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Add padding byte validation during extended private key de-serialization to match bip 32's spec #1871

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thecodefactory merged 1 commit into from
Jun 9, 2026
+13 −1
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Add padding byte validation during extended private key de-serialization to match bip 32's spec #1871

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6 changes: 5 additions & 1 deletion src/wallet/keys/hd_private.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -145,13 +145,17 @@ hd_private hd_private::from_key(const hd_key& key, uint64_t prefixes) NOEXCEPT
const auto parent = source.read_4_bytes_big_endian();
const auto child = source.read_4_bytes_big_endian();
const auto chain = source.read_hash();
source.skip_byte();
const auto padding = source.read_byte();

@thecodefactory thecodefactory Jun 9, 2026

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It's been a long time since I've looked at this, but isn't the skip_byte or padding check only required in the hardened key case? Quick glance I'm not seeing that distinction, so just curious if you can point me to that if it's correct.

@KY-U KY-U Jun 9, 2026

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You're correct, the 0x00 byte used as HMAC input is only required for hardened derivation, we can see that in hd_private::derive_private().

But this skip_byte() is not in derivation. It is in parsing serialized extended private key data, where BIP32 always encodes private key data as 0x00 || ser256(k), regardless of whether the extended private key is a master key, a hardened child, or a non-hardened child.

I've updated the title to clarify that the byte-reading change relates to the serialized format and not key derivation. Thanks for pointing that out.

@thecodefactory thecodefactory Jun 9, 2026

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Thank you for satisfying my curiosity, makes a lot more sense now!

const auto secret = source.read_hash();

// Validate the prefix against the provided value.
if (prefix != to_prefix(prefixes))
return {};

// Validate the key padding.
if (padding != 0x00)
return {};

const hd_lineage lineage
{
prefixes,
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8 changes: 8 additions & 0 deletions test/wallet/keys/hd_private.cpp
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Expand Up @@ -137,6 +137,14 @@ BOOST_AUTO_TEST_CASE(hd_private__constructor__null_key__decodes_to_invalid)
BOOST_REQUIRE(!xprv_null);
}

BOOST_AUTO_TEST_CASE(hd_private__constructor__nonzero_private_key_padding__invalid)
{
static const auto encoded = "tprvAk8SVSmAT8FFvWjCNUB546H9tsmRk5bii5WZvSY2zSnxSg4uuLFspzTZffwYUoYXqsP7WDdHTRS92WJzeXTki31ftCwzYCEnLAPt9a7aJyV";

const hd_private xprv_invalid(encoded);
BOOST_REQUIRE(!xprv_invalid);
}

BOOST_AUTO_TEST_CASE(hd_private__to_public__from_invalid_private__invalid)
{
// the 11...14rcJhr is a serialization of a null key;
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