fix: omit <Transforms> element when no transforms are specified

src/signed-xml.ts

@@ -427,12 +427,16 @@ export class SignedXml {
     return this.getCanonXml([this.canonicalizationAlgorithm], signedInfo[0], c14nOptions);
   }
 
-  private getCanonReferenceXml(doc: Document, ref: Reference, node: Node) {
+  private getCanonReferenceXml(_doc: Document, ref: Reference, node: Node) {
     /**
-     * Search for ancestor namespaces before canonicalization.
+     * Derive ancestor namespaces from the specific node being digested, not by
+     * re-running ref.xpath. findAncestorNs uses only the first XPath match, so
+     * when multiple references are created for the same xpath pattern (e.g. via
+     * addAllReferences), references beyond the first are digested with the wrong
+     * ancestor namespace scope.
      */
-    if (Array.isArray(ref.transforms)) {
-      ref.ancestorNamespaces = utils.findAncestorNs(doc, ref.xpath, this.namespaceResolver);
+    if (isDomNode.isElementNode(node)) {
+      ref.ancestorNamespaces = utils.findAncestorNsForNode(node);
     }
 
     const c14nOptions = {
@@ -819,10 +823,6 @@ export class SignedXml {
       throw new Error("digestAlgorithm is required");
     }
 
-    if (!utils.isArrayHasLength(transforms)) {
-      throw new Error("transforms must contain at least one transform algorithm");
-    }
-
     this.references.push({
       xpath,
       transforms,
@@ -1155,32 +1155,36 @@ export class SignedXml {
           referenceElem.setAttribute("Type", ref.type);
         }
 
-        const transformsElem = signatureDoc.createElementNS(
-          signatureNamespace,
-          `${currentPrefix}Transforms`,
-        );
-
-        for (const trans of ref.transforms || []) {
-          const transform = this.findCanonicalizationAlgorithm(trans);
-          const transformElem = signatureDoc.createElementNS(
+        if (utils.isArrayHasLength(ref.transforms)) {
+          const transformsElem = signatureDoc.createElementNS(
             signatureNamespace,
-            `${currentPrefix}Transform`,
+            `${currentPrefix}Transforms`,
           );
-          transformElem.setAttribute("Algorithm", transform.getAlgorithmName());
 
-          if (utils.isArrayHasLength(ref.inclusiveNamespacesPrefixList)) {
-            const inclusiveNamespacesElem = signatureDoc.createElementNS(
-              transform.getAlgorithmName(),
-              "InclusiveNamespaces",
+          for (const trans of ref.transforms) {
+            const transform = this.findCanonicalizationAlgorithm(trans);
+            const transformElem = signatureDoc.createElementNS(
+              signatureNamespace,
+              `${currentPrefix}Transform`,
             );
-            inclusiveNamespacesElem.setAttribute(
-              "PrefixList",
-              ref.inclusiveNamespacesPrefixList.join(" "),
-            );
-            transformElem.appendChild(inclusiveNamespacesElem);
+            transformElem.setAttribute("Algorithm", transform.getAlgorithmName());
+
+            if (utils.isArrayHasLength(ref.inclusiveNamespacesPrefixList)) {
+              const inclusiveNamespacesElem = signatureDoc.createElementNS(
+                transform.getAlgorithmName(),
+                "InclusiveNamespaces",
+              );
+              inclusiveNamespacesElem.setAttribute(
+                "PrefixList",
+                ref.inclusiveNamespacesPrefixList.join(" "),
+              );
+              transformElem.appendChild(inclusiveNamespacesElem);
+            }
+
+            transformsElem.appendChild(transformElem);
           }
 
-          transformsElem.appendChild(transformElem);
+          referenceElem.appendChild(transformsElem);
         }
 
         // Get the canonicalized XML
@@ -1201,7 +1205,6 @@ export class SignedXml {
         );
         digestValueElem.textContent = digestAlgorithm.getHash(canonXml);
 
-        referenceElem.appendChild(transformsElem);
         referenceElem.appendChild(digestMethodElem);
         referenceElem.appendChild(digestValueElem);
 
