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Understanding WS-Security


Scott Seely
Microsoft Corporation

October 2002

Applies to:
   Web Services Specifications (WS-Security, WS-Security Addendum)

Summary: This article looks at how to use WS-Security to embed security within the SOAP message itself, exploring the concerns WS-Security addresses: authentication, signatures, and encryption. (14 printed pages)


Parallels in Daily Life

Applying Existing Concepts to SOAP Messages

WS-Security SOAP Header




Before I explain what WS-Security is, I believe that it is important to understand why WS-Security exists at all. Many people new to Web services see SOAP as a way to exchange messages between two endpoints over HTTP. Over HTTP, one can authenticate the caller, sign the message, and encrypt the contents of the message. This makes the message secure in several dimensions: the caller is known, the receiver of the message can verify that the message did not change in transit, and entities watching the wire traffic cannot figure out what data is being exchanged. For those looking at SOAP messaging to solve bigger problems, however, HTTP-based security simply isn't enough. Many of the bigger problems involve sending the message along a path more complicated than request/response or over a transport that does not involve HTTP. The identity, integrity, and security of the message and the caller need to be preserved over multiple hops. More than one encryption key may be used along the route. Trust domains will be crossed. HTTP and its security mechanisms only address point-to-point security. More complex solutions need end-to-end security baked in. WS-Security addresses how to maintain a secure context over a multi-point message path.

Note   This article assumes that you are already familiar with XML Canonicalization, XML Signature, and XML Encryption.

WS-Security addresses security by leveraging existing standards and specifications. This avoids the necessity to define a complete security solution within WS-Security. The industry has solved many of these problems. Kerberos and X.509 address authentication. X.509 also uses existing PKI for key management. XML Encryption and XML Signature describe ways of encrypting and signing the contents of XML messages. XML Canonicalization describes ways of making the XML ready to be signed and encrypted. What WS-Security adds to existing specifications is a framework to embed these mechanisms into a SOAP message. This is done in a transport-neutral fashion.

WS-Security defines a SOAP Header element to carry security-related data. If XML Signature is used, this header can contain the information defined by XML Signature that conveys how the message was signed, the key that was used, and the resulting signature value. Likewise, if an element within the message is encrypted, the encryption information such as that conveyed by XML Encryption can be contained within the WS-Security header. WS-Security does not specify the format of the signature or encryption. Instead, it specifies how one would embed the security information laid out by other specifications within a SOAP message. WS-Security is primarily a specification for an XML-based security metadata container.

What does WS-Security do, beyond leveraging other existing protocols for message authentication, integrity, and privacy? It specifies a mechanism for transferring simple user credentials via the UsernameToken element. To send binary tokens that were used for encryption or signing the message, a BinarySecurityToken is also defined. Within this header, messages can store information about the caller, how the message was signed, and how the message was encrypted. WS-Security presents an end-to-end solution for Web service security by keeping all security information in the SOAP part of the message.

In this article, we will take a look at how to use WS-Security and friends to embed security within the SOAP message itself. We will look at the concerns WS-Security addresses:

  • Authentication
  • Signatures
  • Encryption

This triad addresses the main concerns of security and answers the questions:

  • Who am I authorizing?
  • Was the message modified between hops?
  • Did this message come from whom I think it came from?
  • How do I hide things I only want certain parties to see?

To begin with, let's look at some analogies seen in every day life.

Parallels in Daily Life

To understand what WS-Security is trying to do, I first want to take a look at a real-world parallel. Specifically, when and how do you use credentials in everyday life? After all, in day-to-day living you use credentials all the time. If someone asks you to prove your age, you dig into your wallet and pull out a driver's license. When you go to pay for an item without using currency, a credit card is used to identify you to the credit agency. When crossing a country's border or while in a foreign country, a passport vouches for your identity. All of these items are credentials. They assert that the owner of the credit card, driver's license, or passport is the person named on the document. They do not authenticate your identity, though. Authentication is an action that a document cannot perform. In the world of paper documents, people perform authentication. How does that side of authentication work?

