diff --git a/testkit_test.go b/testkit_test.go
index ab315e5..ae707ac 100644
--- a/testkit_test.go
+++ b/testkit_test.go
@@ -19,11 +19,15 @@ import (
 
 	"filippo.io/age"
 	"filippo.io/age/armor"
+	"filippo.io/age/internal/format"
+	"filippo.io/age/internal/stream"
+	"golang.org/x/crypto/chacha20poly1305"
+	"golang.org/x/crypto/hkdf"
 
 	agetest "c2sp.org/CCTV/age"
 )
 
-func TestVectors(t *testing.T) {
+func forEachVector(t *testing.T, f func(t *testing.T, v *vector)) {
 	tests, err := fs.ReadDir(agetest.Vectors, ".")
 	if err != nil {
 		t.Fatal(err)
@@ -35,25 +39,29 @@ func TestVectors(t *testing.T) {
 			t.Fatal(err)
 		}
 		t.Run(name, func(t *testing.T) {
-			testVector(t, contents)
+			t.Parallel()
+			f(t, parseVector(t, contents))
 		})
 	}
 }
 
-func testVector(t *testing.T, test []byte) {
-	var (
-		expect      string
-		payloadHash *[32]byte
-		identities  []age.Identity
-		armored     bool
-	)
+type vector struct {
+	expect      string
+	payloadHash *[32]byte
+	fileKey     *[16]byte
+	identities  []age.Identity
+	armored     bool
+	file        []byte
+}
 
+func parseVector(t *testing.T, test []byte) *vector {
+	v := &vector{file: test}
 	for {
-		line, rest, ok := bytes.Cut(test, []byte("\n"))
+		line, rest, ok := bytes.Cut(v.file, []byte("\n"))
 		if !ok {
 			t.Fatal("invalid test file: no payload")
 		}
-		test = rest
+		v.file = rest
 		if len(line) == 0 {
 			break
 		}
@@ -70,87 +78,194 @@ func testVector(t *testing.T, test []byte) {
 			default:
 				t.Fatal("invalid test file: unknown expect value:", value)
 			}
-			expect = value
+			v.expect = value
 		case "payload":
 			h, err := hex.DecodeString(value)
 			if err != nil {
 				t.Fatal(err)
 			}
-			payloadHash = (*[32]byte)(h)
+			v.payloadHash = (*[32]byte)(h)
+		case "file key":
+			h, err := hex.DecodeString(value)
+			if err != nil {
+				t.Fatal(err)
+			}
+			v.fileKey = (*[16]byte)(h)
 		case "identity":
 			i, err := age.ParseX25519Identity(value)
 			if err != nil {
 				t.Fatal(err)
 			}
-			identities = append(identities, i)
+			v.identities = append(v.identities, i)
 		case "passphrase":
 			i, err := age.NewScryptIdentity(value)
 			if err != nil {
 				t.Fatal(err)
 			}
-			identities = append(identities, i)
+			v.identities = append(v.identities, i)
 		case "armored":
-			armored = true
-		case "file key":
-			// Ignored.
+			v.armored = true
 		case "comment":
 			t.Log(value)
 		default:
 			t.Fatal("invalid test file: unknown header key:", key)
 		}
 	}
+	return v
+}
 
