reverse: use per-purpose nonce generation

Also pull all the deterministic nonce code into fusefrontend_reverse
to greatly simplify the normal code path.

5 files changed, 46 insertions, 11 deletions

diff --git a/internal/fusefrontend_reverse/reverse_longnames.go b/internal/fusefrontend_reverse/reverse_longnames.go
index 6409d95..b3d21c9 100644
--- a/internal/fusefrontend_reverse/reverse_longnames.go
+++ b/internal/fusefrontend_reverse/reverse_longnames.go
@@ -84,7 +84,7 @@ func (rfs *reverseFS) newNameFile(relPath string) (nodefs.File, fuse.Status) {
 	if err != nil {
 		return nil, fuse.ToStatus(err)
 	}
-	dirIV := derivePathIV(cDir)
+	dirIV := derivePathIV(cDir, ivPurposeDirIV)
 	e, err := rfs.findLongnameParent(pDir, dirIV, longname)
 	if err != nil {
 		return nil, fuse.ToStatus(err)
diff --git a/internal/fusefrontend_reverse/rfile.go b/internal/fusefrontend_reverse/rfile.go
index 8c5e5f3..2656b7b 100644
--- a/internal/fusefrontend_reverse/rfile.go
+++ b/internal/fusefrontend_reverse/rfile.go
@@ -2,6 +2,7 @@ package fusefrontend_reverse
 
 import (
 	"bytes"
+	"encoding/binary"
 	"fmt"
 	"io"
 	"os"
@@ -20,6 +21,8 @@ type reverseFile struct {
 	fd *os.File
 	// File header (contains the IV)
 	header contentenc.FileHeader
+	// IV for block 0
+	block0IV []byte
 	// Content encryption helper
 	contentEnc *contentenc.ContentEnc
 }
@@ -33,7 +36,7 @@ func (rfs *reverseFS) NewFile(relPath string, flags uint32) (nodefs.File, fuse.S
 	if err != nil {
 		return nil, fuse.ToStatus(err)
 	}
-	id := derivePathIV(relPath)
+	id := derivePathIV(relPath, ivPurposeFileID)
 	header := contentenc.FileHeader{
 		Version: contentenc.CurrentVersion,
 		Id:      id,
@@ -42,6 +45,7 @@ func (rfs *reverseFS) NewFile(relPath string, flags uint32) (nodefs.File, fuse.S
 		File:       nodefs.NewDefaultFile(),
 		fd:         fd,
 		header:     header,
+		block0IV:   derivePathIV(relPath, ivPurposeBlock0IV),
 		contentEnc: rfs.contentEnc,
 	}, fuse.OK
 }
@@ -52,6 +56,26 @@ func (rf *reverseFile) GetAttr(*fuse.Attr) fuse.Status {
 	return fuse.ENOSYS
 }
 
+// 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)
+		outBuf.Write(outBlock)
+	}
+	return outBuf.Bytes()
+}
+
 // readBackingFile: read from the backing plaintext file, encrypt it, return the
 // ciphertext.
 // "off" ... ciphertext offset (must be >= HEADER_LEN)
@@ -71,7 +95,7 @@ func (rf *reverseFile) readBackingFile(off uint64, length uint64) (out []byte, e
 	plaintext = plaintext[0:n]
 
 	// Encrypt blocks
-	ciphertext := rf.contentEnc.EncryptBlocks(plaintext, blocks[0].BlockNo, rf.header.Id, contentenc.ReverseDeterministicNonce)
+	ciphertext := rf.encryptBlocks(plaintext, blocks[0].BlockNo, rf.header.Id, rf.block0IV)
 
 	// Crop down to the relevant part
 	lenHave := len(ciphertext)
diff --git a/internal/fusefrontend_reverse/rfs.go b/internal/fusefrontend_reverse/rfs.go
index 8411461..1b5e812 100644
--- a/internal/fusefrontend_reverse/rfs.go
+++ b/internal/fusefrontend_reverse/rfs.go
@@ -254,7 +254,7 @@ func (rfs *reverseFS) OpenDir(cipherPath string, context *fuse.Context) ([]fuse.
 	nVirtual := 1
 
