From c6dacd6f913b4c6eb7a8917af49190dce32db108 Mon Sep 17 00:00:00 2001
From: Jakob Unterwurzacher
Date: Tue, 8 Dec 2015 16:13:29 +0100
Subject: Add EME filename encryption & enable it by default

---
 cryptfs/names_core.go | 134 ++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 134 insertions(+)
 create mode 100644 cryptfs/names_core.go

(limited to 'cryptfs/names_core.go')

diff --git a/cryptfs/names_core.go b/cryptfs/names_core.go
new file mode 100644
index 0000000..0f2e5b3
--- /dev/null
+++ b/cryptfs/names_core.go
@@ -0,0 +1,134 @@
+package cryptfs
+
+// Filename encryption / decryption functions
+
+import (
+	"crypto/aes"
+	"crypto/cipher"
+	"encoding/base64"
+	"errors"
+	"fmt"
+
+	"github.com/rfjakob/eme"
+)
+
+// DecryptName - decrypt base64-encoded encrypted filename "cipherName"
+// The used encryption is either CBC or EME, depending on the "EMENames" argument.
+//
+// This function is exported because it allows for a very efficient readdir
+// implementation (read IV once, decrypt all names using this function).
+func (be *CryptFS) DecryptName(cipherName string, iv []byte, EMENames bool) (string, error) {
+	return be.decryptName(cipherName, iv, EMENames)
+}
+
+// decryptName - decrypt base64-encoded encrypted filename "cipherName".
+// The used encryption is either CBC or EME, depending on the "EMENames" argument.
+func (be *CryptFS) decryptName(cipherName string, iv []byte, EMENames bool) (string, error) {
+
+	// Make sure relative symlinks still work after encryption
+	// by passing these through unchanged
+	if cipherName == "." || cipherName == ".." {
+		return cipherName, nil
+	}
+
+	bin, err := base64.URLEncoding.DecodeString(cipherName)
+	if err != nil {
+		return "", err
+	}
+
+	if len(bin)%aes.BlockSize != 0 {
+		return "", fmt.Errorf("Decoded length %d is not a multiple of the AES block size", len(bin))
+	}
+
+	if EMENames {
+		bin = eme.Transform(be.blockCipher, iv, bin, eme.DirectionDecrypt)
+	} else {
+		cbc := cipher.NewCBCDecrypter(be.blockCipher, iv)
+		cbc.CryptBlocks(bin, bin)
+	}
+
+	bin, err = be.unPad16(bin)
+	if err != nil {
+		return "", err
+	}
+
+	plain := string(bin)
+	return plain, err
+}
+
+// encryptName - encrypt "plainName", return base64-encoded "cipherName64"
+// The used encryption is either CBC or EME, depending on the "EMENames" argument.
+func (be *CryptFS) encryptName(plainName string, iv []byte, EMENames bool) (cipherName64 string) {
+
+	// Make sure relative symlinks still work after encryption
+	// by passing these trough unchanged
+	if plainName == "." || plainName == ".." {
+		return plainName
+	}
+
+	bin := []byte(plainName)
+	bin = be.pad16(bin)
+
+	if EMENames {
+		bin = eme.Transform(be.blockCipher, iv, bin, eme.DirectionEncrypt)
+	} else {
+		cbc := cipher.NewCBCEncrypter(be.blockCipher, iv)
+		cbc.CryptBlocks(bin, bin)
+	}
+
+	cipherName64 = base64.URLEncoding.EncodeToString(bin)
+	return cipherName64
+}
+
+// pad16 - pad filename to 16 byte blocks using standard PKCS#7 padding
+// https://tools.ietf.org/html/rfc5652#section-6.3
+func (be *CryptFS) pad16(orig []byte) (padded []byte) {
+	oldLen := len(orig)
+	if oldLen == 0 {
+		panic("Padding zero-length string makes no sense")
+	}
+	padLen := aes.BlockSize - oldLen%aes.BlockSize
+	if padLen == 0 {
+		padLen = aes.BlockSize
+	}
+	newLen := oldLen + padLen
+	padded = make([]byte, newLen)
+	copy(padded, orig)
+	padByte := byte(padLen)
+	for i := oldLen; i < newLen; i++ {
+		padded[i] = padByte
+	}
+	return padded
+}
+
+// unPad16 - remove padding
+func (be *CryptFS) unPad16(orig []byte) ([]byte, error) {
+	oldLen := len(orig)
+	if oldLen%aes.BlockSize != 0 {
+		return nil, errors.New("Unaligned size")
+	}
+	// The last byte is always a padding byte
+	padByte := orig[oldLen-1]
+	// The padding byte's value is the padding length
+	padLen := int(padByte)
+	// Padding must be at least 1 byte
+	if padLen <= 0 {
+		return nil, errors.New("Padding cannot be zero-length")
+	}
+	// Larger paddings make no sense
+	if padLen > aes.BlockSize {
+		return nil, errors.New("Padding cannot be larger than 16")
+	}
+	// All padding bytes must be identical
+	for i := oldLen - padLen; i < oldLen; i++ {
+		if orig[i] != padByte {
+			return nil, errors.New(fmt.Sprintf("Padding byte at i=%d is invalid", i))
+		}
+	}
+	newLen := oldLen - padLen
+	// Padding an empty string makes no sense
+	if newLen == 0 {
+		return nil, errors.New("Unpadded length is zero")
+	}
+	return orig[0:newLen], nil
+}
-- 
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