than can be built from all the atoms in the solar system, crunching away for thousands of years. Of course, it is possible that someone could come up with algorithms for computing keys that are much more clever than brute force. For example, the RSA algorithm (the encryption algorithm invented by Rivest, Shamir, and Adleman) depends on the difficulty of factoring large numbers. For the last 20 years, many of the best mathematicians have tried to come up with good factoring algorithms, but so far with no success. For that reason, most cryptographers believe that keys with a "modulus" of 2,000 bits or more are currently completely safe from any attack. DSA is believed to be similarly secure.

Figure 9-12 illustrates how the process works in practice.

Figure 9-12

Figure 9-12 Public key signature exchange with DSA

Suppose Alice wants to send Bob a message, and Bob wants to know this message came from Alice and not an impostor. Alice writes the message and then signs the message digest with her private key. Bob gets a copy of her public key. Bob then applies the public key to verify the signature. If the verification passes, then Bob can be assured of two facts:

  • The original message has not been altered.
  • The message was signed by Alice, the holder of the private key that matches the public key that Bob used for verification.

You can see why security for private keys is all-important. If someone steals Alice's private key or if a government can require her to turn it over, then she is in trouble. The thief or a government agent can impersonate her by sending messages, money transfer instructions, and so on, that others will believe came from Alice.

The X.509 Certificate Format

To take advantage of public key cryptography, the public keys must be distributed. One of the most common distribution formats is called X.509. Certificates in the X.509 format are widely used by VeriSign, Microsoft, Netscape, and many other companies, for signing e-mail messages, authenticating program code, and certifying many other kinds of data. The X.509 standard is part of the X.500 series of recommendations for a directory service by the international telephone standards body, the CCITT.

The precise structure of X.509 certificates is described in a formal notation, called "abstract syntax notation #1" or ASN.1. Figure 9-13 shows the ASN.1 definition of version 3 of the X.509 format. The exact syntax is not important for us, but, as you can see, ASN.1 gives a precise definition of the structure of a certificate file. The basic encoding rules, or BER, and a variation, called distinguished encoding rules (DER) describe precisely how to save this structure in a binary file. That is, BER and DER describe how to encode integers, character strings, bit strings, and constructs such as SEQUENCE, CHOICE, and OPTIONAL.

Figure 9-13. ASN.1 definition of X.509v3

[Certificate  ::=  SEQUENCE  {
        tbsCertificate        TBSCertificate,
        signatureAlgorithm    AlgorithmIdentifier,
        signature             BIT STRING  }

   TBSCertificate  ::=  SEQUENCE  {
        version         [0]   EXPLICIT Version DEFAULT v1,
        serialNumber          CertificateSerialNumber,
        signature             AlgorithmIdentifier,
        issuer                Name,
        validity              Validity,
        subject               Name,
        subjectPublicKeyInfo  SubjectPublicKeyInfo,
        issuerUniqueID  [1]  IMPLICIT UniqueIdentifier OPTIONAL,
                              -- If present, version must be v2 or v3
        subjectUniqueID [2]  IMPLICIT UniqueIdentifier OPTIONAL,
                             -- If present, version must be v2 or v3
        extensions      [3]  EXPLICIT Extensions OPTIONAL
                            -- If present, version must be v3
        }

   Version  ::=  INTEGER  {  v1(0), v2(1), v3(2)  }

   CertificateSerialNumber  ::=  INTEGER

   Validity ::= SEQUENCE {
        notBefore      CertificateValidityDate,
        notAfter       CertificateValidityDate }

   CertificateValidityDate ::= CHOICE {
        utcTime        UTCTime,
        generalTime    GeneralizedTime }

   UniqueIdentifier  ::=  BIT STRING

   SubjectPublicKeyInfo  ::=  SEQUENCE  {
        algorithm             AlgorithmIdentifier,
        subjectPublicKey      BIT STRING  }

   Extensions  ::=  SEQUENCE OF Extension

   Extension  ::=  SEQUENCE  {
        extnID      OBJECT IDENTIFIER,
        critical    BOOLEAN DEFAULT FALSE,
        extnValue   OCTET STRING  }

Verifying a Signature

The JDK comes with the keytool program, which is a command-line tool to generate and manage a set of certificates. We expect that ultimately the functionality of this tool will be embedded in other, more user-friendly programs. But right now, we use keytool to show how Alice can sign a document and send it to Bob, and how Bob can verify that the document really was signed by Alice and not an imposter.

The keytool program manages keystores, databases of certificates and private/public key pairs. Each entry in the keystore has an alias. Here is how Alice creates a keystore, alice.certs, and generates a key pair with alias alice.

keytool -genkeypair -keystore alice.certs -alias alice

When creating or opening a keystore, you are prompted for a keystore password. For this example, just use secret. If you were to use the keytool-generated keystore for any serious purpose, you would need to choose a good password and safeguard this file.

