age: add DecryptReaderAt

age.go

@@ -252,13 +252,12 @@ func (e *NoIdentityMatchError) Unwrap() []error {
 }
 
 // Decrypt decrypts a file encrypted to one or more identities.
-//
-// It returns a Reader reading the decrypted plaintext of the age file read
-// from src. All identities will be tried until one successfully decrypts the file.
+// All identities will be tried until one successfully decrypts the file.
 // Native, non-interactive identities are tried before any other identities.
 //
-// If no identity matches the encrypted file, the returned error will be of type
-// [NoIdentityMatchError].
+// Decrypt returns a Reader reading the decrypted plaintext of the age file read
+// from src. If no identity matches the encrypted file, the returned error will
+// be of type [NoIdentityMatchError].
 func Decrypt(src io.Reader, identities ...Identity) (io.Reader, error) {
 	hdr, payload, err := format.Parse(src)
 	if err != nil {
@@ -278,6 +277,58 @@ func Decrypt(src io.Reader, identities ...Identity) (io.Reader, error) {
 	return stream.NewDecryptReader(streamKey(fileKey, nonce), payload)
 }
 
+// DecryptReaderAt decrypts a file encrypted to one or more identities.
+// All identities will be tried until one successfully decrypts the file.
+// Native, non-interactive identities are tried before any other identities.
+//
+// DecryptReaderAt takes an underlying [io.ReaderAt] and its total encrypted
+// size, and returns a ReaderAt of the decrypted plaintext and the plaintext
+// size. These can be used for example to instantiate an [io.SectionReader],
+// which implements [io.Reader] and [io.Seeker]. Note that ReaderAt by
+// definition disregards the seek position of src.
+//
+// The ReadAt method of the returned ReaderAt can be called concurrently.
+// The ReaderAt will internally cache the most recently decrypted chunk.
+// DecryptReaderAt reads and decrypts the final chunk before returning,
+// to authenticate the plaintext size.
+//
+// If no identity matches the encrypted file, the returned error will be of
+// type [NoIdentityMatchError].
+func DecryptReaderAt(src io.ReaderAt, encryptedSize int64, identities ...Identity) (io.ReaderAt, int64, error) {
+	srcReader := io.NewSectionReader(src, 0, encryptedSize)
+	hdr, payload, err := format.Parse(srcReader)
+	if err != nil {
+		return nil, 0, fmt.Errorf("failed to read header: %w", err)
+	}
+	buf := &bytes.Buffer{}
+	if err := hdr.Marshal(buf); err != nil {
+		return nil, 0, fmt.Errorf("failed to serialize header: %w", err)
+	}
+
+	fileKey, err := decryptHdr(hdr, identities...)
+	if err != nil {
+		return nil, 0, err
+	}
+
+	nonce := make([]byte, streamNonceSize)
+	if _, err := io.ReadFull(payload, nonce); err != nil {
+		return nil, 0, fmt.Errorf("failed to read nonce: %w", err)
+	}
+
+	payloadOffset := int64(buf.Len()) + int64(len(nonce))
+	payloadSize := encryptedSize - payloadOffset
+	plaintextSize, err := stream.PlaintextSize(payloadSize)
+	if err != nil {
+		return nil, 0, err
+	}
+	payloadReaderAt := io.NewSectionReader(src, payloadOffset, payloadSize)
+	r, err := stream.NewDecryptReaderAt(streamKey(fileKey, nonce), payloadReaderAt, payloadSize)
+	if err != nil {
+		return nil, 0, err
+	}
+	return r, plaintextSize, nil
+}
+
 func decryptHdr(hdr *format.Header, identities ...Identity) ([]byte, error) {
 	if len(identities) == 0 {
 		return nil, errors.New("no identities specified")

internal/inspect/inspect.go

@@ -10,7 +10,6 @@ import (
 	"filippo.io/age/armor"
 	"filippo.io/age/internal/format"
 	"filippo.io/age/internal/stream"
-	"golang.org/x/crypto/chacha20poly1305"
 )
 
 type Metadata struct {
@@ -88,9 +87,9 @@ func Inspect(r io.Reader, fileSize int64) (*Metadata, error) {
 		}
 		data.Sizes.Armor = tr.count - fileSize
 	}
-	data.Sizes.Overhead = streamOverhead(fileSize - data.Sizes.Header)
-	if data.Sizes.Overhead > fileSize-data.Sizes.Header {
-		return nil, fmt.Errorf("payload too small to be a valid age file")
+	data.Sizes.Overhead, err = streamOverhead(fileSize - data.Sizes.Header)
+	if err != nil {
+		return nil, fmt.Errorf("failed to compute stream overhead: %w", err)
 	}
 	data.Sizes.MinPayload = fileSize - data.Sizes.Header - data.Sizes.Overhead
 	data.Sizes.MaxPayload = data.Sizes.MinPayload
@@ -114,13 +113,15 @@ func (tr *trackReader) Read(p []byte) (int, error) {
 	return n, err
 }
 
-func streamOverhead(payloadSize int64) int64 {
+func streamOverhead(payloadSize int64) (int64, error) {
 	const streamNonceSize = 16
-	const encChunkSize = stream.ChunkSize + chacha20poly1305.Overhead
-	payloadSize -= streamNonceSize
-	if payloadSize <= 0 {
-		return streamNonceSize
+	if payloadSize < streamNonceSize {
+		return 0, fmt.Errorf("encrypted size too small: %d", payloadSize)
 	}
-	chunks := (payloadSize + encChunkSize - 1) / encChunkSize
-	return streamNonceSize + chunks*chacha20poly1305.Overhead
+	encryptedSize := payloadSize - streamNonceSize
+	plaintextSize, err := stream.PlaintextSize(encryptedSize)
+	if err != nil {
+		return 0, err
+	}
+	return payloadSize - plaintextSize, nil
 }

