Add snapshot merkle trees to the reference POUF.

This adds the data formats and hash requiements for
snapshot merkle trees used in the reference implementation.

Signed-off-by: Marina Moore <mnm678@gmail.com>

POUFs/reference-POUF/pouf1.md

@@ -22,6 +22,8 @@ This POUF uses a subset of the JSON object format, with floating-point numbers o
 
 In this POUF, metadata files are hosted on the repository using HTTP. The filenames for these files are ROLE.json where ROLE is the associated role name (root, targets, snapshot, or timestamp). A client downloads these files by HTTP post request. The location of the repository is preloaded onto the clients.
 
+Snapshot Merkle trees in this implementation will use sha256 to compute the hash of each node.
+
 ## Message Handler Table
 
 This table lists the message handlers supported by the reference implementation.
@@ -336,6 +338,7 @@ The timestamp file is signed by a timestamp key.  It indicates the
          "spec_version" : SPEC_VERSION,
          "version" : VERSION,
          "expires" : EXPIRES,
+         ("merkle_root": ROOT_HASH),
          "meta" : METAFILES
        }
 
@@ -361,6 +364,8 @@ The timestamp file is signed by a timestamp key.  It indicates the
      HASH is the hexdigest of the cryptographic function computed on the
      snapshot.json metadata file.
 
+     ROOT_HASH is the hash of the Merkle tree's root node.
+
 ### mirrors.json
 The mirrors.json file is signed by the mirrors role.  It indicates which
    mirrors are active and believed to be mirroring specific parts of the
@@ -401,6 +406,22 @@ This behavior can be modified by the client code that uses the framework to,
 for example, randomly select from the listed mirrors.
 
 
+### Snapshot Merkle metadata
+
+Snapsot Merkle metadata is not signed. It lists version information for a metadata file, and a path through the Merkle tree to verify this information.
+
+```
+{ “leaf_contents”: {METAFILES},
+  “merkle_path”: {INDEX:HASH}
+  “path_directions”:{INDEX:DIR}
+}
+```
+
+Where `METAFILES` is the version information as defined for snapshot metadata,
+`INDEX` provides the ordering of nodes, `HASH` is the sha256 hash of the sibling node,
+and `DIR` is a `1` or `0` that indicates whether the given node is a left or right sibling.
+
+
 # Security Audit
 This profile was included in TUF security audits available at https://theupdateframework.github.io/audits.html.