A Transform without an Algorithm attribute is skipped instead of rejected #622

Problem

loadReference skips a Transform element that has no Algorithm attribute instead of rejecting it (src/signed-xml.ts#L767-L773):

const transformAttr = utils.findAttr(transform, "Algorithm");

if (transformAttr) {
  transforms.push(transformAttr.value);
}

The XMLDSig schema makes the attribute required (§4.4.3.4):

<complexType name="TransformType" mixed="true">
  ...
  <attribute name="Algorithm" type="anyURI" use="required"/>
</complexType>

A DigestMethod without Algorithm already throws could not find Algorithm attribute in node .... A Transform without it names a step that cannot be identified, and verifying as though the element were absent digests something the signer never described.

On 6.3.2, a signature whose SignedInfo contains <Transforms><Transform/><Transform Algorithm="...enveloped-signature"/><Transform Algorithm="...xml-exc-c14n#"/></Transforms>, signed over that SignedInfo, passes checkSignature(). The loaded reference lists only the two transforms that have an Algorithm.

The element is inside SignedInfo, so only the signer can produce it. This is a conformance problem, not a bypass.

Proposal

Throw, as for DigestMethod. That rejects documents that verify today, so it belongs in 7.0.