[Acme] Hash algorithms for challenges

[Logan Widick <[e-mail]>]

I noticed that the challenges in the draft seem to be tied to the same
hash algorithm (SHA-256). Would it be possible to have the the server
include an array of supported hash algorithms, and then have the
client specify which one of the server's supported algorithms is used
in the response? This way, the hash algorithms can be changed when
needed without changing the challenges themselves.

For example, consider the following directory, modified to include the
server's supported hash algorithms:
{
  "new-nonce": "https://example.com/acme/new-nonce",
  //other standard URIs here
  "meta": {
    "terms-of-service": "https://example.com/acme/terms",
    "website": "https://www.example.com/",
    "caa-identities": ["example.com"],
    "hash-algorithms": ["SHA-256", "SHA-512"]
},

Alternatively, the server's hash algorithms could be included in the
challenges, like this:
{
   "type": "http-01",
   "url": "https://example.com/authz/1234/0",
   "token": "DGyRejmCefe7v4NfDGDKfA",
   "hash-algorithms":["SHA-256", "SHA-512"]
}

Then, the client indicates the preferred hash algorithm (from the
server's list) in the response:
{
  "protected": base64url({
    "alg": "ES256",
    "kid": "https://example.com/acme/acct/1",
    "nonce": "Q_s3MWoqT05TrdkM2MTDcw",
    "url": "https://example.com/acme/authz/asdf/0"
  }),
  "payload": base64url({
    "type": "http-01",
    "keyAuthorization": "IlirfxKKXA...vb29HhjjLPSggwiE",
    "hash-algorithm": "SHA-256"
  }),
  "signature": "9cbg5JO1Gf5YLjjz...SpkUfcdPai9uVYYQ"
}

For compatibility with existing implementations, the default hash
algorithm if none is specified could be SHA-256.

Sincerely,

Logan Widick