xml_crypto - Dart API docs

xml-crypto

Port of awesome Node.js library - xml-crypto - in Dart.

An xml digital signature library for Dart.

Install

Install:

dart pub add xml_crypto

Supported Algorithms

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

HMAC-SHA1 http://www.w3.org/2000/09/xmldsig#hmac-sha1

to enable HMAC-SHA1, do:

import 'package:xml_crypto/xml_crypto.dart';

SignedXml.enableHMAC();

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.

by default the following algorithms are used:

Canonicalization/Transformation Algorithm: Exclusive Canonicalization http://www.w3.org/2001/10/xml-exc-c14n#

Hashing Algorithm: SHA1 digest http://www.w3.org/2000/09/xmldsig#sha1

Signature Algorithm: RSA-SHA1 http://www.w3.org/2000/09/xmldsig#rsa-sha1

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

Signing Xml documents

When signing a xml document you can specify the following properties on a SignedXml instance to customize the signature process:

sign.signingKey - required a Uint8List containing your private key
sign.keyInfoProvider - optional a key info provider instance, see customizing algorithms for an implementation example
sign.signatureAlgorithm - optional one of the supported signature algorithms. Ex: sign.signatureAlgorithm = "http://www.w3.org/2001/04/xmldsig-more#rsa-sha256";
sign.canonicalizationAlgorithm - optional one of the supported canonicalization algorithms. Ex: sign.canonicalizationAlgorithm = "http://www.w3.org/2001/10/xml-exc-c14n#WithComments";

Use this code:

	import 'dart:io';
	import 'package:xml_crypto/xml_crypto.dart';

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

	final sig = SignedXml()
		..addReference("//*[local-name()='book']")
		..signingKey = File("client.pem").readAsBytesSync()
		..computeSignature(xml);
	File("signed.xml").writeAsStringSync(sig.signedXml);

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:

To generate a <X509Data></X509Data> element in the signature you must provide a key info implementation, see customizing algorithms for an example.

Verifying Xml documents

When verifying a xml document you must specify the following properties on a SignedXml instance:

sign.keyInfoProvider - required a key info provider instance containing your certificate, see customizing algorithms for an implementation example

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

dart pub add xml

Example:

	import 'dart:io';
	import 'package:xml_crypto/xml_crypto.dart';

	final xml = File("signed.xml").readAsStringSync()
	var doc = new dom().parseFromString(xml)    

	var signature = select(doc, "//*[local-name(.)='Signature' and namespace-uri(.)='http://www.w3.org/2000/09/xmldsig#']")[0]
	final sig = SignedXml()
		..keyInfoProvider = FileKeyInfo("client_public.pem")
		..loadSignature(signature);
	final res = sig.checkSignature(xml);
	if (!res) print(sig.validationErrors);

if the verification process fails sig.validationErrors will have the errors.

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

	final elem = select(doc, "/xpath_to_interesting_element");
	final uri = sig.references[0].uri; // might not be 0 - depending on the document you verify
	final id = (uri[0] == '#') ? uri.substring(1) : uri;
	if (elem.getAttribute('ID') != id && elem.getAttribute('Id') != id && elem.getAttribute('id') != id)
		throw 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:

final option = {
	'implicitTransforms': ['http://www.w3.org/TR/2001/REC-xml-c14n-20010315']};
final sig = SignedXml('', option)
	..keyInfoProvider = FileKeyInfo("client_public.pem")
	..loadSignature(signature);
final res = sig.checkSignature(xml);

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

API

SignedXml

The SignedXml constructor provides an abstraction for sign and verify xml documents. The object is constructed using new SignedXml([String idMode]) where:

idMode - if the value of "wssecurity" is passed it will create/validate id's with the ws-security namespace.

API

A SignedXml object provides the following methods:

To sign xml documents:

void addReference(String? xpath, [List<String>? transforms, String? 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
void computeSignature(String xml, {Map<String, dynamic>? options}) - compute the signature of the given xml where:
- xml - a string containing a xml document
- options - a map 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
String get signedXml - returns the original xml document with the signature in it, must be called only after computeSignature
String get signatureXml - returns just the signature part, must be called only after computeSignature
String get originalXmlWithIds - returns the original xml with Id attributes added on relevant elements (required for validation), must be called only after computeSignature

To verify xml documents:

void loadSignature(dynamic signatureXml) - loads the signature where:
- signatureXml - a string or XmlNode object (xml) containing the xml representation of the signature
bool checkSignature(String xml) - validates the given xml document and returns true if the validation was successful, sig.validationErrors will have the validation errors if any, where:
- xml - a string containing a xml document

FileKeyInfo

A basic key info provider implementation using File(file).readAsBytesSync(), is constructed using new FileKeyInfo([file]) where:

file - a path to a pem encoded certificate

See verifying xml documents for an example usage

Customizing Algorithms

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

First import some modules:

import 'package:xml_crypto/xml_crypto.dart';

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

A key info provider is used to extract and construct the key and the KeyInfo xml section. Implement it if you want to create a signature with a KeyInfo section, or you want to read your key in a different way then the default file read option.

