- services/signing-service/pkg/httpapi exposes NewTestHandler so sibling
modules can build the real signing-service in-process (same pattern
transparency-log already uses).
- services/admin-api/internal/attest/interop_signing_test.go drives the real
attest.HTTPSigner against the real signing-service handler and asserts the
end-to-end ADR-0005 round-trip: envelope shape, keyid, content_hash
self-consistency across canonicalisers, Ed25519 signature verification
with the server's public key, and canonical-payload byte equality.
Closes the "Interop-Test: admin-api attest.HTTPSigner-Client erfolgreich
gegen diesen Server" acceptance criterion from DEV-599.
Co-Authored-By: Paperclip <noreply@paperclip.ing>
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