stupidgcm: introduce stupidAEADCommon and use for both chacha & gcm

Nice deduplication and brings the GCM decrypt speed up to par.

internal/speed$ benchstat old new
name                old time/op   new time/op   delta
StupidGCM-4          4.71µs ± 0%   4.66µs ± 0%   -0.99%  (p=0.008 n=5+5)
StupidGCMDecrypt-4   5.77µs ± 1%   4.51µs ± 0%  -21.80%  (p=0.008 n=5+5)

name                old speed     new speed     delta
StupidGCM-4         870MB/s ± 0%  879MB/s ± 0%   +1.01%  (p=0.008 n=5+5)
StupidGCMDecrypt-4  710MB/s ± 1%  908MB/s ± 0%  +27.87%  (p=0.008 n=5+5)

13 files changed, 280 insertions, 402 deletions

diff --git a/internal/stupidgcm/chacha.go b/internal/stupidgcm/chacha.go
new file mode 100644
index 0000000..37f7e1f
--- /dev/null
+++ b/internal/stupidgcm/chacha.go
@@ -0,0 +1,35 @@
+// +build !without_openssl
+
+package stupidgcm
+
+import (
+	"crypto/cipher"
+	"log"
+
+	"golang.org/x/crypto/chacha20poly1305"
+)
+
+/*
+#include <openssl/evp.h>
+*/
+import "C"
+
+type stupidChacha20poly1305 struct {
+	stupidAEADCommon
+}
+
+// Verify that we satisfy the cipher.AEAD interface
+var _ cipher.AEAD = &stupidChacha20poly1305{}
+
+func newChacha20poly1305(key []byte) *stupidChacha20poly1305 {
+	if len(key) != chacha20poly1305.KeySize {
+		log.Panicf("Only %d-byte keys are supported, you passed %d bytes", chacha20poly1305.KeySize, len(key))
+	}
+	return &stupidChacha20poly1305{
+		stupidAEADCommon{
+			key:              append([]byte{}, key...), // private copy
+			openSSLEVPCipher: C.EVP_chacha20_poly1305(),
+			nonceSize:        chacha20poly1305.NonceSize,
+		},
+	}
+}
diff --git a/internal/stupidgcm/stupidchacha_test.go b/internal/stupidgcm/chacha_test.go
index 513b68f..513b68f 100644
--- a/internal/stupidgcm/stupidchacha_test.go
+++ b/internal/stupidgcm/chacha_test.go
diff --git a/internal/stupidgcm/common.go b/internal/stupidgcm/common.go
new file mode 100644
index 0000000..3788315
--- /dev/null
+++ b/internal/stupidgcm/common.go
@@ -0,0 +1,68 @@
+package stupidgcm
+
+import (
+	"log"
+)
+
+/*
+#include <openssl/evp.h>
+*/
+import "C"
+
+type stupidAEADCommon struct {
+	wiped            bool
+	key              []byte
+	openSSLEVPCipher *C.EVP_CIPHER
+	nonceSize        int
+}
+
+// Overhead returns the number of bytes that are added for authentication.
+//
+// Part of the cipher.AEAD interface.
+func (c *stupidAEADCommon) Overhead() int {
+	return tagLen
+}
+
+// NonceSize returns the required size of the nonce / IV
+//
+// Part of the cipher.AEAD interface.
+func (c *stupidAEADCommon) NonceSize() int {
+	return c.nonceSize
+}
+
+// Seal encrypts "in" using "iv" and "authData" and append the result to "dst"
+//
+// Part of the cipher.AEAD interface.
+func (c *stupidAEADCommon) Seal(dst, iv, in, authData []byte) []byte {
+	return openSSLSeal(c, dst, iv, in, authData)
+}
+
+// Open decrypts "in" using "iv" and "authData" and append the result to "dst"
+//
+// Part of the cipher.AEAD interface.
+func (c *stupidAEADCommon) Open(dst, iv, in, authData []byte) ([]byte, error) {
+	return openSSLOpen(c, dst, iv, in, authData)
+}
+
+// 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 (c *stupidAEADCommon) Wipe() {
+	key := c.key
+	c.wiped = true
+	c.key = nil
+	for i := range key {
+		key[i] = 0
+	}
+}
+
+func (c *stupidAEADCommon) Wiped() bool {
+	if c.wiped {
+		return true
+	}
+	if len(c.key) != keyLen {
+		log.Panicf("wrong key length %d", len(c.key))
+	}
+	return false
+}
diff --git a/internal/stupidgcm/common_test.go b/internal/stupidgcm/common_test.go
index 8123ce2..a8080ca 100644
--- a/internal/stupidgcm/common_test.go
+++ b/internal/stupidgcm/common_test.go
@@ -162,25 +162,26 @@ func testCorruption(t *testing.T, c cipher.AEAD) {
 	}
 }
 
