Mention the proofValue and nonce values must be base64 encoded

[TimoGlastra]

As the title suggests. It is not clear from reading the spec that the values must be base64 encoded

[kdenhartog]

Agree the encoding should be normatively stated. I think it may be useful for us to look at multibase encoding or base64url no padding as well which is discussed in #16

We should first decide what encoding format we want to do and then make sure that it's commonly used.

@msporny is this something that we want to define at the LDS layer? E.g. all suites must use the same encoding format?

[OR13]

This is stale, proofValue must be multibase.