[bitcoin-dev] New serialization/encoding format for key material

Wed May 30 06:30:25 UTC 2018

The idea to add birthdate and gap limit sounds very good and addresses lots
of problems users are facing.

However, adding birthday to keys breaks two basic properties

- Visually Comparing two keys to find if they are same (Important)
- Different wallet software could set different birthday/gap limit.
creating different xpub/xprv for the same set of mathematically derived
individual keys. This removes the decoupling between key and wallet metadata

In fact, same could be argued to add birthday to WIF private key format to
let wallet discover funds faster.

Is it possible to have a serialization so that in the encoding, the key
part is still visually the same ?

On Tue, May 29, 2018 at 5:30 PM, <
bitcoin-dev-request at lists.linuxfoundation.org> wrote:

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> Today's Topics:
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>    1. New serialization/encoding format for key material
>       (Jonas Schnelli)
>
>
> ----------------------------------------------------------------------
>
> Message: 1
> Date: Tue, 29 May 2018 11:13:37 +0200
> From: Jonas Schnelli <dev at jonasschnelli.ch>
> To: Bitcoin Protocol Discussion
>         <bitcoin-dev at lists.linuxfoundation.org>
> Subject: [bitcoin-dev] New serialization/encoding format for key
>         material
> Message-ID: <03557E21-8CFC-4822-8494-F4A78E23860B at jonasschnelli.ch>
> Content-Type: text/plain; charset="utf-8"
>
> Hi
>
> Extended public and private keys are defined in BIP32 [1].
>
> Encoded extended private keys should not be confused with a wallet ?seed?
> (proposals like BIP39) while they can also partially serve the purpose to
> ?seed? a wallet (there may be an overlap in the use-case).
>
> Recovering a wallet by its extended private master key (xpriv; may or may
> not
> be at depth 0) is a complex task with risks of failing to recover all
> available
> funds.
>
> It may be reasonable to consider that recovering a wallet purely based on
> the
> existence of an extended private master key is a forensic funds recovery
> process and should probably be the last resort in case of a backup-recovery
> situation. A simple example here is, that it was/is possible to have used
> an
> xpriv (referring to extended private master key) in production that is/was
> used
> to derive BIP45 based P2SH multisig addresses (1of1, used by Bitpays BWS
> for
> while), later used for bare BIP45ish multisig 1of1 as well as for P2PKH
> after
> BIP44 & vanilla BIP32 P2WPKH (m/0?/k?).
> I?m not aware of any wallet that would recover 100% of those funds,
> leading to
> the risk that forwarding the unspents and destroying the extended master
> key
> may result in coins forever lost.
>
> The case above may be an edge case, but I?m generally under the assumption
> that
> recovering funds based on the sole existence of an xpriv (or seed) without
> further
> metadata is a fragile concept.
>
> Second, the missing birthday-metadata tend to lead to non-optimal
> blockchain
> scans (eventually increased p2p traffic). Recovering funds can take hours.
>
> Additionally, the BIP44 gap limit seems to be a weak construct. The
> current gap
> limit in BIP44 is set to 20 [2] which basically means, handing out more
> then 20
> incoming payment requests (addresses) results in taking the risks that
> funds
> may be destroyed (or at least not detected) during a recovery.
> The Gap limit value may also depend on the use case, but the current
> proposals
> do not allow to set an arbitrary value. High load merchants very likely
> need a
> different gap limit value then individuals create a transaction once a
> year.
>
> During creation time of an xpriv/xpub, it is impossible to know if the
> created
> xpriv will be used for an unforeseen derivation scheme. Future proposals
> may
> want to limit an extended key to a single derivation scheme.
>
>
> This is an early draft in order to allow discussion that may lead to a
> possible
> proposal.
> This proposals could also make BIP 178 obsolete since it can be replace the
> WIF[3] standard.
>
>
> Thanks for feedback
> /jonas
>
>
> ------------------------------------
>
>
> Titel
> ######
> Bech32 encoded key material including metadata
>
> Abstract
> ########
> An error tolerant encoding format for key material up to 520bits with a