When you present a driver's license or passport, a person reading the documents performs a few different actions to verify that the documents are real and that you are the rightful owner of those documents:

  • Both documents contain a picture of the registered holder of the document as well as other identifying characteristics: height, weight, and eye color. The person reading the document can make sure you look like the person shown and described by the document.
  • The documents expire on a regular basis. This is done so that up to date descriptive data is on the document.
  • The documents contain a signature that can be compared against the signature of the person presenting the documents. The difficulty of accurately reproducing another person's signature makes this a reasonable way to check identity when used in combination with the description of the person.

These documents have other important properties as well. They have marking that allow someone to quickly verify that the documents are genuine. The documents themselves are granted by organizations that we trust to provide valid identity information: local and national governments. I mentioned credit cards too. These are different from a driver's license or passport.

  • They are issued by banks, not governments.
  • They only contain a signature and a name to identify the card holder.
  • They can only be used to verify identity in the presence of another supporting document such as a government issued ID.

What does something like a credit card do then?

Credit cards typically rely on only using a signature for authentication. Some cards include photos to make authentication a little more solid. Because of the weak authentication provided by credit cards, many establishments will ask to see a government issued ID with the credit card. In terms of security, when you present the credit card, you assert that you have the right to charge goods and services and that the organization that gave you the credit card will pay the merchant. The merchant can validate your identity as the valid cardholder by comparing your government-issued photo ID to your physical person. (Of course, if you perform the transaction over the phone or the Internet, this part of my argument falls apart, but it suffices to state that other mechanisms exist to protect you in these arenas as well.)

Applying Existing Concepts to SOAP Messages

WS-Security seeks to move a lot of these concepts about identification and authorization into the world of SOAP messaging. In order to do something meaningful with a SOAP message, that message must contain information that does the following things:

  • Identify the entity or entities involved with the message.
  • Prove that the entities have the correct group memberships.
  • Prove that the entities have the correct set of access rights.
  • Prove that the message has not changed.

Finally, we also want a mechanism that would hide information from unauthorized parties. In the world of personal identification, I prove who I am with my driver's license or passport. I prove that I have certain rights through membership cards. In my wallet, I have cards that allow me to charge goods and services, check out books from the library, direct medical bills to my insurance provider, and receive discounts at local grocery stores. WS-Security allows me to apply the same concepts to SOAP messages. Using security tokens to identify the caller and assert its rights, a message could convey the following information:

  • Caller identity: I am Joe User.
  • Group membership: I am a ColdRooster.com developer.
  • Rights assertions: Because I am a ColdRooster.com developer, I can create databases and add Web applications to the ColdRooster.com machines.

To create a message that can create a new database on the ColdRooster.com servers using an authentication technology such as Kerberos, the application would have to acquire a number of security tokens. To start with, the application creating the message would need to acquire a security token that identifies it as acting on behalf of Joe User. Joe User provides that token when he logs in via a username/password or by using a smart card. Assuming that the security infrastructure uses Kerberos, the environment Joe is using has a Key Distribution Center that grants Joe a Ticket Granting Ticket (TGT) when he logs in. When Joe decides to create a new database on ColdRooster.com, the environment goes to a Ticket Granting Service and requests a Service Ticket that shows that Joe has the right to create a new database on ColdRooster.com. The environment takes that Service Ticket (ST) and presents it to the database server at ColdRooster.com. That database server validates the ticket and then allows Joe to create the new process.

WS-Security seeks to encapsulate the security interactions described above within a set of SOAP Headers. WS-Security handles credential management in two ways. It defines a special element, UsernameToken, to pass the username and password if the Web service is using custom authentication. WS-Security also provides a place to provide binary authentication tokens such as Kerberos Tickets and X.509 Certifications: BinarySecurityToken.

Figure 1 depicts what will become a fairly common message flow.

Figure 1. Typical message flow.

