pathiv: move block IV algorithm into this package

This was implemented in fusefrontend_reverse, but we need it
in fusefrontend as well. Move the algorithm into pathiv.BlockIV().

internal/fusefrontend_reverse/rfile.go

@@ -2,7 +2,6 @@ package fusefrontend_reverse
 
 import (
 	"bytes"
-	"encoding/binary"
 	"io"
 	"os"
 	"syscall"
@@ -95,18 +94,13 @@ func (rf *reverseFile) GetAttr(*fuse.Attr) fuse.Status {
 // encryptBlocks - encrypt "plaintext" into a number of ciphertext blocks.
 // "plaintext" must already be block-aligned.
 func (rf *reverseFile) encryptBlocks(plaintext []byte, firstBlockNo uint64, fileID []byte, block0IV []byte) []byte {
-	nonce := make([]byte, len(block0IV))
-	copy(nonce, block0IV)
-	block0IVlow := binary.BigEndian.Uint64(block0IV[8:])
-	nonceLow := nonce[8:]
-
 	inBuf := bytes.NewBuffer(plaintext)
 	var outBuf bytes.Buffer
 	bs := int(rf.contentEnc.PlainBS())
 	for blockNo := firstBlockNo; inBuf.Len() > 0; blockNo++ {
-		binary.BigEndian.PutUint64(nonceLow, block0IVlow+blockNo)
 		inBlock := inBuf.Next(bs)
-		outBlock := rf.contentEnc.EncryptBlockNonce(inBlock, blockNo, fileID, nonce)
+		iv := pathiv.BlockIV(block0IV, blockNo)
+		outBlock := rf.contentEnc.EncryptBlockNonce(inBlock, blockNo, fileID, iv)
 		outBuf.Write(outBlock)
 	}
 	return outBuf.Bytes()

internal/pathiv/pathiv.go

@@ -2,6 +2,7 @@ package pathiv
 
 import (
 	"crypto/sha256"
+	"encoding/binary"
 
 	"github.com/rfjakob/gocryptfs/internal/nametransform"
 )
@@ -42,3 +43,15 @@ func DeriveFile(path string) (fileIVs FileIVs) {
 	fileIVs.Block0IV = Derive(path, PurposeBlock0IV)
 	return fileIVs
 }
+
+// BlockIV returns the block IV for block number "blockNo". "block0iv" is the block
+// IV of block #0.
+func BlockIV(block0iv []byte, blockNo uint64) []byte {
+	iv := make([]byte, len(block0iv))
+	copy(iv, block0iv)
+	// Add blockNo to one half of the iv
+	lowBytes := iv[8:]
+	lowInt := binary.BigEndian.Uint64(lowBytes)
+	binary.BigEndian.PutUint64(lowBytes, lowInt+blockNo)
+	return iv
+}

internal/pathiv/pathiv_test.go

@@ -0,0 +1,28 @@
+package pathiv
+
+import (
+	"bytes"
+	"encoding/hex"
+	"testing"
+)
+
+// TestBlockIV makes sure we don't change the block iv derivation algorithm "BlockIV()"
+// inadvertedly.
+func TestBlockIV(t *testing.T) {
+	b0 := make([]byte, 16)
+	b0x := BlockIV(b0, 0)
+	if !bytes.Equal(b0, b0x) {
+		t.Errorf("b0x should be equal to b0")
+	}
+	b27 := BlockIV(b0, 0x27)
+	expected, _ := hex.DecodeString("00000000000000000000000000000027")
+	if !bytes.Equal(b27, expected) {
+		t.Error()
+	}
+	bff := bytes.Repeat([]byte{0xff}, 16)
+	b28 := BlockIV(bff, 0x28)
+	expected, _ = hex.DecodeString("ffffffffffffffff0000000000000027")
+	if !bytes.Equal(b28, expected) {
+		t.Errorf("\nhave=%s\nwant=%s", hex.EncodeToString(b28), hex.EncodeToString(expected))
+	}
+}