diff --git a/internal/archiver/archiver_test.go b/internal/archiver/archiver_test.go
index 01e9d9e74..293de9152 100644
--- a/internal/archiver/archiver_test.go
+++ b/internal/archiver/archiver_test.go
@@ -32,6 +32,7 @@ type Rdr interface {
 func benchmarkChunkEncrypt(b testing.TB, buf, buf2 []byte, rd Rdr, key *crypto.Key) {
 	rd.Seek(0, 0)
 	ch := chunker.New(rd, testPol)
+	nonce := crypto.NewRandomNonce()
 
 	for {
 		chunk, err := ch.Next(buf)
@@ -42,12 +43,10 @@ func benchmarkChunkEncrypt(b testing.TB, buf, buf2 []byte, rd Rdr, key *crypto.K
 
 		rtest.OK(b, err)
 
-		// reduce length of buf
 		rtest.Assert(b, uint(len(chunk.Data)) == chunk.Length,
 			"invalid length: got %d, expected %d", len(chunk.Data), chunk.Length)
 
-		_, err = key.Encrypt(buf2, chunk.Data)
-		rtest.OK(b, err)
+		_ = key.Seal(buf2[:0], nonce, chunk.Data, nil)
 	}
 }
 
@@ -71,6 +70,7 @@ func BenchmarkChunkEncrypt(b *testing.B) {
 
 func benchmarkChunkEncryptP(b *testing.PB, buf []byte, rd Rdr, key *crypto.Key) {
 	ch := chunker.New(rd, testPol)
+	nonce := crypto.NewRandomNonce()
 
 	for {
 		chunk, err := ch.Next(buf)
@@ -78,8 +78,7 @@ func benchmarkChunkEncryptP(b *testing.PB, buf []byte, rd Rdr, key *crypto.Key)
 			break
 		}
 
-		// reduce length of chunkBuf
-		key.Encrypt(chunk.Data, chunk.Data)
+		_ = key.Seal(chunk.Data[:0], nonce, chunk.Data, nil)
 	}
 }
 
diff --git a/internal/checker/checker.go b/internal/checker/checker.go
index c557ab475..efef7ae94 100644
--- a/internal/checker/checker.go
+++ b/internal/checker/checker.go
@@ -724,15 +724,15 @@ func checkPack(ctx context.Context, r restic.Repository, id restic.ID) error {
 			continue
 		}
 
-		n, err := r.Key().Decrypt(buf, buf)
+		nonce, ciphertext := buf[:r.Key().NonceSize()], buf[r.Key().NonceSize():]
+		plaintext, err := r.Key().Open(ciphertext[:0], nonce, ciphertext, nil)
 		if err != nil {
 			debug.Log("  error decrypting blob %v: %v", blob.ID.Str(), err)
 			errs = append(errs, errors.Errorf("blob %v: %v", i, err))
 			continue
 		}
-		buf = buf[:n]
 
-		hash := restic.Hash(buf)
+		hash := restic.Hash(plaintext)
 		if !hash.Equal(blob.ID) {
 			debug.Log("  Blob ID does not match, want %v, got %v", blob.ID.Str(), hash.Str())
 			errs = append(errs, errors.Errorf("Blob ID does not match, want %v, got %v", blob.ID.Str(), hash.Str()))
diff --git a/internal/crypto/crypto.go b/internal/crypto/crypto.go
index aaa42b54f..b56a9da2e 100644
--- a/internal/crypto/crypto.go
+++ b/internal/crypto/crypto.go
@@ -147,7 +147,9 @@ func NewRandomKey() *Key {
 	return k
 }
 
-func newIV() []byte {
+// NewRandomNonce returns a new random nonce. It panics on error so that the
+// program is safely terminated.
+func NewRandomNonce() []byte {
 	iv := make([]byte, ivSize)
 	n, err := rand.Read(iv)
 	if n != ivSize || err != nil {
@@ -233,91 +235,134 @@ func (k *EncryptionKey) Valid() bool {
 // holds the plaintext.
 var ErrInvalidCiphertext = errors.New("invalid ciphertext, same slice used for plaintext")
 
