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

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

[Jonas Schnelli]

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

[Russell O'Connor]

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On Tue, May 29, 2018 at 5:13 AM, Jonas Schnelli via bitcoin-dev <
bitcoin-dev@lists.linuxfoundation.org> wrote:

> Hi
>
> 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)
>

In a 33 byte compressed public key, only 1 bit from the first byte conveys
information.  The other 7 bits can be discarded.  This will allow you to
reduce the bech32 encoded result by one character.

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

[Pieter Wuille]

On Fri, Jun 15, 2018 at 8:54 AM, Russell O'Connor
<roconnor@blockstream.io> wrote:
>
>> For codes designed for length 341 (the first length enough to support
>> 512 bits of data):
>> * correct 1 error = 3 checksum characters
>> * correct 2 errors = 7 checksum characters
>> * correct 3 errors = 11 checksum characters
>> * correct 4 errors = 15 checksum characters
>> * correct 5 errors = 19 checksum characters
>> * ...
>> * correct 7 errors = 26 checksum characters (~ length * 1.25)
>> * correct 13 errors = 51 checksum characters (~ length * 1.5)
>> * correct 28 errors = 102 checksum characters (~ length * 2)
>>
>> So it really boils down to a trade-off between length of the code, and
>> recovery properties.
>
>
> At the risk of making the proposal more complex, I wonder if it might be
> better to support multiple checksum variants?  The trade-off between code
> length and recovery seems to be largely determined by the user's medium of
> storage, which is likely to vary from person to person.  I personally would
> probably be interested in the 51 or even 102 character checksums variants.

Here are some more numbers then. It's important to note that the
number of correctable errors includes errors inside the checksum
characters themselves. So if you want to aim for a certain percentage
of correctable characters, the numbers go up much more dramatically.

For codes restricted to 341 characters total (including the checksum
characters), and assuming 103 data characters (enough for 512 bits):
* With 26 checksum characters (adding 25%, 20% of overall string), 7
errors can be corrected (5% of overall string)
* With 62 checksum characters (adding 60%, 38% of overall string), 17
errors can be corrected (10% of overall string)
* With 116 checksum characters (adding 113%, 53% of overall string),
33 errors can be corrected (15% of overall string)
* With 195 checksum characters (adding 189%, 65% of overall string),
60 errors can be corrected (20% of overall string)

For codes restricted to 1023 characters total (including the checksum
characters), and assuming 103 data characters (enough for 512 bits):
* With 27 checksum characters (adding 26%, 21% of overall string), 7
errors can be corrected (5% of overall string)
* With 64 checksum characters (adding 62%, 38% of overall string), 17
errors can be corrected (10% of overall string)
* With 127 checksum characters (adding 123%, 57% of overall string),
36 errors can be corrected (15% of overall string)
* With 294 checksum characters (adding 285%, 74% of overall string),
80 errors can be corrected (20% of overall string)
* With 920 checksum characters (adding 893%, 90% of overall string),
255 errors can be corrected (25% of overall string)

I'll gladly construct reference source code for any of these.

Cheers,

-- 
Pieter

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

[Russell O'Connor]

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> For codes designed for length 341 (the first length enough to support
> 512 bits of data):
> * correct 1 error = 3 checksum characters
> * correct 2 errors = 7 checksum characters
> * correct 3 errors = 11 checksum characters
> * correct 4 errors = 15 checksum characters
> * correct 5 errors = 19 checksum characters
> * ...
> * correct 7 errors = 26 checksum characters (~ length * 1.25)
> * correct 13 errors = 51 checksum characters (~ length * 1.5)
> * correct 28 errors = 102 checksum characters (~ length * 2)
>
> So it really boils down to a trade-off between length of the code, and
> recovery properties.
>

At the risk of making the proposal more complex, I wonder if it might be
better to support multiple checksum variants?  The trade-off between code
length and recovery seems to be largely determined by the user's medium of
storage, which is likely to vary from person to person.  I personally would
probably be interested in the 51 or even 102 character checksums variants.

