XML Reference Guide

Mar 14, 2003

␡

⎙ Print

< Back Page 171 of 278 Next >

To the uninitiated -- that's just about anybody who doesn't eat, drink, and sleep electronic security -- XML Signature can be very confusing. The recommendation is filled with talk of canonicalization, keystores, digests, hashes, and lots of other stuff that just doesn't come up in everyday programming. The result is that until you see it in action, it can be very hard to put the pieces together into something that seems intelligable.

Fortunately, once you do see it in action, it's not that difficult to get your head around. Here's the basic idea:

You have data of some sort. You want someone to vouch for it. That someone wants to make sure that the data you're looking at is the same data he or she vouched for. XML Signature let's you do that.

Here's how it works:

The person vouching for the data -- let's call that person the "signer" -- has some sort of certificate to prove that they're really them. It's like when you go to a notary public to sign a document and she takes a photocopy of your driver's license to make sure you're really you. That certificate can be any number of things, such as a private key or an X.509 certificate. The more you're worried about the signer, the more you need to worry about that certificate, or "key". If you're only worried about making sure the data hasn't changed since it was signed, the key only has to exist. For example, Java comes with a program called keytool that generates certficates.

The signer then takes a program, and signs the data. The process of signing the data creates a "message digest" that refers back to the data and packages it all up. Any change in the data means a change in the digest, so when the recipient -- let's call that person the "verifier" -- gets the data, he or she can check to make sure that the digest still matches the message. If it does, then the verifier knows that the data is intact.

That's the general idea, so let's look at how it actually happens. For this example, I used IBM's XML Security Suite, a set of Java classes that implements a number of XML and Web Services security-related specifications.

First, I had to generate a key to sign the data with. I did that using the keytool.exe application located in Java's bin directory:

keytool -genkey -dname "CN=Nick Chase, OU=InformIT, O=Pearson, 
L=NPR, S=Florida, C=US" -alias nick -storepass mystorepassword -keypass mykeypassword

Next I created a very simple XML document to sign:

<theroot>
<thedata>data!</thedata>
</theroot>

Next I used one of the sample applications, dsig.SampleSign2, to actually sign the data:

java -cp .;xml4j.jar;xss4j.jar;are.jar dsig.SampleSign2 nick mystorepassword 
mykeypassword -embxml stuff.xml > signature.xml

The jar files are part of the XML Security Suite distribution, and the SampleSign2 application takes the id for the certificate and the certificate passwords to start with, and then the resource (or resources) to sign, finally sending the to the file signature.xml. In this case, I've chosen to embed the resource right into the signature file, but it's also possible to have external resources.

If we look at the signature.xml file, we can see it all come together:

<Signature xmlns="http://www.w3.org/2000/09/xmldsig#">
  <SignedInfo>
    <CanonicalizationMethod Algorithm="http://www.w3.org/TR/2001/REC-xml-c14n-20010315"></CanonicalizationMethod>
    <SignatureMethod Algorithm="http://www.w3.org/2000/09/xmldsig#dsa-sha1"></SignatureMethod>
    <Reference URI="#Res0">
      <Transforms>
        <Transform Algorithm="http://www.w3.org/TR/2001/REC-xml-c14n-20010315"></Transform>
      </Transforms>
      <DigestMethod Algorithm="http://www.w3.org/2000/09/xmldsig#sha1"></DigestMethod>
      <DigestValue>psLjBQtzI7t7wiozRfLoNdaIb08=</DigestValue>
    </Reference>
  </SignedInfo>
  <SignatureValue>
    ZimJc+beK3HLLlpPcxh0kvBIPYB6YQs+as6SqXID4pY0tyF6qWQ3KQ==
  </SignatureValue>
  <KeyInfo>
    <KeyValue>
      <DSAKeyValue>
        <P>
          /X9TgR11EilS30qcLuzk5/YRt1I870QAwx4/gLZRJmlFXUAiUftZPY1Y+r/F9bow9s
          ubVWzXgTuAHTRv8mZgt2uZUKWkn5/oBHsQIsJPu6nX/rfGG/g7V+fGqKYVDwT7g/bT
          xR7DAjVUE1oWkTL2dfOuK2HXKu/yIgMZndFIAcc=
        </P>
        <Q>l2BQjxUjC8yykrmCouuEC/BYHPU=</Q>
        <G>
          9+GghdabPd7LvKtcNrhXuXmUr7v6OuqC+VdMCz0HgmdRWVeOutRZT+ZxBxCBgLRJFn
          Ej6EwoFhO3zwkyjMim4TwWeotUfI0o4KOuHiuzpnWRbqN/C/ohNWLx+2J6ASQ7zKTx
          vqhRkImog9/hWuWfBpKLZl6Ae1UlZAFMO/7PSSo=
        </G>
        <Y>
          HPnTW4UAvR4MIYOqym1zdLFlzNREQpKCxOMbiSvD04es7RONiBECT6OtlMWAZ9k/9N
          gOK/HmxObSC5inAjXN2omNZzBburQuJAmRs/G2nwRp49yLRNHWq1922jCYoZ6iFmNA
          iraKSIOYTS1LnfGEEgl7zNc/XDsArRtZ+LBdrZo=
        </Y>
      </DSAKeyValue>
    </KeyValue>
    <X509Data>
      <X509IssuerSerial>
        <X509IssuerName>CN=Nick Chase,OU=InformIT,O=Pearson,L=New Port Richey,ST=Florida,C=US</X509IssuerName>
        <X509SerialNumber>1089899916</X509SerialNumber>
      </X509IssuerSerial>
      <X509SubjectName>CN=Nick Chase,OU=InformIT,O=Pearson,L=New Port Richey,ST=Florida,C=US</X509SubjectName>
      <X509Certificate>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      </X509Certificate>
    </X509Data>
  </KeyInfo>
  <dsig:Object xmlns="" xmlns:dsig="http://www.w3.org/2000/09/xmldsig#" Id="Res0"><theroot>

