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Web3 Secret Storage Definition

This documents version 3 of the Web3 Secret Storage Definition.

Definition

The actual encoding and decoding of the file remains largely unchanged from version 1, except that the crypto algorithm is no longer fixed to AES-128-CBC (AES-128-CTR is now the minimal requirement). Most of the meanings/algorithm are similar to version 1, except mac, which is given as the SHA3 (keccak-256) of the concatenation of the last 16 bytes of the derived key together with the full ciphertext.

Secret key files are stored directly in ~/.web3/keystore (for Unix-like systems) and ~/AppData/Web3/keystore (for Windows). They may be named anything, but a good convention is <uuid>.json, where <uuid> is the 128-bit UUID given to the secret key (a privacy-preserving proxy for the secret key's address).

All such files have an associated password. To derive a given .json file's secret key, first derive the file's encryption key; this is done through taking the file's password and passing it through a key derivation function as described by the kdf key. KDF-dependent static and dynamic parameters to the KDF function are described in kdfparams key.

PBKDF2 must be supported by all minimally-compliant implementations, denoted though:

  • kdf: pbkdf2

For PBKDF2, the kdfparams include:

  • prf: Must be hmac-sha256 (may be extended in the future);
  • c: number of iterations;
  • salt: salt passed to PBKDF;
  • dklen: length for the derived key. Must be >= 32.

One the file's key has been derived, it should be verified through the derivation of the MAC. The MAC should be calculated as the SHA3 (keccak-256) hash of the byte array formed as the concatenations of the second-leftmost 16 bytes of the derived key with the ciphertext key's contents, i.e.:

KECCAK(DK[16..31] ++ <ciphertext>)

(where ++ is the concatenation operator)

This value should be compared to the contents of the mac key; if they are different, an alternative password should be requested (or the operation cancelled).

After the file's key has been verified, the cipher text (the ciphertext key in the file) may be decrypted using the symmetric encryption algorithm specified by the cipher key and parameterised through the cipherparams key. If the derived key size and the algorithm's key size are mismatched, the zero padded, rightmost bytes of the derived key should be used as the key to the algorithm.

All minimally-compliant implementations must support the AES-128-CTR algorithm, denoted through:

  • cipher: aes-128-ctr

This cipher takes the following parameters, given as keys to the cipherparams key:

  • iv: 128-bit initialisation vector for the cipher.

The key for the cipher is the leftmost 16 bytes of the derived key, i.e. DK[0..15]

The creation/encryption of a secret key should be essentially the reverse of these instructions. Make sure the uuid, salt and iv are actually random.

In addition to the version field, which should act as a "hard" identifier of version, implementations may also use minorversion to track smaller, non-breaking changes to the format.

Test Vectors

Details:

  • Address: 008aeeda4d805471df9b2a5b0f38a0c3bcba786b
  • ICAP: XE542A5PZHH8PYIZUBEJEO0MFWRAPPIL67
  • UUID: 3198bc9c-6672-5ab3-d9954942343ae5b6
  • Password: testpassword
  • Secret: 7a28b5ba57c53603b0b07b56bba752f7784bf506fa95edc395f5cf6c7514fe9d

PBKDF2-SHA-256

Test vector using AES-128-CTR and PBKDF2-SHA-256:

File contents of ~/.web3/keystore/3198bc9c-6672-5ab3-d9954942343ae5b6.json:

{
    "crypto" : {
        "cipher" : "aes-128-ctr",
        "cipherparams" : {
            "iv" : "6087dab2f9fdbbfaddc31a909735c1e6"
        },
        "ciphertext" : "5318b4d5bcd28de64ee5559e671353e16f075ecae9f99c7a79a38af5f869aa46",
        "kdf" : "pbkdf2",
        "kdfparams" : {
            "c" : 262144,
            "dklen" : 32,
            "prf" : "hmac-sha256",
            "salt" : "ae3cd4e7013836a3df6bd7241b12db061dbe2c6785853cce422d148a624ce0bd"
        },
        "mac" : "517ead924a9d0dc3124507e3393d175ce3ff7c1e96529c6c555ce9e51205e9b2"
    },
    "id" : "3198bc9c-6672-5ab3-d995-4942343ae5b6",
    "version" : 3
}