> minimal
> amount of metadata.
>
> Motivation
> ##########
> (See above; intro text)
>
>
> Specification
> #############
>
> ## Serialization format
>
> 1 bit version bit
> 15 bits (bit 1 to 16) key-birthday (0-32767)
> (12 bit gap limit)
> 3 or 5 bits script type
> 256 or 512 or 520 bits key material
> = Total 275, 545, 553 bits
>
> The initial version bit allows extending the serialization-format in
> future.
> The encoding format must hint the total length and thus allow to calculate
> the
> length of the key material.
>
> The total length for 256 or 512 bit key material is optimised for Bech32
> (power
> of 5).
>
> ### Key material
> If the key material length is 520 bits, it must contain an extended public
> key
> If the key material length is 512 bits, it must contain an extended
> private key
> Key material length other then 256, 512, 520 bits and invalid.
>
> If 520 bits are present, first 256 bits are the BIP32 chain code, to
> second 264
> bits (33 bytes) define the public key (according to BIP32)
>
> If 512 bits are present, first 256 bits are the BIP32 chain code, to
> second 256
> bits define the private key
>
> If 256 bits are present, those bits represent a pure private key (or seed)
>
> ### Key birthday
> A 15 bit timestamp expressed in days since genesis (valid up to ~2098). The
> birthday must be set to the first possible derivation of the according
> extended
> key, if unknown, the used seed birthday must be used. If both unknown, 0
> (16x0bit) must be used.
>
> ### Gap limit delta
> 12 bits, results in a possible range from 0 to 4095.
>
> If the total decoded serialization length is 275 bits (decode) or if the
> key
> material is 256 bits (encode), the gap limit must not be present.
>
> The base gap limit value is 20 (to disallow insane gap limits). The final
> gap
> limit is the base value + the gap limit delta stored in those 12 bits.
> Key derivation gap limit must not be exceeded when deriving child keys and
> must
> be respected during transaction rescans.
> Child key derivation must not be possible if gap limit is hit.
>
> ### Script type restriction
> 3 or 5 bits (range 0-7 / 0-31)
> 0 no restriction
> 1 P2PKH compressed
> 2 P2PKH | P2SH
> 3 P2WPKH P2WSH nested in P2SH
> 4 P2WPKH | P2WSH
>
> If the total decoded serialization length is 275 bits (decode) or if the
> key
> material is 256 bits (encode), 3 bits are used for the script type. 5 bits
> are
> used for key material with the size of 512, 520 bits.
>
> If the script type restriction is set, the according extended key must
> only be
> used to derive addresses with the selected script type.
> This does not stands in contradiction to derivation path proposals ([4]).
> It
> does allow to derive and encode an extended key at a keypath where users
> assume
> restricted script types in derivation due to other supported proposals.
>
>
> Encoding
> ########
>
> Bech32 must be used as encoding format (see the Bech32 rational [5]).
> Encoding
> 545 or 553 bits (results in 109 resp. 111 x 5 bits) will exceed the Bech32
> property of a
> guaranteed detection of 4 errors (only 3 are).
> It is possible that there are more efficient BCH codes, especially for
> encoding
> extended private keys. Since a Bech32 implementation needs to be present in
> modern Bitcoin software, re-using Bech32 will allow to migrate to this
> proposal
> with a minimal implementation effort.
> Forensic, cpu-intense key-recovery (including brute-force techniques) may
> allow
> to recover keys beyond the guaranteed error detection limits.
>
> Bech32 HRPs
> Mainnet Private Extended: xp
> Mainnet Public Extended: xpu
> Testnet Private Extended: tp
> Testnet Public Extended: tpu
> Mainnet Key: pk-
> Testnet Key: tk-
>
> Compatibility
> ###########
> Only new software will be able to use these serialization and encoding
> format.
>
> References
> ##########
>
>
> [1] https://github.com/bitcoin/bips/blob/master/bip-0032.mediawiki <
> https://github.com/bitcoin/bips/blob/master/bip-0032.mediawiki>
> [2] https://github.com/bitcoin/bips/blob/master/bip-0044.mediawiki
> [3] https://github.com/bitcoin/bips/blob/master/bip-0178.mediawiki
> [4] https://github.com/bitcoin/bips/blob/master/bip-0049.mediawiki
> [5] https://github.com/bitcoin/bips/blob/master/bip-0173.
> mediawiki#rationale
>
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