In this commit, we publicly export the `word` field as it makes it
easier to programmatically interact with the package when attempting to
re-derive proper `cipherseed` instances. We also add a new `Index` field
as well to provide additional context for programmatic manipulating of
seeds.
In this commit, we add a new package implementing the aezeed cipher
seed scheme. This is a new scheme developed that aims to overcome the
two major short comings of BIP39: a lack of a version, and a lack of a
wallet birthday. A lack a version means that wallets may not
necessarily know *how* to re-derive addresses during the recovery
process. A lack of a birthday means that wallets don’t know how far
back to look in the chain to ensure that they derive *all* the proper
user addresses.
The aezeed scheme addresses these two drawbacks and adds a number of
desirable features. First, we start with the following plaintext seed:
{1 byte internal version || 2 byte timestamp || 16 bytes of entropy}.
The version field is for wallets to be able to know *how* to re-derive
the keys of the wallet.
The 2 byte timestamp is expressed in Bitcoin Days Genesis, meaning that
the number of days since the timestamp in Bitcoin’s genesis block. This
allow us to save space, and also avoid using a wasteful level of
granularity. With the currently, this can express time up until 2188.
Finally, the entropy is raw entropy that should be used to derive
wallet’s HD root.
Next, we’ll take the plaintext seed described above and encipher it to
procure a final cipher text. We’ll then take this cipher text (the
CipherSeed) and encode that using a 24-word mnemonic. The enciphering
process takes a user defined passphrase. If no passphrase is provided,
then the string “aezeed” will be used.
To encipher a plaintext seed (19 bytes) to arrive at an enciphered
cipher seed (33 bytes), we apply the following operations:
* First we take the external version an append it to our buffer. The
external version describes *how* we encipher. For the first version
(version 0), we’ll use scrypt(n=32768, r=8, p=1) and aezeed.
* Next, we’ll use scrypt (with the version 9 params) to generate a
strong key for encryption. We’ll generate a 32-byte key using 5 bytes
as a salt. The usage of the salt is meant to make the creation of
rainbow tables infeasible.
* Next, the enciphering process. We use aezeed, modern AEAD with
nonce-misuse resistance properties. The important trait we exploit is
that it’s an *arbitrary input length block cipher*. Additionally, it
has what’s essentially a configurable MAC size. In our scheme we’ll use
a value of 4, which acts as a 32-bit checksum. We’ll encrypt with our
generated seed, and use an AD of (version || salt). We'll them compute a
checksum over all the data, using crc-32, appending the result to the
end.
* Finally, we’ll encode this 33-byte cipher text using the default
world list of BIP 39 to produce 24 english words.
The `aezeed` cipher seed scheme has a few cool properties, notably:
* The mnemonic itself is a cipher text, meaning leaving it in
plaintext is advisable if the user also set a passphrase. This is in
contrast to BIP 39 where the mnemonic alone (without a passphrase) may
be sufficient to steal funds.
* A cipherseed can be modified to *change* the passphrase. This
means that if the users wants a stronger passphrase, they can decipher
(with the old passphrase), then encipher (with a new passphrase).
Compared to BIP 39, where if the users used a passphrase, since the
mapping is one way, they can’t change the passphrase of their existing
HD key chain.
* A cipher seed can be *upgraded*. Since we have an external version,
offline tools can be provided to decipher using the old params, and
encipher using the new params. In the future if we change ciphers,
change scrypt, or just the parameters of scrypt, then users can easily
upgrade their seed with an offline tool.
* We're able to verify that a user has input the incorrect passphrase,
and that the user has input the incorrect mnemonic independently.
