new API for derive_key

blake3.go

@@ -246,19 +246,6 @@ func New(size int, key []byte) *Hasher {
 	return newHasher(keyWords, flagKeyedHash, size)
 }
 
-// NewFromDerivedKey returns a Hasher whose key was derived from the supplied
-// context string.
-func NewFromDerivedKey(size int, ctx string) *Hasher {
-	const derivedKeyLen = 32
-	h := newHasher(iv, flagDeriveKeyContext, derivedKeyLen)
-	h.Write([]byte(ctx))
-	key := make([]byte, derivedKeyLen)
-	h.Sum(key[:0])
-	var keyWords [8]uint32
-	bytesToWords(key, keyWords[:])
-	return newHasher(keyWords, flagDeriveKeyMaterial, size)
-}
-
 func (h *Hasher) addChunkChainingValue(cv [8]uint32, totalChunks uint64) {
 	// This chunk might complete some subtrees. For each completed subtree,
 	// its left child will be the current top entry in the CV stack, and
@@ -356,6 +343,20 @@ func Sum512(b []byte) [64]byte {
 	return out
 }
 
+// DeriveKey derives a subkey from ctx and srcKey.
+func DeriveKey(subKey []byte, ctx string, srcKey []byte) {
+	// construct the derivation Hasher
+	const derivationIVLen = 32
+	h := newHasher(iv, flagDeriveKeyContext, 32)
+	h.Write([]byte(ctx))
+	var derivationIV [8]uint32
+	bytesToWords(h.Sum(make([]byte, 0, derivationIVLen)), derivationIV[:])
+	h = newHasher(derivationIV, flagDeriveKeyMaterial, len(subKey))
+	// derive the subKey
+	h.Write(srcKey)
+	h.Sum(subKey[:0])
+}
+
 // ensure that Hasher implements hash.Hash
 var _ hash.Hash = (*Hasher)(nil)
 

blake3_test.go

@@ -52,10 +52,10 @@ func TestVectors(t *testing.T) {
 		}
 		// derive key
 		const ctx = "BLAKE3 2019-12-27 16:29:52 test vectors context"
-		h = blake3.NewFromDerivedKey(len(vec.DeriveKey)/2, ctx)
-		h.Write(in)
-		if out := toHex(h.Sum(nil)); out != vec.DeriveKey {
-			t.Errorf("output did not match test vector:\n\texpected: %v...\n\t     got: %v...", vec.DeriveKey[:10], out[:10])
+		subKey := make([]byte, len(vec.DeriveKey)/2)
+		blake3.DeriveKey(subKey, ctx, in)
+		if out := toHex(subKey); out != vec.DeriveKey {
+			t.Errorf("output did not match test vector:\n\texpected: %v...\n\t     got: %v...", vec.DeriveKey[:10], subKey[:10])
 		}
 		// XOF should produce identical results, even when outputting 7 bytes at a time
 		h = blake3.New(len(vec.Hash)/2, nil)