Add 'hdderive' RPC, a generic BIP32 calculator

[sipa]

This adds a 'hdderive' RPC command, which allows arbitrary BIP32 chains/addresses to be computed.

It supports starting from a seed, or from an xpub/xprv specification, and computes for the provided root or any derivation thereof the depth, index, chaincode, public key, address, fingerprint, parent fingerprint, private key, extended public key, and extended private key.

Note that this does not add HD functionality to the wallet itself.

[luke-jr]

This is pretty stateless. What do you think of making/using a shared library with stuff like this?

[sipa]

I'm sure tons of such libraries already exist.

[sipa]

You're right of course, but it's pretty nice to have this available in the client (perhaps mostly inside the GUI; for RPC you can use a python library or something easily too), and the code is already there.

[laanwj]

Edit: I take back my recommendation to not merge. The point about confusion still stands, the solution may be just to document better that this is not part of wallet functionality:

Also offering per-key functionality unrelated to the wallet is bound to confuse people with the current wallet. People will try to use this, then possibly importprivkey, then forget about change addresses that still get generated the random way.

[sipa]

Three points from discussing this in #bitcoin-dev:

• Let's wait until some form of deterministic wallets are implemented as well, so this feature doesn't confuse people into thinking their wallets are deterministic.
• Move this to rcpmisc, as it's a utility that may be useful without having a live wallet.
• Document clearly in the help message that this is only a utility, and does not interact with the wallet.

[jgarzik]

I think this is more appropriate outside bitcoind, in an in-tree utility.

PR #4332 adds the "bitcoin-tx" utility. I was also thinking about adding simple "bitcoin-script" and "bitcoin-key" utilities along similar lines. hdderive would find a perfect home among such utilities.

In general, I would prefer to move all "utility RPCs" completely outside bitcoind. e.g. with #4332 we can eliminate "createrawtransaction", "decoderawtransaction" and other RPCs that touch neither wallet nor blockchain.

[sipa]

Agree.

[laanwj]

Agree @jgarzik

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[jgarzik]

Closing this, with the presumption that a similar feature will be added to a utility like bitcoin-tx (and a PR opened for that at that time)