 	// Encrypt names
-	dirIV := derivePathIV(cipherPath)
+	dirIV := derivePathIV(cipherPath, ivPurposeDirIV)
 	for i := range entries {
 		var cName string
 		// ".gocryptfs.reverse.conf" in the root directory is mapped to "gocryptfs.conf"
@@ -296,9 +296,9 @@ func (rfs *reverseFS) Readlink(cipherPath string, context *fuse.Context) (string
 	if rfs.args.PlaintextNames {
 		return plainTarget, fuse.OK
 	}
-	nonce := derivePathIV(cipherPath)
+	nonce := derivePathIV(cipherPath, ivPurposeSymlinkIV)
 	// Symlinks are encrypted like file contents and base64-encoded
-	cBinTarget := rfs.contentEnc.EncryptBlock([]byte(plainTarget), 0, nil, contentenc.ExternalNonce, nonce)
+	cBinTarget := rfs.contentEnc.EncryptBlockNonce([]byte(plainTarget), 0, nil, nonce)
 	cTarget := base64.URLEncoding.EncodeToString(cBinTarget)
 	return cTarget, fuse.OK
 }
diff --git a/internal/fusefrontend_reverse/rpath.go b/internal/fusefrontend_reverse/rpath.go
index 55fb481..ca8c442 100644
--- a/internal/fusefrontend_reverse/rpath.go
+++ b/internal/fusefrontend_reverse/rpath.go
@@ -19,9 +19,20 @@ func saneDir(path string) string {
 	return d
 }
 
-// derivePathIV derives an IV from an encrypted path by hashing it
-func derivePathIV(path string) []byte {
-	hash := sha256.Sum256([]byte(path))
+type ivPurposeType string
+
+const (
+	ivPurposeDirIV     ivPurposeType = "DIRIV"
+	ivPurposeFileID    ivPurposeType = "FILEID"
+	ivPurposeSymlinkIV ivPurposeType = "SYMLINKIV"
+	ivPurposeBlock0IV  ivPurposeType = "BLOCK0IV"
+)
+
+// derivePathIV derives an IV from an encrypted path by hashing it with sha256
+func derivePathIV(path string, purpose ivPurposeType) []byte {
+	// Use null byte as separator as it cannot occour in the path
+	extended := []byte(path + "\000" + string(purpose))
+	hash := sha256.Sum256(extended)
 	return hash[:nametransform.DirIVLen]
 }
 
@@ -43,7 +54,7 @@ func (rfs *reverseFS) decryptPath(relPath string) (string, error) {
 		// Start at the top and recurse
 		currentDir := filepath.Join(parts[:i]...)
 		nameType := nametransform.NameType(part)
-		dirIV := derivePathIV(currentDir)
+		dirIV := derivePathIV(currentDir, ivPurposeDirIV)
 		var transformedPart string
 		if nameType == nametransform.LongNameNone {
 			transformedPart, err = rfs.nameTransform.DecryptName(part, dirIV)
diff --git a/internal/fusefrontend_reverse/virtualfile.go b/internal/fusefrontend_reverse/virtualfile.go
index 351cbdc..2447e16 100644
--- a/internal/fusefrontend_reverse/virtualfile.go
+++ b/internal/fusefrontend_reverse/virtualfile.go
@@ -14,7 +14,7 @@ func (rfs *reverseFS) newDirIVFile(cRelPath string) (nodefs.File, fuse.Status) {
 	if err != nil {
 		return nil, fuse.ToStatus(err)
 	}
-	return rfs.NewVirtualFile(derivePathIV(cDir), absDir)
+	return rfs.NewVirtualFile(derivePathIV(cDir, ivPurposeDirIV), absDir)
 }
 
 type virtualFile struct {