On the top of my head, probably BIP39[1]. It's the reason why we can use mnemonic phrase backups(those 12-24 word backups that are always recommended that you store offline) with our SPV[2] wallets like Electrum today.

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[1] https://github.com/bitcoin/bips/blob/master/bip-0039.mediawiki
[2] https://en.bitcoinwiki.org/wiki/Simplified_Payment_Verification