The Security Token service might be Kerberos, PKI, or a username/password validation service. This service may not be Web service-based. Indeed, a Kerberos service ticket granting service might be accessed through the Kerberos protocols using operating system security functions. Once the client gets the tokens it wants to use in the message, the client will embed those tokens within the message. The client should sign the message with a piece of data that only they know. The server will be able to deduce the signature in a number of ways. If the client is using a UsernameToken for authentication, the client should send a hashed password and sign the message using that password. The server will be able to verify that the client sent the message if the signatures it generates for the message match the signatures contained in the message.

When using X.509 certificates, the message can be signed using the private key. The message should contain the certificate in a BinarySecurityToken. When using X.509, anyone who knows the X.509 public key can verify the signature. Finally, when using Kerberos, the message could be signed or encrypted with a session key embedded in the Kerberos ticket. Because the Kerberos ticket will be keyed for the receiver of the token, only the receiver will be able to decrypt the ticket, discover the session key, and verify the signature.

It is critical that SOAP messages be signed or encrypted if authentication is important. Why? It isn't enough that a valid identity token is added to a message. These tokens can be lifted from a valid message and added to messages used by attackers. There needs to be evidence that the identity used in the message created the message. Without using XML Signature and signing the message, you cannot tell that the message has not been changed or that the identity token has not been abused.

At this point, I think you understand what WS-Security does. Let's dig a little deeper and look at how.

WS-Security SOAP Header

Starting in this section and continuing throughout the rest of the article, I will be using XML snippets. So that I don't have to show XML Namespace declarations all over and muddy the snippets, I will use the following XML Namespaces:

Table 1: XML Namespaces

Namespace Description Namespace URI
Xs XML Schema http://www.w3.org/2001/XMLSchema
Wsse WS-Security http://schemas.xmlsoap.org/ws/2002/07/secext
Wsu Utility elements http://schemas.xmlsoap.org/ws/2002/07/utility
Soap SOAP elements http://schemas.xmlsoap.org/soap/envelope/

The WS-Security specification defines a new SOAP header. To understand what the WS-Security SOAP header contains, I think it would be helpful to look at the schema fragment for the element first.

<xs:element name="Security">
        <xs:any processContents="lax" 
            minOccurs="0" maxOccurs="unbounded">
        <xs:anyAttribute processContents="lax"/>

As you can see, the Security header element allows any XML element or attribute to live within it. This allows the header to adapt to whatever security mechanisms your application needs. If this sounds a little odd, think about how the SOAP header and body work. The header and body both can contain a collection of XML elements. The SOAP specification makes few claims about the contents of these elements other than the fact that they cannot contain XML processing instructions.

WS-Security needs this type of structure because of what the header must do. It must be able to carry multiple security tokens to identify the caller's rights and identity. If the message is signed, the header must contain information about how it was signed and where the information regarding the key is stored. The key may be in the message or stored elsewhere and merely referenced. Finally, information about encryption must also be able to be carried in this header.

So, how does an intermediary know which WS-Security header it owns? A SOAP message may contain multiple WS-Security headers. Each header is identified by a unique actor. No two WS-Security headers can use the same actor or omit the actor. This makes it easy for intermediaries to identify which WS-Security headers contain the information they need. Of course, the intermediary does need to know which actor URI it handles. Associating a URI with an actor and making sure that the intermediary knows what to do is something that must be handled via programming. The actor attribute in any SOAP header is meant to say "this header is meant for any endpoint acting in the capacity indicated by the actor URI." What gives that URI meaning? The team that architects the Web service gives meaning to the URI. This means that an intermediary may act in varying capacities. As a result, that intermediary may consume zero, one, or more headers. Yes, it may even consume multiple security headers.

WS-Security Addendum

After evaluating WS-Security for a little while, a number of items came out that needed to be made clearer for security in particular. Also, additional items needed to be specified for Web services in general. The parts of the addendum that apply to security are covered throughout the article. In this section, I want to take a look at two items that are not specific to security: wsu:Id and wsu:Timestamp. The addendum specifies exactly what these two items do and how they should be used.