-type Wiper interface {
-	Wipe()
-}
-
 func testWipe(t *testing.T, c cipher.AEAD) {
 	switch c2 := c.(type) {
 	case *StupidGCM:
 		c2.Wipe()
-		if c2.key != nil {
-			t.Fatal("key is not nil")
+		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")
+			}
 		}
 	case *stupidChacha20poly1305:
 		c2.Wipe()
-		if !c2.wiped {
+		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")
+				t.Fatal("c2._key is not zeroed")
 			}
 		}
 	case *stupidXchacha20poly1305:
diff --git a/internal/stupidgcm/gcm.go b/internal/stupidgcm/gcm.go
new file mode 100644
index 0000000..439e7a7
--- /dev/null
+++ b/internal/stupidgcm/gcm.go
@@ -0,0 +1,45 @@
+// +build !without_openssl
+
+// Package stupidgcm is a thin wrapper for OpenSSL's GCM encryption and
+// decryption functions. It only support 32-byte keys and 16-bit IVs.
+package stupidgcm
+
+// #include <openssl/evp.h>
+import "C"
+
+import (
+	"crypto/cipher"
+	"log"
+)
+
+const (
+	// BuiltWithoutOpenssl indicates if openssl been disabled at compile-time
+	BuiltWithoutOpenssl = false
+
+	keyLen = 32
+	ivLen  = 16
+	tagLen = 16
+)
+
+// StupidGCM implements the cipher.AEAD interface
+type StupidGCM struct {
+	stupidAEADCommon
+}
+
+// Verify that we satisfy the interface
+var _ cipher.AEAD = &StupidGCM{}
+
+// New returns a new cipher.AEAD implementation..
+func New(keyIn []byte, forceDecode bool) cipher.AEAD {
+	if len(keyIn) != keyLen {
+		log.Panicf("Only %d-byte keys are supported", keyLen)
+	}
+	return &StupidGCM{
+		stupidAEADCommon{
+			// Create a private copy of the key
+			key:              append([]byte{}, keyIn...),
+			openSSLEVPCipher: C.EVP_aes_256_gcm(),
+			nonceSize:        ivLen,
+		},
+	}
+}
diff --git a/internal/stupidgcm/stupidgcm_test.go b/internal/stupidgcm/gcm_test.go
index 5323afa..5323afa 100644
--- a/internal/stupidgcm/stupidgcm_test.go
+++ b/internal/stupidgcm/gcm_test.go
diff --git a/internal/stupidgcm/openssl.go b/internal/stupidgcm/openssl.go
new file mode 100644
index 0000000..d57d100
--- /dev/null
+++ b/internal/stupidgcm/openssl.go
@@ -0,0 +1,108 @@
+package stupidgcm
+
+import (
+	"fmt"
+	"log"
+)
+
+/*
+#include "openssl_aead.h"
+#cgo pkg-config: libcrypto
+*/
+import "C"
+
+func openSSLSeal(a *stupidAEADCommon, dst, iv, in, authData []byte) []byte {
+	if a.Wiped() {
+		panic("BUG: tried to use wiped key")
+	}
+	if len(iv) != a.NonceSize() {
+		log.Panicf("Only %d-byte IVs are supported, you passed %d bytes", a.NonceSize(), len(iv))
+	}
+	if len(in) == 0 {
+		log.Panic("Zero-length input data is not supported")
+	}
+
+	// If the "dst" slice is large enough we can use it as our output buffer
+	outLen := len(in) + tagLen
+	var buf []byte
+	inplace := false
+	if cap(dst)-len(dst) >= outLen {
+		inplace = true
+		buf = dst[len(dst) : len(dst)+outLen]
+	} else {
+		buf = make([]byte, outLen)
+	}
+
+	res := int(C.openssl_aead_seal(a.openSSLEVPCipher,
+		(*C.uchar)(&in[0]),
+		C.int(len(in)),
+		(*C.uchar)(&authData[0]),
+		C.int(len(authData)),
+		(*C.uchar)(&a.key[0]),
+		C.int(len(a.key)),
+		(*C.uchar)(&iv[0]),
+		C.int(len(iv)),
+		(*C.uchar)(&buf[0]),
+		C.int(len(buf))))
+
+	if res != outLen {
+		log.Panicf("expected length %d, got %d", outLen, res)
+	}
+
+	if inplace {
+		return dst[:len(dst)+outLen]
+	}
+	return append(dst, buf...)
+}
+
+func openSSLOpen(a *stupidAEADCommon, dst, iv, in, authData []byte) ([]byte, error) {
+	if a.Wiped() {
+		panic("BUG: tried to use wiped key")
+	}
+	if len(iv) != a.NonceSize() {
+		log.Panicf("Only %d-byte IVs are supported, you passed %d bytes", a.NonceSize(), len(iv))
+	}
+	if len(in) <= tagLen {
+		return nil, fmt.Errorf("stupidChacha20poly1305: input data too short (%d bytes)", len(in))
+	}
+
+	// If the "dst" slice is large enough we can use it as our output buffer
+	outLen := len(in) - tagLen
+	var buf []byte
+	inplace := false
+	if cap(dst)-len(dst) >= outLen {
+		inplace = true
+		buf = dst[len(dst) : len(dst)+outLen]
+	} else {
+		buf = make([]byte, len(in)-tagLen)
+	}
+
+	ciphertext := in[:len(in)-tagLen]
+	tag := in[len(in)-tagLen:]
+
+	res := int(C.openssl_aead_open(a.openSSLEVPCipher,
+		(*C.uchar)(&ciphertext[0]),
+		C.int(len(ciphertext)),
+		(*C.uchar)(&authData[0]),
+		C.int(len(authData)),
+		(*C.uchar)(&tag[0]),
+		C.int(len(tag)),
+		(*C.uchar)(&a.key[0]),
+		C.int(len(a.key)),
+		(*C.uchar)(&iv[0]),
+		C.int(len(iv)),
+		(*C.uchar)(&buf[0]),
+		C.int(len(buf))))
+
+	if res < 0 {
+		return nil, ErrAuth
+	}
+	if res != outLen {
+		log.Panicf("unexpected length %d", res)
+	}
+
+	if inplace {
+		return dst[:len(dst)+outLen], nil
+	}
+	return append(dst, buf...), nil
+}
diff --git a/internal/stupidgcm/chacha.c b/internal/stupidgcm/openssl_aead.c
index 05d68af..9dc6866 100644
--- a/internal/stupidgcm/chacha.c
+++ b/internal/stupidgcm/openssl_aead.c
@@ -1,4 +1,4 @@
-#include "chacha.h"
+#include "openssl_aead.h"
 #include <openssl/evp.h>
 #include <stdio.h>
 //#cgo pkg-config: libcrypto
@@ -9,24 +9,12 @@ static void panic(const char* const msg)
     __builtin_trap();
 }
 