-// Encrypt encrypts and authenticates data. Stored in ciphertext is IV || Ciphertext ||
-// MAC. Encrypt returns the new ciphertext slice, which is extended when
-// necessary. ciphertext and plaintext may not point to (exactly) the same
-// slice or non-intersecting slices.
-func (k *Key) Encrypt(ciphertext []byte, plaintext []byte) ([]byte, error) {
+// validNonce checks that nonce is not all zero.
+func validNonce(nonce []byte) bool {
+	var sum byte
+	for _, b := range nonce {
+		sum |= b
+	}
+	return sum > 0
+}
+
+// statically ensure that *Key implements crypto/cipher.AEAD
+var _ cipher.AEAD = &Key{}
+
+// NonceSize returns the size of the nonce that must be passed to Seal
+// and Open.
+func (k *Key) NonceSize() int {
+	return ivSize
+}
+
+// Overhead returns the maximum difference between the lengths of a
+// plaintext and its ciphertext.
+func (k *Key) Overhead() int {
+	return macSize
+}
+
+// sliceForAppend takes a slice and a requested number of bytes. It returns a
+// slice with the contents of the given slice followed by that many bytes and a
+// second slice that aliases into it and contains only the extra bytes. If the
+// original slice has sufficient capacity then no allocation is performed.
+//
+// taken from the stdlib, crypto/aes/aes_gcm.go
+func sliceForAppend(in []byte, n int) (head, tail []byte) {
+	if total := len(in) + n; cap(in) >= total {
+		head = in[:total]
+	} else {
+		head = make([]byte, total)
+		copy(head, in)
+	}
+	tail = head[len(in):]
+	return
+}
+
+// Seal encrypts and authenticates plaintext, authenticates the
+// additional data and appends the result to dst, returning the updated
+// slice. The nonce must be NonceSize() bytes long and unique for all
+// time, for a given key.
+//
+// The plaintext and dst may alias exactly or not at all. To reuse
+// plaintext's storage for the encrypted output, use plaintext[:0] as dst.
+func (k *Key) Seal(dst, nonce, plaintext, additionalData []byte) []byte {
+	if !k.Valid() {
+		panic("key is invalid")
+	}
+
+	if len(additionalData) > 0 {
+		panic("additional data is not supported")
+	}
+
+	if len(nonce) != ivSize {
+		panic("incorrect nonce length")
+	}
+
+	if !validNonce(nonce) {
+		panic("nonce is invalid")
+	}
+
+	ret, out := sliceForAppend(dst, len(plaintext)+k.Overhead())
+
+	c, err := aes.NewCipher(k.EncryptionKey[:])
+	if err != nil {
+		panic(fmt.Sprintf("unable to create cipher: %v", err))
+	}
+	e := cipher.NewCTR(c, nonce)
+	e.XORKeyStream(out, plaintext)
+
+	mac := poly1305MAC(out[:len(plaintext)], nonce, &k.MACKey)
+	copy(out[len(plaintext):], mac)
+
+	return ret
+}
+
+// Open decrypts and authenticates ciphertext, authenticates the
+// additional data and, if successful, appends the resulting plaintext
+// to dst, returning the updated slice. The nonce must be NonceSize()
+// bytes long and both it and the additional data must match the
+// value passed to Seal.
+//
+// The ciphertext and dst may alias exactly or not at all. To reuse
+// ciphertext's storage for the decrypted output, use ciphertext[:0] as dst.
+//
+// Even if the function fails, the contents of dst, up to its capacity,
+// may be overwritten.
+func (k *Key) Open(dst, nonce, ciphertext, additionalData []byte) ([]byte, error) {
 	if !k.Valid() {
 		return nil, errors.New("invalid key")
 	}
 
-	ciphertext = ciphertext[:cap(ciphertext)]
-
-	// test for same slice, if possible
-	if len(plaintext) > 0 && len(ciphertext) > 0 && &plaintext[0] == &ciphertext[0] {
-		return nil, ErrInvalidCiphertext
+	// check parameters
+	if len(nonce) != ivSize {
+		panic("incorrect nonce length")
 	}
 
-	// extend ciphertext slice if necessary
-	if len(ciphertext) < len(plaintext)+Extension {
-		ext := len(plaintext) + Extension - len(ciphertext)
-		ciphertext = append(ciphertext, make([]byte, ext)...)
-	}
-
-	iv := newIV()
-	copy(ciphertext, iv[:])
-
-	c, err := aes.NewCipher(k.EncryptionKey[:])
-	if err != nil {
-		panic(fmt.Sprintf("unable to create cipher: %v", err))
-	}
-	e := cipher.NewCTR(c, ciphertext[:ivSize])
-	e.XORKeyStream(ciphertext[ivSize:], plaintext)
-
-	// truncate to only cover iv and actual ciphertext
-	ciphertext = ciphertext[:ivSize+len(plaintext)]
-
-	mac := poly1305MAC(ciphertext[ivSize:], ciphertext[:ivSize], &k.MACKey)
-	ciphertext = append(ciphertext, mac...)
-
-	return ciphertext, nil
-}
-
-// Decrypt verifies and decrypts the ciphertext. Ciphertext must be in the form
-// IV || Ciphertext || MAC. plaintext and ciphertext may point to (exactly) the
-// same slice.
-func (k *Key) Decrypt(plaintext []byte, ciphertextWithMac []byte) (int, error) {
-	if !k.Valid() {
-		return 0, errors.New("invalid key")
+	if !validNonce(nonce) {
+		return nil, errors.New("nonce is invalid")
 	}
 