@@ -1272,7 +1275,7 @@ export class SignedXml {
     const canonXml = node.cloneNode(true); // Deep clone
     let transformedXml: Node | string = canonXml;
 
-    transforms.forEach((transformName) => {
+    (transforms ?? []).forEach((transformName) => {
       if (isDomNode.isNodeLike(transformedXml)) {
         // If, after processing, `transformedNode` is a string, we can't do anymore transforms on it
         const transform = this.findCanonicalizationAlgorithm(transformName);
@@ -1287,6 +1290,27 @@ export class SignedXml {
       //if only y is the node to sign then a string would be <p:y/> without the definition of the p namespace. probably xmldom toString() should have added it.
     });
 
+    // When the transform chain produces a DOM node (including the no-transform
+    // case), apply C14N so that the digest is computed over a canonical byte
+    // sequence. This mirrors what loadReference does on the verification side:
+    // it appends C14N when the transform list is empty or ends with
+    // enveloped-signature, ensuring signing and verification agree on the bytes.
+    if (typeof transformedXml === "string") {
+      return transformedXml;
+    }
+
+    // When there are no transforms, the XMLDSig processing model requires the
+    // node-set to be serialized via C14N before digesting. This mirrors what
+    // loadReference already does on the verification side (it appends C14N when
+    // the transform list is empty), so that signing and verification compute the
+    // same digest.
+    if (!utils.isArrayHasLength(transforms)) {
+      const c14n = this.findCanonicalizationAlgorithm(
+        "http://www.w3.org/TR/2001/REC-xml-c14n-20010315",
+      );
+      return String(c14n.process(transformedXml, options));
+    }
+
     return transformedXml.toString();
   }
 

src/types.ts

@@ -127,7 +127,9 @@ export interface Reference {
   xpath?: string;
 
   // An array of transforms to be applied to the data before signing.
-  transforms: ReadonlyArray<CanonicalizationOrTransformAlgorithmType>;
+  // When absent or empty, no Transforms element is emitted and the referenced
+  // node is digested directly (after C14N, per the XMLDSig processing model).
+  transforms?: ReadonlyArray<CanonicalizationOrTransformAlgorithmType>;
 
   // The algorithm used to calculate the digest value of the data.
   digestAlgorithm: HashAlgorithmType;

src/utils.ts

@@ -236,6 +236,49 @@ function isElementSubset(docSubset: Node[]): docSubset is Element[] {
   return docSubset.every((node) => isDomNode.isElementNode(node));
 }
 
+/** Deduplicate and filter ancestor namespaces for a given subset element. */
+function buildAncestorNsForElement(element: Element): NamespacePrefix[] {
+  const ancestorNs = collectAncestorNamespaces(element);
+  const ancestorNsWithoutDuplicate: NamespacePrefix[] = [];
+  for (let i = 0; i < ancestorNs.length; i++) {
+    let notOnTheList = true;
+    for (const v in ancestorNsWithoutDuplicate) {
+      if (ancestorNsWithoutDuplicate[v].prefix === ancestorNs[i].prefix) {
+        notOnTheList = false;
+        break;
+      }
+    }
+
+    if (notOnTheList) {
+      ancestorNsWithoutDuplicate.push(ancestorNs[i]);
+    }
+  }
+
+  // Remove namespaces which are already declared in the subset with the same prefix
+  const returningNs: NamespacePrefix[] = [];
+  const subsetNsPrefix = findNSPrefix(element);
+  for (const ns of ancestorNsWithoutDuplicate) {
+    if (ns.prefix !== subsetNsPrefix) {
+      returningNs.push(ns);
+    }
+  }
+
+  return returningNs;
+}
+
+/**
+ * Extract ancestor namespaces for a specific element node.
+ * Prefer this over `findAncestorNs` when the target element is already known,
+ * since `findAncestorNs` re-executes an XPath and uses the first match —
+ * which is incorrect when multiple nodes are referenced individually.
+ *
+ * @param element - The element whose ancestor namespace declarations to collect
+ * @returns i.e. [{prefix: "saml", namespaceURI: "urn:oasis:names:tc:SAML:2.0:assertion"}]
+ */
+export function findAncestorNsForNode(element: Element): NamespacePrefix[] {
+  return buildAncestorNsForElement(element);
+}
+
 /**
  * Extract ancestor namespaces in order to import it to root of document subset
  * which is being canonicalized for non-exclusive c14n.
@@ -264,33 +307,7 @@ export function findAncestorNs(
     throw new Error("Document subset must be list of elements");
   }
 