-	var in io.Reader = bytes.NewReader(test)
-	if armored {
+func TestVectors(t *testing.T) {
+	forEachVector(t, testVector)
+}
+
+func testVector(t *testing.T, v *vector) {
+	var in io.Reader = bytes.NewReader(v.file)
+	if v.armored {
 		in = armor.NewReader(in)
 	}
-	r, err := age.Decrypt(in, identities...)
+	r, err := age.Decrypt(in, v.identities...)
 	if err != nil && strings.HasSuffix(err.Error(), "bad header MAC") {
-		if expect == "HMAC failure" {
+		if v.expect == "HMAC failure" {
 			t.Log(err)
 			return
 		}
-		t.Fatalf("expected %s, got HMAC error", expect)
+		t.Fatalf("expected %s, got HMAC error", v.expect)
 	} else if e := new(armor.Error); errors.As(err, &e) {
-		if expect == "armor failure" {
+		if v.expect == "armor failure" {
 			t.Log(err)
 			return
 		}
-		t.Fatalf("expected %s, got: %v", expect, err)
+		t.Fatalf("expected %s, got: %v", v.expect, err)
 	} else if _, ok := err.(*age.NoIdentityMatchError); ok {
-		if expect == "no match" {
+		if v.expect == "no match" {
 			t.Log(err)
 			return
 		}
-		t.Fatalf("expected %s, got: %v", expect, err)
+		t.Fatalf("expected %s, got: %v", v.expect, err)
 	} else if err != nil {
-		if expect == "header failure" {
+		if v.expect == "header failure" {
 			t.Log(err)
 			return
 		}
-		t.Fatalf("expected %s, got: %v", expect, err)
-	} else if expect != "success" && expect != "payload failure" &&
-		expect != "armor failure" {
-		t.Fatalf("expected %s, got success", expect)
+		t.Fatalf("expected %s, got: %v", v.expect, err)
+	} else if v.expect != "success" && v.expect != "payload failure" &&
+		v.expect != "armor failure" {
+		t.Fatalf("expected %s, got success", v.expect)
 	}
 	out, err := io.ReadAll(r)
-	if err != nil && expect == "success" {
-		t.Fatalf("expected %s, got: %v", expect, err)
+	if err != nil && v.expect == "success" {
+		t.Fatalf("expected %s, got: %v", v.expect, err)
 	} else if err != nil {
 		t.Log(err)
-		if expect == "armor failure" {
+		if v.expect == "armor failure" {
 			if e := new(armor.Error); !errors.As(err, &e) {
 				t.Errorf("expected armor.Error, got %T", err)
 			}
 		}
-		if payloadHash != nil && sha256.Sum256(out) != *payloadHash {
+		if v.payloadHash != nil && sha256.Sum256(out) != *v.payloadHash {
 			t.Error("partial payload hash mismatch")
 		}
 		return
-	} else if expect != "success" {
-		t.Fatalf("expected %s, got success", expect)
+	} else if v.expect != "success" {
+		t.Fatalf("expected %s, got success", v.expect)
 	}
-	if sha256.Sum256(out) != *payloadHash {
+	if sha256.Sum256(out) != *v.payloadHash {
 		t.Error("payload hash mismatch")
 	}
 }
+
+// TestVectorsRoundTrip checks that any (valid) armor, header, and/or STREAM
+// payload in the test vectors re-encodes identically.
+func TestVectorsRoundTrip(t *testing.T) {
+	forEachVector(t, testVectorRoundTrip)
+}
+
+func testVectorRoundTrip(t *testing.T, v *vector) {
+	if v.armored {
+		t.Run("armor", func(t *testing.T) {
+			payload, err := io.ReadAll(armor.NewReader(bytes.NewReader(v.file)))
+			if err != nil {
+				// If the error is unexpected, it will be caught by TestVectors.
+				t.Skip(err)
+			}
+			buf := &bytes.Buffer{}
+			w := armor.NewWriter(buf)
+			if _, err := w.Write(payload); err != nil {
+				t.Fatal(err)
+			}
+			if err := w.Close(); err != nil {
+				t.Fatal(err)
+			}
+			// Armor format is not perfectly strict: CRLF ↔ LF and trailing and
+			// leading spaces are allowed and won't round-trip.
+			expect := bytes.Replace(v.file, []byte("\r\n"), []byte("\n"), -1)
+			expect = bytes.TrimSpace(expect)
+			expect = append(expect, '\n')
+			if !bytes.Equal(buf.Bytes(), expect) {
+				t.Error("got a different armor encoding")
+			}
+		})
+		// Armor tests are not interesting beyond their armor encoding.
+		return
+	}
+
+	hdr, p, err := format.Parse(bytes.NewReader(v.file))
+	if err != nil {
+		// If the error is unexpected, it will be caught by TestVectors.
+		t.Skip(err)
+	}
+	payload, err := io.ReadAll(p)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	t.Run("header", func(t *testing.T) {
+		buf := &bytes.Buffer{}
+		if err := hdr.Marshal(buf); err != nil {
+			t.Fatal(err)
+		}
+		buf.Write(payload)
+		if !bytes.Equal(buf.Bytes(), v.file) {
+			t.Error("got a different header+payload encoding")
+		}
+	})
+
+	if v.fileKey != nil && len(payload) > 16 {
+		t.Run("STREAM", func(t *testing.T) {
+			nonce, payload := payload[:16], payload[16:]
+			key := streamKey(v.fileKey[:], nonce)
+			r, err := stream.NewReader(key, bytes.NewReader(payload))
+			if err != nil {
+				t.Fatal(err)
+			}
+			plaintext, err := io.ReadAll(r)
+			if err != nil {
+				// If the error is unexpected, it will be caught by TestVectors.
+				t.Skip(err)
+			}
+			buf := &bytes.Buffer{}
+			w, err := stream.NewWriter(key, buf)
+			if err != nil {
+				t.Fatal(err)
+			}
+			if _, err := w.Write(plaintext); err != nil {
+				t.Fatal(err)
+			}
+			if err := w.Close(); err != nil {
+				t.Fatal(err)
+			}
+			if !bytes.Equal(buf.Bytes(), payload) {
+				t.Error("got a different STREAM ciphertext")
+			}
+		})
+	}
+}
+
+func streamKey(fileKey, nonce []byte) []byte {
+	h := hkdf.New(sha256.New, fileKey, nonce, []byte("payload"))
+	streamKey := make([]byte, chacha20poly1305.KeySize)
+	if _, err := io.ReadFull(h, streamKey); err != nil {
+		panic("age: internal error: failed to read from HKDF: " + err.Error())
+	}
+	return streamKey
+}