Go treats underscores specially

In case there's ever a platform called helpers or protocol, we don't
want to be doing this.

noise-helpers.go

@@ -0,0 +1,98 @@
+package main
+
+import (
+	"crypto/hmac"
+	"crypto/rand"
+	"crypto/subtle"
+	"golang.org/x/crypto/blake2s"
+	"golang.org/x/crypto/curve25519"
+	"hash"
+)
+
+/* KDF related functions.
+ * HMAC-based Key Derivation Function (HKDF)
+ * https://tools.ietf.org/html/rfc5869
+ */
+
+func HMAC1(sum *[blake2s.Size]byte, key, in0 []byte) {
+	mac := hmac.New(func() hash.Hash {
+		h, _ := blake2s.New256(nil)
+		return h
+	}, key)
+	mac.Write(in0)
+	mac.Sum(sum[:0])
+}
+
+func HMAC2(sum *[blake2s.Size]byte, key, in0, in1 []byte) {
+	mac := hmac.New(func() hash.Hash {
+		h, _ := blake2s.New256(nil)
+		return h
+	}, key)
+	mac.Write(in0)
+	mac.Write(in1)
+	mac.Sum(sum[:0])
+}
+
+func KDF1(t0 *[blake2s.Size]byte, key, input []byte) {
+	HMAC1(t0, key, input)
+	HMAC1(t0, t0[:], []byte{0x1})
+	return
+}
+
+func KDF2(t0, t1 *[blake2s.Size]byte, key, input []byte) {
+	var prk [blake2s.Size]byte
+	HMAC1(&prk, key, input)
+	HMAC1(t0, prk[:], []byte{0x1})
+	HMAC2(t1, prk[:], t0[:], []byte{0x2})
+	setZero(prk[:])
+	return
+}
+
+func KDF3(t0, t1, t2 *[blake2s.Size]byte, key, input []byte) {
+	var prk [blake2s.Size]byte
+	HMAC1(&prk, key, input)
+	HMAC1(t0, prk[:], []byte{0x1})
+	HMAC2(t1, prk[:], t0[:], []byte{0x2})
+	HMAC2(t2, prk[:], t1[:], []byte{0x3})
+	setZero(prk[:])
+	return
+}
+
+func isZero(val []byte) bool {
+	acc := 1
+	for _, b := range val {
+		acc &= subtle.ConstantTimeByteEq(b, 0)
+	}
+	return acc == 1
+}
+
+func setZero(arr []byte) {
+	for i := range arr {
+		arr[i] = 0
+	}
+}
+
+/* curve25519 wrappers */
+
+func newPrivateKey() (sk NoisePrivateKey, err error) {
+	// clamping: https://cr.yp.to/ecdh.html
+	_, err = rand.Read(sk[:])
+	sk[0] &= 248
+	sk[31] &= 127
+	sk[31] |= 64
+	return
+}
+
+func (sk *NoisePrivateKey) publicKey() (pk NoisePublicKey) {
+	apk := (*[NoisePublicKeySize]byte)(&pk)
+	ask := (*[NoisePrivateKeySize]byte)(sk)
+	curve25519.ScalarBaseMult(apk, ask)
+	return
+}
+
+func (sk *NoisePrivateKey) sharedSecret(pk NoisePublicKey) (ss [NoisePublicKeySize]byte) {
+	apk := (*[NoisePublicKeySize]byte)(&pk)
+	ask := (*[NoisePrivateKeySize]byte)(sk)
+	curve25519.ScalarMult(&ss, ask, apk)
+	return ss
+}