-  // Remove duplicate on ancestor namespace
-  const ancestorNs = collectAncestorNamespaces(docSubset[0]);
-  const ancestorNsWithoutDuplicate: NamespacePrefix[] = [];
-  for (let i = 0; i < ancestorNs.length; i++) {
-    let notOnTheList = true;
-    for (const v in ancestorNsWithoutDuplicate) {
-      if (ancestorNsWithoutDuplicate[v].prefix === ancestorNs[i].prefix) {
-        notOnTheList = false;
-        break;
-      }
-    }
-
-    if (notOnTheList) {
-      ancestorNsWithoutDuplicate.push(ancestorNs[i]);
-    }
-  }
-
-  // Remove namespaces which are already declared in the subset with the same prefix
-  const returningNs: NamespacePrefix[] = [];
-  const subsetNsPrefix = findNSPrefix(docSubset[0]);
-  for (const ancestorNs of ancestorNsWithoutDuplicate) {
-    if (ancestorNs.prefix !== subsetNsPrefix) {
-      returningNs.push(ancestorNs);
-    }
-  }
-
-  return returningNs;
+  return buildAncestorNsForElement(docSubset[0]);
 }
 
 export function validateDigestValue(digest, expectedDigest) {

test/signature-unit-tests.spec.ts

@@ -908,8 +908,7 @@ describe("Signature unit tests", function () {
             ref.uri,
             `wrong uri for index ${i}. expected: ${expectedUri} actual: ${ref.uri}`,
           ).to.equal(expectedUri);
-          expect(ref.transforms.length).to.equal(1);
-          expect(ref.transforms[0]).to.equal("http://www.w3.org/2001/10/xml-exc-c14n#");
+          expect(ref.transforms).to.deep.equal(["http://www.w3.org/2001/10/xml-exc-c14n#"]);
           expect(ref.digestValue).to.equal(digests[i]);
           expect(ref.digestAlgorithm).to.equal("http://www.w3.org/2000/09/xmldsig#sha1");
         }
@@ -1074,6 +1073,108 @@ describe("Signature unit tests", function () {
     expect(URI.value, `uri should be empty but instead was ${URI.value}`).to.equal("");
   });
 