The Id attribute uses the XML Schema ID type. This element was added to simplify processing for Web service intermediaries as well as receivers. The value of this attribute must not be duplicated elsewhere in the document. The addendum does not go into more detail about how the element should be used other than as a unique identifier in GXA specifications. The door is left wide open to allow other specifications to restrict the usage of Id.


A common concern in message-oriented systems relates to the timeliness of data. If the data is too old, it may get thrown out. If two contradicting messages arrive, the related timestamps may be used to decide which message gets executed and which one is ignored. To handle the time-related issues that showed up in WS-Security and that will show up in other GXA specifications, the wsu:Timestamp element, along with a few helper elements, was defined.

The interesting events in a message's life are the time it was created, the time the sender wants the message to expire, and the time that the message was received. By knowing the creation and expiration time, a receiver can decide if the data is new enough for its own use or if the data has become so stale that the message should be discarded. The elements that convey this data are:

  • wsu:Created: Contains the time that the message was created.
  • wsu:Expires: Set by a sender or intermediary, this identifies when the message expires.
  • wsu:Received: Explains when the message was received by a particular intermediary.

All of the above elements may appear independently or as part of a wsu:Timestamp element. Each may contain a wsu:Id attribute to uniquely identify the item. By default, these timestamps express the time as an xs:dateTime type. To allow flexibility for other, nonstandard time stamps that may be meaningful in other problem domains, each of these items also contains an attribute named ValueType. This attribute does not need to appear if the time is expressed as xs:dateTime.

The wsu:Received element allows for two extra attributes not found on wsu:Created or wsu:Expires. The element can express the URI of the actor it is related to, using the Actor attribute and the amount of delay, in milliseconds, caused by the actor using the Delay attribute.

As I mentioned, you can use the wsu:Received, wsu:Created, and wsu:Expires elements within other structures. For example, it may be common to see the wsu:Created element to indicate when a particular element was added to the message. When indicating more information about a message and using more than one of these elements at a time, the elements can be wrapped inside of a wsu:Timestamp element. Each of three elements may only appear once within a timestamp. The timestamp may used on the message as a whole, in which case it appears as a child of the soap:Header node. For example, a message could indicate it was valid for the next five minutes using the following wsu:Timestamp header.

    <wsu:Created wsu:Id=
    <wsu:Expires wsu:Id=

At this point, I believe that you have enough background information to dig into how authentication, signing, and encryption work with WS-Security.


WS-Security provides for an infinite number of ways to validate a user. The specification addresses three methods from that infinite number:

  • Username/Password
  • PKI through X.509 Certificates
  • Kerberos

In this section, we will look at how each of these authentication methods works and how that information is encoded into a SOAP message.


One of the most common ways to pass around caller credentials is to use a username and password combination. This is a technique used in HTTP Basic and Digest authentication. As a matter of fact, if you are familiar with how HTTP Digest authentication works, you will feel right at home with this authentication mechanism. To pass user credentials in this manner, WS-Security has defined the UsernameToken element. Schema for the element is as follows:

<xs:element name="UsernameToken">
            <xs:element ref="Username"/>
            <xs:element ref="Password" minOccurs="0"/>
        <xs:attribute name="Id" type="xs:ID"/>
        <xs:anyAttribute namespace="##other"/>

This schema fragment references two other types: Username and Password. These two types are essentially strings that may contain extra attributes as needed. Password contains an attribute named Type that indicates how the password is being passed around. A password can be passed as plain text or in digest format. When passing a UsernameToken in a SOAP message, the XML may come across as something like this:

    <wsse:Password Type="wsse:PasswordText">password</wsse:Password>

What you see here is an example of the password being sent as plain text. This particular solution looks pretty easy to break. If you want the password sent in a more secure manner, you can send it is as a digest hash.