internal/inspect/inspect_test.go

@@ -0,0 +1,46 @@
+package inspect
+
+import (
+	"fmt"
+	"testing"
+
+	"filippo.io/age/internal/stream"
+)
+
+func TestStreamOverhead(t *testing.T) {
+	tests := []struct {
+		payloadSize int64
+		want        int64
+		wantErr     bool
+	}{
+		{payloadSize: 0, wantErr: true},
+		{payloadSize: 15, wantErr: true},
+		{payloadSize: 16, wantErr: true},
+		{payloadSize: 16 + 15, wantErr: true},
+		{payloadSize: 16 + 16, want: 16 + 16}, // empty plaintext
+		{payloadSize: 16 + 1 + 16, want: 16 + 16},
+		{payloadSize: 16 + stream.ChunkSize + 16, want: 16 + 16},
+		{payloadSize: 16 + stream.ChunkSize + 16 + 1, wantErr: true},
+		{payloadSize: 16 + stream.ChunkSize + 16 + 15, wantErr: true},
+		{payloadSize: 16 + stream.ChunkSize + 16 + 16, wantErr: true}, // empty final chunk
+		{payloadSize: 16 + stream.ChunkSize + 16 + 1 + 16, want: 16 + 16 + 16},
+	}
+	for _, tt := range tests {
+		name := "payloadSize=" + fmt.Sprint(tt.payloadSize)
+		t.Run(name, func(t *testing.T) {
+			got, gotErr := streamOverhead(tt.payloadSize)
+			if gotErr != nil {
+				if !tt.wantErr {
+					t.Errorf("streamOverhead() failed: %v", gotErr)
+				}
+				return
+			}
+			if tt.wantErr {
+				t.Fatal("streamOverhead() succeeded unexpectedly")
+			}
+			if got != tt.want {
+				t.Errorf("streamOverhead() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}

internal/stream/stream.go

@@ -8,15 +8,42 @@ package stream
 import (
 	"bytes"
 	"crypto/cipher"
+	"encoding/binary"
 	"errors"
 	"fmt"
 	"io"
+	"sync/atomic"
 
 	"golang.org/x/crypto/chacha20poly1305"
 )
 
 const ChunkSize = 64 * 1024
 
+func EncryptedChunkCount(encryptedSize int64) (int64, error) {
+	chunks := (encryptedSize + encChunkSize - 1) / encChunkSize
+
+	plaintextSize := encryptedSize - chunks*chacha20poly1305.Overhead
+	expChunks := (plaintextSize + ChunkSize - 1) / ChunkSize
+	// Empty plaintext, the only case that allows (and requires) an empty chunk.
+	if plaintextSize == 0 {
+		expChunks = 1
+	}
+	if expChunks != chunks {
+		return 0, fmt.Errorf("invalid encrypted payload size: %d", encryptedSize)
+	}
+
+	return chunks, nil
+}
+
+func PlaintextSize(encryptedSize int64) (int64, error) {
+	chunks, err := EncryptedChunkCount(encryptedSize)
+	if err != nil {
+		return 0, err
+	}
+	plaintextSize := encryptedSize - chunks*chacha20poly1305.Overhead
+	return plaintextSize, nil
+}
+
 type DecryptReader struct {
 	a   cipher.AEAD
 	src io.Reader
@@ -135,6 +162,12 @@ func incNonce(nonce *[chacha20poly1305.NonceSize]byte) {
 	panic("stream: chunk counter wrapped around")
 }
 
+func nonceForChunk(chunkIndex int64) *[chacha20poly1305.NonceSize]byte {
+	var nonce [chacha20poly1305.NonceSize]byte
+	binary.BigEndian.PutUint64(nonce[3:11], uint64(chunkIndex))
+	return &nonce
+}
+
 func setLastChunkFlag(nonce *[chacha20poly1305.NonceSize]byte) {
 	nonce[len(nonce)-1] = lastChunkFlag
 }
@@ -312,3 +345,102 @@ func (r *EncryptReader) feedBuffer() error {
 
 	return nil
 }
+
+type DecryptReaderAt struct {
+	a      cipher.AEAD
+	src    io.ReaderAt
+	size   int64
+	chunks int64
+	cache  atomic.Pointer[cachedChunk]
+}
+
+type cachedChunk struct {
+	off  int64
+	data []byte
+}
+
+func NewDecryptReaderAt(key []byte, src io.ReaderAt, size int64) (*DecryptReaderAt, error) {
+	aead, err := chacha20poly1305.New(key)
+	if err != nil {
+		return nil, err
+	}
+
+	// Check that size is valid by decrypting the final chunk.
+	chunks, err := EncryptedChunkCount(size)
+	if err != nil {
+		return nil, err
+	}
+	finalChunkIndex := chunks - 1
+	finalChunkOff := finalChunkIndex * encChunkSize
+	finalChunkSize := size - finalChunkOff
+	finalChunk := make([]byte, finalChunkSize)
+	if _, err := src.ReadAt(finalChunk, finalChunkOff); err != nil {
+		return nil, fmt.Errorf("failed to read final chunk: %w", err)
+	}
+	nonce := nonceForChunk(finalChunkIndex)
+	setLastChunkFlag(nonce)
+	plaintext, err := aead.Open(finalChunk[:0], nonce[:], finalChunk, nil)
+	if err != nil {
+		return nil, fmt.Errorf("failed to decrypt and authenticate final chunk: %w", err)
+	}
+	cache := &cachedChunk{off: finalChunkOff, data: plaintext}
+
+	plaintextSize := size - chunks*chacha20poly1305.Overhead
+	r := &DecryptReaderAt{a: aead, src: src, size: plaintextSize, chunks: chunks}
+	r.cache.Store(cache)
+	return r, nil
+}
+
+func (r *DecryptReaderAt) ReadAt(p []byte, off int64) (n int, err error) {
+	if off < 0 || off > r.size {
+		return 0, fmt.Errorf("offset out of range [0:%d]: %d", r.size, off)
+	}
+	if len(p) == 0 {
+		return 0, nil
+	}
+	chunk := make([]byte, encChunkSize)
+	for len(p) > 0 && off < r.size {
+		chunkIndex := off / ChunkSize
+		chunkOff := chunkIndex * encChunkSize
+		encSize := r.size + r.chunks*chacha20poly1305.Overhead
+		chunkSize := min(encSize-chunkOff, encChunkSize)
+
+		cached := r.cache.Load()
+		var plaintext []byte
+		if cached != nil && cached.off == chunkOff {
+			plaintext = cached.data
+		} else {
+			nn, err := r.src.ReadAt(chunk[:chunkSize], chunkOff)
+			if err == io.EOF {
+				if int64(nn) != chunkSize {
+					err = io.ErrUnexpectedEOF
+				} else {
+					err = nil
+				}
+			}
+			if err != nil {
+				return n, fmt.Errorf("failed to read chunk at offset %d: %w", chunkOff, err)
+			}
+			nonce := nonceForChunk(chunkIndex)
+			if chunkIndex == r.chunks-1 {
+				setLastChunkFlag(nonce)
+			}
+			plaintext, err = r.a.Open(chunk[:0], nonce[:], chunk[:chunkSize], nil)
+			if err != nil {
+				return n, fmt.Errorf("failed to decrypt and authenticate chunk at offset %d: %w", chunkOff, err)
+			}
+			r.cache.Store(&cachedChunk{off: chunkOff, data: plaintext})
+		}
+
+		plainChunkOff := int(off - chunkIndex*ChunkSize)
+		copySize := min(len(plaintext)-plainChunkOff, len(p))
+		copy(p, plaintext[plainChunkOff:plainChunkOff+copySize])
+		p = p[copySize:]
+		off += int64(copySize)
+		n += copySize
+	}
+	if off == r.size {
+		return n, io.EOF
+	}
+	return n, nil
+}

internal/stream/stream_test.go

@@ -10,6 +10,7 @@ import (
 	"fmt"
 	"io"
 	"testing"
+	"testing/iotest"
 
