Add file header (on-disk-format change)

Format: [ "Version" uint16 big endian ] [ "Id" 16 random bytes ]

Quoting SECURITY.md:

* Every file has a header that contains a 16-byte random *file id*
* Each block uses the file id and its block number as GCM *authentication data*
 * This means the position of the blocks is protected as well. The blocks
   can not be reordered or copied between different files without
   causing an decryption error.

1 files changed, 56 insertions, 0 deletions

diff --git a/cryptfs/file_header.go b/cryptfs/file_header.go
new file mode 100644
index 0000000..3fd7266
--- /dev/null
+++ b/cryptfs/file_header.go
@@ -0,0 +1,56 @@
+package cryptfs
+
+// Per-file header
+//
+// Format: [ "Version" uint16 big endian ] [ "Id" 16 random bytes ]
+
+import (
+	"encoding/binary"
+	"fmt"
+)
+
+const (
+	HEADER_CURRENT_VERSION  = 1 // Current on-disk-format version
+	HEADER_VERSION_LEN = 2 // uint16
+	HEADER_ID_LEN = 16 // 128 bit random file id
+	HEADER_LEN = HEADER_VERSION_LEN + HEADER_ID_LEN // Total header length
+)
+
+type FileHeader struct {
+	Version uint16
+	Id []byte
+}
+
+// Pack - serialize fileHeader object
+func (h *FileHeader) Pack() []byte {
+	if len(h.Id) != HEADER_ID_LEN || h.Version != HEADER_CURRENT_VERSION {
+		panic("FileHeader object not properly initialized")
+	}
+	buf := make([]byte, HEADER_LEN)
+	binary.BigEndian.PutUint16(buf[0:HEADER_VERSION_LEN], h.Version)
+	copy(buf[HEADER_VERSION_LEN:], h.Id)
+	return buf
+
+}
+
+// ParseHeader - parse "buf" into fileHeader object
+func ParseHeader(buf []byte) (*FileHeader, error) {
+	if len(buf) != HEADER_LEN {
+		return nil, fmt.Errorf("ParseHeader: invalid length: got %d, want %d", len(buf), HEADER_LEN)
+	}
+	var h FileHeader
+	h.Version = binary.BigEndian.Uint16(buf[0:HEADER_VERSION_LEN])
+	if h.Version != HEADER_CURRENT_VERSION {
+		return nil, fmt.Errorf("ParseHeader: invalid version: got %d, want %d", h.Version, HEADER_CURRENT_VERSION)
+	}
+	h.Id = buf[HEADER_VERSION_LEN:]
+	return &h, nil
+}
+
+// RandomHeader - create new fileHeader object with random Id
+func RandomHeader() *FileHeader {
+	var h FileHeader
+	h.Version = HEADER_CURRENT_VERSION
+	h.Id = RandBytes(HEADER_ID_LEN)
+	return &h
+}