Add drafty skeleton

1 files changed, 271 insertions, 0 deletions

diff --git a/internal/merkle/merkle.go b/internal/merkle/merkle.go
new file mode 100644
index 0000000..872364f
--- /dev/null
+++ b/internal/merkle/merkle.go
@@ -0,0 +1,271 @@
+// 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
+package merkle
+
+import (
+	"crypto/sha256"
+	"fmt"
+)
+
+// HashEmptyTree computes the hash of an empty tree.  See RFC 6162, §2.1:
+//
+//	MTH({}) = SHA-256()
+func HashEmptyTree() [sha256.Size]byte {
+	return sha256.Sum256(nil)
+}
+
+// HashLeafNode computes the hash of a leaf's data.  See RFC 6162, §2.1:
+//
+//	MTH({d(0)}) = SHA-256(0x00 || d(0))
+func HashLeafNode(data []byte) (hash [sha256.Size]byte) {
+	h := sha256.New()
+	h.Write([]byte{0x00})
+	h.Write(data)
+	copy(hash[:], h.Sum(nil))
+	return
+}
+
+// HashInteriorNode computes the hash of an interior node.  See RFC 6962, §2.1:
+//
+//	MTH(D[n]) = SHA-256(0x01 || MTH(D[0:k]) || MTH(D[k:n])
+func HashInteriorNode(left, right [sha256.Size]byte) (hash [sha256.Size]byte) {
+	h := sha256.New()
+	h.Write([]byte{0x01})
+	h.Write(left[:])
+	h.Write(right[:])
+	copy(hash[:], h.Sum(nil))
+	return
+}
+
+// inclusion implements the algorithm specified in RFC 9162, Section 2.1.3.2.
+// In addition, the caller is allowed to confirm right-node subtree hashes.
+func inclusion(leaf [sha256.Size]byte, index, size uint64, proof [][sha256.Size]byte,
+	confirmRoot func([sha256.Size]byte) error, confirmHash func([sha256.Size]byte) error) error {
+	// Step 1
+	if index >= size {
+		return fmt.Errorf("leaf index must be in [%d, %d]", 0, size-1)
+	}
+
+	// Step 2
+	fn := index
+	sn := size - 1
+
+	// Step 3
+	r := leaf
+
+	// Step 4
+	for i, p := range proof {
+		// Step 4a
+		if sn == 0 {
+			return fmt.Errorf("reached tree head with %d remaining proof hash(es)", len(proof[i:]))
+		}
+
+		// Step 4b
+		if isLSB(fn) || fn == sn {
+			// Step 4b, i
+			r = HashInteriorNode(p, r)
+
+			// Step 4b, ii
+			if !isLSB(fn) {
+				for {
+					fn = rshift(fn)
+					sn = rshift(sn)
+
+					if isLSB(fn) || fn == 0 {
+						break
+					}
+				}
+			}
+		} else {
+			// Step 4b, i
+			r = HashInteriorNode(r, p)
+
+			// Extension: allow the caller to confirm right-node subtree hashes
+			if err := confirmHash(p); err != nil {
+				return fmt.Errorf("subtree index %d: %v", fn, err)
+			}
+		}
+
+		// Step 4c
+		fn = rshift(fn)
+		sn = rshift(sn)
+	}
+
+	// Step 5
+	if sn != 0 {
+		return fmt.Errorf("stopped at subtree with index %d due to missing proof hashes", fn)
+	}
+	return confirmRoot(r)
+}
+
+// consistency implements the algorithm specified in RFC 9162, §2.1.4.2
+func consistency(oldSize, newSize uint64, oldRoot, newRoot [sha256.Size]byte, proof [][sha256.Size]byte) error {
+	// Step 1
+	if len(proof) == 0 {
+		return fmt.Errorf("need at least one proof hash")
+	}
+
+	// Step 2
+	if isPOW2(oldSize) {
+		proof = append([][sha256.Size]byte{oldRoot}, proof...)