-static const EVP_CIPHER* getEvpCipher(enum aeadType cipherId)
-{
-    switch (cipherId) {
-    case aeadTypeChacha:
-        return EVP_chacha20_poly1305();
-    case aeadTypeGcm:
-        return EVP_aes_256_gcm();
-    }
-    panic("unknown cipherId");
-    return NULL;
-}
-
 // We only support 16-byte tags
 static const int supportedTagLen = 16;
 
 // https://wiki.openssl.org/index.php/EVP_Authenticated_Encryption_and_Decryption#Authenticated_Encryption_using_GCM_mode
-int aead_seal(
-    const enum aeadType cipherId,
+int openssl_aead_seal(
+    const EVP_CIPHER* evpCipher,
     const unsigned char* const plaintext,
     const int plaintextLen,
     const unsigned char* const authData,
@@ -38,8 +26,6 @@ int aead_seal(
     unsigned char* const ciphertext,
     const int ciphertextBufLen)
 {
-    const EVP_CIPHER* evpCipher = getEvpCipher(cipherId);
-
     // Create scratch space "ctx"
     EVP_CIPHER_CTX* ctx = EVP_CIPHER_CTX_new();
     if (!ctx) {
@@ -111,8 +97,8 @@ int aead_seal(
     return ciphertextLen;
 }
 
