polyseed

Description:

polyseed

Polyseed #
A dart implementation of polyseed.
Features #

16 mnemonic words (36% shorter than the original 25-word seed)
embedded wallet birthday to optimize restoring from the seed
supports encryption by a passphrase
can store up to 3 custom bits
advanced checksum based on a polynomial code
seeds are incompatible between different coins

Supported languages:

English
Japanese
Korean
Spanish
French
Italian
Czech
Portuguese
Chinese (Simplified)
Chinese (Traditional)

For languages based on the latin alphabet, just the first 4 characters of each word need to be provided when restoring a
seed. French and Spanish seeds can be input with or without accents. Wordlists are based
on BIP-39 with a few minor changes.
Encoding #
Each word contains 11 bits of information. The data are encoded as follows:



word #
contents




1
checksum (11 bits)


2-6
secret seed (10 bits) + features (1 bit)


7-16
secret seed (10 bits) + birthday (1 bit)



In total, there are 11 bits for the checksum, 150 bits for the secret seed, 5 feature bits and 10 birthday bits. Because
the feature and birthday bits are non-random, they are spread over the 15 data words so that two different mnemonic
phrases are unlikely to have the same word in the same position.
Checksum #
The mnemonic phrase can be treated as a polynomial over GF(2048), which enables the use of an efficient Reed-Solomon
error correction code with one check word. All single-word errors can be detected and all single-word erasures can be
corrected without false positives.
To prevent the seed from being accidentally used with a different cryptocurrency, a coin flag is XORed with the second
word after the checksum is calculated. Checksum validation will fail unless the wallet software XORs the same coin flag
with the second word when restoring.
Feature bits #
There are 5 feature bits in the phrase. The first 2 bits are for internal use (one bit is used to indicate a seed
encrypted by a passphrase and the other bit is reserved for a future update of the key derivation function). The
remaining 3 bits are reserved for library users and can be enabled and accessed through the API. The library requires
reserved bits to be zero (if not, UnsupportedSeedFeatureException is thrown).
Wallet birthday #
The mnemonic phrase stores the approximate date when the wallet was created. This allows the seed to be generated
offline without access to the blockchain. Wallet software can easily convert a date to the corresponding block height
when restoring a seed.
The wallet birthday has a resolution of 2629746 seconds (1/12 of the average Gregorian year). All dates between November
2021 and February 2107 can be represented.
Secret seed #
Polyseed was designed for the 128-bit security level. This corresponds to the security of the ed25519 elliptic curve,
which requires about 2126 operations to break a key.
The private key is derived from the 150-bit secret seed using PBKDF2-HMAC-SHA256 with 10000 iterations. The KDF
parameters were selected to allow for the key to be derived by hardware wallets. Key generation is domain-separated by
the wallet birthday month, seed features and the coin flag.
The size of the secret seed and the domain separation parameters provide a comfortable security margin
against multi-target attacks.