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Diffstat (limited to 'internal/stupidgcm/xchacha.go')
-rw-r--r--internal/stupidgcm/xchacha.go111
1 files changed, 111 insertions, 0 deletions
diff --git a/internal/stupidgcm/xchacha.go b/internal/stupidgcm/xchacha.go
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+++ b/internal/stupidgcm/xchacha.go
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+// 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 {
+ // array instead of byte slice like
+ // `struct xchacha20poly1305` in x/crypto/chacha20poly1305
+ 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 tagLen
+}
+
+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")
+ }
+
+ hKey, _ := chacha20.HChaCha20(x.key[:], nonce[0:16])
+ c := newChacha20poly1305(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")
+ }
+
+ hKey, _ := chacha20.HChaCha20(x.key[:], nonce[0:16])
+ c := newChacha20poly1305(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
+ }
+}