-int aead_open(
-    const enum aeadType cipherId,
+int openssl_aead_open(
+    const EVP_CIPHER* evpCipher,
     const unsigned char* const ciphertext,
     const int ciphertextLen,
     const unsigned char* const authData,
@@ -126,8 +112,6 @@ int aead_open(
     unsigned char* const plaintext,
     const int plaintextBufLen)
 {
-    const EVP_CIPHER* evpCipher = getEvpCipher(cipherId);
-
     // Create scratch space "ctx"
     EVP_CIPHER_CTX* ctx = EVP_CIPHER_CTX_new();
     if (!ctx) {
diff --git a/internal/stupidgcm/chacha.h b/internal/stupidgcm/openssl_aead.h
index a5eac04..6a818b6 100644
--- a/internal/stupidgcm/chacha.h
+++ b/internal/stupidgcm/openssl_aead.h
@@ -1,10 +1,7 @@
-enum aeadType {
-    aeadTypeChacha = 1,
-    aeadTypeGcm = 2,
-};
+#include <openssl/evp.h>
 
-int aead_seal(
-    const enum aeadType cipherId,
+int openssl_aead_seal(
+    const EVP_CIPHER* evpCipher,
     const unsigned char* const plaintext,
     const int plaintextLen,
     const unsigned char* const authData,
@@ -16,8 +13,8 @@ int aead_seal(
     unsigned char* const ciphertext,
     const int ciphertextBufLen);
 
