rclone/docs/content/crypt.md

---
title: "Crypt"
description: "Encryption overlay remote"
date: "2016-07-28"
---

<i class="fa fa-lock"></i>Crypt
----------------------------------------

The `crypt` remote encrypts and decrypts another remote.

To use it first set up the underlying remote following the config
instructions for that remote.  You can also use a local pathname
instead of a remote which will encrypt and decrypt from that directory
which might be useful for encrypting onto a USB stick for example.

First check your chosen remote is working - we'll call it
`remote:path` in these docs.  Note that anything inside `remote:path`
will be encrypted and anything outside won't.  This means that if you
are using a bucket based remote (eg S3, B2, swift) then you should
probably put the bucket in the remote `s3:bucket`. If you just use
`s3:` then rclone will make encrypted bucket names too (if using file
name encryption) which may or may not be what you want.

Now configure `crypt` using `rclone config`. We will call this one
`secret` to differentiate it from the `remote`.

```
No remotes found - make a new one
n) New remote
s) Set configuration password
q) Quit config
n/s/q> n   
name> secret
Type of storage to configure.
Choose a number from below, or type in your own value
 1 / Amazon Drive
   \ "amazon cloud drive"
 2 / Amazon S3 (also Dreamhost, Ceph, Minio)
   \ "s3"
 3 / Backblaze B2
   \ "b2"
 4 / Dropbox
   \ "dropbox"
 5 / Encrypt/Decrypt a remote
   \ "crypt"
 6 / Google Cloud Storage (this is not Google Drive)
   \ "google cloud storage"
 7 / Google Drive
   \ "drive"
 8 / Hubic
   \ "hubic"
 9 / Local Disk
   \ "local"
10 / Microsoft OneDrive
   \ "onedrive"
11 / Openstack Swift (Rackspace Cloud Files, Memset Memstore, OVH)
   \ "swift"
12 / Yandex Disk
   \ "yandex"
Storage> 5
Remote to encrypt/decrypt.
remote> remote:path
How to encrypt the filenames.
Choose a number from below, or type in your own value
 1 / Don't encrypt the file names.  Adds a ".bin" extension only.
   \ "off"
 2 / Encrypt the filenames see the docs for the details.
   \ "standard"
filename_encryption> 2
Password or pass phrase for encryption.
y) Yes type in my own password
g) Generate random password
y/g> y
Enter the password:
password:
Confirm the password:
password:
Password or pass phrase for salt. Optional but recommended.
Should be different to the previous password.
y) Yes type in my own password
g) Generate random password
n) No leave this optional password blank
y/g/n> g
Password strength in bits.
64 is just about memorable
128 is secure
1024 is the maximum
Bits> 128
Your password is: JAsJvRcgR-_veXNfy_sGmQ
Use this password?
y) Yes
n) No
y/n> y
Remote config
--------------------
[secret]
remote = remote:path
filename_encryption = standard
password = CfDxopZIXFG0Oo-ac7dPLWWOHkNJbw
password2 = HYUpfuzHJL8qnX9fOaIYijq0xnVLwyVzp3y4SF3TwYqAU6HLysk
--------------------
y) Yes this is OK
e) Edit this remote
d) Delete this remote
y/e/d> y
```

**Important** The password is stored in the config file is lightly
obscured so it isn't immediately obvious what it is.  It is in no way
secure unless you use config file encryption.

A long passphrase is recommended, or you can use a random one.  Note
that if you reconfigure rclone with the same passwords/passphrases
elsewhere it will be compatible - all the secrets used are derived
from those two passwords/passphrases.

Note that rclone does not encrypt
  * file length - this can be calcuated within 16 bytes
  * modification time - used for syncing

## Example ##

To test I made a little directory of files using "standard" file name
encryption.

```
plaintext/
├── file0.txt
├── file1.txt
└── subdir
    ├── file2.txt
    ├── file3.txt
    └── subsubdir
        └── file4.txt
```

Copy these to the remote and list them back

```
$ rclone -q copy plaintext secret:
$ rclone -q ls secret:
        7 file1.txt
        6 file0.txt
        8 subdir/file2.txt
       10 subdir/subsubdir/file4.txt
        9 subdir/file3.txt
```

Now see what that looked like when encrypted

```
$ rclone -q ls remote:path
       55 hagjclgavj2mbiqm6u6cnjjqcg
       54 v05749mltvv1tf4onltun46gls
       57 86vhrsv86mpbtd3a0akjuqslj8/dlj7fkq4kdq72emafg7a7s41uo
       58 86vhrsv86mpbtd3a0akjuqslj8/7uu829995du6o42n32otfhjqp4/b9pausrfansjth5ob3jkdqd4lc
       56 86vhrsv86mpbtd3a0akjuqslj8/8njh1sk437gttmep3p70g81aps
```