 	"filippo.io/age/internal/stream"
 	"golang.org/x/crypto/chacha20poly1305"
@@ -20,13 +21,16 @@ const cs = stream.ChunkSize
 func TestRoundTrip(t *testing.T) {
 	for _, length := range []int{0, 1000, cs - 1, cs, cs + 1, cs + 100, 2 * cs, 2*cs + 500} {
 		for _, stepSize := range []int{512, 600, 1000, cs - 1, cs, cs + 1} {
-			t.Run(fmt.Sprintf("len=%d,step=%d", length, stepSize),
-				func(t *testing.T) { testRoundTrip(t, stepSize, length) })
+			t.Run(fmt.Sprintf("len=%d,step=%d", length, stepSize), func(t *testing.T) {
+				testRoundTrip(t, stepSize, length)
+			})
 		}
 	}
+
 	length, stepSize := 2*cs+500, 1
-	t.Run(fmt.Sprintf("len=%d,step=%d", length, stepSize),
-		func(t *testing.T) { testRoundTrip(t, stepSize, length) })
+	t.Run(fmt.Sprintf("len=%d,step=%d", length, stepSize), func(t *testing.T) {
+		testRoundTrip(t, stepSize, length)
+	})
 }
 
 func testRoundTrip(t *testing.T, stepSize, length int) {
@@ -34,85 +38,753 @@ func testRoundTrip(t *testing.T, stepSize, length int) {
 	if _, err := rand.Read(src); err != nil {
 		t.Fatal(err)
 	}
-	buf := &bytes.Buffer{}
+	key := make([]byte, chacha20poly1305.KeySize)
+	if _, err := rand.Read(key); err != nil {
+		t.Fatal(err)
+	}
+	var ciphertext []byte
+
+	t.Run("EncryptWriter", func(t *testing.T) {
+		buf := &bytes.Buffer{}
+		w, err := stream.NewEncryptWriter(key, buf)
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		var n int
+		for n < length {
+			b := min(length-n, stepSize)
+			nn, err := w.Write(src[n : n+b])
+			if err != nil {
+				t.Fatal(err)
+			}
+			if nn != b {
+				t.Errorf("Write returned %d, expected %d", nn, b)
+			}
+			n += nn
+
+			nn, err = w.Write(src[n:n])
+			if err != nil {
+				t.Fatal(err)
+			}
+			if nn != 0 {
+				t.Errorf("Write returned %d, expected 0", nn)
+			}
+		}
+		if err := w.Close(); err != nil {
+			t.Error("Close returned an error:", err)
+		}
+
+		ciphertext = buf.Bytes()
+	})
+
+	t.Run("DecryptReader", func(t *testing.T) {
+		r, err := stream.NewDecryptReader(key, bytes.NewReader(ciphertext))
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		var n int
+		readBuf := make([]byte, stepSize)
+		for n < length {
+			nn, err := r.Read(readBuf)
+			if err != nil {
+				t.Fatalf("Read error at index %d: %v", n, err)
+			}
+
+			if !bytes.Equal(readBuf[:nn], src[n:n+nn]) {
+				t.Errorf("wrong data at indexes %d - %d", n, n+nn)
+			}
+
+			n += nn
+		}
+
+		t.Run("TestReader", func(t *testing.T) {
+			if length > 1000 && testing.Short() {
+				t.Skip("skipping slow iotest.TestReader on long input")
+			}
+			r, _ := stream.NewDecryptReader(key, bytes.NewReader(ciphertext))
+			if err := iotest.TestReader(r, src); err != nil {
+				t.Error("iotest.TestReader error on DecryptReader:", err)
+			}
+		})
+	})
+
+	t.Run("DecryptReaderAt", func(t *testing.T) {
+		rAt, err := stream.NewDecryptReaderAt(key, bytes.NewReader(ciphertext), int64(len(ciphertext)))
+		if err != nil {
+			t.Fatal(err)
+		}
+		rr := io.NewSectionReader(rAt, 0, int64(len(ciphertext)))
+
+		var n int
+		readBuf := make([]byte, stepSize)
+		for n < length {
+			nn, err := rr.Read(readBuf)
+			if n+nn == length && err == io.EOF {
+				err = nil
+			}
+			if err != nil {
+				t.Fatalf("ReadAt error at index %d: %v", n, err)
+			}
+
+			if !bytes.Equal(readBuf[:nn], src[n:n+nn]) {
+				t.Errorf("wrong data at indexes %d - %d", n, n+nn)
+			}
+
+			n += nn
+		}
+
+		t.Run("TestReader", func(t *testing.T) {
+			if length > 1000 && testing.Short() {
+				t.Skip("skipping slow iotest.TestReader on long input")
+			}
+			rr := io.NewSectionReader(rAt, 0, int64(len(src)))
+			if err := iotest.TestReader(rr, src); err != nil {
+				t.Error("iotest.TestReader error on DecryptReaderAt:", err)
+			}
+		})
+	})
+
+	t.Run("EncryptReader", func(t *testing.T) {
+		er, err := stream.NewEncryptReader(key, bytes.NewReader(src))
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		var n int
+		readBuf := make([]byte, stepSize)
+		for {
+			nn, err := er.Read(readBuf)
+			if nn == 0 && err == io.EOF {
+				break
+			} else if err != nil {
+				t.Fatalf("EncryptReader Read error at index %d: %v", n, err)
+			}
+
+			if !bytes.Equal(readBuf[:nn], ciphertext[n:n+nn]) {
+				t.Errorf("EncryptReader wrong data at indexes %d - %d", n, n+nn)
+			}
+
+			n += nn
+		}
+		if n != len(ciphertext) {
+			t.Errorf("EncryptReader read %d bytes, expected %d", n, len(ciphertext))
+		}
+
+		t.Run("TestReader", func(t *testing.T) {
+			if length > 1000 && testing.Short() {
+				t.Skip("skipping slow iotest.TestReader on long input")
+			}
+			er, _ := stream.NewEncryptReader(key, bytes.NewReader(src))
+			if err := iotest.TestReader(er, ciphertext); err != nil {
+				t.Error("iotest.TestReader error on EncryptReader:", err)
+			}
+		})
+	})
+}
+
+// trackingReaderAt wraps an io.ReaderAt and tracks whether ReadAt was called.
+type trackingReaderAt struct {
+	r      io.ReaderAt
+	called bool
+}
+
+func (t *trackingReaderAt) ReadAt(p []byte, off int64) (int, error) {
+	t.called = true
+	return t.r.ReadAt(p, off)
+}
+
+func (t *trackingReaderAt) reset() {
+	t.called = false
+}
+
+func TestDecryptReaderAt(t *testing.T) {
 	key := make([]byte, chacha20poly1305.KeySize)
 	if _, err := rand.Read(key); err != nil {
 		t.Fatal(err)
 	}
 
