BIP32: specify the maximum depth of 255 #2322

pull fametrano wants to merge 2 commits into bitcoin:master from fametrano:bip32-max-depth changing 1 files +12 −0

fametrano commented at 7:41 AM on October 2, 2026: contributor

BIP32 serializes the depth as one byte but does not say what happens when deriving past depth 255. Bitcoin Core refuses, while several libraries let the depth overflow (bitcoin/bitcoin#32201). In that issue @sipa said a hard limit is better for interoperability and that he would add a note to the BIP: https://github.com/bitcoin/bitcoin/issues/32201#issuecomment-3847331033

I added that note: deriving a child of a depth-255 key must fail. I also added test vector 6, with the keys at depth 255 on the chain m/0/0/.../0 (255 levels). I checked them with Bitcoin Core 31.1 and btclib-wallet, and both refuse to derive any child of them.

I left out the derivation path parsing rules raised in bitcoin/bitcoin#35306. I am happy to add them here if reviewers prefer.

Made with my usual tools: a computer, the Internet and an LLM. The mistakes, as usual, are all mine.

BIP32: specify the maximum depth of 255

The depth is serialized as one byte. Deriving a child of a depth-255
key must fail, as Bitcoin Core already does.

ca1cb3351d

BIP32: add test vectors for the maximum depth

Test vector 6 gives the keys at depth 255 on the chain m/0/.../0.
Deriving any child of them must fail.

fe2bd469d0

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