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core/commit.go

@@ -0,0 +1,123 @@
+//
+// Copyright Coinbase, Inc. All Rights Reserved.
+//
+// SPDX-License-Identifier: Apache-2.0
+//
+
+package core
+
+import (
+	"crypto/hmac"
+	crand "crypto/rand"
+	"crypto/sha256"
+	"crypto/subtle"
+	"encoding/json"
+	"fmt"
+	"hash"
+)
+
+// Size of random values and hash outputs are determined by our hash function
+const Size = sha256.Size
+
+type (
+	// Commitment to a given message which can be later revealed.
+	// This is sent to and held by a verifier until the corresponding
+	// witness is provided.
+	Commitment []byte
+
+	// Witness is sent to and opened by the verifier. This proves that
+	// committed message hasn't been altered by later information.
+	Witness struct {
+		Msg []byte
+		r   [Size]byte
+	}
+
+	// witnessJSON is used for un/marshaling.
+	witnessJSON struct {
+		Msg []byte
+		R   [Size]byte
+	}
+)
+
+// MarshalJSON encodes Witness in JSON
+func (w Witness) MarshalJSON() ([]byte, error) {
+	return json.Marshal(witnessJSON{w.Msg, w.r})
+}
+
+// UnmarshalJSON decodes JSON into a Witness struct
+func (w *Witness) UnmarshalJSON(data []byte) error {
+	witness := &witnessJSON{}
+	err := json.Unmarshal(data, witness)
+	if err != nil {
+		return err
+	}
+	w.Msg = witness.Msg
+	w.r = witness.R
+	return nil
+}
+
+// Commit to a given message. Uses SHA256 as the hash function.
+func Commit(msg []byte) (Commitment, *Witness, error) {
+	// Initialize our decommitment
+	d := Witness{msg, [Size]byte{}}
+
+	// Generate a random nonce of the required length
+	n, err := crand.Read(d.r[:])
+	// Ensure no errors retrieving nonce
+	if err != nil {
+		return nil, nil, err
+	}
+
+	// Ensure we read all the bytes expected
+	if n != Size {
+		return nil, nil, fmt.Errorf(
+			"failed to read %v bytes from crypto/rand: received %v bytes",
+			Size,
+			n,
+		)
+	}
+	// Compute the commitment: HMAC(Sha2, msg, key)
+	c, err := ComputeHMAC(sha256.New, msg, d.r[:])
+	if err != nil {
+		return nil, nil, err
+	}
+	return c, &d, nil
+}
+
+// Open a commitment and return true if the commitment/decommitment pair are valid.
+// reference: spec.§2.4: Commitment Scheme
+func Open(c Commitment, d Witness) (bool, error) {
+	// Ensure commitment is well-formed.
+	if len(c) != Size {
+		return false, fmt.Errorf("invalid commitment, wrong length. %v != %v", len(c), Size)
+	}
+
+	// Re-compute the commitment: HMAC(Sha2, msg, key)
+	cʹ, err := ComputeHMAC(sha256.New, d.Msg, d.r[:])
+	if err != nil {
+		return false, err
+	}
+	return subtle.ConstantTimeCompare(cʹ, c) == 1, nil
+}
+
+// ComputeHMAC computes HMAC(hash_fn, msg, key)
+// Takes in a hash function to use for HMAC
+func ComputeHMAC(f func() hash.Hash, msg []byte, k []byte) ([]byte, error) {
+	if f == nil {
+		return nil, fmt.Errorf("hash function cannot be nil")
+	}
+
+	mac := hmac.New(f, k)
+	w, err := mac.Write(msg)
+
+	if w != len(msg) {
+		return nil, fmt.Errorf(
+			"bytes written to hash doesn't match expected: %v != %v",
+			w,
+			len(msg),
+		)
+	} else if err != nil {
+		return nil, err
+	}
+	return mac.Sum(nil), nil
+}