xml-crypto
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xml-crypto

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An xml digital signature library for node. Xml encryption is coming soon. Written in pure javascript!

For more information visit my blog or my twitter.

Install

Install with npm:

npm install xml-crypto

A pre requisite it to have openssl installed and its /bin to be on the system path. I used version 1.0.1c but it should work on older versions too.

Supported Algorithms

Canonicalization and Transformation Algorithms

Hashing Algorithms

Signature Algorithms

HMAC-SHA1 is also available but it is disabled by default

to enable HMAC-SHA1, call enableHMAC() on your instance of SignedXml.

This will enable HMAC and disable digital signature algorithms. Due to key confusion issues, it is risky to have both HMAC-based and public key digital signature algorithms enabled at same time.

You are able to extend xml-crypto with custom algorithms.

Signing Xml documents

When signing a xml document you can pass the following options to the SignedXml constructor to customize the signature process:

  • privateKey - [required] a Buffer or pem encoded String containing your private key
  • publicCert - [optional] a Buffer or pem encoded String containing your public key
  • signatureAlgorithm - [required] one of the supported signature algorithms. Ex: sign.signatureAlgorithm = "http://www.w3.org/2001/04/xmldsig-more#rsa-sha256"
  • canonicalizationAlgorithm - [required] one of the supported canonicalization algorithms. Ex: sign.canonicalizationAlgorithm = "http://www.w3.org/2001/10/xml-exc-c14n#WithComments"

Use this code:

var SignedXml = require("xml-crypto").SignedXml,
  fs = require("fs");

var xml = "<library>" + "<book>" + "<name>Harry Potter</name>" + "</book>" + "</library>";

var sig = new SignedXml({ privateKey: fs.readFileSync("client.pem") });
sig.addReference({
  xpath: "//*[local-name(.)='book']",
  digestAlgorithm: "http://www.w3.org/2000/09/xmldsig#sha1",
  transforms: ["http://www.w3.org/2001/10/xml-exc-c14n#"],
});
sig.canonicalizationAlgorithm = "http://www.w3.org/2001/10/xml-exc-c14n#";
sig.signatureAlgorithm = "http://www.w3.org/2000/09/xmldsig#rsa-sha1";
sig.computeSignature(xml);
fs.writeFileSync("signed.xml", sig.getSignedXml());

The result will be:

<library>
  <book Id="_0">
    <name>Harry Potter</name>
  </book>
  <Signature xmlns="http://www.w3.org/2000/09/xmldsig#">
    <SignedInfo>
      <CanonicalizationMethod Algorithm="http://www.w3.org/2001/10/xml-exc-c14n#" />
      <SignatureMethod Algorithm="http://www.w3.org/2000/09/xmldsig#rsa-sha1" />
      <Reference URI="#_0">
        <Transforms>
          <Transform Algorithm="http://www.w3.org/2001/10/xml-exc-c14n#" />
        </Transforms>
        <DigestMethod Algorithm="http://www.w3.org/2000/09/xmldsig#sha1" />
        <DigestValue>cdiS43aFDQMnb3X8yaIUej3+z9Q=</DigestValue>
      </Reference>
    </SignedInfo>
    <SignatureValue>vhWzpQyIYuncHUZV9W...[long base64 removed]...</SignatureValue>
  </Signature>
</library>

Note:

If you set the publicCert and the getKeyInfoContent properties, a <KeyInfo></KeyInfo> element with the public certificate will be generated in the signature:

<Signature xmlns="http://www.w3.org/2000/09/xmldsig#">
  <SignedInfo>
    ...[signature info removed]...
  </SignedInfo>
  <SignatureValue>vhWzpQyIYuncHUZV9W...[long base64 removed]...</SignatureValue>
  <KeyInfo>
    <X509Data>
      <X509Certificate>MIIGYjCCBJagACCBN...[long base64 removed]...</X509Certificate>
    </X509Data>
  </KeyInfo>
</Signature>

For getKeyInfoContent, a default implementation SignedXml.getKeyInfoContent is available.

To customize this see customizing algorithms for an example.

Verifying Xml documents

When verifying a xml document you can pass the following options to the SignedXml constructor to customize the verify process:

  • publicCert - [optional] your certificate as a string, a string of multiple certs in PEM format, or a Buffer
  • privateKey - [optional] your private key as a string or a Buffer - used for verifying symmetrical signatures (HMAC)

The certificate that will be used to check the signature will first be determined by calling this.getCertFromKeyInfo(), which function you can customize as you see fit. If that returns null, then publicCert is used. If that is null, then privateKey is used (for symmetrical signing applications).

