Home > Articles > Security > Network Security

Cryptography with Java

This chapter is from the book

Message Digest

Message digests, also known as message fingerprints or secure hash, are computed by applying a one-way hash function over the data bits comprising the message. Any modification in the original message, either intentional or unintentional, will most certainly result in a change of the digest value. Also, it is computationally impossible to derive the original message from the digest value. These properties make digests ideal for detecting changes in a given message. Compute the digest before storing or transmitting the message and then compute the digest after loading or receiving the message. If the digest values match then one can be sure with good confidence that the message has not changed. However, this scheme fails if a malicious interceptor has access to both the original message and its digest. In this case the interceptor could easily alter the message, compute the digest of the modified message and replace the original digest with the new one. The solution, as we see in the next section, is to secure the message digest by encrypting it with a secret key.

A common use of message digests is to securely store and validate passwords. The basic idea is that you never store the password in clear-text. Compute the message digest of the password and store the digest value. To verify the password, compute its digest and match it with the stored value. If both values are equal, the verification succeeds. This way no one, not even the administrator, gets to know your password. A side effect of this mechanism is that you cannot get back a forgotten password. This is not really as bad as it sounds, for you can always get it changed to a temporary password by an administrator, and then change it to something that only you know.

Message digests of messages stored in byte arrays are computed using engine class java.security.MessageDigest. The following program illustrates this.

Listing 3-7. Computing message digest

// File: src\jsbook\ch3\ComputeDigest.java
import java.security.MessageDigest;
import java.io.FileInputStream;

public class ComputeDigest {
  public static void main(String[] unused) throws Exception{
  String datafile = "ComputeDigest.java";

  MessageDigest md = MessageDigest.getInstance("SHA1");
  FileInputStream fis = new FileInputStream(datafile);
  byte[] dataBytes = new byte[1024];
  int nread = fis.read(dataBytes);
  while (nread > 0) {
    md.update(dataBytes, 0, nread);
    nread = fis.read(dataBytes);
  byte[] mdbytes = md.digest();
  System.out.println("Digest(in hex):: " + Util.byteArray2Hex(mdbytes));

A concrete, algorithm-specific MessageDigest object is created following the general pattern of all engine classes. The invocation of update() method computes the digest value and the digest() call completes the computation. It is possible to make multiple invocations of update(byte[] bytes) before calling the digest() method, thus avoiding the need to accumulate the complete message in a single buffer, if the original message happens to be fragmented over more than one buffer or cannot be kept completely in main memory. This is likely to be the case if the data bytes are being read from a huge file in fixed size buffers. In fact, convenience classes DigestInputStream and DigestOutputStream, both in the package java.security, exist to compute the digest as the bytes flow through the associated streams.

The verification or check for integrity of the message is done by computing the digest value and comparing this with the original digest for size and content equality. Class MessageDigest even includes static method isEqual(byte[] digestA, byte[] digestB) to perform this task.

Theoretically, because a much larger set of messages get mapped to a much smaller set of digest values, it is possible that two or more messages will have the same digest value. For example, the set of 1 KB messages has a total of 2(8*1024) distinct messages. If the size of the digest value is 128 then there are only 2128 different digest values possible. What it means is that there are, on the average, 2(8*1024—128) different 1KB messages with the same digest value. However, a brute-force search for a message that results in a given digest value would still require examining, on the average, 2127 messages. The problem becomes a bit simpler if one were to look for any pair of messages that give rise to the same digest value, requiring, on the average, only 264 attempts. This is known as the birthday attack, deriving its name from a famous mathematics puzzle, whose result can be stated as: there is more than a 50 percent chance that you will find someone with the same birthday as yours in a party of 183 persons. However, this number drops to 23 for any pair to have the same day as their birthday.

The providers bundled with J2SE v1.4 support two message digest algorithms: SHA (Secure Hash Algorithm) and MD5. SHA, also known as SHA-1, produces a message digest of 160 bits. It is a FIPS (Federal Information Processing Standard) approved standard. In August 2002, NIST announced three more FIPS approved standards for computing message digest: SHA-256, SHA-384 and SHA-512. These algorithms use a digest value of 256, 384 and 512 bits respectively, and hence provide much better protection against brute-force attacks. MD5 produces only 128 bits as message digest, and is considerably weaker.

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information

To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.


Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.


If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information

Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.


This site is not directed to children under the age of 13.


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information

If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information

Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents

California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure

Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact

Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice

We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020