diff --git a/internal/siv_aead/correctness_test.go b/internal/siv_aead/correctness_test.go
index c271970..a9885e7 100644
--- a/internal/siv_aead/correctness_test.go
+++ b/internal/siv_aead/correctness_test.go
@@ -8,7 +8,31 @@ import (
 	"github.com/jacobsa/crypto/siv"
 )
 
-func TestAll(t *testing.T) {
+// Test all supported key lengths
+func TestKeyLens(t *testing.T) {
+	keyLens := []int{32, 48, 64}
+	nonce := make([]byte, 16)
+	plaintext := []byte("foobar")
+	for _, keyLen := range keyLens {
+		key := make([]byte, keyLen)
+		a := New(key)
+		ciphertext2 := a.Seal(nil, nonce, plaintext, nil)
+
+		ciphertext, err := siv.Encrypt(nil, key, plaintext, [][]byte{nil, nonce})
+		if err != nil {
+			t.Error(err)
+		} else if o := len(ciphertext) - len(plaintext); o != a.Overhead() {
+			t.Errorf("keyLen=%d, actual overhead: %d\n", keyLen, o)
+		}
+		if !bytes.Equal(ciphertext, ciphertext2) {
+			t.Errorf("siv and siv_aead produce different results")
+		}
+	}
+
+}
+
+// Test using a 32-byte key
+func TestK32(t *testing.T) {
 	key := bytes.Repeat([]byte{1}, 32)
 	nonce := bytes.Repeat([]byte{2}, 16)
 	plaintext := []byte{1, 2, 3, 4, 5, 6, 7, 8, 9}
@@ -64,3 +88,61 @@ func TestAll(t *testing.T) {
 		t.Error("should have failed")
 	}
 }
+
+// Test using a 64-byte key
+func TestK64(t *testing.T) {
+	key := bytes.Repeat([]byte{1}, 64)
+	nonce := bytes.Repeat([]byte{2}, 16)
+	plaintext := []byte{1, 2, 3, 4, 5, 6, 7, 8, 9}
+	aData := make([]byte, 24)
+	// Compare siv and siv_aead results
+	sResult, err := siv.Encrypt(nonce, key, plaintext, [][]byte{aData, nonce})
+	if err != nil {
+		t.Fatal(err)
+	}
+	a := New(key)
+	aResult := a.Seal(nonce, nonce, plaintext, aData)
+	if !bytes.Equal(sResult, aResult) {
+		t.Errorf("siv and siv_aead produce different results")
+	}
+	expectedResult, _ := hex.DecodeString(
+		"02020202020202020202020202020202317b316f67c3ad336c01c9a01b4c5e552ba89e966bc4c1ade1")
+	if !bytes.Equal(aResult, expectedResult) {
+		t.Errorf(hex.EncodeToString(aResult))
+	}
+	// Verify overhead
+	overhead := len(aResult) - len(plaintext) - len(nonce)
+	if overhead != a.Overhead() {
+		t.Errorf("Overhead() returns a wrong value")
+	}
+	// Decrypt
+	p1, err := a.Open(nil, aResult[:16], aResult[16:], aData)
+	if err != nil {
+		t.Error(err)
+	}
+	if !bytes.Equal(plaintext, p1) {
+		t.Errorf("wrong plaintext")
+	}
+	// Decrypt and append
+	dst := []byte{0xaa, 0xbb, 0xcc}
+	p2, err := a.Open(dst, aResult[:16], aResult[16:], aData)
+	if err != nil {
+		t.Error(err)
+	}
+	p2e := append(dst, plaintext...)
+	if !bytes.Equal(p2e, p2) {
+		t.Errorf("wrong plaintext: %s", hex.EncodeToString(p2))
+	}
+	// Decrypt corrupt
+	aResult[17] = 0
+	_, err = a.Open(nil, aResult[:16], aResult[16:], aData)
+	if err == nil {
+		t.Error("should have failed")
+	}
+	// Decrypt and append corrupt
+	aResult[17] = 0
+	_, err = a.Open(dst, aResult[:16], aResult[16:], aData)
+	if err == nil {
+		t.Error("should have failed")
+	}
+}
diff --git a/internal/siv_aead/siv_aead.go b/internal/siv_aead/siv_aead.go
index 21106a5..a0ed882 100644
--- a/internal/siv_aead/siv_aead.go
+++ b/internal/siv_aead/siv_aead.go
@@ -22,11 +22,7 @@ func (s *sivAead) NonceSize() int {
 }
 
 func (s *sivAead) Overhead() int {
-	// RFC5297:
-	// [...] the key length used by AES in CTR and S2V is len(K)/2 and will
-	// each be either 128 bits, 192 bits, or 256 bits.
-	return len(s.key) / 2
-
+	return 16
 }
 
 // Seal - encrypt "in" using "nonce" and "authData" and append the result to "dst"