PWDv1Challenge

How it works

The idea behind the PWDv1 challenge is, that the server should be able to verify that a client is in possession of the master password before handing over the encrypted keychain(s). To archive this, a salted hash (referred to as secret) of the master password is sent to the server when the challenge is created. A client trying to open a session must be able to compute the hash before it can continue. The secret is not stored on the server. It is used to encrypt the SSEv2 keychain and discarded afterwards. The server assumes that a salt is correct if it can decrypt the keychain with it.

The PWDv1 challenge is implemented using libsodium. The standard implementation of the PWDv1 challenge can be found in the passwords-client package.

Structure

Property	Type	Description
salts	Array	An array with the salts to compute the secret
salts[0]	string	A 512 byte hexadecimal encoded string with the password salt.
salts[1]	string	A 128 byte hexadecimal encoded string with the key for the generic hash.
salts[2]	string	A 32 byte hexadecimal encoded string with the password hash salt.
secret	string	A 64 byte hexadecimal encoded string with the secret. This is only present in the request to create the challenge

Solving the challenge

The master password is required to solve the challenge. It must be at least 12 and no more than 128 characters long. The provided salts need to be decoded before they can be used.

First, crypto_generichash must be used to compute the generic hash. The hash size is crypto_generichash_BYTES_MAX. The message consists of the user provided master password with the password salt (salt 0) appended. The key is the generic hash (salt 1) with the length crypto_generichash_KEYBYTES_MAX.

With the generic hash, the secret can be computed using crypto_pwhash. The key length is crypto_box_SEEDBYTES. The password is the generic hash. The salt is the password hash salt (salt 3) with the length crypto_pwhash_SALTBYTES. The ops limit is crypto_pwhash_OPSLIMIT_INTERACTIVE. The mem limit is crypto_pwhash_MEMLIMIT_INTERACTIVE. The algorithm is crypto_pwhash_ALG_ARGON2ID13 which is currently crypto_pwhash_ALG_DEFAULT.

The resulting key must then be encoded as a hexadecimal string and be sent to the server.

Creating the challenge

The master password is required to create the challenge. It must be at least 12 and no more than 128 characters long.

The password salt (salt 0) must be generated with 256 bytes of random data. The generic hash key (salt 1) must be generated with crypto_generichash_KEYBYTES_MAX bytes of random data. The password hash salt (salt 2) must be generated with crypto_pwhash_SALTBYTES bytes of random data. Libsodium provides the function randombytes_buf to generate secure random data.

Important: Always generate new salts when creating a new challenge and never re-use previously used salts.

First, crypto_generichash must be used to compute the generic hash. The hash size is crypto_generichash_BYTES_MAX. The message consists of the user provided master password with the password salt (salt 0) appended. The key is the generic hash (salt 1) with the length crypto_generichash_KEYBYTES_MAX.

With the generic hash, the secret can be computed using crypto_pwhash. The key length is crypto_box_SEEDBYTES. The password is the generic hash. The salt is the password hash salt (salt 3) with the length crypto_pwhash_SALTBYTES. The ops limit is crypto_pwhash_OPSLIMIT_INTERACTIVE. The mem limit is crypto_pwhash_MEMLIMIT_INTERACTIVE. The algorithm is crypto_pwhash_ALG_ARGON2ID13 which is currently crypto_pwhash_ALG_DEFAULT.

The challenge has to be sent to the server as an object with two keys. The salts property is an Array with the three salts encoded as hexadecimal strings. The secret property is the secret encoded as hexadecimal string.