 	// check for plausible length
-	if len(ciphertextWithMac) < Extension {
-		return 0, errors.Errorf("trying to decrypt invalid data: ciphertext too small")
+	if len(ciphertext) < k.Overhead() {
+		return nil, errors.Errorf("trying to decrypt invalid data: ciphertext too small")
 	}
 
-	// check buffer length for plaintext
-	plaintextLength := len(ciphertextWithMac) - Extension
-	if len(plaintext) < plaintextLength {
-		return 0, errors.Errorf("plaintext buffer too small, %d < %d", len(plaintext), plaintextLength)
-	}
-
-	// extract mac
-	l := len(ciphertextWithMac) - macSize
-	ciphertextWithIV, mac := ciphertextWithMac[:l], ciphertextWithMac[l:]
-
-	// extract iv
-	iv, ciphertext := ciphertextWithIV[:ivSize], ciphertextWithIV[ivSize:]
+	l := len(ciphertext) - macSize
+	ct, mac := ciphertext[:l], ciphertext[l:]
 
 	// verify mac
-	if !poly1305Verify(ciphertext, iv, &k.MACKey, mac) {
-		return 0, ErrUnauthenticated
+	if !poly1305Verify(ct, nonce, &k.MACKey, mac) {
+		return nil, ErrUnauthenticated
 	}
 
-	if len(ciphertext) != plaintextLength {
-		panic("plaintext and ciphertext lengths do not match")
-	}
+	ret, out := sliceForAppend(dst, len(ct))
 
-	// decrypt data
 	c, err := aes.NewCipher(k.EncryptionKey[:])
 	if err != nil {
 		panic(fmt.Sprintf("unable to create cipher: %v", err))
 	}
-	e := cipher.NewCTR(c, iv)
-	e.XORKeyStream(plaintext, ciphertext)
+	e := cipher.NewCTR(c, nonce)
+	e.XORKeyStream(out, ct)
 
-	return plaintextLength, nil
+	return ret, nil
 }
 
 // Valid tests if the key is valid.
diff --git a/internal/crypto/crypto_int_test.go b/internal/crypto/crypto_int_test.go
index 3ace3f393..769f34d1e 100644
--- a/internal/crypto/crypto_int_test.go
+++ b/internal/crypto/crypto_int_test.go
@@ -113,46 +113,80 @@ func TestCrypto(t *testing.T) {
 			MACKey:        tv.skey,
 		}
 
-		msg, err := k.Encrypt(msg, tv.plaintext)
-		if err != nil {
-			t.Fatal(err)
-		}
+		nonce := NewRandomNonce()
+		ciphertext := k.Seal(msg[0:], nonce, tv.plaintext, nil)
 
 		// decrypt message
-		buf := make([]byte, len(tv.plaintext))
-		n, err := k.Decrypt(buf, msg)
+		buf := make([]byte, 0, len(tv.plaintext))
+		buf, err := k.Open(buf, nonce, ciphertext, nil)
 		if err != nil {
 			t.Fatal(err)
 		}
-		buf = buf[:n]
 
-		// change mac, this must fail
-		msg[len(msg)-8] ^= 0x23
-
-		if _, err = k.Decrypt(buf, msg); err != ErrUnauthenticated {
-			t.Fatal("wrong MAC value not detected")
+		if !bytes.Equal(buf, tv.plaintext) {
+			t.Fatalf("wrong plaintext returned")
 		}
 
+		// change mac, this must fail
+		ciphertext[len(ciphertext)-8] ^= 0x23
+
+		if _, err = k.Open(buf[:0], nonce, ciphertext, nil); err != ErrUnauthenticated {
+			t.Fatal("wrong MAC value not detected")
+		}
 		// reset mac
-		msg[len(msg)-8] ^= 0x23
+		ciphertext[len(ciphertext)-8] ^= 0x23
+
+		// tamper with nonce, this must fail
+		nonce[2] ^= 0x88
+		if _, err = k.Open(buf[:0], nonce, ciphertext, nil); err != ErrUnauthenticated {
+			t.Fatal("tampered nonce not detected")
+		}
+		// reset nonce
+		nonce[2] ^= 0x88
 
 		// tamper with message, this must fail
-		msg[16+5] ^= 0x85
-
-		if _, err = k.Decrypt(buf, msg); err != ErrUnauthenticated {
+		ciphertext[16+5] ^= 0x85
+		if _, err = k.Open(buf[:0], nonce, ciphertext, nil); err != ErrUnauthenticated {
 			t.Fatal("tampered message not detected")
 		}
 