+  for (const { label, transforms } of [
+    { label: "omitted transforms property", transforms: undefined },
+    { label: "empty transforms array", transforms: [] as string[] },
+  ]) {
+    it(`omits Transforms element when no transforms are specified (${label})`, function () {
+      const xml = "<root><x Id='ref1'/></root>";
+      const sig = new SignedXml();
+      sig.privateKey = fs.readFileSync("./test/static/client.pem");
+      sig.addReference({
+        xpath: "//*[local-name(.)='x']",
+        digestAlgorithm: "http://www.w3.org/2001/04/xmlenc#sha256",
+        uri: "#ref1",
+        digestValue: "",
+        inclusiveNamespacesPrefixList: [],
+        isEmptyUri: false,
+        ...(transforms !== undefined ? { transforms } : {}),
+      });
+      sig.canonicalizationAlgorithm = "http://www.w3.org/TR/2001/REC-xml-c14n-20010315";
+      sig.signatureAlgorithm = "http://www.w3.org/2001/04/xmldsig-more#rsa-sha256";
+      sig.computeSignature(xml);
+      const signedXml = sig.getSignedXml();
+      const doc = new xmldom.DOMParser().parseFromString(signedXml);
+      const transformNodes = xpath.select("//*[local-name(.)='Transforms']", doc);
+      expect(transformNodes, "Transforms element should be absent when no transforms specified").to
+        .have.length(0);
+    });
+  }
+
+  it("signs and verifies correctly with no transforms (round-trip)", function () {
+    const xml = "<root><x Id='ref1'/></root>";
+    const sig = new SignedXml();
+    sig.privateKey = fs.readFileSync("./test/static/client.pem");
+    sig.publicCert = fs.readFileSync("./test/static/client_public.pem");
+    sig.addReference({
+      xpath: "//*[local-name(.)='x']",
+      digestAlgorithm: "http://www.w3.org/2001/04/xmlenc#sha256",
+      uri: "#ref1",
+      digestValue: "",
+      inclusiveNamespacesPrefixList: [],
+      isEmptyUri: false,
+    });
+    sig.canonicalizationAlgorithm = "http://www.w3.org/TR/2001/REC-xml-c14n-20010315";
+    sig.signatureAlgorithm = "http://www.w3.org/2001/04/xmldsig-more#rsa-sha256";
+    sig.computeSignature(xml);
+    const signedXml = sig.getSignedXml();
+
+    const doc = new xmldom.DOMParser().parseFromString(signedXml);
+    const sigNode = xpath.select1(
+      "//*[local-name(.)='Signature' and namespace-uri(.)='http://www.w3.org/2000/09/xmldsig#']",
+      doc,
+    );
+    isDomNode.assertIsNodeLike(sigNode);
+
+    const verifySig = new SignedXml();
+    verifySig.publicCert = fs.readFileSync("./test/static/client_public.pem");
+    verifySig.loadSignature(sigNode);
+    const result = verifySig.checkSignature(signedXml);
+    expect(result, "expected signature to verify successfully").to.be.true;
+  });
+
+  it("correctly canonicalizes no-transform references under different ancestor namespace scopes", function () {
+    // Two <item> elements live under different namespace scopes. Without the
+    // fix, findAncestorNs(doc, ref.xpath) always uses the first XPath match
+    // (item1's scope), so item2 is digested with the wrong ancestor namespaces
+    // and verification fails.
+    const xml =
+      "<root>" +
+      "<section xmlns:ns1='http://ns1.example.com'><item Id='item1'>one</item></section>" +
+      "<section xmlns:ns2='http://ns2.example.com'><item Id='item2'>two</item></section>" +
+      "</root>";
+    const sig = new SignedXml();
+    sig.privateKey = fs.readFileSync("./test/static/client.pem");
+    sig.publicCert = fs.readFileSync("./test/static/client_public.pem");
+    for (const id of ["item1", "item2"]) {
+      sig.addReference({
+        xpath: `//*[@Id='${id}']`,
+        digestAlgorithm: "http://www.w3.org/2001/04/xmlenc#sha256",
+        uri: `#${id}`,
+        digestValue: "",
+        inclusiveNamespacesPrefixList: [],
+        isEmptyUri: false,
+      });
+    }
+    sig.canonicalizationAlgorithm = "http://www.w3.org/TR/2001/REC-xml-c14n-20010315";
+    sig.signatureAlgorithm = "http://www.w3.org/2001/04/xmldsig-more#rsa-sha256";
+    sig.computeSignature(xml);
+    const signedXml = sig.getSignedXml();
+
+    const doc = new xmldom.DOMParser().parseFromString(signedXml);
+    const sigNode = xpath.select1(
+      "//*[local-name(.)='Signature' and namespace-uri(.)='http://www.w3.org/2000/09/xmldsig#']",
+      doc,
+    );
+    isDomNode.assertIsNodeLike(sigNode);
+
+    const verifySig = new SignedXml();
+    verifySig.publicCert = fs.readFileSync("./test/static/client_public.pem");
+    verifySig.loadSignature(sigNode);
+    const result = verifySig.checkSignature(signedXml);
+    expect(result, "expected signature to verify successfully").to.be.true;
+  });
+
   it("signer appends signature to a non-existing reference node", function () {
     const xml = "<root><name>xml-crypto</name><repository>github</repository></root>";
     const sig = new SignedXml();