Canonical XML 1.0 without comments, the form XMLDSig digests.

OTP ships no xmerl_c14n, so this is written here. Output is compared byte for byte against xmllint --c14n in the test suite: a canonicalisation that disagrees with the reference by one byte produces a valid-looking signature that SEFAZ rejects, and the failure gives no hint why.

What the specification asks for

Empty elements become a start and end tag pair, attribute values and text are escaped to a fixed set, namespace declarations come before attributes, and both are sorted — namespaces by prefix, attributes by namespace URI then local name.

The apex rule

A subtree being signed is canonicalised out of its document, so the top element must render every namespace it merely inherited. infNFe carries no xmlns of its own — the declaration sits on NFe — and omitting it there is the classic reason an NF-e signature verifies locally and is refused by SEFAZ. Descendants render only what differs from what an ancestor already rendered.

This is Canonical XML 1.0, not the exclusive variant: every in-scope namespace is rendered, whether or not the subtree uses it.