When generating a key, you are prompted for the following information:

Enter keystore password:  secret
Reenter new password:  secret
What is your first and last name?
  [Unknown]:  Alice Lee
What is the name of your organizational unit?
  [Unknown]:  Engineering Department
What is the name of your organization?
  [Unknown]:  ACME Software
What is the name of your City or Locality?
  [Unknown]:  San Francisco
What is the name of your State or Province?
  [Unknown]:  CA
What is the two-letter country code for this unit?
  [Unknown]:  US
Is <CN=Alice Lee, OU=Engineering Department, O=ACME Software, L=San Francisco, ST=CA, C=US> cor-
rect?
  [no]:  yes

The keytool uses X.500 distinguished names, with components Common Name (CN), Organizational Unit (OU), Organization (O), Location (L), State (ST), and Country (C) to identify key owners and certificate issuers.

Finally, specify a key password, or press ENTER to use the keystore password as the key password.

Suppose Alice wants to give her public key to Bob. She needs to export a certificate file:

keytool -exportcert -keystore alice.certs -alias alice -file alice.cer

Now Alice can send the certificate to Bob. When Bob receives the certificate, he can print it:

keytool -printcert -file alice.cer

The printout looks like this:

Owner: CN=Alice Lee, OU=Engineering Department, O=ACME Software, L=San Francisco, ST=CA, C=US
Issuer: CN=Alice Lee, OU=Engineering Department, O=ACME Software, L=San Francisco, ST=CA, C=US
Serial number: 470835ce
Valid from: Sat Oct 06 18:26:38 PDT 2007 until: Fri Jan 04 17:26:38 PST 2008
Certificate fingerprints:
         MD5:  BC:18:15:27:85:69:48:B1:5A:C3:0B:1C:C6:11:B7:81
         SHA1: 31:0A:A0:B8:C2:8B:3B:B6:85:7C:EF:C0:57:E5:94:95:61:47:6D:34
         Signature algorithm name: SHA1withDSA
         Version: 3

If Bob wants to check that he got the right certificate, he can call Alice and verify the certificate fingerprint over the phone.

Once Bob trusts the certificate, he can import it into his keystore.

keytool -importcert -keystore bob.certs -alias alice -file alice.cer

Now Alice can start sending signed documents to Bob. The jarsigner tool signs and verifies JAR files. Alice simply adds the document to be signed into a JAR file.

jar cvf document.jar document.txt

Then she uses the jarsigner tool to add the signature to the file. She needs to specify the keystore, the JAR file, and the alias of the key to use.

jarsigner -keystore alice.certs document.jar alice

When Bob receives the file, he uses the -verify option of the jarsigner program.

jarsigner -verify -keystore bob.certs document.jar

Bob does not need to specify the key alias. The jarsigner program finds the X.500 name of the key owner in the digital signature and looks for matching certificates in the keystore.

If the JAR file is not corrupted and the signature matches, then the jarsigner program prints

jar verified.

Otherwise, the program displays an error message.

The Authentication Problem

Suppose you get a message from your friend Alice, signed with her private key, using the method we just showed you. You might already have her public key, or you can easily get it by asking her for a copy or by getting it from her web page. Then, you can verify that the message was in fact authored by Alice and has not been tampered with. Now, suppose you get a message from a stranger who claims to represent a famous software company, urging you to run the program that is attached to the message. The stranger even sends you a copy of his public key so you can verify that he authored the message. You check that the signature is valid. This proves that the message was signed with the matching private key and that it has not been corrupted.

Be careful: You still have no idea who wrote the message. Anyone could have generated a pair of public and private keys, signed the message with the private key, and sent the signed message and the public key to you. The problem of determining the identity of the sender is called the authentication problem.

The usual way to solve the authentication problem is simple. Suppose the stranger and you have a common acquaintance you both trust. Suppose the stranger meets your acquaintance in person and hands over a disk with the public key. Your acquaintance later meets you, assures you that he met the stranger and that the stranger indeed works for the famous software company, and then gives you the disk (see Figure 9-14). That way, your acquaintance vouches for the authenticity of the stranger.

Figure 9-14

Figure 9-14 Authentication through a trusted intermediary

In fact, your acquaintance does not actually need to meet you. Instead, he can use his private key to sign the stranger's public key file (see Figure 9-15).

Figure 9-15

Figure 9-15 Authentication through a trusted intermediary's signature

When you get the public key file, you verify the signature of your friend, and because you trust him, you are confident that he did check the stranger's credentials before applying his signature.

However, you might not have a common acquaintance. Some trust models assume that there is always a "chain of trust"—a chain of mutual acquaintances—so that you trust every member of that chain. In practice, of course, that isn't always true. You might trust your friend, Alice, and you know that Alice trusts Bob, but you don't know Bob and aren't sure that you trust him. Other trust models assume that there is a benevolent big brother in whom we all trust. The best known of these companies is VeriSign, Inc. (http://www.verisign.com).

You will often encounter digital signatures that are signed by one or more entities who will vouch for the authenticity, and you will need to evaluate to what degree you trust the authenticators. You might place a great deal of trust in VeriSign, perhaps because you saw their logo on many web pages or because you heard that they require multiple people with black attaché cases to come together into a secure chamber whenever new master keys are to be minted.

