POST /acme/chall/prV_B7yEyA4 HTTP/1.1
Host: example.com
Content-Type: application/jose+json

{
    "protected": base64url({
        "alg": "ES256",
        "kid": "https://example.com/acme/acct/evOfKhNU60wg",
        "nonce": "Q_s3MWoqT05TrdkM2MTDcw",
        "url": "https://example.com/acme/chall/prV_B7yEyA4"
    }),
    "payload": base64url({}),
    "signature": "9cbg5JO1Gf5YLjjz...SpkUfcdPai9uVYYQ"
}