 		// test decryption
 		p := make([]byte, len(tv.ciphertext))
-		n, err = k.Decrypt(p, tv.ciphertext)
+		nonce, ciphertext = tv.ciphertext[:16], tv.ciphertext[16:]
+		p, err = k.Open(p[:0], nonce, ciphertext, nil)
 		if err != nil {
 			t.Fatal(err)
 		}
-		p = p[:n]
 
 		if !bytes.Equal(p, tv.plaintext) {
 			t.Fatalf("wrong plaintext: expected %q but got %q\n", tv.plaintext, p)
 		}
 	}
 }
+
+func TestNonceVadlid(t *testing.T) {
+	nonce := make([]byte, ivSize)
+
+	if validNonce(nonce) {
+		t.Error("null nonce detected as valid")
+	}
+
+	for i := 0; i < 100; i++ {
+		nonce = NewRandomNonce()
+		if !validNonce(nonce) {
+			t.Errorf("random nonce not detected as valid: %02x", nonce)
+		}
+	}
+}
+
+func BenchmarkNonceValid(b *testing.B) {
+	nonce := NewRandomNonce()
+
+	b.ResetTimer()
+
+	for i := 0; i < b.N; i++ {
+		if !validNonce(nonce) {
+			b.Fatal("nonce is invalid")
+		}
+	}
+}
diff --git a/internal/crypto/crypto_test.go b/internal/crypto/crypto_test.go
index f1a7b7994..47debe896 100644
--- a/internal/crypto/crypto_test.go
+++ b/internal/crypto/crypto_test.go
@@ -24,18 +24,17 @@ func TestEncryptDecrypt(t *testing.T) {
 
 	for _, size := range tests {
 		data := rtest.Random(42, size)
-		buf := make([]byte, size+crypto.Extension)
+		buf := make([]byte, 0, size+crypto.Extension)
 
-		ciphertext, err := k.Encrypt(buf, data)
-		rtest.OK(t, err)
-		rtest.Assert(t, len(ciphertext) == len(data)+crypto.Extension,
+		nonce := crypto.NewRandomNonce()
+		ciphertext := k.Seal(buf[:0], nonce, data, nil)
+		rtest.Assert(t, len(ciphertext) == len(data)+k.Overhead(),
 			"ciphertext length does not match: want %d, got %d",
 			len(data)+crypto.Extension, len(ciphertext))
 
-		plaintext := make([]byte, len(ciphertext))
-		n, err := k.Decrypt(plaintext, ciphertext)
+		plaintext := make([]byte, 0, len(ciphertext))
+		plaintext, err := k.Open(plaintext[:0], nonce, ciphertext, nil)
 		rtest.OK(t, err)
-		plaintext = plaintext[:n]
 		rtest.Assert(t, len(plaintext) == len(data),
 			"plaintext length does not match: want %d, got %d",
 			len(data), len(plaintext))
@@ -52,8 +51,9 @@ func TestSmallBuffer(t *testing.T) {
 	_, err := io.ReadFull(rand.Reader, data)
 	rtest.OK(t, err)
 
-	ciphertext := make([]byte, size/2)
-	ciphertext, err = k.Encrypt(ciphertext, data)
+	ciphertext := make([]byte, 0, size/2)
+	nonce := crypto.NewRandomNonce()
+	ciphertext = k.Seal(ciphertext[:0], nonce, data, nil)
 	// this must extend the slice
 	rtest.Assert(t, cap(ciphertext) > size/2,
 		"expected extended slice, but capacity is only %d bytes",
@@ -61,9 +61,8 @@ func TestSmallBuffer(t *testing.T) {
 
 	// check for the correct plaintext
 	plaintext := make([]byte, len(ciphertext))
-	n, err := k.Decrypt(plaintext, ciphertext)
+	plaintext, err = k.Open(plaintext[:0], nonce, ciphertext, nil)
 	rtest.OK(t, err)
-	plaintext = plaintext[:n]
 	rtest.Assert(t, bytes.Equal(plaintext, data),
 		"wrong plaintext returned")
 }
@@ -78,37 +77,169 @@ func TestSameBuffer(t *testing.T) {
 
 	ciphertext := make([]byte, 0, size+crypto.Extension)
 
-	ciphertext, err = k.Encrypt(ciphertext, data)
-	rtest.OK(t, err)
+	nonce := crypto.NewRandomNonce()
+	ciphertext = k.Seal(ciphertext, nonce, data, nil)
 
 	// use the same buffer for decryption
-	n, err := k.Decrypt(ciphertext, ciphertext)
+	ciphertext, err = k.Open(ciphertext[:0], nonce, ciphertext, nil)
 	rtest.OK(t, err)
-	ciphertext = ciphertext[:n]
 	rtest.Assert(t, bytes.Equal(ciphertext, data),
 		"wrong plaintext returned")
 }
 
