feat: add computeSignatureAsync() and checkSignatureAsync(), fix asynchronous verification through a callback, and deprecate the callback form

[cjbarth]

Refs #546, #571, #595.

In 6.x the only way to sign with an asynchronous signature algorithm is the callback form of computeSignature(). #571 removes that form in 7.0, in the same release that adds its replacement. Deprecating it here, which is all this PR first did, told those callers to stop and gave them nowhere to go before the major.

So this PR now does three things:

1. Ships the replacement in 6.x: computeSignatureAsync() and checkSignatureAsync().
2. Fixes checkSignature(xml, callback), which has rejected every signature from an asynchronous algorithm since 6.1.0.
3. Deprecates the callback form, naming the replacement.

1. Promise entry points

• computeSignatureAsync(xml, options?) resolves with the SignedXml instance. It waits for the algorithm's getSignatureAsync(), and calls getSignature() on an algorithm that has none.
• checkSignatureAsync(xml) resolves true for a valid signature, resolves false where checkSignature() returns false, and rejects where it throws. It waits for verifySignatureAsync(), and calls verifySignature() on an algorithm that has none.
• SignatureAlgorithm gains optional getSignatureAsync() and verifySignatureAsync().

The names and signatures are #571's, so code moved to them on 6.x runs unchanged on 7.0.

Two properties worth a reviewer's eye:

• checkSignatureAsync() publishes signed references only once the algorithm has answered true, and treats any other answer as an invalid signature.
• computeSignatureAsync() rejects an algorithm that returns no signature. That is what an algorithm written for the callback form does when it is called without a callback, and without the check the document would be written with <SignatureValue>undefined</SignatureValue>.

Left for 7.0, as in #571:

• getHashAsync() and validateElementAgainstReferencesAsync().
• Making getSignature() and verifySignature() optional. In 6.x the type still requires them, and the README says an algorithm that only works asynchronously can have them throw.
• The error that names the Async entry point when a synchronous one meets such an algorithm.

2. checkSignature(xml, callback) with an asynchronous algorithm

#489, released in 6.1.0, replaced the call that passed the callback to verifySignature() with a synchronous one, so that the result could decide whether to publish signedReferences. An algorithm that answers only through its callback has had every valid signature rejected since. Nothing in #489 discusses it, and no test used the callback form.

With the released packages and this branch, checking a valid signature:

verifySignature() of the algorithm	6.0.1	6.1.0 to 6.3.3	this PR
answers through the callback	callback(null, true)	invalid signature error, or the algorithm's own throw	callback(null, true)
returns its verdict and ignores the callback	callback never called	callback(null, true)	callback(null, true)
built-in	callback(null, true)	callback(null, true)	callback(null, true)

Given a callback, checkSignature() passes one to the algorithm again, and also takes a boolean the algorithm returns, so both kinds of algorithm work. Only the first answer counts. signedReferences is published when the algorithm answers true and not before, so #489's defence stands.

An invalid signature still reaches the callback as the invalid signature error, as it has since 6.1.0. 6.0.1 called back with (null, false).

One thing moves for the built-in algorithms: an error thrown inside verifySignature(), such as a key it cannot read, reaches the callback, as it did in 6.0.1. 6.1.0 to 6.3.3 threw it from checkSignature().

3. Deprecation

• The callback form of computeSignature() and checkSignature(). The three overloads carry @deprecated, and passing a callback prints the DeprecationWarning XML_CRYPTO_CALLBACK_FORM, once per process. Both name computeSignatureAsync() and checkSignatureAsync().
• The callback overloads of SignatureAlgorithm.getSignature() and verifySignature(), tagged @deprecated in favor of getSignatureAsync() and verifySignatureAsync().
• createOptionalCallbackFunction and ErrorFirstCallback. The package root exports a deprecated copy of each, so the library's own use of them does not warn. createOptionalCallbackFunction() prints XML_CRYPTO_CREATE_OPTIONAL_CALLBACK_FUNCTION when called.

README

• "Asynchronous signing" is now "Asynchronous signing and verification" and shows the promise API, signing and verifying.
• A new subsection, "Moving from the callback form", gives the migration as before-and-after code, for a built-in algorithm and for an asynchronous one.
• The API list has the two new methods, and "Deprecated ahead of 7.0" points to the migration.

Tests

• New API: 13 tests in test/async-entry-points.spec.ts. I broke the new code eight ways (a truthy verdict accepted, references published before the algorithm answers, the Async method ignored, and so on) and each change fails at least one of them.
• Callback form: no test goes through it, because it is deprecated. The fix in section 2 was checked by script against the released 6.0.1 and 6.3.3 and against this branch, with seven kinds of algorithm against a valid signature and three against an invalid one; the table above is from those runs. That leaves the callback path of checkSignature() uncovered in CI.
• Warnings: none tested, like the other deprecations. The places where existing tests use a deprecated form carry eslint-disable-next-line deprecation/deprecation.

