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LESSONS

BIP 85: many wallets from one backup

BIP 85 derives independent child seeds from one master seed, so a single metal backup can recreate your hot wallet, a family member's wallet and a business wallet. What it gives you, what it does not, and how to use it on a signer.

6 MIN READ
COURSE · Keys, addresses and ScriptSTAGE 5 · MASTERLESSON 7 OF 9
A master seed derives independent child seeds through the BIP 85 path.
LEVEL · Advanced
IN THIS LESSON · 7 OF 9
BY THE END YOU WILL BE ABLE TO ANSWER
  • What does BIP 85 derive from the master seed?
  • What does a compromised BIP 85 child reveal?
  • What must you record alongside the master backup?
  • You derived child seeds at indexes 0, 1 and 2 for spending, a relative and a business. Your phone dies. From the master seed and your index list, how many of the three can you recreate?

Then prove it: a 3-question quiz · a challenge. The lesson marks itself complete at 100%.

After this lesson you can: derive a child seed from your master seed on a signer, load it into a hot wallet, and explain why one backup now covers both.

The problem it solves

Every wallet you create is another seed to back up. A phone wallet for spending, a wallet for a relative you are helping, a small business wallet: three seeds, three plates, three things to lose. BIP 85 (Ethan Kosakovsky, 2020) uses the BIP 32 tree you met in the previous lesson to derive new, independent seeds from your master seed at a special path: m/83696968'/39'/0'/12'/0' means "BIP 85, application BIP 39, English, 12 words, index 0". Change the index and you get another unrelated-looking 12 words. Back up the master once; every child can be re-derived from it forever.

What it gives and what it does not

Derivation diagram: master seed to BIP 85 path to three child seeds for three wallets.
One master on metal; children re-derived whenever needed.

  • The children are independent: knowing one child seed tells you nothing about the master or the other children. A compromised hot wallet does not expose your cold storage.
  • The master is everything: whoever has it has every child. It stays on metal and on the signer, never on a phone.
  • It is deterministic: the same master and index always give the same child, on any device that implements the standard.
  • It does not replace a passphrase or multisig; it replaces having many backups.

Doing it

Two columns: what BIP 85 replaces and what it does not replace.
Fewer backups, not fewer responsibilities.

BIP85: Segregated Bitcoin Accounts From One Seed · BTC SessionsWatch on YouTube ↗
MINI CHALLENGEWhich child?

You derived child seeds at indexes 0, 1 and 2 for spending, a relative and a business. Your phone dies. From the master seed and your index list, how many of the three can you recreate?

wallets
CHECK YOURSELF
1/3 What does BIP 85 derive from the master seed?
2/3 What does a compromised BIP 85 child reveal?
3/3 What must you record alongside the master backup?
pick an answer for each question

SOURCES

  1. BIP 85: Deterministic Entropy From BIP32 Keychains
  2. BIP 32: Hierarchical Deterministic Wallets
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