-int aead_open(
-    const enum aeadType cipherId,
+int openssl_aead_open(
+    const EVP_CIPHER* evpCipher,
     const unsigned char* const ciphertext,
     const int ciphertextLen,
     const unsigned char* const authData,
diff --git a/internal/stupidgcm/stupidchacha.go b/internal/stupidgcm/stupidchacha.go
deleted file mode 100644
index 5073aa3..0000000
--- a/internal/stupidgcm/stupidchacha.go
+++ /dev/null
@@ -1,156 +0,0 @@
-// +build !without_openssl
-
-package stupidgcm
-
-import (
-	"crypto/cipher"
-	"fmt"
-	"log"
-
-	"golang.org/x/crypto/chacha20poly1305"
-)
-
-/*
-#include <openssl/evp.h>
-#include "chacha.h"
-#cgo pkg-config: libcrypto
-*/
-import "C"
-
-type stupidChacha20poly1305 struct {
-	key   [chacha20poly1305.KeySize]byte
-	wiped bool
-}
-
-// Verify that we satisfy the cipher.AEAD interface
-var _ cipher.AEAD = &stupidChacha20poly1305{}
-
-func newChacha20poly1305(key []byte) cipher.AEAD {
-	if len(key) != chacha20poly1305.KeySize {
-		log.Panicf("Only %d-byte keys are supported, you passed %d bytes", chacha20poly1305.KeySize, len(key))
-	}
-	ret := new(stupidChacha20poly1305)
-	copy(ret.key[:], key)
-	return ret
-}
-
-// NonceSize returns the required size of the nonce / IV.
-func (g *stupidChacha20poly1305) NonceSize() int {
-	return chacha20poly1305.NonceSize
-}
-
-// Overhead returns the number of bytes that are added for authentication.
-func (g *stupidChacha20poly1305) Overhead() int {
-	return tagLen
-}
-
-// Seal encrypts "in" using "iv" and "authData" and append the result to "dst"
-func (g *stupidChacha20poly1305) Seal(dst, iv, in, authData []byte) []byte {
-	if g.wiped {
-		panic("BUG: tried to use wiped key")
-	}
-	if len(iv) != g.NonceSize() {
-		log.Panicf("Only %d-byte IVs are supported, you passed %d bytes", g.NonceSize(), len(iv))
-	}
-	if len(in) == 0 {
-		log.Panic("Zero-length input data is not supported")
-	}
-	if len(g.key) != chacha20poly1305.KeySize {
-		log.Panicf("Wrong key length: %d. Key has been wiped?", len(g.key))
-	}
-
-	// If the "dst" slice is large enough we can use it as our output buffer
-	outLen := len(in) + tagLen
-	var buf []byte
-	inplace := false
-	if cap(dst)-len(dst) >= outLen {
-		inplace = true
-		buf = dst[len(dst) : len(dst)+outLen]
-	} else {
-		buf = make([]byte, outLen)
-	}
-
-	C.aead_seal(C.aeadTypeChacha,
-		(*C.uchar)(&in[0]),
-		C.int(len(in)),
-		(*C.uchar)(&authData[0]),
-		C.int(len(authData)),
-		(*C.uchar)(&g.key[0]),
-		C.int(len(g.key)),
-		(*C.uchar)(&iv[0]),
-		C.int(len(iv)),
-		(*C.uchar)(&buf[0]),
-		C.int(len(buf)))
-
-	if inplace {
-		return dst[:len(dst)+outLen]
-	}
-	return append(dst, buf...)
-}
-
-// Open decrypts "in" using "iv" and "authData" and append the result to "dst"
-func (g *stupidChacha20poly1305) Open(dst, iv, in, authData []byte) ([]byte, error) {
-	if g.wiped {
-		panic("BUG: tried to use wiped key")
-	}
-	if len(iv) != g.NonceSize() {
-		log.Panicf("Only %d-byte IVs are supported", g.NonceSize())
-	}
-	if len(g.key) != chacha20poly1305.KeySize {
-		log.Panicf("Wrong key length: %d. Key has been wiped?", len(g.key))
-	}
-	if len(in) <= tagLen {
-		return nil, fmt.Errorf("stupidChacha20poly1305: input data too short (%d bytes)", len(in))
-	}
-
-	// If the "dst" slice is large enough we can use it as our output buffer
-	outLen := len(in) - tagLen
-	var buf []byte
-	inplace := false
-	if cap(dst)-len(dst) >= outLen {
-		inplace = true
-		buf = dst[len(dst) : len(dst)+outLen]
-	} else {
-		buf = make([]byte, len(in)-tagLen)
-	}
-
-	ciphertext := in[:len(in)-tagLen]
-	tag := in[len(in)-tagLen:]
-
-	res := int(C.aead_open(C.aeadTypeChacha,
-		(*C.uchar)(&ciphertext[0]),
-		C.int(len(ciphertext)),
-		(*C.uchar)(&authData[0]),
-		C.int(len(authData)),
-		(*C.uchar)(&tag[0]),
-		C.int(len(tag)),
-		(*C.uchar)(&g.key[0]),
-		C.int(len(g.key)),
-		(*C.uchar)(&iv[0]),
-		C.int(len(iv)),
-		(*C.uchar)(&buf[0]),
-		C.int(len(buf))))
-
-	if res < 0 {
-		return nil, ErrAuth
-	}
-	if res != outLen {
-		log.Panicf("unexpected length %d", res)
-	}
-
-	if inplace {
-		return dst[:len(dst)+outLen], nil
-	}
-	return append(dst, buf...), nil
-}
-
-// 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 *stupidChacha20poly1305) Wipe() {
-	g.wiped = true
-	for i := range g.key {
-		g.key[i] = 0
-	}
-}
diff --git a/internal/stupidgcm/stupidgcm.go b/internal/stupidgcm/stupidgcm.go
deleted file mode 100644
index 46b6b86..0000000
--- a/internal/stupidgcm/stupidgcm.go
+++ /dev/null
@@ -1,204 +0,0 @@
-// +build !without_openssl
-
-// Package stupidgcm is a thin wrapper for OpenSSL's GCM encryption and
-// decryption functions. It only support 32-byte keys and 16-bit IVs.
-package stupidgcm
-
-// #include <openssl/evp.h>
-// #include "chacha.h"
-// #cgo pkg-config: libcrypto
-import "C"
-
-import (
-	"crypto/cipher"
-	"fmt"
-	"log"
-	"unsafe"
-)
-
-const (
-	// BuiltWithoutOpenssl indicates if openssl been disabled at compile-time
-	BuiltWithoutOpenssl = false
-
-	keyLen = 32
-	ivLen  = 16
-	tagLen = 16
-)
-
-// StupidGCM implements the cipher.AEAD interface
-type StupidGCM struct {
-	key         []byte
-	forceDecode bool
-}
-
-// Verify that we satisfy the cipher.AEAD interface