	class MyKeyInfo implements KeyInfoProvider {
		@override
		String getKeyInfo(Uint8List? signingKey, String? prefix) {
			prefix = prefix ?? '';
			prefix = prefix.isNotEmpty ? prefix + ':' : prefix;
			return '<' + prefix + "X509Data></" + prefix + 'X509Data>';
		}

		@override
		//you can use the keyInfo parameter to extract the key in any way you want
		Uint8List? getKey(String? keyInfo) => File("key.pem").readAsBytesSync();
	}

A custom hash algorithm is used to calculate digests. Implement it if you want a hash other than the default SHA1.

	class MyDigest implements HashAlgorithm {
		@override
		String getHash(String xml) => 'the base64 hash representation of the given xml string'

		@override
		String get algorithmName => 'http://myDigestAlgorithm';
	}

A custom signing algorithm. The default is RSA-SHA1

	class MySignatureAlgorithm implements SignatureAlgorithm {
		/*sign the given SignedInfo using the key. return base64 signature value*/
		@override
		String getSignature(String xml, Uint8List signingKey, [CalculateSignatureCallback? callback])
		=> 'signature of signedInfo as base64...';

		@override
		bool verifySignature(String xml, Uint8List key, String signatureValue, [ValidateSignatureCallback? callback]) 
		=> true;

		@override
		String get algorithmName => 'http://mySigningAlgorithm';
	}

Custom transformation algorithm. The default is exclusive canonicalization.

	class MyTransformation implements CanonicalizationAlgorithm<String> {
		/*given a node (from the xml module) return its canonical representation (as string)*/
		@override
		String process(XmlNode node, [Map<String, dynamic> options = const {}]) {
			//you should apply your transformation before returning
			return node.toString();
		}

		@override
		String get algorithmName => 'http://myTransformation';
	}

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

	class MyCanonicalization implements CanonicalizationAlgorithm<String> {
		/*given a node (from the xml module) return its canonical representation (as string)*/
		@override
		String process(XmlNode node, [Map<String, dynamic> options = const {}]) {
			//you should apply your transformation before returning
			return '< x/>';
		}

		@override
		String get algorithmName => '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.cashAlgorithms["http://myDigestAlgorithm"] = MyDigest();
	SignedXml.signatureAlgorithms["http://mySigningAlgorithm"] = MySignatureAlgorithm();

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

	void signXml(String xml, String xpath, String key, String dest) {
		final sig = SignedXml();

		/*configure the signature object to use the custom algorithms*/
		sig.signatureAlgorithm = "http://mySignatureAlgorithm";
		sig.keyInfoProvider = MyKeyInfo();
		sig.canonicalizationAlgorithm = "http://MyCanonicalization";
		sig.addReference("//*[local-name()='x']", ["http://MyTransformation"], "http://myDigestAlgorithm");

		sig.signingKey = File(key).readAsBytesSync();
		sig.addReference(xpath);
		sig.computeSignature(xml);
		File(dest).writeAsStringSync(sig.signedXml);
	}

	void main() {
		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 (or drop me a mail).

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.

	class AsyncSignatureAlgorithm implements SignatureAlgorithm {
		@override
		String getSignature(String xml, Uint8List signingKey, [CalculateSignatureCallback? callback]) {
			final rsa = RSAPrivateKey.fromPEM(utf8.decode(signingKey));
			final res = rsa.signSsaPkcs1v15ToBase64(utf8.encode(xml), hasher: EmsaHasher.sha1);
			//Do some asynchronous things here
			callback?.call(null, res);
			return '';
		}

		@override
		bool verifySignature(String xml, Uint8List key, String signatureValue, [ValidateSignatureCallback? callback]) => true;

		@override
		String get algorithmName => 'http://www.w3.org/2000/09/xmldsig#rsa-sha1';
	}

	SignedXml.signatureAlgorithms["http://asyncSignatureAlgorithm"] = AsyncSignatureAlgorithm();
	final sig = SignedXml();
	sig.signatureAlgorithm = "http://asyncSignatureAlgorithm";
	sig.computeSignature(xml, opts: opts, callback: (err, _) {
		final signedResponse = sig.signedXml;
	});

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

X.509 / Key formats

PEM encoded certificates are supported. So to sign an xml use key.pem that looks like this (only the begining 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 yourcert.pfx -out 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.

import 'dart:io';

import 'package:xml_crypto/xml_crypto.dart';

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

final sig = SignedXml();
sig.addReference("//*[local-name()='book']");
sig.signingKey = File("client.pem").readAsBytesSync();
sig.computeSignature(xml, opts: {
  '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)

import 'dart:io';

import 'package:xml_crypto/xml_crypto.dart';

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

final sig = SignedXml();
sig.addReference("//*[local-name()='book']");
sig.signingKey = File("client.pem").readAsBytesSync();
sig.computeSignature(xml, opts: {
	"location": {
		"reference": "//*[local-name()='book']",
		"action": "after" //This will place the signature after the book element
	}
});

more examples coming soon

Development

The test framework is test. To run tests use:

$> dart test

More information

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License

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

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