+	// Create plaintext spanning exactly 3 chunks: 2 full chunks + partial third
+	// Chunk 0: [0, cs)
+	// Chunk 1: [cs, 2*cs)
+	// Chunk 2: [2*cs, 2*cs+500)
+	plaintextSize := 2*cs + 500
+	plaintext := make([]byte, plaintextSize)
+	if _, err := rand.Read(plaintext); err != nil {
+		t.Fatal(err)
+	}
+
+	// Encrypt
+	buf := &bytes.Buffer{}
 	w, err := stream.NewEncryptWriter(key, buf)
 	if err != nil {
 		t.Fatal(err)
 	}
-
-	var n int
-	for n < length {
-		b := min(length-n, stepSize)
-		nn, err := w.Write(src[n : n+b])
-		if err != nil {
-			t.Fatal(err)
-		}
-		if nn != b {
-			t.Errorf("Write returned %d, expected %d", nn, b)
-		}
-		n += nn
-
-		nn, err = w.Write(src[n:n])
-		if err != nil {
-			t.Fatal(err)
-		}
-		if nn != 0 {
-			t.Errorf("Write returned %d, expected 0", nn)
-		}
+	if _, err := w.Write(plaintext); err != nil {
+		t.Fatal(err)
 	}
-
 	if err := w.Close(); err != nil {
-		t.Error("Close returned an error:", err)
+		t.Fatal(err)
 	}
+	ciphertext := buf.Bytes()
 
-	t.Logf("buffer size: %d", buf.Len())
-	ciphertext := bytes.Clone(buf.Bytes())
+	// Create tracking ReaderAt
+	tracker := &trackingReaderAt{r: bytes.NewReader(ciphertext)}
 
-	r, err := stream.NewDecryptReader(key, buf)
+	// Create DecryptReaderAt (this reads and caches the final chunk)
+	ra, err := stream.NewDecryptReaderAt(key, tracker, int64(len(ciphertext)))
 	if err != nil {
 		t.Fatal(err)
 	}
+	tracker.reset()
 
-	n = 0
-	readBuf := make([]byte, stepSize)
-	for n < length {
-		nn, err := r.Read(readBuf)
-		if err != nil {
-			t.Fatalf("Read error at index %d: %v", n, err)
+	// Helper to check reads
+	checkRead := func(name string, off int64, size int, wantN int, wantEOF bool, wantSrcRead bool) {
+		t.Helper()
+		tracker.reset()
+		p := make([]byte, size)
+		n, err := ra.ReadAt(p, off)
+
+		if wantEOF {
+			if err != io.EOF {
+				t.Errorf("%s: got err=%v, want EOF", name, err)
+			}
+		} else {
+			if err != nil {
+				t.Errorf("%s: got err=%v, want nil", name, err)
+			}
 		}
 
-		if !bytes.Equal(readBuf[:nn], src[n:n+nn]) {
-			t.Errorf("wrong data at indexes %d - %d", n, n+nn)
+		if n != wantN {
+			t.Errorf("%s: got n=%d, want %d", name, n, wantN)
 		}
 
-		n += nn
+		if tracker.called != wantSrcRead {
+			t.Errorf("%s: src.ReadAt called=%v, want %v", name, tracker.called, wantSrcRead)
+		}
+
+		// Verify data correctness
+		if n > 0 && off >= 0 && off < int64(plaintextSize) {
+			end := int(off) + n
+			if end > plaintextSize {
+				end = plaintextSize
+			}
+			if !bytes.Equal(p[:n], plaintext[off:end]) {
+				t.Errorf("%s: data mismatch", name)
+			}
+		}
 	}
 
