Mnemonic phrase is a list of words that store all the information needed for the recovery of a Bitcoin wallet. Usually, a wallet generates a mnemonic backup phrase by itself, so that the user could write it down on paper. If the user’s computer is broken or its hard disk is damaged, he will be able to upload the same wallet and use the paper backup copy to get his Bitcoins back. As every owner of a mnemonic phrase gets an access to the wallet, it must be kept very carefully.
Anybody else who discovers the phrase can steal the bitcoins. It must be kept secret, for example it must not be typed into any website. Mnemonic phrases are an excellent way of backing up and storing bitcoin so all good wallets use them.
What is a mnemonic phrase?
A mnemonic phrase, mnemonic recovery phrase or mnemonic seed is a list of words which store all the information needed to recover a bitcoin wallet. A wallet will typically generate a mnemonic backup phrase and instruct the user to write it down on paper. If the user's computer breaks or their hard drive becomes corrupted, they can download the same wallet again and use the paper backup to get their bitcoins back.
How it works
The software of a wallet uses a whole set of words taken from a vocabulary; at that every word is assigned to a specific number. A mnemonic phrase can be modified into a number, which is used as a seed that generates all pairs of keys for this wallet. The English list of words for the BIP39 standard has 2048 words. If a phrase consists of 12 words, then the number of possible combination is 2048 to the 12th power, or 2 to the 132nd power, i.e. the phrase will have 132 bits of safety.
Theoretically, one can think out the mnemonic phrase by his own, but it is not safe, because a human cannot do a great job with the random generation. The best way to secure one’s wallet is to take a mnemonic phrase generated by the program code.
Two-factor mnemonic phrases
Many wallets support the creation of a two-factor mnemonic phrase. In this case, the wallet generates a mnemonic phrase and requests a password or a code word by the user. Then, both the mnemonic phrase and the word / password will be needed to restore the wallet. The password can be written down together with the mnemonic phrase, but it would be more effective to remember it, so that a really two-factor recovery system could be achieved: for unblocking Bitcoins, you will need “what you have” plus “what you know”.
Storage of mnemonic sentence
The majority of people write the phrases on paper, but they can be kept in many other ways: remembered, written down on the margin of a book, engraved on metal and so on. For storing on paper, it would be better to write the phase with a pencil instead of a pen. The note should be kept in the darkness, avoiding extremely high or low temperature and humidity.
Some people divide their phrases: they keep six words in one place, another six – in another one. This is less than effective because discovering one half of the phrase makes finding another half easier. One more ineffective, but frequently used way: to add words to the phrase that are significant for you personally, to embarrass the plotter. As the vocabulary of words used for the generation of a mnemonic phrase is known, it won’t make any difficulties to cut off words that don’t belong to it.
An example of a mnemonic phrase is:
witch collapse practice feed shame open despair creek road again ice least
The order is important.
- BIP39 mnemonic phrase standard
- BIP39 wordlists
- Electrum old-style wordlist