3 files changed, 142 insertions, 1 deletions
diff --git a/internal/stupidgcm/common_test.go b/internal/stupidgcm/common_test.go
index cf555b0..8123ce2 100644
--- a/internal/stupidgcm/common_test.go
+++ b/internal/stupidgcm/common_test.go
@@ -183,8 +183,18 @@ func testWipe(t *testing.T, c cipher.AEAD) {
 				t.Fatal("c2.key is not zeroed")
 			}
 		}
+	case *stupidXchacha20poly1305:
+		c2.Wipe()
+		if !c2.wiped {
+			t.Error("c2.wiped is not set")
+		}
+		for _, v := range c2.key {
+			if v != 0 {
+				t.Fatal("c2.key is not zeroed")
+			}
+		}
 	default:
-		t.Fatalf("BUG: unhandled type %t", c2)
+		t.Fatalf("BUG: unhandled type %T", c2)
 	}
 }
 
diff --git a/internal/stupidgcm/stupidxchacha.go b/internal/stupidgcm/stupidxchacha.go
new file mode 100644
index 0000000..9f2ac2f
--- /dev/null
+++ b/internal/stupidgcm/stupidxchacha.go
@@ -0,0 +1,111 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+//
+// Copied from
+// https://github.com/golang/crypto/blob/32db794688a5a24a23a43f2a984cecd5b3d8da58/chacha20poly1305/xchacha20poly1305.go
+// and adapted for stupidgcm by @rfjakob.
+
+package stupidgcm
+
+import (
+	"crypto/cipher"
+	"errors"
+
+	"golang.org/x/crypto/chacha20"
+	"golang.org/x/crypto/chacha20poly1305"
+)
+
+type stupidXchacha20poly1305 struct {
+	key   [chacha20poly1305.KeySize]byte
+	wiped bool
+}
+
+// NewXchacha20poly1305 returns a XChaCha20-Poly1305 AEAD that uses the given 256-bit key.
+//
+// XChaCha20-Poly1305 is a ChaCha20-Poly1305 variant that takes a longer nonce,
+// suitable to be generated randomly without risk of collisions. It should be
+// preferred when nonce uniqueness cannot be trivially ensured, or whenever
+// nonces are randomly generated.
+func NewXchacha20poly1305(key []byte) cipher.AEAD {
+	if len(key) != chacha20poly1305.KeySize {
+		panic("bad key length")
+	}
+	ret := new(stupidXchacha20poly1305)
+	copy(ret.key[:], key)
+	return ret
+}
+
+func (*stupidXchacha20poly1305) NonceSize() int {
+	return chacha20poly1305.NonceSizeX
+}
+
+func (*stupidXchacha20poly1305) Overhead() int {
+	return 16
+}
+
+func (x *stupidXchacha20poly1305) Seal(dst, nonce, plaintext, additionalData []byte) []byte {
+	if x.wiped {
+		panic("BUG: tried to use wiped key")
+	}
+	if len(nonce) != chacha20poly1305.NonceSizeX {
+		panic("bad nonce length passed to Seal")
+	}
+
+	// XChaCha20-Poly1305 technically supports a 64-bit counter, so there is no
+	// size limit. However, since we reuse the ChaCha20-Poly1305 implementation,
+	// the second half of the counter is not available. This is unlikely to be
+	// an issue because the cipher.AEAD API requires the entire message to be in
+	// memory, and the counter overflows at 256 GB.
+	if uint64(len(plaintext)) > (1<<38)-64 {
+		panic("plaintext too large")
+	}
+
+	c := new(stupidChacha20poly1305)
+	hKey, _ := chacha20.HChaCha20(x.key[:], nonce[0:16])
+	copy(c.key[:], hKey)
+	defer c.Wipe()
+
+	// The first 4 bytes of the final nonce are unused counter space.
+	cNonce := make([]byte, chacha20poly1305.NonceSize)
+	copy(cNonce[4:12], nonce[16:24])
+
+	return c.Seal(dst, cNonce[:], plaintext, additionalData)
+}
+
+func (x *stupidXchacha20poly1305) Open(dst, nonce, ciphertext, additionalData []byte) ([]byte, error) {
+	if x.wiped {
+		panic("BUG: tried to use wiped key")
+	}
+	if len(nonce) != chacha20poly1305.NonceSizeX {
+		panic("bad nonce length passed to Open")
+	}
+	if len(ciphertext) < 16 {
+		return nil, errors.New("message too short")
+	}
+	if uint64(len(ciphertext)) > (1<<38)-48 {
+		panic("ciphertext too large")
+	}
+
+	c := new(stupidChacha20poly1305)
+	hKey, _ := chacha20.HChaCha20(x.key[:], nonce[0:16])
+	copy(c.key[:], hKey)
+	defer c.Wipe()
+
+	// The first 4 bytes of the final nonce are unused counter space.
+	cNonce := make([]byte, chacha20poly1305.NonceSize)
+	copy(cNonce[4:12], nonce[16:24])
+
+	return c.Open(dst, cNonce[:], ciphertext, additionalData)
+}
+
+// Wipe tries to wipe the key from memory by overwriting it with zeros.
+//
+// This is not bulletproof due to possible GC copies, but
+// still raises the bar for extracting the key.
+func (g *stupidXchacha20poly1305) Wipe() {
+	g.wiped = true
+	for i := range g.key {
+		g.key[i] = 0
+	}
+}
diff --git a/internal/stupidgcm/stupidxchacha_test.go b/internal/stupidgcm/stupidxchacha_test.go
new file mode 100644
index 0000000..fdea8b5
--- /dev/null
+++ b/internal/stupidgcm/stupidxchacha_test.go
@@ -0,0 +1,20 @@
+// +build !without_openssl
+
+package stupidgcm
+
+import (
+	"testing"
+
+	"golang.org/x/crypto/chacha20poly1305"
+)
+
+func TestStupidXchacha20poly1305(t *testing.T) {
+	key := randBytes(32)
+	c := NewXchacha20poly1305(key)
+	ref, err := chacha20poly1305.NewX(key)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	testCiphers(t, c, ref)
+}