Add compact umbilical format

[bwesterb]

This is only available for CAs that require umbilical evidence.

Instead of storing the certificates directly in the evidence, we store the sha256 fingerprint.

In each batch, the CA publishes an extra file called umbilical-certificates. This is just a list of X509 certificates together with an index to look them up by hash. The certificate referred to in the evidence doesn't have to appear in the same batch, but could appear in any batch within the validity window.

This deduplicates intermediates, and deduplicates when multiple MTCs are issued using the same umbilical leaf certificate.

Closes #25

[lukevalenta]

Isn't the fact that the key is the SHA-256 hash of the blob used in compressEvidence to derive the key from the blob?

[lukevalenta]

Or do you mean that a cryptographic hash isn't needed? We could add a new function to compute the key from the blob so the caller doesn't need to know that internally it's a SHA-256 hash.

func (h *Handle) Key(blob []byte) []byte {
  var [32]byte key
  _, _ = h.hasher.Write(blob)
  h.hasher.Sum(key[:0])
  h.hasher.Reset()
  return key[:]
}

[bwesterb]

Isn't the fact that the key is the SHA-256 hash of the blob used in compressEvidence to derive the key from the blob?

It's not used in the lookup code.

[lukevalenta]

Shouldn't this package name be frozencerts instead, since we're also compressing end-entity certs? Or just blobs or blobstore since there's really nothing in that package specific to certificates, and it could be re-used for other evidence types in the future.

[bwesterb]

😆 I meant Frozen Content Addressed Storage, but I see now that that is an ambiguous name.