+	}
+
+	// Step 3
+	fn := oldSize - 1
+	sn := newSize - 1
+
+	// Step 4
+	for isLSB(fn) {
+		fn = rshift(fn)
+		sn = rshift(sn)
+	}
+
+	// Step 5
+	fr := proof[0]
+	sr := proof[0]
+
+	// Step 6
+	for i, c := range proof[1:] {
+		// Step 6a
+		if sn == 0 {
+			return fmt.Errorf("reached tree head with %d remaining proof hash(es)", len(proof[i+1:]))
+		}
+
+		// Step 6b
+		if isLSB(fn) || fn == sn {
+			// Step 6b, i
+			fr = HashInteriorNode(c, fr)
+			// Step 6b, ii
+			sr = HashInteriorNode(c, sr)
+			// Step 6b, iii
+			if !isLSB(fn) {
+				for {
+					fn = rshift(fn)
+					sn = rshift(sn)
+
+					if isLSB(fn) || fn == 0 {
+						break
+					}
+				}
+			}
+		} else {
+			// Step 6b, i
+			sr = HashInteriorNode(sr, c)
+		}
+
+		// Step 6c
+		fn = rshift(fn)
+		sn = rshift(sn)
+	}
+
+	// Step 7
+	if sn != 0 {
+		return fmt.Errorf("stopped at subtree with index %d due to missing proof hashes", fn)
+	}
+	if fr != oldRoot {
+		return fmt.Errorf("recomputed old tree head %x is not equal to reference tree head %x", fr[:], oldRoot[:])
+	}
+	if sr != newRoot {
+		return fmt.Errorf("recomputed new tree head %x is not equal to reference tree head %x", sr[:], newRoot[:])
+	}
+	return nil
+}
+
+// VerifyInclusion verifies that a leaf's data is commited at a given index in a
+// reference tree
+func VerifyInclusion(data []byte, index, size uint64, root [sha256.Size]byte, proof [][sha256.Size]byte) error {
+	if size == 0 {
+		return fmt.Errorf("tree size must be larger than zero")
+	}
+
+	confirmHash := func(h [sha256.Size]byte) error { return nil } // No compact range extension
+	confirmRoot := func(r [sha256.Size]byte) error {
+		if r != root {
+			return fmt.Errorf("recomputed tree head %x is not equal to reference tree head %x", r[:], root[:])
+		}
+		return nil
+	}
+	return inclusion(HashLeafNode(data), index, size, proof, confirmRoot, confirmHash)
+}
+
+// VerifyConsistency verifies that an an old tree is consistent with a new tree
+func VerifyConsistency(oldSize, newSize uint64, oldRoot, newRoot [sha256.Size]byte, proof [][sha256.Size]byte) error {
+	checkTree := func(size uint64, root [sha256.Size]byte) error {
+		if size == 0 {
+			if root != HashEmptyTree() {
+				return fmt.Errorf("non-empty tree head %x for size zero", root[:])
+			}
+			if len(proof) != 0 {
+				return fmt.Errorf("non-empty proof with %d hashes for size zero", len(proof))
+			}
+		} else if root == HashEmptyTree() {
+			return fmt.Errorf("empty tree head %x for tree size %d", root[:], size)
+		}
+		return nil
+	}
+
+	if err := checkTree(oldSize, oldRoot); err != nil {
+		return fmt.Errorf("old: %v", err)
+	}
+	if err := checkTree(newSize, newRoot); err != nil {
+		return fmt.Errorf("new: %v", err)
+	}
+	if oldSize == 0 {
+		return nil
+	}
+
+	if oldSize == newSize {
+		if oldRoot != newRoot {
+			return fmt.Errorf("different tree heads %x and %x with equal tree size %d", oldRoot, newRoot, oldSize)
+		}
+		if len(proof) != 0 {
+			return fmt.Errorf("non-empty proof with %d hashes for equal tree size %d", len(proof), oldSize)
+		}
+		return nil
+	}
+	if oldSize > newSize {
+		return fmt.Errorf("old tree size %d must be smaller than or equal to the new tree size %d", oldSize, newSize)
+	}
+
+	return consistency(oldSize, newSize, oldRoot, newRoot, proof)
+}
+
+// isLSB returns true if the least significant bit of num is set
+func isLSB(num uint64) bool {
+	return (num & 1) != 0
+}
+
+// isPOW2 returns true if num is a power of two (1, 2, 4, 8, ...)
+func isPOW2(num uint64) bool {
+	return (num & (num - 1)) == 0
+}
+
+func rshift(num uint64) uint64 {
+	return num >> 1
+}