-var _ cipher.AEAD = &StupidGCM{}
-
-// New returns a new cipher.AEAD implementation..
-func New(keyIn []byte, forceDecode bool) cipher.AEAD {
-	if len(keyIn) != keyLen {
-		log.Panicf("Only %d-byte keys are supported", keyLen)
-	}
-	// Create a private copy of the key
-	key := append([]byte{}, keyIn...)
-	return &StupidGCM{key: key, forceDecode: forceDecode}
-}
-
-// NonceSize returns the required size of the nonce / IV.
-func (g *StupidGCM) NonceSize() int {
-	return ivLen
-}
-
-// Overhead returns the number of bytes that are added for authentication.
-func (g *StupidGCM) Overhead() int {
-	return tagLen
-}
-
-// Seal encrypts "in" using "iv" and "authData" and append the result to "dst"
-func (g *StupidGCM) Seal(dst, iv, in, authData []byte) []byte {
-	if len(iv) != ivLen {
-		log.Panicf("Only %d-byte IVs are supported", ivLen)
-	}
-	if len(in) == 0 {
-		log.Panic("Zero-length input data is not supported")
-	}
-	if len(g.key) != keyLen {
-		log.Panicf("Wrong key length: %d. Key has been wiped?", len(g.key))
-	}
-
-	// If the "dst" slice is large enough we can use it as our output buffer
-	outLen := len(in) + tagLen
-	var buf []byte
-	inplace := false
-	if cap(dst)-len(dst) >= outLen {
-		inplace = true
-		buf = dst[len(dst) : len(dst)+outLen]
-	} else {
-		buf = make([]byte, outLen)
-	}
-
-	C.aead_seal(C.aeadTypeGcm,
-		(*C.uchar)(&in[0]),
-		C.int(len(in)),
-		(*C.uchar)(&authData[0]),
-		C.int(len(authData)),
-		(*C.uchar)(&g.key[0]),
-		C.int(len(g.key)),
-		(*C.uchar)(&iv[0]),
-		C.int(len(iv)),
-		(*C.uchar)(&buf[0]),
-		C.int(len(buf)))
-
-	if inplace {
-		return dst[:len(dst)+outLen]
-	}
-	return append(dst, buf...)
-}
-
-// Open decrypts "in" using "iv" and "authData" and append the result to "dst"
-func (g *StupidGCM) Open(dst, iv, in, authData []byte) ([]byte, error) {
-	if len(iv) != ivLen {
-		log.Panicf("Only %d-byte IVs are supported", ivLen)
-	}
-	if len(g.key) != keyLen {
-		log.Panicf("Wrong key length: %d. Key has been wiped?", len(g.key))
-	}
-	if len(in) <= tagLen {
-		return nil, fmt.Errorf("stupidgcm: input data too short (%d bytes)", len(in))
-	}
-
-	// If the "dst" slice is large enough we can use it as our output buffer
-	outLen := len(in) - tagLen
-	var buf []byte
-	inplace := false
-	if cap(dst)-len(dst) >= outLen {
-		inplace = true
-		buf = dst[len(dst) : len(dst)+outLen]
-	} else {
-		buf = make([]byte, len(in)-tagLen)
-	}
-
-	ciphertext := in[:len(in)-tagLen]
-	tag := in[len(in)-tagLen:]
-
-	// https://wiki.openssl.org/index.php/EVP_Authenticated_Encryption_and_Decryption#Authenticated_Encryption_using_GCM_mode
-
-	// Create scratch space "context"
-	ctx := C.EVP_CIPHER_CTX_new()
-	if ctx == nil {
-		log.Panic("EVP_CIPHER_CTX_new failed")
-	}
-
-	// Set cipher to AES-256
-	if C.EVP_DecryptInit_ex(ctx, C.EVP_aes_256_gcm(), nil, nil, nil) != 1 {
-		log.Panic("EVP_DecryptInit_ex I failed")
-	}
-
-	// Use 16-byte IV
-	if C.EVP_CIPHER_CTX_ctrl(ctx, C.EVP_CTRL_GCM_SET_IVLEN, ivLen, nil) != 1 {
-		log.Panic("EVP_CIPHER_CTX_ctrl EVP_CTRL_GCM_SET_IVLEN failed")
-	}
-
-	// Set key and IV
-	if C.EVP_DecryptInit_ex(ctx, nil, nil, (*C.uchar)(&g.key[0]), (*C.uchar)(&iv[0])) != 1 {
-		log.Panic("EVP_DecryptInit_ex II failed")
-	}
-
-	// Set expected GMAC tag
-	if C.EVP_CIPHER_CTX_ctrl(ctx, C.EVP_CTRL_GCM_SET_TAG, tagLen, (unsafe.Pointer)(&tag[0])) != 1 {
-		log.Panic("EVP_CIPHER_CTX_ctrl failed")
-	}
-
-	// Provide authentication data
-	var resultLen C.int
-	if C.EVP_DecryptUpdate(ctx, nil, &resultLen, (*C.uchar)(&authData[0]), C.int(len(authData))) != 1 {
-		log.Panic("EVP_DecryptUpdate authData failed")
-	}
-	if int(resultLen) != len(authData) {
-		log.Panicf("Unexpected length %d", resultLen)
-	}
-
-	// Decrypt "ciphertext" into "buf"
-	if C.EVP_DecryptUpdate(ctx, (*C.uchar)(&buf[0]), &resultLen, (*C.uchar)(&ciphertext[0]), C.int(len(ciphertext))) != 1 {
-		log.Panic("EVP_DecryptUpdate failed")
-	}
-	if int(resultLen) != len(ciphertext) {
-		log.Panicf("Unexpected length %d", resultLen)
-	}
-
-	// Check GMAC
-	dummy := make([]byte, 16)
-	res := C.EVP_DecryptFinal_ex(ctx, (*C.uchar)(&dummy[0]), &resultLen)
-	if resultLen != 0 {
-		log.Panicf("Unexpected length %d", resultLen)
-	}
-
-	// Free scratch space
-	C.EVP_CIPHER_CTX_free(ctx)
-
-	if res != 1 {
-		// The error code must always be checked by the calling function, because the decrypted buffer
-		// may contain corrupted data that we are returning in case the user forced reads
-		if g.forceDecode {
-			return append(dst, buf...), ErrAuth
-		}
-		return nil, ErrAuth
-	}
-
-	if inplace {
-		return dst[:len(dst)+outLen], nil
-	}
-	return append(dst, buf...), nil
-}
-
-// Wipe tries to wipe the AES key from memory by overwriting it with zeros
-// and setting the reference to nil.
-//
-// This is not bulletproof due to possible GC copies, but
-// still raises the bar for extracting the key.
-func (g *StupidGCM) Wipe() {
-	for i := range g.key {
-		g.key[i] = 0
-	}
-	g.key = nil
-}
diff --git a/internal/stupidgcm/stupidxchacha.go b/internal/stupidgcm/xchacha.go
index 9f2ac2f..d8668dc 100644
--- a/internal/stupidgcm/stupidxchacha.go
+++ b/internal/stupidgcm/xchacha.go
@@ -17,6 +17,8 @@ import (
 )
 