-	er, err := stream.NewEncryptReader(key, bytes.NewReader(src))
+	// Test 1: Read from final chunk (cached by constructor)
+	checkRead("final chunk (cached)", int64(2*cs+100), 100, 100, false, false)
+
+	// Test 2: Read spanning second and third chunk
+	checkRead("span chunks 1-2", int64(cs+cs-50), 100, 100, false, true)
+
+	// Test 3: Read from final chunk again (cached from test 2)
+	// When reading across chunks 1-2 in test 2, the loop processes chunk 1 then chunk 2,
+	// so chunk 2 ends up in the cache.
+	checkRead("final chunk after span", int64(2*cs+200), 100, 100, false, false)
+
+	// Test 4: Read from final chunk again (now cached)
+	checkRead("final chunk (cached again)", int64(2*cs+50), 50, 50, false, false)
+
+	// Test 5: Read from first chunk (not cached)
+	checkRead("first chunk", 0, 100, 100, false, true)
+
+	// Test 6: Read from first chunk again (now cached)
+	checkRead("first chunk (cached)", 50, 100, 100, false, false)
+
+	// Test 7: Read spanning all chunks
+	tracker.reset()
+	p := make([]byte, plaintextSize)
+	n, err := ra.ReadAt(p, 0)
+	if err != io.EOF {
+		t.Errorf("span all: got err=%v, want EOF", err)
+	}
+	if n != plaintextSize {
+		t.Errorf("span all: got n=%d, want %d", n, plaintextSize)
+	}
+	if !bytes.Equal(p, plaintext) {
+		t.Errorf("span all: data mismatch")
+	}
+
+	// Test 8: Read beyond the end (offset > size)
+	tracker.reset()
+	p = make([]byte, 100)
+	n, err = ra.ReadAt(p, int64(plaintextSize+100))
+	if err == nil {
+		t.Error("beyond end: expected error, got nil")
+	}
+	if n != 0 {
+		t.Errorf("beyond end: got n=%d, want 0", n)
+	}
+
+	// Test 9: Read with off = size (should return 0, EOF)
+	tracker.reset()
+	p = make([]byte, 100)
+	n, err = ra.ReadAt(p, int64(plaintextSize))
+	if err != io.EOF {
+		t.Errorf("off=size: got err=%v, want EOF", err)
+	}
+	if n != 0 {
+		t.Errorf("off=size: got n=%d, want 0", n)
+	}
+
+	// Test 10: Read spanning last chunk and beyond
+	tracker.reset()
+	p = make([]byte, 1000) // request more than available
+	n, err = ra.ReadAt(p, int64(2*cs+400))
+	if err != io.EOF {
+		t.Errorf("span last+beyond: got err=%v, want EOF", err)
+	}
+	wantN := 500 - 400 // only 100 bytes available from offset 2*cs+400
+	if n != wantN {
+		t.Errorf("span last+beyond: got n=%d, want %d", n, wantN)
+	}
+	if !bytes.Equal(p[:n], plaintext[2*cs+400:]) {
+		t.Error("span last+beyond: data mismatch")
+	}
+
+	// Test 11: Read spanning second+last chunk and beyond
+	tracker.reset()
+	p = make([]byte, cs+1000) // request more than available
+	n, err = ra.ReadAt(p, int64(cs+100))
+	if err != io.EOF {
+		t.Errorf("span 1-2+beyond: got err=%v, want EOF", err)
+	}
+	wantN = plaintextSize - (cs + 100)
+	if n != wantN {
+		t.Errorf("span 1-2+beyond: got n=%d, want %d", n, wantN)
+	}
+	if !bytes.Equal(p[:n], plaintext[cs+100:]) {
+		t.Error("span 1-2+beyond: data mismatch")
+	}
+
+	// Test 12: Negative offset
+	tracker.reset()
+	p = make([]byte, 100)
+	n, err = ra.ReadAt(p, -1)
+	if err == nil {
+		t.Error("negative offset: expected error, got nil")
+	}
+	if n != 0 {
+		t.Errorf("negative offset: got n=%d, want 0", n)
+	}
+
+	// Test 13: Zero-length read in the middle
+	tracker.reset()
+	p = make([]byte, 0)
+	n, err = ra.ReadAt(p, 100)
+	if err != nil {
+		t.Errorf("zero-length middle: got err=%v, want nil", err)
+	}
+	if n != 0 {
+		t.Errorf("zero-length middle: got n=%d, want 0", n)
+	}
+
+	// Test 14: Zero-length read at end
+	tracker.reset()
+	p = make([]byte, 0)
+	n, err = ra.ReadAt(p, int64(plaintextSize))
+	if err != nil {
+		t.Errorf("zero-length end: got err=%v, want nil", err)
+	}
+	if n != 0 {
+		t.Errorf("zero-length end: got n=%d, want 0", n)
+	}
+
+	// Test 15: Read exactly one chunk at chunk boundary
+	checkRead("exact chunk at boundary", int64(cs), cs, cs, false, true)
+
+	// Test 16: Read one byte at each chunk boundary
+	checkRead("one byte at start", 0, 1, 1, false, true)
+	checkRead("one byte at cs-1", int64(cs-1), 1, 1, false, false) // cached from test 15
+	checkRead("one byte at cs", int64(cs), 1, 1, false, true)
+	checkRead("one byte at 2*cs-1", int64(2*cs-1), 1, 1, false, false) // same chunk
+	checkRead("one byte at 2*cs", int64(2*cs), 1, 1, false, true)
+	checkRead("last byte", int64(plaintextSize-1), 1, 1, true, false) // same chunk, EOF because we reach end
+
+	// Test 17: Read crossing exactly one chunk boundary
+	checkRead("cross boundary 0-1", int64(cs-50), 100, 100, false, true)
+	checkRead("cross boundary 1-2", int64(2*cs-50), 100, 100, false, true)
+}
+
+func TestDecryptReaderAtEmpty(t *testing.T) {
+	key := make([]byte, chacha20poly1305.KeySize)
+	if _, err := rand.Read(key); err != nil {
+		t.Fatal(err)
+	}
+
+	// Create empty encrypted file