-func TestCornerCases(t *testing.T) {
+func encrypt(t testing.TB, k *crypto.Key, data, ciphertext, nonce []byte) []byte {
+	prefixlen := len(ciphertext)
+	ciphertext = k.Seal(ciphertext, nonce, data, nil)
+	if len(ciphertext) != len(data)+k.Overhead()+prefixlen {
+		t.Fatalf("destination slice has wrong length, want %d, got %d",
+			len(data)+k.Overhead(), len(ciphertext))
+	}
+
+	return ciphertext
+}
+
+func decryptNewSliceAndCompare(t testing.TB, k *crypto.Key, data, ciphertext, nonce []byte) {
+	plaintext := make([]byte, 0, len(ciphertext))
+	decryptAndCompare(t, k, data, ciphertext, nonce, plaintext)
+}
+
+func decryptAndCompare(t testing.TB, k *crypto.Key, data, ciphertext, nonce, dst []byte) {
+	prefix := make([]byte, len(dst))
+	copy(prefix, dst)
+
+	plaintext, err := k.Open(dst, nonce, ciphertext, nil)
+	if err != nil {
+		t.Fatalf("unable to decrypt ciphertext: %v", err)
+	}
+
+	if len(data)+len(prefix) != len(plaintext) {
+		t.Fatalf("wrong plaintext returned, want %d bytes, got %d", len(data)+len(prefix), len(plaintext))
+	}
+
+	if !bytes.Equal(plaintext[:len(prefix)], prefix) {
+		t.Fatal("prefix is wrong")
+	}
+
+	if !bytes.Equal(plaintext[len(prefix):], data) {
+		t.Fatal("wrong plaintext returned")
+	}
+}
+
+func TestAppendOpen(t *testing.T) {
+	k := crypto.NewRandomKey()
+	nonce := crypto.NewRandomNonce()
+
+	data := make([]byte, 600)
+	_, err := io.ReadFull(rand.Reader, data)
+	rtest.OK(t, err)
+	ciphertext := encrypt(t, k, data, nil, nonce)
+
+	// we need to test several different cases:
+	//  * destination slice is nil
+	//  * destination slice is empty and has enough capacity
+	//  * destination slice is empty and does not have enough capacity
+	//  * destination slice contains data and has enough capacity
+	//  * destination slice contains data and does not have enough capacity
+
+	// destination slice is nil
+	t.Run("nil", func(t *testing.T) {
+		var plaintext []byte
+		decryptAndCompare(t, k, data, ciphertext, nonce, plaintext)
+	})
+
+	// destination slice is empty and has enough capacity
+	t.Run("empty-large", func(t *testing.T) {
+		plaintext := make([]byte, 0, len(data)+100)
+		decryptAndCompare(t, k, data, ciphertext, nonce, plaintext)
+	})
+
+	// destination slice is empty and does not have enough capacity
+	t.Run("empty-small", func(t *testing.T) {
+		plaintext := make([]byte, 0, len(data)/2)
+		decryptAndCompare(t, k, data, ciphertext, nonce, plaintext)
+	})
+
+	// destination slice contains data and has enough capacity
+	t.Run("prefix-large", func(t *testing.T) {
+		plaintext := make([]byte, 0, len(data)+100)
+		plaintext = append(plaintext, []byte("foobar")...)
+		decryptAndCompare(t, k, data, ciphertext, nonce, plaintext)
+	})
+
+	// destination slice contains data and does not have enough capacity
+	t.Run("prefix-small", func(t *testing.T) {
+		plaintext := make([]byte, 0, len(data)/2)
+		plaintext = append(plaintext, []byte("foobar")...)
+		decryptAndCompare(t, k, data, ciphertext, nonce, plaintext)
+	})
+}
+
+func TestAppendSeal(t *testing.T) {
 	k := crypto.NewRandomKey()
 
-	// nil plaintext should encrypt to the empty string
-	// nil ciphertext should allocate a new slice for the ciphertext
-	c, err := k.Encrypt(nil, nil)
+	data := make([]byte, 600)
+	_, err := io.ReadFull(rand.Reader, data)
 	rtest.OK(t, err)
 
-	rtest.Assert(t, len(c) == crypto.Extension,
-		"wrong length returned for ciphertext, expected 0, got %d",
-		len(c))
+	// we need to test several different cases:
+	//  * destination slice is nil
+	//  * destination slice is empty and has enough capacity
+	//  * destination slice is empty and does not have enough capacity
+	//  * destination slice contains data and has enough capacity
+	//  * destination slice contains data and does not have enough capacity
 
