An interface for verifying that a given string of data was signed by the
private key matching the given public key.
Verifies that the data matches the data that was used to generate the
signature.
| aData | The data to be tested. |
| aSignature | The signature of the data, base64 encoded. |
| aPublicKey | The public part of the key used for signing, DER encoded then base64 encoded. |
| true if the signature matches the data, false if not. |