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|
package cryptfs
import (
"crypto/cipher"
"crypto/aes"
"fmt"
"strings"
"encoding/base64"
"errors"
"os"
)
const (
NONCE_LEN = 12
AUTH_TAG_LEN = 16
DEFAULT_PLAINBS = 4096
ENCRYPT = true
DECRYPT = false
)
type CryptFS struct {
blockCipher cipher.Block
gcm cipher.AEAD
plainBS int64
cipherBS int64
}
func NewCryptFS(key [16]byte) *CryptFS {
b, err := aes.NewCipher(key[:])
if err != nil {
panic(err)
}
g, err := cipher.NewGCM(b)
if err != nil {
panic(err)
}
return &CryptFS{
blockCipher: b,
gcm: g,
plainBS: DEFAULT_PLAINBS,
cipherBS: DEFAULT_PLAINBS + NONCE_LEN + AUTH_TAG_LEN,
}
}
func (fs *CryptFS) NewFile(f *os.File) *CryptFile {
return &CryptFile {
file: f,
gcm: fs.gcm,
plainBS: fs.plainBS,
cipherBS: fs.cipherBS,
}
}
// DecryptName - decrypt filename
func (be *CryptFS) decryptName(cipherName string) (string, error) {
bin, err := base64.URLEncoding.DecodeString(cipherName)
if err != nil {
return "", err
}
if len(bin) % aes.BlockSize != 0 {
return "", errors.New(fmt.Sprintf("Name len=%d is not a multiple of 16", len(bin)))
}
iv := make([]byte, aes.BlockSize) // TODO ?
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 filename
func (be *CryptFS) encryptName(plainName string) string {
bin := []byte(plainName)
bin = be.pad16(bin)
iv := make([]byte, 16) // TODO ?
cbc := cipher.NewCBCEncrypter(be.blockCipher, iv)
cbc.CryptBlocks(bin, bin)
cipherName64 := base64.URLEncoding.EncodeToString(bin)
return cipherName64
}
// TranslatePath - encrypt or decrypt path. Just splits the string on "/"
// and hands the parts to EncryptName() / DecryptName()
func (be *CryptFS) translatePath(path string, op bool) (string, error) {
var err error
// Empty string means root directory
if path == "" {
return path, err
}
// Run operation on each path component
var translatedParts []string
parts := strings.Split(path, "/")
for _, part := range parts {
var newPart string
if op == ENCRYPT {
newPart = be.encryptName(part)
} else {
newPart, err = be.decryptName(part)
if err != nil {
return "", err
}
}
translatedParts = append(translatedParts, newPart)
}
return strings.Join(translatedParts, "/"), err
}
// EncryptPath - encrypt filename or path. Just hands it to TranslatePath().
func (be *CryptFS) EncryptPath(path string) string {
newPath, _ := be.translatePath(path, ENCRYPT)
return newPath
}
// DecryptPath - decrypt filename or path. Just hands it to TranslatePath().
func (be *CryptFS) DecryptPath(path string) (string, error) {
return be.translatePath(path, DECRYPT)
}
// plainSize - calculate plaintext size from ciphertext size
func (be *CryptFS) PlainSize(s int64) int64 {
// Zero sized files stay zero-sized
if s > 0 {
// Number of blocks
n := s / be.cipherBS + 1
overhead := be.cipherBS - be.plainBS
s -= n * overhead
}
return s
}
// 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
}
|