-	// this should decrypt to nil
-	n, err := k.Decrypt(nil, c)
-	rtest.OK(t, err)
-	rtest.Equals(t, 0, n)
+	// destination slice is nil
+	t.Run("nil", func(t *testing.T) {
+		nonce := crypto.NewRandomNonce()
+		var ciphertext []byte
 
-	// test encryption for same slice, this should return an error
-	_, err = k.Encrypt(c, c)
-	rtest.Equals(t, crypto.ErrInvalidCiphertext, err)
+		ciphertext = encrypt(t, k, data, ciphertext, nonce)
+		decryptNewSliceAndCompare(t, k, data, ciphertext, nonce)
+	})
+
+	// destination slice is empty and has enough capacity
+	t.Run("empty-large", func(t *testing.T) {
+		nonce := crypto.NewRandomNonce()
+		ciphertext := make([]byte, 0, len(data)+100)
+
+		ciphertext = encrypt(t, k, data, ciphertext, nonce)
+		decryptNewSliceAndCompare(t, k, data, ciphertext, nonce)
+	})
+
+	// destination slice is empty and does not have enough capacity
+	t.Run("empty-small", func(t *testing.T) {
+		nonce := crypto.NewRandomNonce()
+		ciphertext := make([]byte, 0, len(data)/2)
+
+		ciphertext = encrypt(t, k, data, ciphertext, nonce)
+		decryptNewSliceAndCompare(t, k, data, ciphertext, nonce)
+	})
+
+	// destination slice contains data and has enough capacity
+	t.Run("prefix-large", func(t *testing.T) {
+		nonce := crypto.NewRandomNonce()
+		ciphertext := make([]byte, 0, len(data)+100)
+		ciphertext = append(ciphertext, []byte("foobar")...)
+
+		ciphertext = encrypt(t, k, data, ciphertext, nonce)
+		if string(ciphertext[:6]) != "foobar" {
+			t.Errorf("prefix is missing")
+		}
+		decryptNewSliceAndCompare(t, k, data, ciphertext[6:], nonce)
+	})
+
+	// destination slice contains data and does not have enough capacity
+	t.Run("prefix-small", func(t *testing.T) {
+		nonce := crypto.NewRandomNonce()
+		ciphertext := make([]byte, 0, len(data)/2)
+		ciphertext = append(ciphertext, []byte("foobar")...)
+
+		ciphertext = encrypt(t, k, data, ciphertext, nonce)
+		if string(ciphertext[:6]) != "foobar" {
+			t.Errorf("prefix is missing")
+		}
+		decryptNewSliceAndCompare(t, k, data, ciphertext[6:], nonce)
+	})
 }
 
 func TestLargeEncrypt(t *testing.T) {
@@ -123,10 +254,9 @@ func TestLargeEncrypt(t *testing.T) {
 		_, err := io.ReadFull(rand.Reader, data)
 		rtest.OK(t, err)
 
-		ciphertext, err := k.Encrypt(make([]byte, size+crypto.Extension), data)
-		rtest.OK(t, err)
-
-		plaintext, err := k.Decrypt([]byte{}, ciphertext)
+		nonce := crypto.NewRandomNonce()
+		ciphertext := k.Seal(make([]byte, size+k.Overhead()), nonce, data, nil)
+		plaintext, err := k.Open([]byte{}, nonce, ciphertext, nil)
 		rtest.OK(t, err)
 
 		rtest.Equals(t, plaintext, data)
@@ -139,13 +269,13 @@ func BenchmarkEncrypt(b *testing.B) {
 
 	k := crypto.NewRandomKey()
 	buf := make([]byte, len(data)+crypto.Extension)
+	nonce := crypto.NewRandomNonce()
 
 	b.ResetTimer()
 	b.SetBytes(int64(size))
 
 	for i := 0; i < b.N; i++ {
-		_, err := k.Encrypt(buf, data)
-		rtest.OK(b, err)
+		_ = k.Seal(buf, nonce, data, nil)
 	}
 }
 
@@ -155,17 +285,18 @@ func BenchmarkDecrypt(b *testing.B) {
 
 	k := crypto.NewRandomKey()
 
-	plaintext := make([]byte, size)
-	ciphertext := make([]byte, size+crypto.Extension)
+	plaintext := make([]byte, 0, size)
+	ciphertext := make([]byte, 0, size+crypto.Extension)
+	nonce := crypto.NewRandomNonce()
+	ciphertext = k.Seal(ciphertext, nonce, data, nil)
 