    <wsse:Password Type="wsse:PasswordDigest">
    <wsu:Created xmlns:wsu=

This adds a bit more security because the password is now obscured using a SHA1 hash. The password digest is the concatenation of the nonce plus the creation time plus the password. The nonce is 16 bytes long and is passed along as a base64 encoded value. The way this works is that the client creates the password hash using all of this information plus the password. The receiver verifies the data by getting the plain password and creating the hash again. If the results agree, the password must be correct. This protection does not protect against replay attacks. If you use it, make sure to also include a wsu:Timestamp header with a small enough time window for the created and expired values. Then, sign the wsu:Timestamp elements within the message so that any tampering with the timestamp can be detected. Otherwise, an attacker could use the complete UsernameToken to attack your Web service. To defend against a replay attack, you will also need to include a mechanism that tracks some unique characteristic of the incoming messages. This mechanism needs to save this characteristic in a cache for at least the timeout period of the message.

X.509 Certificates

Another option to use when authenticating users is to simply send around an X.509 certificate. An X.509 certificate tells you exactly who the user is. Using PKI, you can map the certificate to an existing user in your application. Using the certificate on its own would make for some pretty easy replay attacks. As a result, it's a good idea to force the message sender to also sign the message using their private key. That way, when the message gets decrypted, you'll know it is really the user.

When a message does send along an X.509 certificate, it will pass the public version of the certificate in a WS-Security token named BinarySecurityToken. The certificate itself gets sent along as base64 encoded data. The BinarySecurityToken has the following schema:

<xs:element name="BinarySecurityToken">
            <xs:extension base="xs:string">
                <xs:attribute name="Id" type="xs:ID" />
                <xs:attribute name="ValueType" type="xs:QName" />
                <xs:attribute name="EncodingType" type="xs:QName" />
                <xs:anyAttribute namespace="##other" 
                    processContents="strict" />

At its most basic, this item contains a string, a unique identifier, and some information indicating what type of value is included and how it was encoded. The ValueType may be any of the following values, defined by the ValueTypeEnum in the WS-Security schema document:

  • wsse:X509v3: An X.509, version 3 certificate.
  • wsse:Kerberosv5TGT: A ticket granting ticket as defined by section 5.3.1 of the Kerberos specification.
  • wsse:Kerberosv5ST: A service ticket as defined by section 5.3.1 of the Kerberos specification.

If this information on Kerberos doesn't make any sense to you, I'll explain it a little better in the next section. The EncodingType is another enumeration. If it is set to wsse:Base64Binary or wsse:HexBinary. As you might guess, this value simply indicates which encoding method was used. In a WS-Security header, this element would look something like this when passing an X.509 certificate:


Remember, when you use an X.509 certificate, you want to do something else as well, such as sign the message. The signature, created using the certificate's private key, proves that the client is the rightful owner of the certificate. Such a message could be replayed. To help mitigate the problems around replays, you would institute a policy that states how old a message can be before it is ignored. The time should travel in a wsu:Timestamp element that is shipped as a SOAP Header within the message.


To use Kerberos, a user presents a set of credentials such as username/password or an X.509 certificate. If everything checks out, the security system grants the user a ticket granting ticket (TGT). The TGT is an opaque piece of data that the user cannot read but must present in order to access other resources. The user will typically present the TGT in order to get a service ticket (ST). The way the system works is as follows:

  1. A client authenticates to a Key Distribution Center (KDC) and is granted a TGT.
  2. The client takes the TGT and uses it to access a Ticket Granting Service (TGS).
  3. The client requests an ST for a particular network resource. The TGS then issues the ST to the client.
  4. The client presents the ST to the network resource and begins accessing the resource with the permissions the ST indicated.

Kerberos is appealing because it contains a mechanism for the client to prove their identity to a service and for the service to prove their identity to the client. The ST is only good for accessing the one network resource and can be used to discover who the caller is. When presenting a Kerberos ticket in a message, the data needs to be blindly copied into the message itself. WS-Security does not explain how a TGT or ST is obtained.