Note that this retains the directory structure which means you can do this

```
$ rclone -q ls secret:subdir
        8 file2.txt
        9 file3.txt
       10 subsubdir/file4.txt
```

If don't use file name encryption then the remote will look like this
- note the `.bin` extensions added to prevent the cloud provider
attempting to interpret the data.

```
$ rclone -q ls remote:path
       54 file0.txt.bin
       57 subdir/file3.txt.bin
       56 subdir/file2.txt.bin
       58 subdir/subsubdir/file4.txt.bin
       55 file1.txt.bin
```

### File name encryption modes ###

Here are some of the features of the file name encryption modes

Off
  * doesn't hide file names or directory structure
  * allows for longer file names (~246 characters)
  * can use sub paths and copy single files

Standard
  * file names encrypted
  * file names can't be as long (~156 characters)
  * can use sub paths and copy single files
  * directory structure visibile
  * identical files names will have identical uploaded names
  * can use shortcuts to shorten the directory recursion

Cloud storage systems have various limits on file name length and
total path length which you are more likely to hit using "Standard"
file name encryption.  If you keep your file names to below 156
characters in length then you should be OK on all providers.

There may be an even more secure file name encryption mode in the
future which will address the long file name problem.

### Modified time and hashes ###

Crypt stores modification times using the underlying remote so support
depends on that.

Hashes are not stored for crypt.  However the data integrity is
protected by an extremely strong crypto authenticator.

## File formats ##

### File encryption ###

Files are encrypted 1:1 source file to destination object.  The file
has a header and is divided into chunks.

#### Header ####

  * 8 bytes magic string `RCLONE\x00\x00`
  * 24 bytes Nonce (IV)

The initial nonce is generated from the operating systems crypto
strong random number genrator.  The nonce is incremented for each
chunk read making sure each nonce is unique for each block written.
The chance of a nonce being re-used is miniscule.  If you wrote an
exabyte of data (10¹⁸ bytes) you would have a probability of
approximately 2×10⁻³² of re-using a nonce.

#### Chunk ####

Each chunk will contain 64kB of data, except for the last one which
may have less data.  The data chunk is in standard NACL secretbox
format. Secretbox uses XSalsa20 and Poly1305 to encrypt and
authenticate messages.

Each chunk contains:

  * 16 Bytes of Poly1305 authenticator
  * 1 - 65536 bytes XSalsa20 encrypted data

64k chunk size was chosen as the best performing chunk size (the
authenticator takes too much time below this and the performance drops
off due to cache effects above this).  Note that these chunks are
buffered in memory so they can't be too big.

This uses a 32 byte (256 bit key) key derived from the user password.

#### Examples ####

1 byte file will encrypt to

  * 32 bytes header
  * 17 bytes data chunk

49 bytes total

1MB (1048576 bytes) file will encrypt to

  * 32 bytes header
  * 16 chunks of 65568 bytes

1049120 bytes total (a 0.05% overhead).  This is the overhead for big
files.

### Name encryption ###

File names are encrypted segment by segment - the path is broken up
into `/` separated strings and these are encrypted individually.

File segments are padded using using PKCS#7 to a multiple of 16 bytes
before encryption.

They are then encrypted with EME using AES with 256 bit key. EME
(ECB-Mix-ECB) is a wide-block encryption mode presented in the 2003
paper "A Parallelizable Enciphering Mode" by Halevi and Rogaway.

This makes for determinstic encryption which is what we want - the
same filename must encrypt to the same thing otherwise we can't find
it on the cloud storage system.

This means that

  * filenames with the same name will encrypt the same
  * filenames which start the same won't have a common prefix

This uses a 32 byte key (256 bits) and a 16 byte (128 bits) IV both of
which are derived from the user password.

After encryption they are written out using a modified version of
standard `base32` encoding as described in RFC4648.  The standard
encoding is modified in two ways:

  * it becomes lower case (no-one likes upper case filenames!)
  * we strip the padding character `=`

`base32` is used rather than the more efficient `base64` so rclone can be
used on case insensitive remotes (eg Windows, Amazon Drive).

### Key derivation ###

Rclone uses `scrypt` with parameters `N=16384, r=8, p=1` with a an
optional user supplied salt (password2) to derive the 32+32+16 = 80
bytes of key material required.  If the user doesn't supply a salt
then rclone uses an internal one.

`scrypt` makes it impractical to mount a dictionary attack on rclone
encrypted data.  For full protection agains this you should always use
a salt.