Intermediates:

  • Derived key: f06d69cdc7da0faffb1008270bca38f5e31891a3a773950e6d0fea48a7188551
  • MAC Body: e31891a3a773950e6d0fea48a71885515318b4d5bcd28de64ee5559e671353e16f075ecae9f99c7a79a38af5f869aa46
  • MAC 517ead924a9d0dc3124507e3393d175ce3ff7c1e96529c6c555ce9e51205e9b2
  • Cipher key: f06d69cdc7da0faffb1008270bca38f5

Scrypt

Test vector using AES-128-CTR and Scrypt:

{
    "crypto" : {
        "cipher" : "aes-128-ctr",
        "cipherparams" : {
            "iv" : "83dbcc02d8ccb40e466191a123791e0e"
        },
        "ciphertext" : "d172bf743a674da9cdad04534d56926ef8358534d458fffccd4e6ad2fbde479c",
        "kdf" : "scrypt",
        "kdfparams" : {
            "dklen" : 32,
            "n" : 262144,
            "r" : 1,
            "p" : 8,
            "salt" : "ab0c7876052600dd703518d6fc3fe8984592145b591fc8fb5c6d43190334ba19"
        },
        "mac" : "2103ac29920d71da29f15d75b4a16dbe95cfd7ff8faea1056c33131d846e3097"
    },
    "id" : "3198bc9c-6672-5ab3-d995-4942343ae5b6",
    "version" : 3
}

Intermediates:

  • Derived key: fac192ceb5fd772906bea3e118a69e8bbb5cc24229e20d8766fd298291bba6bd
  • MAC Body bb5cc24229e20d8766fd298291bba6bdd172bf743a674da9cdad04534d56926ef8358534d458fffccd4e6ad2fbde479c
  • MAC: 2103ac29920d71da29f15d75b4a16dbe95cfd7ff8faea1056c33131d846e3097
  • Cipher key: fac192ceb5fd772906bea3e118a69e8b

Alterations from Version 1

This version fixes several inconsistencies with the version 1 published here. In brief these are:

  • Capitalisation is unjustified and inconsistent (scrypt lowercase, Kdf mixed-case, MAC uppercase).
  • Address unnecessary and compromises privacy.
  • Salt is intrinsically a parameter of the key derivation function and deserves to be associated with it, not with the crypto in general.
  • SaltLen unnecessary (just derive it from Salt).
  • The key derivation function is given, yet the crypto algorithm is hard specified.
  • Version is intrinsically numeric yet is a string (structured versioning would be possible with a string, but can be considered out of scope for a rarely changing configuration file format).
  • KDF and cipher are notionally sibling concepts yet are organised differently.
  • MAC is calculated through a whitespace agnostic piece of data(!)

Changes have been made to the format to give the following file, functionally equivalent to the example given on the previously linked page:

{
    "crypto": {
        "cipher": "aes-128-cbc",
        "ciphertext": "07533e172414bfa50e99dba4a0ce603f654ebfa1ff46277c3e0c577fdc87f6bb4e4fe16c5a94ce6ce14cfa069821ef9b",
        "cipherparams": {
            "iv": "16d67ba0ce5a339ff2f07951253e6ba8"
        },
        "kdf": "scrypt",
        "kdfparams": {
            "dklen": 32,
            "n": 262144,
            "p": 1,
            "r": 8,
            "salt": "06870e5e6a24e183a5c807bd1c43afd86d573f7db303ff4853d135cd0fd3fe91"
        },
        "mac": "8ccded24da2e99a11d48cda146f9cc8213eb423e2ea0d8427f41c3be414424dd",
        "version": 1
    },
    "id": "0498f19a-59db-4d54-ac95-33901b4f1870",
    "version": 2
}

Alterations from Version 2

Version 2 was an early C++ implementation with a number of bugs. All essentials remain unchanged from it.