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SLIP-0016 : Format for password storage and its encryption

Number:  SLIP-0016
Title:   Format for password storage and its encryption
Type:    Standard
Status:  Final
Authors: Peter Jensen <[email protected]>
Created: 2016-18-02

Abstract

SLIP-0016 describes simple encryption concept for hardware device for secure storage of passwords.

General design

At first, we derive a master key from HW device itself, which is divided in two parts. First part is hashed and used as a name of storage file. Second part is used for primary storage encryption.

Storage file is encrypted JSON object, which contains configuration, tags and separate entries. Each entry has other two encrypted properties derivated from device to provide higher level of security with low risk of leaks.

Design details

Deriving master key

We derive masterKey from hardware device by sending cipherKeyValue with following params:

  • path: m/10016'/0 (hardened path, see BIP32)
  • ENC_KEY: 'Unlock encrypted storage?' (max length message is 256 bytes)
  • ENC_VALUE: '2d650551248d792eabf628f451200d7f51cb63e46aadcbb1038aacb05e8c8aee2d650551248d792eabf628f451200d7f51cb63e46aadcbb1038aacb05e8c8aee' (in hexadecimal (128 /2), max length is 1024 bytes)
  • encrypt: true
  • ask_on_encrypt: true
  • ask_on_decrypt: true
  • iv: unset

JS EXAMPLE:

session.cipherKeyValue(
[(10016 | 0x80000000) >>> 0, 0],
'Unlock encrypted storage?',
'2d650551248d792eabf628f451200d7f51cb63e46aadcbb1038aacb05e8c8aee2d650551248d792eabf628f451200d7f51cb63e46aadcbb1038aacb05e8c8aee',
true, true, true)

CipherKeyValue is defined in SLIP-0011.

Deriving file name

From the first half of master key, we derive the file name for every user/device in the following way: First, we use the HMAC function:

HMAC-SHA256(fileKey, FILENAME_MESS)

where:

  • fileKey is the first half of masterKey (masterKey.substring(0, masterKey.length / 2))
  • FILENAME_MESS is a constant string '5f91add3fa1c3c76e90c90a3bd0999e2bd7833d06a483fe884ee60397aca277a'

The output result is digested to HEX string. After, we append extension '.pswd'

EXAMPLE RESULT: a80387a2222f4360f71fd2165368c6ed91b26287d9bc1ce8be71e64e6b216a4f.pswd

Deriving encryption key and file level encryption

As an encryption key is used the SECOND half (32 bytes) of master key for the first level of data file encryption. Encryption key is in HEX string.

For encrypt/decrypt we are using AES-256-GCM algorithm.

  • Input Vector (IV) is 12 randomly generated bytes
  • GCM is used with full 128-bit autentication tag (authTag)

more info

The result output stored in file is:

  • first 12 bytes of the file is randomly generated IV
  • next 16 bytes is the GCM authTag
  • the rest is output ciphertext

more info

Data format

(Decrypted) data file is serialized JSON object with the following keys:

  • version: for future backwards compatibility and data storage manipulation
  • config: for remembering personal setup of application
  • tags: contain set of labels with their icons (from icomoon set). Default tag is All and it is only tag, unable to edit or delete.
{title:"My social networks", icon:"person", active:"active"}
  • entries: is object of all password entries encrypted second time
{
    "title": "http://wwww.github.com",
    "username": "Satoshi Nakamoto",
    "nonce": "8688105887642a3cbb61889d8762432ef864df107e097d2b19e93c8d808c2e21",
    "note": "public note",
    "password": {},
    "safe_note": {},
    "tags": [1]
}

Entry level encryption

Every entry contains keys from upper example.

  • title: title is represented as string. If given string is matching URL, it will be shown on device as domain without protocol prefix.
  • username: string, will be passed to device, in encryption/decryption process
  • nonce: hidden generated string which is output of cipherKeyValue over Title + Username key and random values
  • password: is buffer array output of plain string and nonce (encryption process described later)
  • safe_note: is also buffer array output of plain string and nonce (also described later)
  • note: is plain UTF8 string
  • tags: is array of Tags key values

Step by step entry encryption:

  1. Generate random 32 bytes buffer and convert to HEX string inadequately called nonce
  2. Set key as 'Unlock ' + title + ' for user ' + username + '?'
  3. Ask device for cipherKeyValue, where path is the same as in the deriving file name, key is described in second step and enc_value is our nonce from the first step. Do not forget to setup properly other three bool values!

EXAMPLE:

session.cipherKeyValue(
[(10016 | 0x80000000) >>> 0, 0], // same path
'Unlock github.com for user Satoshi Nakamoto?',
'2d650551248d792eabf628f451200d7f51cb63e46aadcbb1038aacb05e8c8aee2d650551248d792eabf628f451200d7f51cb63e46aadcbb1038aacb05e8c8aee',
true,   //encrypt? - has to be TRUE in encryption
false,  //askOnEncrypt? is the same in encryption and decryption
true) // askOnDecrypt? we want this becuase otherwise somebody could rob us!
  1. Then we use our famous nonce from the first step in AES-256-GCM algorithm encryption for password string and safe_note string. Process of encryption is the same as in the deriving encryption key and file level encryption. So basically we get some Buffer array output with 12 bytes of IV and 16 bytes of GCM authTag and the rest is cipherText.
  2. Output of each encryption is stored to appropriate keys, just instead of generated nonce we store result from third step ( cipherKeyValue) which we later use for decryption process

Entry decryption

  1. We ask device for the same cipherKeyValue as in encryption process, just instead of nonce, we use our encrypted result and boolean value encrypt? is false!

EXAMPLE:

session.cipherKeyValue(
[(10016 | 0x80000000) >>> 0, 0], // same path
'Unlock github.com for user Satoshi Nakamoto?',
'8688105887642a3cbb61889d8762432ef864df107e097d2b19e93c8d808c2e21',
false,   //encrypt? - has to be FALSE in decryption
false,  //askOnEncrypt? is the same in encryption and decryption
true) // askOnDecrypt? we want this becuase otherwise somebody could rob us!
  1. Other steps are the same as in entry encryption, we just symetrically decrypt values of password and safe_note via AES-256-GCM algorithm. Size of IV and authTag for AES is the same as in encryption. Beware on cipher Key data type - it must be hex. Output is in JSON.

Check example of password reader implementation in Python: pwd_reader.py - there is an example code for decryption.