However, you should have realistic expectations about what is actually being authenticated. The CEO of VeriSign does not personally meet every individual or company representative when authenticating a public key. You can get a "class 1" ID simply by filling out a web form and paying a small fee. The key is mailed to the e-mail address included in the certificate. Thus, you can be reasonably assured that the e-mail address is genuine, but the requestor could have filled in any name and organization. There are more stringent classes of IDs. For example, with a "class 3" ID, VeriSign will require an individual requestor to appear before a notary public, and it will check the financial rating of a corporate requestor. Other authenticators will have different procedures. Thus, when you receive an authenticated message, it is important that you understand what, in fact, is being authenticated.

Certificate Signing

In the section "Verifying a Signature" on page 814, you saw how Alice used a selfsigned certificate to distribute a public key to Bob. However, Bob needed to ensure that the certificate was valid by verifying the fingerprint with Alice.

Suppose Alice wants to send her colleague Cindy a signed message, but Cindy doesn't want to bother with verifying lots of signature fingerprints. Now suppose that there is an entity that Cindy trusts to verify signatures. In this example, Cindy trusts the Information Resources Department at ACME Software.

That department operates a certificate authority (CA). Everyone at ACME has the CA's public key in their keystore, installed by a system administrator who carefully checked the key fingerprint. The CA signs the keys of ACME employees. When they install each other's keys, then the keystore will trust them implicitly because they are signed by a trusted key.

Here is how you can simulate this process. Create a keystore acmesoft.certs. Generate a key par and export the public key:

keytool -genkeypair -keystore acmesoft.certs -alias acmeroot
keytool -exportcert -keystore acmesoft.certs -alias acmeroot -file acmeroot.cer

The public key is exported into a "self-signed" certificate. Then add it to every employee's keystore.

keytool -importcert -keystore cindy.certs -alias acmeroot -file acmeroot.cer

For Alice to send messages to Cindy and to everyone else at ACME Software, she needs to bring her certificate to the Information Resources Department and have it signed. Unfortunately, this functionality is missing in the keytool program. In the book's companion code, we supply a CertificateSigner class to fill the gap. An authorized staff member at ACME Software would verify Alice's identity and generate a signed certificate as follows:

java CertificateSigner -keystore acmesoft.certs -alias acmeroot
   -infile alice.cer -outfile alice_signedby_acmeroot.cer

The certificate signer program must have access to the ACME Software keystore, and the staff member must know the keystore password. Clearly, this is a sensitive operation.

Alice gives the file alice_signedby_acmeroot.cer file to Cindy and to anyone else in ACME Software. Alternatively, ACME Software can simply store the file in a company directory. Remember, this file contains Alice's public key and an assertion by ACME Software that this key really belongs to Alice.

Now Cindy imports the signed certificate into her keystore:

keytool -importcert -keystore cindy.certs -alias alice -file alice_signedby_acmeroot.cer

The keystore verifies that the key was signed by a trusted root key that is already present in the keystore. Cindy is not asked to verify the certificate fingerprint.

Once Cindy has added the root certificate and the certificates of the people who regularly send her documents, she never has to worry about the keystore again.

Certificate Requests

In the preceding section, we simulated a CA with a keystore and the CertificateSigner tool. However, most CAs run more sophisticated software to manage certificates, and they use slightly different formats for certificates. This section shows the added steps that are required to interact with those software packages.

We will use the OpenSSL software package as an example. The software is preinstalled for many Linux systems and Mac OS X, and a Cygwin port is also available. Alternatively, you can download the software at http://www.openssl.org.

To create a CA, run the CA script. The exact location depends on your operating system. On Ubuntu, run

/usr/lib/ssl/misc/CA.pl -newca

This script creates a subdirectory called demoCA in the current directory. The directory contains a root key pair and storage for certificates and certificate revocation lists.

You will want to import the public key into the Java keystore of all employees, but it is in the Privacy Enhanced Mail (PEM) format, not the DER format that the keystore accepts easily. Copy the file demoCA/cacert.pem to a file acmeroot.pem and open that file in a text editor. Remove everything before the line

-----BEGIN CERTIFICATE-----

and after the line

-----END CERTIFICATE-----

Now you can import acmeroot.pem into each keystore in the usual way:

keytool -importcert -keystore cindy.certs -alias alice -file acmeroot.pem

It seems quite incredible that the keytool cannot carry out this editing operation itself.

To sign Alice's public key, you start by generating a certificate request that contains the certificate in the PEM format:

keytool -certreq -keystore alice.store -alias alice -file alice.pem

To sign the certificate, run

openssl ca -in alice.pem -out alice_signedby_acmeroot.pem

As before, cut out everything outside the BEGIN CERTIFICATE/END CERTIFICATE markers from alice_signedby_acmeroot.pem. Then import it into the keystore:

keytool -importcert -keystore cindy.certs -alias alice -file alice_signedby_acmeroot.pem

You use the same steps to have a certificate signed by a public certificate authority such as VeriSign.

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