+	buf := &bytes.Buffer{}
+	w, err := stream.NewEncryptWriter(key, buf)
 	if err != nil {
 		t.Fatal(err)
 	}
-	n = 0
-	for {
-		nn, err := er.Read(readBuf)
-		if nn == 0 && err == io.EOF {
-			break
-		} else if err != nil {
-			t.Fatalf("EncryptReader Read error at index %d: %v", n, err)
-		}
-
-		if !bytes.Equal(readBuf[:nn], ciphertext[n:n+nn]) {
-			t.Errorf("EncryptReader wrong data at indexes %d - %d", n, n+nn)
-		}
-
-		n += nn
+	if err := w.Close(); err != nil {
+		t.Fatal(err)
 	}
-	if n != len(ciphertext) {
-		t.Errorf("EncryptReader read %d bytes, expected %d", n, len(ciphertext))
+	ciphertext := buf.Bytes()
+
+	tracker := &trackingReaderAt{r: bytes.NewReader(ciphertext)}
+	ra, err := stream.NewDecryptReaderAt(key, tracker, int64(len(ciphertext)))
+	if err != nil {
+		t.Fatal(err)
+	}
+	tracker.reset()
+
+	// Test 1: Read from empty file at offset 0
+	p := make([]byte, 100)
+	n, err := ra.ReadAt(p, 0)
+	if err != io.EOF {
+		t.Errorf("empty read: got err=%v, want EOF", err)
+	}
+	if n != 0 {
+		t.Errorf("empty read: got n=%d, want 0", n)
+	}
+
+	// Test 2: Zero-length read from empty file
+	p = make([]byte, 0)
+	n, err = ra.ReadAt(p, 0)
+	if err != nil {
+		t.Errorf("empty zero-length: got err=%v, want nil", err)
+	}
+	if n != 0 {
+		t.Errorf("empty zero-length: got n=%d, want 0", n)
+	}
+
+	// Test 3: Read beyond empty file
+	p = make([]byte, 100)
+	n, err = ra.ReadAt(p, 1)
+	if err == nil {
+		t.Error("empty beyond: expected error, got nil")
+	}
+	if n != 0 {
+		t.Errorf("empty beyond: got n=%d, want 0", n)
+	}
+}
+
+func TestDecryptReaderAtSingleChunk(t *testing.T) {
+	key := make([]byte, chacha20poly1305.KeySize)
+	if _, err := rand.Read(key); err != nil {
+		t.Fatal(err)
+	}
+
+	// Single chunk, not full
+	plaintext := make([]byte, 1000)
+	if _, err := rand.Read(plaintext); err != nil {
+		t.Fatal(err)
+	}
+
+	buf := &bytes.Buffer{}
+	w, err := stream.NewEncryptWriter(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)
+	}
+	ciphertext := buf.Bytes()
+
+	tracker := &trackingReaderAt{r: bytes.NewReader(ciphertext)}
+	ra, err := stream.NewDecryptReaderAt(key, tracker, int64(len(ciphertext)))
+	if err != nil {
+		t.Fatal(err)
+	}
+	tracker.reset()
+
+	// All reads should use cache (final chunk = only chunk)
+	p := make([]byte, 100)
+	n, err := ra.ReadAt(p, 0)
+	if err != nil {
+		t.Errorf("single chunk start: got err=%v, want nil", err)
+	}
+	if n != 100 {
+		t.Errorf("single chunk start: got n=%d, want 100", n)
+	}
+	if tracker.called {
+		t.Error("single chunk start: unexpected src.ReadAt call")
+	}
+	if !bytes.Equal(p[:n], plaintext[:100]) {
+		t.Error("single chunk start: data mismatch")
+	}
+
+	// Read at end
+	n, err = ra.ReadAt(p, 900)
+	if err != io.EOF {
+		t.Errorf("single chunk end: got err=%v, want EOF", err)
+	}
+	if n != 100 {
+		t.Errorf("single chunk end: got n=%d, want 100", n)
+	}
+	if tracker.called {
+		t.Error("single chunk end: unexpected src.ReadAt call")
+	}
+	if !bytes.Equal(p[:n], plaintext[900:]) {
+		t.Error("single chunk end: data mismatch")
+	}
+}
+
+func TestDecryptReaderAtFullChunks(t *testing.T) {
+	key := make([]byte, chacha20poly1305.KeySize)
+	if _, err := rand.Read(key); err != nil {
+		t.Fatal(err)
+	}
+
+	// Exactly 2 full chunks
+	plaintext := make([]byte, 2*cs)
+	if _, err := rand.Read(plaintext); err != nil {
+		t.Fatal(err)
+	}
+
+	buf := &bytes.Buffer{}
+	w, err := stream.NewEncryptWriter(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)
+	}
+	ciphertext := buf.Bytes()
+
+	tracker := &trackingReaderAt{r: bytes.NewReader(ciphertext)}
+	ra, err := stream.NewDecryptReaderAt(key, tracker, int64(len(ciphertext)))
+	if err != nil {
+		t.Fatal(err)
+	}
+	tracker.reset()
+
+	// Read last byte of second chunk (cached)
+	p := make([]byte, 1)
+	n, err := ra.ReadAt(p, int64(2*cs-1))
+	if err != io.EOF {
+		t.Errorf("last byte: got err=%v, want EOF", err)
+	}
+	if n != 1 {
+		t.Errorf("last byte: got n=%d, want 1", n)
+	}
+	if tracker.called {
+		t.Error("last byte: unexpected src.ReadAt call (should be cached)")
+	}
+	if p[0] != plaintext[2*cs-1] {
+		t.Error("last byte: data mismatch")
+	}
+
+	// Read at exactly the boundary between chunks
+	p = make([]byte, 100)
+	n, err = ra.ReadAt(p, int64(cs-50))
+	if err != nil {
+		t.Errorf("boundary: got err=%v, want nil", err)
+	}
+	if n != 100 {
+		t.Errorf("boundary: got n=%d, want 100", n)
+	}
+	if !bytes.Equal(p, plaintext[cs-50:cs+50]) {
+		t.Error("boundary: data mismatch")
+	}
+}
+
+func TestDecryptReaderAtWrongKey(t *testing.T) {
+	key := make([]byte, chacha20poly1305.KeySize)
+	if _, err := rand.Read(key); err != nil {
+		t.Fatal(err)
+	}
+
+	plaintext := make([]byte, 1000)