Example:

new SignedXml({
  publicCert: client_public_pem,
  getCertFromKeyInfo: () => null,
});

You can use any dom parser you want in your code (or none, depending on your usage). This sample uses xmldom, so you should install it first:

npm install @xmldom/xmldom

Example:

var select = require("xml-crypto").xpath,
  dom = require("@xmldom/xmldom").DOMParser,
  SignedXml = require("xml-crypto").SignedXml,
  fs = require("fs");

var xml = fs.readFileSync("signed.xml").toString();
var doc = new dom().parseFromString(xml);

var signature = select(
  doc,
  "//*[local-name(.)='Signature' and namespace-uri(.)='http://www.w3.org/2000/09/xmldsig#']",
)[0];
var sig = new SignedXml({ publicCert: fs.readFileSync("client_public.pem") });
sig.loadSignature(signature);
try {
  var res = sig.checkSignature(xml);
} catch (ex) {
  console.log(ex);
}

In order to protect from some attacks we must check the content we want to use is the one that has been signed:

// Roll your own
const elem = xpath.select("/xpath_to_interesting_element", doc);
const uri = sig.getReferences()[0].uri; // might not be 0; it depends on the document
const id = uri[0] === "#" ? uri.substring(1) : uri;
if (
  elem.getAttribute("ID") != id &&
  elem.getAttribute("Id") != id &&
  elem.getAttribute("id") != id
) {
  throw new Error("The interesting element was not the one verified by the signature");
}

// Get the validated element directly from a reference
const elem = sig.references[0].getValidatedElement(); // might not be 0; it depends on the document
const matchingReference = xpath.select1("/xpath_to_interesting_element", elem);
if (!isDomNode.isNodeLike(matchingReference)) {
  throw new Error("The interesting element was not the one verified by the signature");
}

// Use the built-in method
const elem = xpath.select1("/xpath_to_interesting_element", doc);
try {
  const matchingReference = sig.validateElementAgainstReferences(elem, doc);
} catch {
  throw new Error("The interesting element was not the one verified by the signature");
}

// Use the built-in method with a an xpath expression
try {
  const matchingReference = sig.validateReferenceWithXPath("/xpath_to_interesting_element", doc);
} catch {
  throw new Error("The interesting element was not the one verified by the signature");
}

Note:

The xml-crypto api requires you to supply it separately the xml signature ("<Signature>...</Signature>", in loadSignature) and the signed xml (in checkSignature). The signed xml may or may not contain the signature in it, but you are still required to supply the signature separately.

Caring for Implicit transform

If you fail to verify signed XML, then one possible cause is that there are some hidden implicit transforms(#).
(#) Normalizing XML document to be verified. i.e. remove extra space within a tag, sorting attributes, importing namespace declared in ancestor nodes, etc.

The reason for these implicit transform might come from complex xml signature specification, which makes XML developers confused and then leads to incorrect implementation for signing XML document.

If you keep failing verification, it is worth trying to guess such a hidden transform and specify it to the option as below:

var options = {
  implicitTransforms: ["http://www.w3.org/TR/2001/REC-xml-c14n-20010315"],
  publicCert: fs.readFileSync("client_public.pem"),
};
var sig = new SignedXml(options);
sig.loadSignature(signature);
var res = sig.checkSignature(xml);

You might find it difficult to guess such transforms, but there are typical transforms you can try.

API

xpath

See xpath.js for usage. Note that this is actually using another library as the underlying implementation.

SignedXml

The SignedXml constructor provides an abstraction for sign and verify xml documents. The object is constructed using new SignedXml(options?: SignedXmlOptions) where the possible options are:

  • idMode - default null - if the value of wssecurity is passed it will create/validate id's with the ws-security namespace.
  • idAttribute - string - default Id or ID or id - the name of the attribute that contains the id of the element
  • privateKey - string or Buffer - default null - the private key to use for signing
  • publicCert - string or Buffer - default null - the public certificate to use for verifying
  • signatureAlgorithm - string - the signature algorithm to use
  • canonicalizationAlgorithm - string - default undefined - the canonicalization algorithm to use
  • inclusiveNamespacesPrefixList - string - default null - a list of namespace prefixes to include during canonicalization
  • implicitTransforms - string[] - default [] - a list of implicit transforms to use during verification
  • keyInfoAttributes - object - default {} - a hash of attributes and values attrName: value to add to the KeyInfo node
  • getKeyInfoContent - function - default noop - a function that returns the content of the KeyInfo node
  • getCertFromKeyInfo - function - default SignedXml.getCertFromKeyInfo - a function that returns the certificate from the <KeyInfo /> node

API

A SignedXml object provides the following methods:

To sign xml documents:

  • addReference(xpath, transforms, digestAlgorithm) - adds a reference to a xml element where:
    • xpath - a string containing a XPath expression referencing a xml element
    • transforms - an array of transform algorithms, the referenced element will be transformed for each value in the array
    • digestAlgorithm - one of the supported hashing algorithms
  • computeSignature(xml, [options]) - compute the signature of the given xml where:
    • xml - a string containing a xml document
    • options - an object with the following properties:
      • prefix - adds this value as a prefix for the generated signature tags
      • attrs - a hash of attributes and values attrName: value to add to the signature root node
      • location - customize the location of the signature, pass an object with a reference key which should contain a XPath expression to a reference node, an action key which should contain one of the following values: append, prepend, before, after
      • existingPrefixes - A hash of prefixes and namespaces prefix: namespace that shouldn't be in the signature because they already exist in the xml
  • getSignedXml() - returns the original xml document with the signature in it, must be called only after computeSignature
  • getSignatureXml() - returns just the signature part, must be called only after computeSignature
  • getOriginalXmlWithIds() - returns the original xml with Id attributes added on relevant elements (required for validation), must be called only after computeSignature

To verify xml documents:

  • loadSignature(signatureXml) - loads the signature where:
    • signatureXml - a string or node object (like an xmldom node) containing the xml representation of the signature
  • checkSignature(xml) - validates the given xml document and returns true if the validation was successful

Customizing Algorithms

The following sample shows how to sign a message using custom algorithms.

First import some modules:

var SignedXml = require("xml-crypto").SignedXml,
  fs = require("fs");

Now define the extension point you want to implement. You can choose one or more.

To determine the inclusion and contents of a <KeyInfo /> element, the function this.getKeyInfoContent() is called. There is a default implementation of this. If you wish to change this implementation, provide your own function assigned to the property this.getKeyInfoContent. If you prefer to use the default implementation, assign SignedXml.getKeyInfoContent to this.getKeyInfoContent If there are no attributes and no contents to the <KeyInfo /> element, it won't be included in the generated XML.

To specify custom attributes on <KeyInfo />, add the properties to the .keyInfoAttributes property.

A custom hash algorithm is used to calculate digests. Implement it if you want a hash other than the built-in methods.

function MyDigest() {
  this.getHash = function (xml) {
    return "the base64 hash representation of the given xml string";
  };

  this.getAlgorithmName = function () {
    return "http://myDigestAlgorithm";
  };
}

A custom signing algorithm.

function MySignatureAlgorithm() {
  /*sign the given SignedInfo using the key. return base64 signature value*/
  this.getSignature = function (signedInfo, privateKey) {
    return "signature of signedInfo as base64...";
  };

  this.getAlgorithmName = function () {
    return "http://mySigningAlgorithm";
  };
}

Custom transformation algorithm.

function MyTransformation() {
  /*given a node (from the xmldom module) return its canonical representation (as string)*/
  this.process = function (node) {
    //you should apply your transformation before returning
    return node.toString();
  };

  this.getAlgorithmName = function () {
    return "http://myTransformation";
  };
}

Custom canonicalization is actually the same as custom transformation. It is applied on the SignedInfo rather than on references.

function MyCanonicalization() {
  /*given a node (from the xmldom module) return its canonical representation (as string)*/
  this.process = function (node) {
    //you should apply your transformation before returning
    return "< x/>";
  };

  this.getAlgorithmName = function () {
    return "http://myCanonicalization";
  };
}

Now you need to register the new algorithms:

/*register all the custom algorithms*/

signedXml.CanonicalizationAlgorithms["http://MyTransformation"] = MyTransformation;
signedXml.CanonicalizationAlgorithms["http://MyCanonicalization"] = MyCanonicalization;
signedXml.HashAlgorithms["http://myDigestAlgorithm"] = MyDigest;
signedXml.SignatureAlgorithms["http://mySigningAlgorithm"] = MySignatureAlgorithm;

Now do the signing. Note how we configure the signature to use the above algorithms:

function signXml(xml, xpath, key, dest) {
  var options = {
    publicCert: fs.readFileSync("my_public_cert.pem", "latin1"),
    privateKey: fs.readFileSync(key),
    /*configure the signature object to use the custom algorithms*/
    signatureAlgorithm: "http://mySignatureAlgorithm",
    canonicalizationAlgorithm: "http://MyCanonicalization",
  };

  var sig = new SignedXml(options);

  sig.addReference({
    xpath: "//*[local-name(.)='x']",
    transforms: ["http://MyTransformation"],
    digestAlgorithm: "http://myDigestAlgorithm",
  });

  sig.addReference({
    xpath,
    transforms: ["http://MyTransformation"],
    digestAlgorithm: "http://myDigestAlgorithm",
  });
  sig.canonicalizationAlgorithm = "http://www.w3.org/2001/10/xml-exc-c14n#";
  sig.signatureAlgorithm = "http://www.w3.org/2000/09/xmldsig#rsa-sha1";
  sig.computeSignature(xml);
  fs.writeFileSync(dest, sig.getSignedXml());
}

var xml = "<library>" + "<book>" + "<name>Harry Potter</name>" + "</book>";
("</library>");

signXml(xml, "//*[local-name(.)='book']", "client.pem", "result.xml");

You can always look at the actual code as a sample.

