Register an ONDC subscriber on the Registry and implement Ed25519 request signing with BLAKE-512 digests, /on_subscribe challenge decryption, and v2.0 lookup verification.

Documented steps

1. Generate an Ed25519 signing key pair (signing_public_key / signing_private_key, base64) and a SEPARATE X25519 encryption key pair (encryption_public_key in ASN.1 DER then base64) using libsodium.
2. Build the /subscribe payload: subscriber_id (your registered FQDN), subscriber_url, callback_url, signing_public_key, encryption_public_key, unique_key_id, valid_from, valid_until, and ops_no (1=BAP, 2=BPP, 4=combined).
3. POST the payload to the environment-specific registry: staging.registry.ondc.org/subscribe, preprod.registry.ondc.org/ondc/subscribe, or prod.registry.ondc.org/subscribe.
4. The registry calls your callback_url /on_subscribe with {subscriber_id, challenge}. Derive a shared secret via X25519 ECDH between your encryption_private_key and ONDC's published encryption public key, AES-decrypt the challenge, and return the plaintext synchronously.
5. For every outbound protocol call, compute a BLAKE-512 digest of the raw request body.
6. Build the signing string as "(created): <ts>\n(expires): <ts>\ndigest: BLAKE-512=<digest>" and sign it with your Ed25519 private key, base64-encoding the signature.
7. Attach the header: Authorization: Signature keyId="{subscriber_id}|{unique_key_id}|ed25519",algorithm="ed25519",created="<ts>",expires="<ts>",headers="(created)(expires)digest",signature="<sig>".
8. When a call is routed via a Beckn Gateway, expect an additional X-Gateway-Authorization header with the same structure signed by the gateway's own subscriber_id. Peer-to-peer calls carry only Authorization.
9. To verify an inbound call: parse keyId into subscriber_id|unique_key_id|algorithm, fetch that subscriber's signing_public_key from the registry lookup, recompute the BLAKE-512 digest, verify the Ed25519 signature, and check created/expires against local time. NACK with an unauthorized code if the key cannot be resolved.
10. For peer discovery use /v2.0/lookup with the same Signature-style Authorization header. The older /vlookup (signing search_parameters as country|domain|type|city|subscriber_id) is deprecated.
11. Get your FQDN whitelisted by ONDC (stated processing time 6-48 hours) with a valid OCSP-capable SSL certificate before any /subscribe request will be accepted.
12. Official docs: https://github.com/ONDC-Official/developer-docs/blob/main/registry/signing-verification.md | https://github.com/ONDC-Official/developer-docs/blob/main/registry/Onboarding%20of%20Participants.md