+	if _, err := rand.Read(plaintext); err != nil {
+		t.Fatal(err)
+	}
+
+	buf := &bytes.Buffer{}
+	w, err := stream.NewEncryptWriter(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)
+	}
+	ciphertext := buf.Bytes()
+
+	// Try to decrypt with wrong key
+	wrongKey := make([]byte, chacha20poly1305.KeySize)
+	if _, err := rand.Read(wrongKey); err != nil {
+		t.Fatal(err)
+	}
+
+	_, err = stream.NewDecryptReaderAt(wrongKey, bytes.NewReader(ciphertext), int64(len(ciphertext)))
+	if err == nil {
+		t.Error("wrong key: expected error, got nil")
+	}
+}
+
+func TestDecryptReaderAtInvalidSize(t *testing.T) {
+	key := make([]byte, chacha20poly1305.KeySize)
+	if _, err := rand.Read(key); err != nil {
+		t.Fatal(err)
+	}
+
+	plaintext := make([]byte, 1000)
+	if _, err := rand.Read(plaintext); err != nil {
+		t.Fatal(err)
+	}
+
+	buf := &bytes.Buffer{}
+	w, err := stream.NewEncryptWriter(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)
+	}
+	ciphertext := buf.Bytes()
+
+	// Wrong size (too small)
+	_, err = stream.NewDecryptReaderAt(key, bytes.NewReader(ciphertext), int64(len(ciphertext)-1))
+	if err == nil {
+		t.Error("wrong size (small): expected error, got nil")
+	}
+
+	// Wrong size (too large)
+	_, err = stream.NewDecryptReaderAt(key, bytes.NewReader(ciphertext), int64(len(ciphertext)+1))
+	if err == nil {
+		t.Error("wrong size (large): expected error, got nil")
+	}
+
+	// Size that would imply empty final chunk (invalid)
+	// This would be: one full encrypted chunk + just overhead
+	invalidSize := int64(cs + chacha20poly1305.Overhead + chacha20poly1305.Overhead)
+	_, err = stream.NewDecryptReaderAt(key, bytes.NewReader(make([]byte, invalidSize)), invalidSize)
+	if err == nil {
+		t.Error("invalid size (empty final chunk): expected error, got nil")
+	}
+}
+
+func TestDecryptReaderAtTruncated(t *testing.T) {
+	key := make([]byte, chacha20poly1305.KeySize)
+	if _, err := rand.Read(key); err != nil {
+		t.Fatal(err)
+	}
+
+	plaintext := make([]byte, 2*cs+500)
+	if _, err := rand.Read(plaintext); err != nil {
+		t.Fatal(err)
+	}
+
+	buf := &bytes.Buffer{}
+	w, err := stream.NewEncryptWriter(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)
+	}
+	ciphertext := buf.Bytes()
+
+	// Truncate ciphertext but lie about size
+	truncated := ciphertext[:len(ciphertext)-100]
+	_, err = stream.NewDecryptReaderAt(key, bytes.NewReader(truncated), int64(len(ciphertext)))
+	if err == nil {
+		t.Error("truncated: expected error, got nil")
+	}
+}
+
+func TestDecryptReaderAtTruncatedChunk(t *testing.T) {
+	key := make([]byte, chacha20poly1305.KeySize)
+	if _, err := rand.Read(key); err != nil {
+		t.Fatal(err)
+	}
+
+	// Create 4 chunks: 3 full + 1 partial
+	plaintext := make([]byte, 3*cs+500)
+	if _, err := rand.Read(plaintext); err != nil {
+		t.Fatal(err)
+	}
+
+	buf := &bytes.Buffer{}
+	w, err := stream.NewEncryptWriter(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)
+	}
+	ciphertext := buf.Bytes()
+
+	// Truncate to 3 chunks (remove the actual final chunk)
+	// The third chunk was NOT encrypted with the last chunk flag,
+	// so decryption should fail when we try to use it as the final chunk.
+	encChunkSize := cs + 16 // ChunkSize + Overhead
+	truncatedSize := int64(3 * encChunkSize)
+	truncated := ciphertext[:truncatedSize]
+
+	_, err = stream.NewDecryptReaderAt(key, bytes.NewReader(truncated), truncatedSize)
+	if err == nil {
+		t.Error("truncated at chunk boundary: expected error, got nil")
+	}
+}
+
+func TestDecryptReaderAtCorrupted(t *testing.T) {
+	key := make([]byte, chacha20poly1305.KeySize)
+	if _, err := rand.Read(key); err != nil {
+		t.Fatal(err)
+	}
+
+	plaintext := make([]byte, 2*cs+500)
+	if _, err := rand.Read(plaintext); err != nil {
+		t.Fatal(err)
+	}
+
+	buf := &bytes.Buffer{}
+	w, err := stream.NewEncryptWriter(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)
+	}
+	ciphertext := bytes.Clone(buf.Bytes())
+
+	// Corrupt final chunk - should fail in constructor
+	corruptedFinal := bytes.Clone(ciphertext)
+	corruptedFinal[len(corruptedFinal)-10] ^= 0xFF
+	_, err = stream.NewDecryptReaderAt(key, bytes.NewReader(corruptedFinal), int64(len(corruptedFinal)))
+	if err == nil {
+		t.Error("corrupted final: expected error, got nil")
+	}
+
+	// Corrupt first chunk - should fail on read
+	corruptedFirst := bytes.Clone(ciphertext)
+	corruptedFirst[10] ^= 0xFF
+	ra, err := stream.NewDecryptReaderAt(key, bytes.NewReader(corruptedFirst), int64(len(corruptedFirst)))
+	if err != nil {
+		t.Fatalf("corrupted first constructor: unexpected error: %v", err)
+	}
+	p := make([]byte, 100)
+	_, err = ra.ReadAt(p, 0)
+	if err == nil {
+		t.Error("corrupted first read: expected error, got nil")
 	}
 }