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

[Pieter Wuille]

On Sun, Jun 3, 2018 at 2:30 PM, Jonas Schnelli <dev@jonasschnelli.ch> wrote:
>> If there is interest, I can construct a code + implementation for any
>> of these in a few days probably, once the requirements are clear.
>
> Yes. Please.

Here is an example BCH code for base32 data which adds 27 checksum
characters, and can correct up to 7 errors occurring in strings up to
length 1023 (including the checksum characters themselves):
https://gist.github.com/sipa/d62f94faa1dcfd9ee4012d4c88955ba6

It can encode sequences of integers (between 0 and 31):

ref.py encode 13 7 22 23 11 29 21 15 3 26 20 26 4 7 6 11 19 1 6 8 31 13 4 19

> d8khta40r656y8xtnpxgldyne96vsfr83uch908se82g98rmnaa

Decode it again:

ref.py decode d8khta40r656y8xtnpxgldyne96vsfr83uch908se82g98rmnaa

> Decoded: 13 7 22 23 11 29 21 15 3 26 20 26 4 7 6 11 19 1 6 8 31 13 4 19

Or correct errors:

ref.py decode d8khta50r656y8xtmpxhlcyne96vsfr84udh908se82g98rmnat

> Errors found: d8khta?0r656y8xt?px?l?yne96vsfr8?u?h908se82g98rmna?
> Correction:   d8khta40r656y8xtnpxgldyne96vsfr83uch908se82g98rmnaa
> Decoded: 13 7 22 23 11 29 21 15 3 26 20 26 4 7 6 11 19 1 6 8 31 13 4 19

The code above is just a randomly picked BCH code, and has no special
properties beyond the ones it is designed for.

I can easily generate similar code for BCH codes with different properties.

Cheers,

-- 
Pieter

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

[Jonas Schnelli]

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> I have some concerns about the use of Bech32. It is designed for
> detecting 3 errors up to length 1023 (but is then picked specifically
> to support 4 errors up to length 89). However, for error correction
> this translates to just being able to efficiently correct 1 error
> (3/2, rounded down) up to length 1023. You can of course always try
> all combinations of up to N changes to the input (for any N), testing
> the checksum, and comparing the results against the UTXO set or other
> wallet information that may have been recovered. However, the checksum
> at best gives you a small constant speedup here, not a fundamentally
> improved way for recovery.

Thanks Peter

I removed the part in the proposals that made false claims about the error
correction or cpu-intense key recovery.

I wrote some test code and figured out that my Core i7 machine can
do 31’775 operations per seconds of a addr-derivation-comparison
(bech32 decode, bip32 ckd, hash160, Base58check).
This is non-optimized code running non-parallelized.

Just in case someone wants to do more math here.

Without knowing to much about BCHs, ideally there would be a code that
includes the fact that computational costs for error correction can be very
high during a disaster recovery and that we can probably assume that the
user can provide a derivation element like a used address or pubkey.

Deriving one million child keys and comparing them against an address
table will take less than a minute on consumer systems.

> * correct 7 errors = 26 checksum characters (~ length * 1.25)
> 
> So it really boils down to a trade-off between length of the code, and
> recovery properties.

I think 5% error correction (7 errors at 555bits) with a 26 char checksum is
probably an acceptable tradeoff.

Resulting string with 26 checksum chars (mockup):
xp1qqqqqq8z4rsgv54z9a92yla4m2yrsqdlwdl7gn6qldvwkuh3zrg66z8ad2snf832tgaxcuv3kmwugzl5x8wtnkj2q3a03ky0kg8p7dvv4czpjqgvv4zgnvv4zgnvv4zgnvv4zgngn
(140 chars)

Versus the bech32 (6 char checksum):
xp1qqqqqq8z4rsgv54z9a92yla4m2yrsqdlwdl7gn6qldvwkuh3zrg66z8ad2snf832tgaxcuv3kmwugzl5x8wtnkj2q3a03ky0kg8p7dvv4czpjqgvv4zgn
(120 chars)

Versus an xpriv:
xprv9wHokC2KXdTSpEepFcu53hMDUHYfAtTaLEJEMyxBPAMf78hJg17WhL5FyeDUQH5KWmGjGgEb2j74gsZqgupWpPbZgP6uFmP8MYEy5BNbyET
(111 chars)

Not sure if the additional 20 characters make the UX worse.
Typing in +20 chars in a disaster recovery is probably acceptable.

