sealEnvelope()
sealEnvelope(string  $plainData) : \CryptoManana\DataStructures\EnvelopeData
                        Seals the envelope with the secured information inside.
Parameters
| string | $plainData | The plain message information. | 
Throws
- \Exception
- Validation errors. 
Returns
\CryptoManana\DataStructures\EnvelopeData —The sealed envelope object.