<thedata>data!</thedata>
</theroot></dsig:Object>
</Signature>

Starting at the bottom, the file defines the object that I've signed, including it in the document and assigning it an ID of Res0 so we can refer back to it later. Above that, we have the KeyInfo element, which includes information about the keys and certificates I used to sign the document. Next up, we have the actual signature. Finally, above that, we have the information on what was actually signed.

This last bit takes a bit more looking into, so let's repeat it here:

  <SignedInfo>
    <CanonicalizationMethod Algorithm="http://www.w3.org/TR/2001/REC-xml-c14n-20010315"></CanonicalizationMethod>
    <SignatureMethod Algorithm="http://www.w3.org/2000/09/xmldsig#dsa-sha1"></SignatureMethod>
    <Reference URI="#Res0">
      <Transforms>
        <Transform Algorithm="http://www.w3.org/TR/2001/REC-xml-c14n-20010315"></Transform>
      </Transforms>
      <DigestMethod Algorithm="http://www.w3.org/2000/09/xmldsig#sha1"></DigestMethod>
      <DigestValue>psLjBQtzI7t7wiozRfLoNdaIb08=</DigestValue>
    </Reference>
  </SignedInfo>

First off, you have to remember that this process is extremely sensitive to changes in the document. For example, just changing the spacing in an XML document is still a change, and the signature would be invalid. Therefore, before signing a document, you need to make sure it's in "canonical" format, so here we're specifying what needs to be done to the document, and how to do it. We're also specifying the type of signature we're dealing with.

Next we've got the actual resource being signed, described by the Reference element. In this case, the data's embedded in the signature file, so we're pointing to it through the URL fragment #Res0, which matches the object ID for the data. From there we're specifying what we did with the data -- the transform and the method for creating the digest -- and then the digest itself.

Before I move on to verifying this file, note that you can also sign external resources. For example:

...
  <SignedInfo>
    <CanonicalizationMethod Algorithm="http://www.w3.org/TR/2001/REC-xml-c14n-20010315"></CanonicalizationMethod>
    <SignatureMethod Algorithm="http://www.w3.org/2000/09/xmldsig#dsa-sha1"></SignatureMethod>
    <Reference URI="http://www.nicholaschase.com/testsoap.php">
      <DigestMethod Algorithm="http://www.w3.org/2000/09/xmldsig#sha1"></DigestMethod>
      <DigestValue>pSw2lraC6kX9m85GUwG0a/+p00c=</DigestValue>
    </Reference>
  </SignedInfo>
...

In this case, the data is not included as part of the signature file.

Finally, you want to verify the data to make sure it's intact. The XML Security Suite has a sample called VerifyGUI that verifies the signature. This application takes the data and performs the same canonicalization and transforms on it, then calculates a digest. If the digest matches what's in the signature file, it passes. If not, it returns a message saying so. Don't believe me? Create a signature of your own, change one caracter of the actual data, and run VerifyGUI. You'll see.

So that's it. I have simplified some things in this explanation, but this is what you need to get you started. Check out the resources, or ask a question in the comments for the XML Reference Guide to find out more.

< Back Page 171 of 278 Next >

🔖 Save To Your Account

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.

Privacy Notice

Overview

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.

Surveys

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.

Newsletters

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.

Security

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

Children

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

Marketing

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.

Choice/Opt-out

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.

Links

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

Email Address