-	ciphertext, err := k.Encrypt(ciphertext, data)
-	rtest.OK(b, err)
+	var err error
 
 	b.ResetTimer()
 	b.SetBytes(int64(size))
 
 	for i := 0; i < b.N; i++ {
-		_, err = k.Decrypt(plaintext, ciphertext)
+		_, err = k.Open(plaintext, nonce, ciphertext, nil)
 		rtest.OK(b, err)
 	}
 }
diff --git a/internal/pack/pack.go b/internal/pack/pack.go
index 292f9dcb0..577b4c763 100644
--- a/internal/pack/pack.go
+++ b/internal/pack/pack.go
@@ -75,10 +75,10 @@ func (p *Packer) Finalize() (uint, error) {
 		return 0, err
 	}
 
-	encryptedHeader, err := p.k.Encrypt(nil, hdrBuf.Bytes())
-	if err != nil {
-		return 0, err
-	}
+	encryptedHeader := make([]byte, 0, hdrBuf.Len()+p.k.Overhead()+p.k.NonceSize())
+	nonce := crypto.NewRandomNonce()
+	encryptedHeader = append(encryptedHeader, nonce...)
+	encryptedHeader = p.k.Seal(encryptedHeader, nonce, hdrBuf.Bytes(), nil)
 
 	// append the header
 	n, err := p.wr.Write(encryptedHeader)
@@ -268,15 +268,19 @@ func List(k *crypto.Key, rd io.ReaderAt, size int64) (entries []restic.Blob, err
 		return nil, err
 	}
 
-	n, err := k.Decrypt(buf, buf)
+	if len(buf) < k.NonceSize()+k.Overhead() {
+		return nil, errors.New("invalid header, too small")
+	}
+
+	nonce, buf := buf[:k.NonceSize()], buf[k.NonceSize():]
+	buf, err = k.Open(buf[:0], nonce, buf, nil)
 	if err != nil {
 		return nil, err
 	}
-	buf = buf[:n]
 
 	hdrRd := bytes.NewReader(buf)
 
-	entries = make([]restic.Blob, 0, uint(n)/entrySize)
+	entries = make([]restic.Blob, 0, uint(len(buf))/entrySize)
 
 	pos := uint(0)
 	for {
diff --git a/internal/repository/key.go b/internal/repository/key.go
index e378991cb..781670bad 100644
--- a/internal/repository/key.go
+++ b/internal/repository/key.go
@@ -87,12 +87,11 @@ func OpenKey(ctx context.Context, s *Repository, name string, password string) (
 	}
 
 	// decrypt master keys
-	buf := make([]byte, len(k.Data))
-	n, err := k.user.Decrypt(buf, k.Data)
+	nonce, ciphertext := k.Data[:k.user.NonceSize()], k.Data[k.user.NonceSize():]
+	buf, err := k.user.Open(nil, nonce, ciphertext, nil)
 	if err != nil {
 		return nil, err
 	}
-	buf = buf[:n]
 
 	// restore json
 	k.master = &crypto.Key{}
@@ -221,7 +220,11 @@ func AddKey(ctx context.Context, s *Repository, password string, template *crypt
 		return nil, errors.Wrap(err, "Marshal")
 	}
 
-	newkey.Data, err = newkey.user.Encrypt(nil, buf)
+	nonce := crypto.NewRandomNonce()
+	ciphertext := make([]byte, 0, len(buf)+newkey.user.Overhead()+newkey.user.NonceSize())
+	ciphertext = append(ciphertext, nonce...)
+	ciphertext = newkey.user.Seal(ciphertext, nonce, buf, nil)
+	newkey.Data = ciphertext
 
 	// dump as json
 	buf, err = json.Marshal(newkey)
diff --git a/internal/repository/repack.go b/internal/repository/repack.go
index 3bba26803..3eacea182 100644
--- a/internal/repository/repack.go
+++ b/internal/repository/repack.go
@@ -90,14 +90,13 @@ func Repack(ctx context.Context, repo restic.Repository, packs restic.IDSet, kee
 					h, tempfile.Name(), len(buf), n)
 			}
 
-			n, err = repo.Key().Decrypt(buf, buf)
+			nonce, ciphertext := buf[:repo.Key().NonceSize()], buf[repo.Key().NonceSize():]
+			plaintext, err := repo.Key().Open(ciphertext[:0], nonce, ciphertext, nil)
 			if err != nil {
 				return nil, err
 			}
 
-			buf = buf[:n]
-
-			id := restic.Hash(buf)
+			id := restic.Hash(plaintext)
 			if !id.Equal(entry.ID) {
 				debug.Log("read blob %v/%v from %v: wrong data returned, hash is %v",
 					h.Type, h.ID, tempfile.Name(), id)
@@ -105,7 +104,7 @@ func Repack(ctx context.Context, repo restic.Repository, packs restic.IDSet, kee
 					h, tempfile.Name(), id)
 			}
 
