DISCLAIMER: This is not a "recommend me a product or service" question.
I am interested in getting to know the technologies we can use for signing a JSON document, from the server-side. What are the technologies available? Especially signed by many organizations.
I am not looking for engaging with a specific commercial signing service nor blockchain, but rather to find a technology that allows many organizations to sign a document.
Using SSL certificates for signing sounds like a viable idea, but as certificates have an expiration date, they can present challenges.
Also, the service shall not only sign per request of the signer but also sign again for verifying the authenticity of the document or returning its public key on the fly.