Tighten up checksum types (#282)

Tighten up checksum types, in response to feedback from PRC. Also incorporates feedback from @briandoconnor 's review and the driver projects on today's Cloud WS call.

openapi/data_repository_service.swagger.yaml

@@ -167,23 +167,26 @@ securityDefinitions:
 definitions:
   Checksum:
     type: object
-    required:
-      - checksum
+    required: ['checksum', 'type']
     properties:
       checksum:
         type: string
         description: 'The hex-string encoded checksum for the data'
       type:
         type: string
-        description: |-
-          The digest method used to create the checksum. If left unspecified md5
-          will be assumed.
+        description: >-
+          The digest method used to create the checksum.
+
 
-          possible values:
-          md5               # most blob stores provide a checksum using this
-          etag              # multipart uploads to blob stores
-          sha256
-          sha512
+          The value (e.g. `sha-256`) SHOULD be listed as `Hash Name String` in the https://www.iana.org/assignments/named-information/named-information.xhtml#hash-alg[IANA Named Information Hash Algorithm Registry].
+          Other values MAY be used, as long as implementors are aware of the issues discussed in https://tools.ietf.org/html/rfc6920#section-9.4[RFC6920].
+
+
+          GA4GH may provide more explicit guidance for use of non-IANA-registered algorithms in the future.
+          Until then, if implementors do choose such an algorithm (e.g. because it's implemented by their storage provider), they SHOULD use an existing
+          standard `type` value such as `md5`, `etag`, `crc32c`, `trunc512`, or `sha1`.
+        example:
+          sha-256
   Object:
     type: object
     required: ['id', 'self_uri', 'size', 'created_time', 'checksums']