Checks

• npm run build && npm test && npm run lint pass (570 tests).
• The two new README samples were extracted and run against the build: the document signs through computeSignatureAsync(), verifies through checkSignatureAsync(), and is refused once altered. Neither prints a warning.

🤖 Generated with Claude Code

Summary by CodeRabbit

• New Features
  • Added promise-based signing and verification APIs, with support for asynchronous algorithms and fallback to synchronous algorithms.
  • Verification now reports invalid references as false in both synchronous and asynchronous flows.
• Documentation
  • Added guidance for asynchronous signing and verification, including migration from callback-based algorithms.
  • Marked callback-based signing and verification APIs as deprecated in favor of promise-based alternatives.
• Bug Fixes
  • Signed references are only made available after signature verification succeeds.

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📝 Walkthrough

The change adds promise-based signing and verification entry points, with asynchronous algorithm methods and synchronous fallback. Reference state is published after successful signature verification. Callback APIs and related declarations are marked deprecated, with migration guidance and tests for the new entry points.

Changes: Asynchronous signing and verification

Layer / File(s)	Summary
Async signing contract and flow src/types.ts, src/signed-xml.ts, test/async-entry-points.spec.ts, README.md	Signature algorithms can implement getSignatureAsync(). computeSignatureAsync() uses it when available and otherwise uses synchronous signing. Tests cover successful signing, fallback, options, and failures.
Async verification and reference acceptance src/types.ts, src/signed-xml.ts, test/async-entry-points.spec.ts, README.md	Signature algorithms can implement verifySignatureAsync(). checkSignatureAsync() validates references before verification and publishes them only after successful verification. Tests cover fallback, digest mismatch, and verification failures.
Callback deprecation and migration README.md, src/index.ts, src/signed-xml.ts, test/signature-integration-tests.spec.ts, test/signature-unit-tests.spec.ts, test/types-tests.spec.ts	Callback forms of computeSignature() and checkSignature() emit deprecation warnings and are marked for removal in version 7.0. The package adds deprecated exports for createOptionalCallbackFunction and ErrorFirstCallback<T>. Documentation describes migration; existing callback tests suppress deprecation lint warnings.

Priority: ➖ Normal

Estimated code review effort: 3 (Moderate) | ~25 minutes

Change: Feature

Sequence Diagram(s)

sequenceDiagram
  participant Caller
  participant SignedXml
  participant SignatureAlgorithm
  Caller->>SignedXml: call computeSignatureAsync or checkSignatureAsync
  SignedXml->>SignatureAlgorithm: request async operation or use synchronous method
  SignatureAlgorithm-->>SignedXml: return signature or verification verdict
  SignedXml-->>Caller: resolve with result

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Suggested reviewers: shunkica

Merge Risk: ⚪ Minimal · up to fd774

The change adds promise-based signing and verification entry points and deprecates the callback forms. No merge-blocking issue was found in the supplied evidence.

Pre-merge checks: 4 · 1

Failed checks: 1 warning

Check name	Status	Explanation	Resolution
Docstring Coverage		Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 4 functions across 7 files. (1 skipped: 1 …	Write docstrings for the functions missing them to satisfy the coverage threshold.

Passed checks: 4 passed

Check name	Status	Explanation
Description Check		Check skipped - CodeRabbit’s high-level summary is enabled.
Title check		The title accurately summarizes the main changes: it adds the asynchronous entry points, fixes callback-based asynchronous verification, and deprecates callback APIs. It is specific and related to the…
Linked Issues check		Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check		Check skipped because no linked issues were found for this pull request.

Full details: Docstring Coverage

Explanation

Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 4 functions across 7 files. (1 skipped: 1 unsupported.)

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Actionable comments posted: 1

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Inline comments:
Review comments at @src/types.ts:
- Around line 218-219: Update the deprecation text on the affected declaration
in `src/types.ts` to retain the 7.0 removal notice without referencing
`checkSignature()` or its callback form.

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📒 Files selected for processing (7)

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[codecov]

Codecov Report

❌ Patch coverage is 72.22222% with 20 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.76%. Comparing base (8a7e264) to head (fd774b0).

Files with missing lines	Patch %	Lines
src/signed-xml.ts	71.42%	16 Missing and 4 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #657      +/-   ##
==========================================
- Coverage   86.05%   85.76%   -0.29%     
==========================================
  Files           9        9              
  Lines        1341     1391      +50     
  Branches      346      359      +13     
==========================================
+ Hits         1154     1193      +39     
- Misses        106      116      +10     
- Partials       81       82       +1     

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