From 895d5fea41177e444c18f4fdc820fffa5f67d5bf Mon Sep 17 00:00:00 2001
From: Rasmus Dahlberg <rasmus@rgdd.se>
Date: Sat, 9 Dec 2023 17:08:45 +0100
Subject: Add drafty skeleton

---
 internal/merkle/compact.go | 115 +++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 115 insertions(+)
 create mode 100644 internal/merkle/compact.go

(limited to 'internal/merkle/compact.go')

diff --git a/internal/merkle/compact.go b/internal/merkle/compact.go
new file mode 100644
index 0000000..6eeabd0
--- /dev/null
+++ b/internal/merkle/compact.go
@@ -0,0 +1,115 @@
+// BSD 2-Clause License
+//
+// Copyright (c) 2022, the ct authors
+// All rights reserved.
+//
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are met:
+//
+//  1. Redistributions of source code must retain the above copyright notice, this
+//     list of conditions and the following disclaimer.
+//
+//  2. Redistributions in binary form must reproduce the above copyright notice,
+//     this list of conditions and the following disclaimer in the documentation
+//     and/or other materials provided with the distribution.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+// AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+// IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+// DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
+// FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+// DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+// SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+// CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+// OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+//
+// From:
+// https://gitlab.torproject.org/rgdd/ct/-/tree/main/pkg/merkle
+//
+// The only difference is that leaf hashes rather than leaf data are passed as
+// input to TreeHeadFromRangeProof, thus also changing the nodes() helper.
+package merkle
+
+import (
+	"crypto/sha256"
+	"fmt"
+)
+
+// node represents a subtree at some level and a particular index
+type node struct {
+	index uint64
+	hash  [sha256.Size]byte
+}
+
+// nodes returns a list of consecutive leaf hashes
+func nodes(index uint64, leafHashes [][sha256.Size]byte) (n []node) {
+	for i, lh := range leafHashes {
+		n = append(n, node{index + uint64(i), lh})
+	}
+	return
+}
+
+// compactRange outputs the minimal number of fixed subtree hashes given a
+// non-empty list of consecutive leaves that start from a non-zero index.  For a
+// definition of this algorithm, see the end of ../../doc/tlog_algorithms.md.
+func compactRange(nodes []node) [][sha256.Size]byte {
+	// Step 1
+	var hashes [][sha256.Size]byte
+
+	// Step 2
+	for len(nodes) > 1 {
+		// Step 2a
+		if xor(nodes[1].index, 1) != nodes[0].index {
+			hashes = append(hashes, nodes[0].hash)
+			nodes = nodes[1:]
+		}
+
+		// Step 2b; Step 2c;        Step 2c(iii)
+		for i := 0; i < len(nodes); i++ {
+			// Step 2c(i)
+			if i+1 != len(nodes) {
+				nodes[i].hash = HashInteriorNode(nodes[i].hash, nodes[i+1].hash)
+				nodes = append(nodes[:i+1], nodes[i+2:]...)
+			}
+
+			// Step 2c(ii)
+			nodes[i].index = rshift(nodes[i].index)
+		}
+	}
+
+	// Step 3
+	return append(hashes, nodes[0].hash)
+}
+
+// TreeHeadFromRangeProof computes a tree head at size n=len(leafHashes)+index
+// if given a list of leaf hashes at indices index,...,n-1 as well as an
+// inclusion proof for the first leaf in the tree of size n.  This allows a
+// verifier to check inclusion of one or more log entries with a single
+// inclusion proof.
+func TreeHeadFromRangeProof(leafHashes [][sha256.Size]byte, index uint64, proof [][sha256.Size]byte) (root [sha256.Size]byte, err error) {
+	var cr [][sha256.Size]byte
+	confirmHash := func(h [sha256.Size]byte) error {
+		if h != cr[0] {
+			return fmt.Errorf("aborted due incorrect right-node subtree hash")
+		}
+		cr = cr[1:]
+		return nil
+	}
+	copyRoot := func(r [sha256.Size]byte) error {
+		root = r
+		return nil
+	}
+
+	if len(leafHashes) == 0 {
+		return [sha256.Size]byte{}, fmt.Errorf("need at least one leaf to recompute tree head from proof")
+	}
+	if len(leafHashes) > 1 {
+		cr = compactRange(nodes(index+1, leafHashes[1:]))
+	}
+	return root, inclusion(leafHashes[0], index, index+uint64(len(leafHashes)), proof, copyRoot, confirmHash)
+}
+
+func xor(a, b uint64) uint64 {
+	return a ^ b
+}
-- 
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