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Snapshot merkle POUF #133

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@mnm678 mnm678 commented Feb 3, 2021

Add description of snapshot Merkle metadata to the reference implementation POUF, based on the description of snapshot Merkle trees in #125.

This pr is a draft, and should be merged only if/when theupdateframework/python-tuf#1113 is incorporated into the reference implementation.

…atino POUF

This commit adds metadata definitions and algorithm requirements for
snapshot Merkle trees in the reference implementation.

Signed-off-by: Marina Moore <[email protected]>
Add the procedures for computing leafs, nodes, and
path_directions used by the reference implementation

Signed-off-by: Marina Moore <[email protected]>
Snapshot Merkle trees in this implementation will use sha256 to compute the digest of each node, and will use the following procedures for computing node digests:
* A leaf digest is the sha256 hash of the cannonical json encoding of its `leaf_contents`.
* An internal node's digest is the sha256 hash of its left child's digest + it's right child's digest, using utf-8 encoding.
* The `path_directions` and `merkle_path` for each snapshot Merkle metadata file provide information needed to reconstruct the Merkle tree. For each node in the tree, starting with the given leaf node, the next `path_directions` will be -1 if the corresponding `merkle_path` is a right sibling of the current node, or 1 if it is a left sibling. So, a `path_direction` of -1 means that the parent node's digest will be the hash of the current node's digest + the next `merkle_path` digest (as the `merkle_path` is a right sibling).
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This part would probably benefit from a diagram.

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