Asynchronous signing and verification

If the private key is not stored locally, and you wish to use a signing server or Hardware Security Module (HSM) to sign documents, you can create a custom signing algorithm that uses an asynchronous callback.

function AsyncSignatureAlgorithm() {
  this.getSignature = function (signedInfo, privateKey, callback) {
    var signer = crypto.createSign("RSA-SHA1");
    signer.update(signedInfo);
    var res = signer.sign(privateKey, "base64");
    //Do some asynchronous things here
    callback(null, res);
  };
  this.getAlgorithmName = function () {
    return "http://www.w3.org/2000/09/xmldsig#rsa-sha1";
  };
}

var sig = new SignedXml({ signatureAlgorithm: "http://asyncSignatureAlgorithm" });
sig.SignatureAlgorithms["http://asyncSignatureAlgorithm"] = AsyncSignatureAlgorithm;
sig.signatureAlgorithm = "http://asyncSignatureAlgorithm";
sig.canonicalizationAlgorithm = "http://www.w3.org/2001/10/xml-exc-c14n#";
sig.computeSignature(xml, opts, function (err) {
  var signedResponse = sig.getSignedXml();
});

The function sig.checkSignature may also use a callback if asynchronous verification is needed.

X.509 / Key formats

Xml-Crypto internally relies on node's crypto module. This means pem encoded certificates are supported. So to sign an xml use key.pem that looks like this (only the beginning of the key content is shown):

-----BEGIN PRIVATE KEY-----
MIICdwIBADANBgkqhkiG9w0...
-----END PRIVATE KEY-----

And for verification use key_public.pem:

-----BEGIN CERTIFICATE-----
MIIBxDCCAW6gAwIBAgIQxUSX...
-----END CERTIFICATE-----

Converting .pfx certificates to pem

If you have .pfx certificates you can convert them to .pem using openssl:

openssl pkcs12 -in c:\certs\yourcert.pfx -out c:\certs\cag.pem

Then you could use the result as is for the purpose of signing. For the purpose of validation open the resulting .pem with a text editor and copy from -----BEGIN CERTIFICATE----- to -----END CERTIFICATE----- (including) to a new text file and save it as .pem.

Examples

how to sign a root node (coming soon)

how to add a prefix for the signature

Use the prefix option when calling computeSignature to add a prefix to the signature.

var SignedXml = require("xml-crypto").SignedXml,
  fs = require("fs");

var xml = "<library>" + "<book>" + "<name>Harry Potter</name>" + "</book>" + "</library>";

var sig = new SignedXml({ privateKey: fs.readFileSync("client.pem") });
sig.addReference({
  xpath: "//*[local-name(.)='book']",
  digestAlgorithm: "http://www.w3.org/2000/09/xmldsig#sha1",
  transforms: ["http://www.w3.org/2001/10/xml-exc-c14n#"],
});
sig.canonicalizationAlgorithm = "http://www.w3.org/2001/10/xml-exc-c14n#";
sig.signatureAlgorithm = "http://www.w3.org/2000/09/xmldsig#rsa-sha1";
sig.computeSignature(xml, {
  prefix: "ds",
});

how to specify the location of the signature

Use the location option when calling computeSignature to move the signature around. Set action to one of the following:

  • append(default) - append to the end of the xml document
  • prepend - prepend to the xml document
  • before - prepend to a specific node (use the referenceNode property)
  • after - append to specific node (use the referenceNode property)
var SignedXml = require("xml-crypto").SignedXml,
  fs = require("fs");

var xml = "<library>" + "<book>" + "<name>Harry Potter</name>" + "</book>" + "</library>";

var sig = new SignedXml({ privateKey: fs.readFileSync("client.pem") });
sig.addReference({
  xpath: "//*[local-name(.)='book']",
  digestAlgorithm: "http://www.w3.org/2000/09/xmldsig#sha1",
  transforms: ["http://www.w3.org/2001/10/xml-exc-c14n#"],
});
sig.canonicalizationAlgorithm = "http://www.w3.org/2001/10/xml-exc-c14n#";
sig.signatureAlgorithm = "http://www.w3.org/2000/09/xmldsig#rsa-sha1";
sig.computeSignature(xml, {
  location: { reference: "//*[local-name(.)='book']", action: "after" }, //This will place the signature after the book element
});

more examples (coming soon)

Development

The testing framework we use is Mocha with Chai as the assertion framework.

To run tests use:

npm test

More information

Visit my blog or my twitter

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License

This project is licensed under the MIT License. See the LICENSE file for more info.

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