When a message is signed, it is nearly impossible to tamper with the message. Message signing does not protect the message itself from external parties seeing its contents. Using the signature, the receiver of the SOAP message can know that the signed elements have not changed en route. You should use XML Signature to sign messages whenever possible. Why? XML Signature already handles a number of the tougher items to figure out. WS-Security simply explains how to use signing to prove that the message has not changed. All three of the authentication mechanisms mentioned above provides a way to sign the message so that you can be sure of two things:

  • The user identified by the X.509 certificate, UsernameToken, or Kerberos ticket signed the message.
  • The message has not been tampered with since it was signed.

Each of the methods provides a secret that can be used to sign the message. X.509 allows the sender to sign the message using their private key. Kerberos provides a session key that the sender creates and transmits in the ticket. Only the intended receiver of that message can read the ticket, discover the session key, and verify the authenticity of the signature. Finally, the UsernameToken could be signed using the password.

The signature is generated using XML Signature. To sign a simple message such as "Hello World," almost every element in the message needs to be individually signed. wsu:Timestamp presents an interesting problem because an intermediary may add a wsu:Received element to wsu:Timestamp. Every time an element changes, the signature needs to be updated or else things won't look right. Why? If the content changes, the signature should not match. Within a SOAP Message, the signatures and required extra data add quite a bit of extra information.


There are times when proving the message sender's identity and showing that the message was not changed is not enough. If you send a credit card number or bank account number in a plain-text but signed manner, an attacker can actually verify that no other attackers have changed the contents of the message. As a result, they have a high confidence that the data is valid. That's no good for you, is it? Instead, you'd like the data encrypted in such a way that only the intended message recipient can read the message. Anyone watching the wire exchange should remain oblivious to the contents of the message. As with signing messages, the WS-Security specification does the right thing and adopts a standard that already exists and does the job of encryption well. That's right, they incorporated XML Encryption.

When you encrypt data, you can choose to use either symmetric or asymmetric encryption. Symmetric encryption requires a shared secret. That is, the key that is used to encrypt the message is the same key that you would use to decrypt the message. Symmetric encryption is good if you control both endpoints and can trust the people and applications that use the keys. Symmetric encryption does have a problem with key distribution. At some point in time, the key needs to be sent to the receiver. How do you do this? Do you ship a disk in the mail or negotiate the key when it is needed? Both options will work.

If you need to send data using easily distributed keys, look to asymmetric encryption. X.509 certificates allow for this. The endpoint receiving the data can publicly post its certificate and allow anyone and everyone to encrypt information using the public key. Only the receiver will know the private key. Because of this, only the receiver can take the encrypted data and turn it back into something readable.

So, what would an encrypted message look like? If you are using Triple-DES, both the sender and receiver would have to exchange the key in some secure manner. The symmetric key can be hidden inside a Kerberos ticket, or exchanged out of band. The WS-Security-based message with embedded XML Encryption information would look something like this:

<?xml version="1.0" encoding="utf-8" ?>
        <wsse:Security soap:mustUnderstand="1" >
        URI="#EncryptedContent-f6f50b24-3458-41d3-aac4-390f476f2e51" />
        URI="#EncryptedContent-666b184a-a388-46cc-a9e3-06583b9d43b6" />
            <xenc:EncryptionMethod Algorithm=
                "http://www.w3.org/2001/04/xmlenc#tripledes-cbc" />
            <KeyInfo xmlns="http://www.w3.org/2000/09/xmldsig#">
                <KeyName>Symmetric Key</KeyName>
                >InmSSXQcBV5UiT...  Y7RVZQqnPpZYMg==</xenc:CipherValue>

The preceding message contains information about what data was encrypted as well as how the encryption was performed. For anyone who does not have access to the key, the cipher text inside the soap:Body cannot be decrypted.

When performing asymmetric encryption, the private key needs to be known to the receiver of the message in order to decrypt that message. Exchanging the public key has to be figured out ahead of time.


WS-Security allows for a SOAP message to identify the caller, sign the message, and encrypt message contents. Whenever possible, existing specifications are reused to reduce the amount of invention required to securely deliver a SOAP message. Because all of the information is delivered within the message itself, the message becomes transport neutral. The message would be secure if it was delivered by HTTP, e-mail, or on CD-ROM.


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