 type stupidXchacha20poly1305 struct {
+	// array instead of byte slice like
+	// `struct xchacha20poly1305` in x/crypto/chacha20poly1305
 	key   [chacha20poly1305.KeySize]byte
 	wiped bool
 }
@@ -41,7 +43,7 @@ func (*stupidXchacha20poly1305) NonceSize() int {
 }
 
 func (*stupidXchacha20poly1305) Overhead() int {
-	return 16
+	return tagLen
 }
 
 func (x *stupidXchacha20poly1305) Seal(dst, nonce, plaintext, additionalData []byte) []byte {
@@ -61,9 +63,8 @@ func (x *stupidXchacha20poly1305) Seal(dst, nonce, plaintext, additionalData []b
 		panic("plaintext too large")
 	}
 
-	c := new(stupidChacha20poly1305)
 	hKey, _ := chacha20.HChaCha20(x.key[:], nonce[0:16])
-	copy(c.key[:], hKey)
+	c := newChacha20poly1305(hKey)
 	defer c.Wipe()
 
 	// The first 4 bytes of the final nonce are unused counter space.
@@ -87,9 +88,8 @@ func (x *stupidXchacha20poly1305) Open(dst, nonce, ciphertext, additionalData []
 		panic("ciphertext too large")
 	}
 
-	c := new(stupidChacha20poly1305)
 	hKey, _ := chacha20.HChaCha20(x.key[:], nonce[0:16])
-	copy(c.key[:], hKey)
+	c := newChacha20poly1305(hKey)
 	defer c.Wipe()
 
 	// The first 4 bytes of the final nonce are unused counter space.
diff --git a/internal/stupidgcm/stupidxchacha_test.go b/internal/stupidgcm/xchacha_test.go
index fdea8b5..fdea8b5 100644
--- a/internal/stupidgcm/stupidxchacha_test.go
+++ b/internal/stupidgcm/xchacha_test.go