> Antoine Pitrou added the comment:
> 
> I won't claim to know/understand the specifics, but "message payload in base64" actually sounds reasonable to me, if far from optimal (both from readibility and space overhead POV) :-).

It is indeed a working solution.  I do though think that communities that previously used XML would accept base64-encoded messages.  It becomes really messy when applied to counter-signed messages like the following:

{
  "@context": "http://xmlns.webpki.org/wcpp-payment-demo",
  "@qualifier": "AuthData",
  "paymentRequest": 
    {
      "commonName": "Demo Merchant",
      "amount": 8600550,
      "currency": "USD",
      "referenceId": "#1000000",
      "dateTime": "2014-12-18T13:39:35Z",
      "signature": 
        {
          "algorithm": "RS256",
          "signerCertificate": 
            {
              "issuer": "CN=Merchant Network Sub CA5,C=DE",
              "serialNumber": "1413983542582",
              "subject": "CN=Demo Merchant,2.5.4.5=#1306383936333235,C=DE"
            },
          "certificatePath": 
            [
              "MIIDQzCCAiugAwIBAgIGAUk3_J02M...eMGlY734U3NasQfAhTUhxrdDbphEvsWTc",
              "MIIEPzCCAiegAwIBAgIBBTANBgkqh...gU1IyRGA7IbdHOeDB2RUpsXloU2QKfLrk"
            ],
          "value": "Ny4Qe6FQhd5_qcSc3xiH8Kt7tIZ9Z...9LEjC6_Rulg_G20fGxJ-wzezFpsAGbmuFQg"
        }
    },
  "domainName": "merchant.com",
  "cardType": "SuperCard",
  "pan": "1618342124919252",
  "dateTime": "2014-12-18T13:40:59Z",
  "signature": 
    {
      "algorithm": "RS256",
      "signerCertificate": 
        {
          "issuer": "CN=Mybank Client Root CA1,C=US",
          "serialNumber": "1413983550045",
          "subject": "CN=The Cardholder,2.5.4.5=#13083935363733353232"
        },
      "certificatePath": ["MIIENzCCAh-gAwIBAgIGAUk3_LpdM...IGcN1md5feo5DndNnV8D0UM-oBRkUDDFiWlhCU"],
      "value": "wyUcFcHmvM5ZozZKOEwOQkYic0D7M...S_HbaPGau5KfZjCaksvb5U1lYZaXxP8kAbuGPQ"
    }
}