-			_, err = repo.SaveBlob(ctx, entry.Type, buf, entry.ID)
+			_, err = repo.SaveBlob(ctx, entry.Type, plaintext, entry.ID)
 			if err != nil {
 				return nil, err
 			}
diff --git a/internal/repository/repository.go b/internal/repository/repository.go
index 161eb8734..9bf686548 100644
--- a/internal/repository/repository.go
+++ b/internal/repository/repository.go
@@ -79,13 +79,13 @@ func (r *Repository) LoadAndDecrypt(ctx context.Context, t restic.FileType, id r
 		return nil, errors.Errorf("load %v: invalid data returned", h)
 	}
 
-	// decrypt
-	n, err := r.decryptTo(buf, buf)
+	nonce, ciphertext := buf[:r.key.NonceSize()], buf[r.key.NonceSize():]
+	plaintext, err := r.key.Open(ciphertext[:0], nonce, ciphertext, nil)
 	if err != nil {
 		return nil, err
 	}
 
-	return buf[:n], nil
+	return plaintext, nil
 }
 
 // sortCachedPacks moves all cached pack files to the front of blobs.
@@ -156,20 +156,22 @@ func (r *Repository) loadBlob(ctx context.Context, id restic.ID, t restic.BlobTy
 		}
 
 		// decrypt
-		n, err = r.decryptTo(plaintextBuf, plaintextBuf)
+		nonce, ciphertext := plaintextBuf[:r.key.NonceSize()], plaintextBuf[r.key.NonceSize():]
+		plaintext, err := r.key.Open(ciphertext[:0], nonce, ciphertext, nil)
 		if err != nil {
 			lastError = errors.Errorf("decrypting blob %v failed: %v", id, err)
 			continue
 		}
-		plaintextBuf = plaintextBuf[:n]
 
 		// check hash
-		if !restic.Hash(plaintextBuf).Equal(id) {
+		if !restic.Hash(plaintext).Equal(id) {
 			lastError = errors.Errorf("blob %v returned invalid hash", id)
 			continue
 		}
 
-		return len(plaintextBuf), nil
+		// move decrypted data to the start of the provided buffer
+		copy(plaintextBuf[0:], plaintext)
+		return len(plaintext), nil
 	}
 
 	if lastError != nil {
@@ -210,11 +212,12 @@ func (r *Repository) SaveAndEncrypt(ctx context.Context, t restic.BlobType, data
 	ciphertext := getBuf()
 	defer freeBuf(ciphertext)
 
+	ciphertext = ciphertext[:0]
+	nonce := crypto.NewRandomNonce()
+	ciphertext = append(ciphertext, nonce...)
+
 	// encrypt blob
-	ciphertext, err := r.Encrypt(ciphertext, data)
-	if err != nil {
-		return restic.ID{}, err
-	}
+	ciphertext = r.key.Seal(ciphertext, nonce, data, nil)
 
 	// find suitable packer and add blob
 	var pm *packerManager
@@ -266,10 +269,11 @@ func (r *Repository) SaveJSONUnpacked(ctx context.Context, t restic.FileType, it
 // storage hash.
 func (r *Repository) SaveUnpacked(ctx context.Context, t restic.FileType, p []byte) (id restic.ID, err error) {
 	ciphertext := restic.NewBlobBuffer(len(p))
-	ciphertext, err = r.Encrypt(ciphertext, p)
-	if err != nil {
-		return restic.ID{}, err
-	}
+	ciphertext = ciphertext[:0]
+	nonce := crypto.NewRandomNonce()
+	ciphertext = append(ciphertext, nonce...)
+
+	ciphertext = r.key.Seal(ciphertext, nonce, p, nil)
 
 	id = restic.Hash(ciphertext)
 	h := restic.Handle{Type: t, Name: id.String()}
@@ -522,26 +526,6 @@ func (r *Repository) init(ctx context.Context, password string, cfg restic.Confi
 	return err
 }
 
-// decrypt authenticates and decrypts ciphertext and stores the result in
-// plaintext.
-func (r *Repository) decryptTo(plaintext, ciphertext []byte) (int, error) {
-	if r.key == nil {
-		return 0, errors.New("key for repository not set")
-	}
-
-	return r.key.Decrypt(plaintext, ciphertext)
-}
-
-// Encrypt encrypts and authenticates the plaintext and saves the result in
-// ciphertext.
-func (r *Repository) Encrypt(ciphertext, plaintext []byte) ([]byte, error) {
-	if r.key == nil {
-		return nil, errors.New("key for repository not set")
-	}
-
-	return r.key.Encrypt(ciphertext, plaintext)
-}
-
 // Key returns the current master key.
 func (r *Repository) Key() *crypto.Key {
 	return r.key