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How to generate a BIP39 mnemonic from an Ed25519 (Ed25519+libsodium, with 8 for salt, 48 for key plus 32 bytes) encrypted private key? I have an Ed25519 encrypted private key (for which I know the password). I would like to create a BIP39 mnemonic from it.

This is to import a private key generated elsewhere into a Nano Ledger S wallet. I get this is not recommended, but I am aware of the risks and want to make it an exercise.

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migrated from security.stackexchange.com Jul 10 '18 at 19:41

This question came from our site for information security professionals.

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    $\begingroup$ This is so crypto-currency specific, I'm thinking that this does not belong here. $\endgroup$ – schroeder Jul 10 '18 at 19:41
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An Ed25519 private key can come in any of several typical forms—almost always either just a 32-byte or 64-byte string. Which form you have or need depends on what software or protocol you are using it with.

For, e.g., a BIP32-style hierarchical wallet, for instance, you almost certainly need the full 64-byte string encoding the PRF secret and the secret scalar. I don't know about the Nano Ledger S: if you want a specific answer about it, you'll have to point to specific documentation about the specific protocol that the Nano Ledger S implements.

If you already have an Ed25519 private key, then you can't generate a BIP39 mnemonic from it because BIP39 is a system for generating keys by way of generating mnemonics from which the keys are derived. Inventing a BIP39 mnemonic for a private key not generated from one would be an amazing feat of cryptography demonstrating an unbelievably structured preimage attack on SHA-256.

If you want to generate an Ed25519 private key by way of a mnemonic, then you could in principle use BIP39 for that. Whether the software you're using supports it, I can't say.

If all you want is a memorable representation of an Ed25519 private key or any other (say) 32-byte string, you could take a (say) 256-entry word list, and map each octet to a word, yielding a 32-word mnemonic. If 32 words is too long, you could use a 65536-entry word list and map a 32-byte string to a 16-word mnemonic. That's not BIP39, but it is a potentially memorable way you could back up an Ed25519 private key.

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  • $\begingroup$ Updated question to specify better the type of Ed25519 $\endgroup$ – Gaia Jul 11 '18 at 1:18
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    $\begingroup$ @Gaia I'm not really sure what you mean by ‘Ed25519+libsodium, with 8 for salt, 48 for key plus 32 bytes’, but in any case, the main point of the answer remains unchanged: BIP39 doesn't help you to make a mnemonic for an existing key; it only helps you to simultaneously generate a mnemonic and a key. $\endgroup$ – Squeamish Ossifrage Jul 11 '18 at 1:26
  • $\begingroup$ I actually don't need the seed phrase anymore (see updated question) $\endgroup$ – Gaia Jul 11 '18 at 1:43
  • $\begingroup$ @Gaia OK…can you revert it to the original question, and ask a new question? This is a completely different question now—I answered ‘Can I create a BIP39 mnemonic for an existing key?’ but your question is now ‘Can I extract a BIP32 private key from an existing Ed25519 key format?’. $\endgroup$ – Squeamish Ossifrage Jul 11 '18 at 2:08
  • $\begingroup$ done. thank you for your patience. new q is at crypto.stackexchange.com/questions/60697 $\endgroup$ – Gaia Jul 11 '18 at 3:03

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