From d9fc652df0957e464d83c87a164ee2b70cb9e4ee Mon Sep 17 00:00:00 2001
From: Jakob Unterwurzacher
Date: Mon, 26 Sep 2016 23:06:40 +0200
Subject: siv_aead: add AES-SIV AEAD wrapper

---
 internal/siv_aead/benchmark.bash      |  7 ++++
 internal/siv_aead/correctness_test.go | 66 +++++++++++++++++++++++++++++++++++
 internal/siv_aead/performance_test.go |  1 +
 internal/siv_aead/siv_aead.go         | 59 +++++++++++++++++++++++++++++++
 4 files changed, 133 insertions(+)
 create mode 100755 internal/siv_aead/benchmark.bash
 create mode 100644 internal/siv_aead/correctness_test.go
 create mode 100644 internal/siv_aead/performance_test.go
 create mode 100644 internal/siv_aead/siv_aead.go

diff --git a/internal/siv_aead/benchmark.bash b/internal/siv_aead/benchmark.bash
new file mode 100755
index 0000000..40b57b3
--- /dev/null
+++ b/internal/siv_aead/benchmark.bash
@@ -0,0 +1,7 @@
+#!/bin/bash
+
+set -eu
+
+cd "$(dirname "$0")"
+
+../stupidgcm/benchmark.bash
diff --git a/internal/siv_aead/correctness_test.go b/internal/siv_aead/correctness_test.go
new file mode 100644
index 0000000..c271970
--- /dev/null
+++ b/internal/siv_aead/correctness_test.go
@@ -0,0 +1,66 @@
+package siv_aead
+
+import (
+	"bytes"
+	"encoding/hex"
+	"testing"
+
+	"github.com/jacobsa/crypto/siv"
+)
+
+func TestAll(t *testing.T) {
+	key := bytes.Repeat([]byte{1}, 32)
+	nonce := bytes.Repeat([]byte{2}, 16)
+	plaintext := []byte{1, 2, 3, 4, 5, 6, 7, 8, 9}
+	aData := make([]byte, 24)
+	// Compare siv and siv_aead results
+	sResult, err := siv.Encrypt(nonce, key, plaintext, [][]byte{aData, nonce})
+	if err != nil {
+		t.Fatal(err)
+	}
+	a := New(key)
+	aResult := a.Seal(nonce, nonce, plaintext, aData)
+	if !bytes.Equal(sResult, aResult) {
+		t.Errorf("siv and siv_aead produce different results")
+	}
+	expectedResult, _ := hex.DecodeString(
+		"02020202020202020202020202020202ad7a4010649a84d8c1dd5f752e935eed57d45b8b10008f3834")
+	if !bytes.Equal(aResult, expectedResult) {
+		t.Errorf(hex.EncodeToString(aResult))
+	}
+	// Verify overhead
+	overhead := len(aResult) - len(plaintext) - len(nonce)
+	if overhead != a.Overhead() {
+		t.Errorf("Overhead() returns a wrong value")
+	}
+	// Decrypt
+	p1, err := a.Open(nil, aResult[:16], aResult[16:], aData)
+	if err != nil {
+		t.Error(err)
+	}
+	if !bytes.Equal(plaintext, p1) {
+		t.Errorf("wrong plaintext")
+	}
+	// Decrypt and append
+	dst := []byte{0xaa, 0xbb, 0xcc}
+	p2, err := a.Open(dst, aResult[:16], aResult[16:], aData)
+	if err != nil {
+		t.Error(err)
+	}
+	p2e := append(dst, plaintext...)
+	if !bytes.Equal(p2e, p2) {
+		t.Errorf("wrong plaintext: %s", hex.EncodeToString(p2))
+	}
+	// Decrypt corrupt
+	aResult[17] = 0
+	_, err = a.Open(nil, aResult[:16], aResult[16:], aData)
+	if err == nil {
+		t.Error("should have failed")
+	}
+	// Decrypt and append corrupt
+	aResult[17] = 0
+	_, err = a.Open(dst, aResult[:16], aResult[16:], aData)
+	if err == nil {
+		t.Error("should have failed")
+	}
+}
diff --git a/internal/siv_aead/performance_test.go b/internal/siv_aead/performance_test.go
new file mode 100644
index 0000000..626024e
--- /dev/null
+++ b/internal/siv_aead/performance_test.go
@@ -0,0 +1 @@
+package siv_aead
diff --git a/internal/siv_aead/siv_aead.go b/internal/siv_aead/siv_aead.go
new file mode 100644
index 0000000..21106a5
--- /dev/null
+++ b/internal/siv_aead/siv_aead.go
@@ -0,0 +1,59 @@
+// Package siv_aead wraps the functions provided by siv
+// in a crypto.AEAD interface.
+package siv_aead
+
+import (
+	"github.com/jacobsa/crypto/siv"
+)
+
+type sivAead struct {
+	key []byte
+}
+
+func New(key []byte) *sivAead {
+	return &sivAead{
+		key: key,
+	}
+}
+
+func (s *sivAead) NonceSize() int {
+	// SIV supports any nonce size, but in gocryptfs we exclusively use 16.
+	return 16
+}
+
+func (s *sivAead) Overhead() int {
+	// RFC5297:
+	// [...] the key length used by AES in CTR and S2V is len(K)/2 and will
+	// each be either 128 bits, 192 bits, or 256 bits.
+	return len(s.key) / 2
+
+}
+
+// Seal - encrypt "in" using "nonce" and "authData" and append the result to "dst"
+func (s *sivAead) Seal(dst, nonce, plaintext, authData []byte) []byte {
+	if len(nonce) != 16 {
+		// SIV supports any nonce size, but in gocryptfs we exclusively use 16.
+		panic("nonce must be 16 bytes long")
+	}
+	// https://github.com/jacobsa/crypto/blob/master/siv/encrypt.go#L48:
+	// As per RFC 5297 section 3, you may use this function for nonce-based
+	// authenticated encryption by passing a nonce as the last associated
+	// data element.
+	associated := [][]byte{authData, nonce}
+	out, err := siv.Encrypt(dst, s.key, plaintext, associated)
+	if err != nil {
+		panic(err)
+	}
+	return out
+}
+
+// Open - decrypt "in" using "nonce" and "authData" and append the result to "dst"
+func (s *sivAead) Open(dst, nonce, ciphertext, authData []byte) ([]byte, error) {
+	if len(nonce) != 16 {
+		// SIV supports any nonce size, but in gocryptfs we exclusively use 16.
+		panic("nonce must be 16 bytes long")
+	}
+	associated := [][]byte{authData, nonce}
+	dec, err := siv.Decrypt(s.key, ciphertext, associated)
+	return append(dst, dec...), err
+}
-- 
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