diff --git a/merkletree/collapsed_tree.go b/merkletree/collapsed_tree.go
new file mode 100644
index 0000000..e138207
--- /dev/null
+++ b/merkletree/collapsed_tree.go
@@ -0,0 +1,98 @@
+// Copyright (C) 2022 Opsmate, Inc.
+//
+// This Source Code Form is subject to the terms of the Mozilla
+// Public License, v. 2.0. If a copy of the MPL was not distributed
+// with this file, You can obtain one at http://mozilla.org/MPL/2.0/.
+//
+// This software is distributed WITHOUT A WARRANTY OF ANY KIND.
+// See the Mozilla Public License for details.
+
+package merkletree
+
+import (
+	"encoding/json"
+	"fmt"
+)
+
+type CollapsedTree struct {
+	nodes []Hash
+	size  uint64
+}
+
+func calculateNumNodes(size uint64) int {
+	numNodes := 0
+	for size > 0 {
+		numNodes += int(size & 1)
+		size >>= 1
+	}
+	return numNodes
+}
+
+func EmptyCollapsedTree() *CollapsedTree {
+	return &CollapsedTree{nodes: []Hash{}, size: 0}
+}
+
+func NewCollapsedTree(nodes []Hash, size uint64) (*CollapsedTree, error) {
+	if len(nodes) != calculateNumNodes(size) {
+		return nil, fmt.Errorf("nodes has wrong length (should be %d, not %d)", calculateNumNodes(size), len(nodes))
+	}
+	return &CollapsedTree{nodes: nodes, size: size}, nil
+}
+
+func CloneCollapsedTree(source *CollapsedTree) *CollapsedTree {
+	nodes := make([]Hash, len(source.nodes))
+	copy(nodes, source.nodes)
+	return &CollapsedTree{nodes: nodes, size: source.size}
+}
+
+func (tree *CollapsedTree) Add(hash Hash) {
+	tree.nodes = append(tree.nodes, hash)
+	tree.size++
+	size := tree.size
+	for size%2 == 0 {
+		left, right := tree.nodes[len(tree.nodes)-2], tree.nodes[len(tree.nodes)-1]
+		tree.nodes = tree.nodes[:len(tree.nodes)-2]
+		tree.nodes = append(tree.nodes, HashChildren(left, right))
+		size /= 2
+	}
+}
+
+func (tree *CollapsedTree) CalculateRoot() Hash {
+	if len(tree.nodes) == 0 {
+		return HashNothing()
+	}
+	i := len(tree.nodes) - 1
+	hash := tree.nodes[i]
+	for i > 0 {
+		i -= 1
+		hash = HashChildren(tree.nodes[i], hash)
+	}
+	return hash
+}
+
+func (tree *CollapsedTree) Size() uint64 {
+	return tree.size
+}
+
+func (tree *CollapsedTree) MarshalJSON() ([]byte, error) {
+	return json.Marshal(map[string]interface{}{
+		"nodes": tree.nodes,
+		"size":  tree.size,
+	})
+}
+
+func (tree *CollapsedTree) UnmarshalJSON(b []byte) error {
+	var rawTree struct {
+		Nodes []Hash `json:"nodes"`
+		Size  uint64 `json:"size"`
+	}
+	if err := json.Unmarshal(b, &rawTree); err != nil {
+		return fmt.Errorf("error unmarshalling Collapsed Merkle Tree: %w", err)
+	}
+	if len(rawTree.Nodes) != calculateNumNodes(rawTree.Size) {
+		return fmt.Errorf("error unmarshalling Collapsed Merkle Tree: nodes has wrong length (should be %d, not %d)", calculateNumNodes(rawTree.Size), len(rawTree.Nodes))
+	}
+	tree.size = rawTree.Size
+	tree.nodes = rawTree.Nodes
+	return nil
+}
diff --git a/merkletree/hash.go b/merkletree/hash.go
new file mode 100644
index 0000000..491472d
--- /dev/null
+++ b/merkletree/hash.go
@@ -0,0 +1,64 @@
+// Copyright (C) 2022 Opsmate, Inc.
+//
+// This Source Code Form is subject to the terms of the Mozilla
+// Public License, v. 2.0. If a copy of the MPL was not distributed
+// with this file, You can obtain one at http://mozilla.org/MPL/2.0/.
+//
+// This software is distributed WITHOUT A WARRANTY OF ANY KIND.
+// See the Mozilla Public License for details.
+
+package merkletree
+
+import (
+	"crypto/sha256"
+	"encoding/base64"
+	"encoding/json"
+	"fmt"
+)
+
+const HashLen = 32
+
+type Hash [HashLen]byte
+
+func (h Hash) Base64String() string {
+	return base64.StdEncoding.EncodeToString(h[:])
+}
+
+func (h Hash) MarshalJSON() ([]byte, error) {
+	return json.Marshal(h[:])
+}
+
+func (h *Hash) UnmarshalJSON(b []byte) error {
+	var hashBytes []byte
+	if err := json.Unmarshal(b, &hashBytes); err != nil {
+		return err
+	}
+	if len(hashBytes) != HashLen {
+		return fmt.Errorf("Merkle Tree hash has wrong length (should be %d bytes long, not %d)", HashLen)
+	}
+	copy(h[:], hashBytes)
+	return nil
+}
+
+func HashNothing() Hash {
+	return sha256.Sum256(nil)
+}
+
+func HashLeaf(leafBytes []byte) Hash {
+	var hash Hash
+	hasher := sha256.New()
+	hasher.Write([]byte{0x00})
+	hasher.Write(leafBytes)
+	hasher.Sum(hash[:0])
+	return hash
+}
+
+func HashChildren(left Hash, right Hash) Hash {
+	var hash Hash
+	hasher := sha256.New()
+	hasher.Write([]byte{0x01})
+	hasher.Write(left[:])
+	hasher.Write(right[:])
+	hasher.Sum(hash[:0])
+	return hash
+}