testkit_test.go

@@ -140,10 +140,16 @@ func parseVector(t *testing.T, test []byte) *vector {
 }
 
 func TestVectors(t *testing.T) {
-	forEachVector(t, testVector)
+	forEachVector(t, func(t *testing.T, v *vector) {
+		var plaintext []byte
+		t.Run("Decrypt", func(t *testing.T) { plaintext = testDecrypt(t, v) })
+		t.Run("DecryptReaderAt", func(t *testing.T) { testDecryptReaderAt(t, v, plaintext) })
+		t.Run("Inspect", func(t *testing.T) { testInspect(t, v, plaintext) })
+		t.Run("RoundTrip", func(t *testing.T) { testVectorRoundTrip(t, v) })
+	})
 }
 
-func testVector(t *testing.T, v *vector) {
+func testDecrypt(t *testing.T, v *vector) []byte {
 	var in io.Reader = bytes.NewReader(v.file)
 	if v.armored {
 		in = armor.NewReader(in)
@@ -152,25 +158,25 @@ func testVector(t *testing.T, v *vector) {
 	if err != nil && strings.HasSuffix(err.Error(), "bad header MAC") {
 		if v.expect == "HMAC failure" {
 			t.Log(err)
-			return
+			return nil
 		}
 		t.Fatalf("expected %s, got HMAC error", v.expect)
 	} else if e := new(armor.Error); errors.As(err, &e) {
 		if v.expect == "armor failure" {
 			t.Log(err)
-			return
+			return nil
 		}
 		t.Fatalf("expected %s, got: %v", v.expect, err)
 	} else if _, ok := err.(*age.NoIdentityMatchError); ok {
 		if v.expect == "no match" {
 			t.Log(err)
-			return
+			return nil
 		}
 		t.Fatalf("expected %s, got: %v", v.expect, err)
 	} else if err != nil {
 		if v.expect == "header failure" {
 			t.Log(err)
-			return
+			return nil
 		}
 		t.Fatalf("expected %s, got: %v", v.expect, err)
 	} else if v.expect != "success" && v.expect != "payload failure" &&
@@ -188,15 +194,77 @@ func testVector(t *testing.T, v *vector) {
 			}
 		}
 		if v.payloadHash != nil && sha256.Sum256(out) != *v.payloadHash {
-			t.Error("partial payload hash mismatch")
+			t.Errorf("partial payload hash mismatch, read %d bytes", len(out))
 		}
-		return
+		return out
 	} else if v.expect != "success" {
 		t.Fatalf("expected %s, got success", v.expect)
 	}
 	if sha256.Sum256(out) != *v.payloadHash {
 		t.Error("payload hash mismatch")
 	}
+	return out
+}
+
+func testDecryptReaderAt(t *testing.T, v *vector, plaintext []byte) {
+	if v.armored {
+		t.Skip("armor.NewReader does not implement ReaderAt")
+	}
+	rAt, s, err := age.DecryptReaderAt(bytes.NewReader(v.file), int64(len(v.file)), v.identities...)
+	switch v.expect {
+	case "success":
+		if err != nil {
+			t.Fatalf("expected success, got: %v", err)
+		}
+		if int64(len(plaintext)) != s {
+			t.Errorf("unexpected size: got %d, want %d", s, len(plaintext))
+		}
+	case "payload failure":
+		// DecryptReaderAt detects some (but not all) payload failures upfront,
+		// either from the size of the payload, or by decrypting the last chunk
+		// to authenticate its size.
+		if err != nil {
+			t.Log(err)
+			return
+		}
+	default:
+		if err != nil {
+			t.Log(err)
+			return
+		}
+		t.Fatalf("expected %s, got success", v.expect)
+	}
+	out, err := io.ReadAll(io.NewSectionReader(rAt, 0, s))
+	if v.expect == "success" {
+		if err != nil {
+			t.Fatalf("expected success, got: %v", err)
+		}
+	} else {
+		if err == nil {
+			t.Fatalf("expected %s, got success", v.expect)
+		}
+		t.Log(err)
+		// We can't check the partial payload hash, because the ReaderAt will
+		// notice errors that a linearly scanning Reader could not. For example,
+		// if there are two final chunks, the linear Reader will decrypt the
+		// first one and then error out on the second, while the ReaderAt will
+		// decrypt the second one to check the size, and then know that the
+		// first chunk could not be the last one. Instead, check that the
+		// prefix, if any, matches.
+		if !bytes.HasPrefix(plaintext, out) {
+			t.Errorf("partial payload prefix mismatch, read %d bytes", len(out))
+		}
+		return
+	}
+	if sha256.Sum256(out) != *v.payloadHash {
+		t.Error("payload hash mismatch")
+	}
+}
+
+func testInspect(t *testing.T, v *vector, plaintext []byte) {
+	if v.expect != "success" {
+		t.Skip("invalid file, can't inspect")
+	}
 	for _, fileSize := range []int64{int64(len(v.file)), -1} {
 		metadata, err := inspect.Inspect(bytes.NewReader(v.file), fileSize)
 		if err != nil {
@@ -211,8 +279,8 @@ func testVector(t *testing.T, v *vector) {
 		if metadata.Sizes.Armor+metadata.Sizes.Header+metadata.Sizes.Overhead+metadata.Sizes.MinPayload != int64(len(v.file)) {
 			t.Errorf("size breakdown does not add up to file size")
 		}
-		if metadata.Sizes.MinPayload != int64(len(out)) {
-			t.Errorf("unexpected payload size: got %d, want %d", metadata.Sizes.MinPayload, len(out))
+		if metadata.Sizes.MinPayload != int64(len(plaintext)) {
+			t.Errorf("unexpected payload size: got %d, want %d", metadata.Sizes.MinPayload, len(plaintext))
 		}
 		if metadata.Sizes.MaxPayload != metadata.Sizes.MinPayload {
 			t.Errorf("unexpected max payload size: got %d, want %d", metadata.Sizes.MaxPayload, metadata.Sizes.MinPayload)
@@ -223,16 +291,12 @@ func testVector(t *testing.T, v *vector) {
 	}
 }
 
-// TestVectorsRoundTrip checks that any (valid) armor, header, and/or STREAM
+// 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 {
 		if v.expect == "armor failure" {
-			t.SkipNow()
+			t.Skip("invalid armor, nothing to round-trip")
 		}
 		t.Run("armor", func(t *testing.T) {
 			payload, err := io.ReadAll(armor.NewReader(bytes.NewReader(v.file)))
@@ -261,7 +325,7 @@ func testVectorRoundTrip(t *testing.T, v *vector) {
 	}
 
 	if v.expect == "header failure" {
-		t.SkipNow()
+		t.Skip("invalid header, nothing to round-trip")
 	}
 	hdr, p, err := format.Parse(bytes.NewReader(v.file))
 	if err != nil {
@@ -283,46 +347,62 @@ func testVectorRoundTrip(t *testing.T, v *vector) {
 		}
 	})
 
-	if v.expect == "success" {
-		t.Run("STREAM", func(t *testing.T) {
-			nonce, payload := payload[:16], payload[16:]
-			key := streamKey(v.fileKey[:], nonce)
-			r, err := stream.NewDecryptReader(key, bytes.NewReader(payload))
-			if err != nil {
-				t.Fatal(err)
-			}
-			plaintext, err := io.ReadAll(r)
-			if err != nil {
-				t.Fatal(err)
-			}
-			buf := &bytes.Buffer{}
-			w, err := stream.NewEncryptWriter(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")
-			}
-			buf.Reset()
-			er, err := stream.NewEncryptReader(key, bytes.NewReader(plaintext))
-			if err != nil {
-				t.Fatal(err)
-			}
-			ciphertext, err := io.ReadAll(er)
-			if err != nil {
-				t.Fatal(err)
-			}
-			if !bytes.Equal(ciphertext, payload) {
-				t.Error("got a different STREAM ciphertext from EncryptReader")
-			}
-		})
+	if v.expect != "success" {
+		return
 	}
+
+	t.Run("STREAM", func(t *testing.T) {
+		nonce, payload := payload[:16], payload[16:]
+		key := streamKey(v.fileKey[:], nonce)
+
+		r, err := stream.NewDecryptReader(key, bytes.NewReader(payload))
+		if err != nil {
+			t.Fatal(err)
+		}
+		plaintext, err := io.ReadAll(r)
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		rAt, err := stream.NewDecryptReaderAt(key, bytes.NewReader(payload), int64(len(payload)))
+		if err != nil {
+			t.Fatal(err)
+		}
+		plaintextAt, err := io.ReadAll(io.NewSectionReader(rAt, 0, int64(len(plaintext))))
+		if err != nil {
+			t.Fatal(err)
+		}
+		if !bytes.Equal(plaintextAt, plaintext) {
+			t.Errorf("got a different plaintext from DecryptReaderAt")
+		}
+
+		buf := &bytes.Buffer{}
+		w, err := stream.NewEncryptWriter(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")
+		}
+
+		er, err := stream.NewEncryptReader(key, bytes.NewReader(plaintext))
+		if err != nil {
+			t.Fatal(err)
+		}
+		ciphertext, err := io.ReadAll(er)
+		if err != nil {
+			t.Fatal(err)
+		}
+		if !bytes.Equal(ciphertext, payload) {
+			t.Error("got a different STREAM ciphertext from EncryptReader")
+		}
+	})
 }
 
 func streamKey(fileKey, nonce []byte) []byte {