> If there is interest, I can construct a code + implementation for any
> of these in a few days probably, once the requirements are clear.


Yes. Please.
Lets first wait for more feedback about the error robustness though.

Thanks
-
Jonas

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

[Pieter Wuille]

On Sun, Jun 3, 2018 at 9:51 AM, Jonas Schnelli via bitcoin-dev
<bitcoin-dev@lists.linuxfoundation.org> wrote:
> Hi
>
> The BIP proposal is now available here:
> https://gist.github.com/jonasschnelli/68a2a5a5a5b796dc9992f432e794d719
>
> Reference C code is available here:
> https://github.com/jonasschnelli/bech32_keys
>
> Feedback, criticism, etc. welcome!

First of all, thanks for working on this.

I have some concerns about the use of Bech32. It is designed for
detecting 3 errors up to length 1023 (but is then picked specifically
to support 4 errors up to length 89). However, for error correction
this translates to just being able to efficiently correct 1 error
(3/2, rounded down) up to length 1023. You can of course always try
all combinations of up to N changes to the input (for any N), testing
the checksum, and comparing the results against the UTXO set or other
wallet information that may have been recovered. However, the checksum
at best gives you a small constant speedup here, not a fundamentally
improved way for recovery.

However, we can design other base32 BCH codes easily with different
properties. As we mostly care about efficient algorithms for recovery
(and not just error detection properties), it seems more important to
have good design strength (as opposed to picking a code from a large
set which happens to have better properties, but without efficient
algorithm, like Bech32).

This is what I find for codes designed for length 93 (the first length
for which efficient codes exist with length long enough to support 256
bits of data):
* correct 1 error = 3 checksum characters
* correct 2 errors = 6 checksum characters
* correct 3 errors = 10 checksum characters
* correct 4 errors = 13 checksum characters
* correct 5 errors = 16 checksum characters
* ...
* correct 8 errors = 26 checksum characters (~ length * 1.5)
* correct 11 errors = 36 checksum characters (~ maximum length without
pushing checksum + data over 93 characters)

For codes designed for length 341 (the first length enough to support
512 bits of data):
* correct 1 error = 3 checksum characters
* correct 2 errors = 7 checksum characters
* correct 3 errors = 11 checksum characters
* correct 4 errors = 15 checksum characters
* correct 5 errors = 19 checksum characters
* ...
* correct 7 errors = 26 checksum characters (~ length * 1.25)
* correct 13 errors = 51 checksum characters (~ length * 1.5)
* correct 28 errors = 102 checksum characters (~ length * 2)

So it really boils down to a trade-off between length of the code, and
recovery properties.

These two sets of codes are distinct (a code designed for length 93
has zero error correction properties when going above 93), so either
we can pick a separate code for the two purposes, or be limited to the
second set.

If there is interest, I can construct a code + implementation for any
of these in a few days probably, once the requirements are clear.

Cheers,

-- 
Pieter

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

[Jonas Schnelli]

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Hi

The BIP proposal is now available here:
https://gist.github.com/jonasschnelli/68a2a5a5a5b796dc9992f432e794d719

Reference C code is available here:
https://github.com/jonasschnelli/bech32_keys

Feedback, criticism, etc. welcome!

Thanks
—
Jonas

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

[Jonas Schnelli]

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Hi

> - 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

What would be the downside of encoding the same key with different metadata (resulting in different "visual strings“)?
If you import it into the same software, it would be trivial to detect it. If you import it into another software, it probably doesn’t matter.

Visual comparing is eventually a broken concept (agree with Greg) and I doubt that this property is important, and IMHO basic metadata seems more important then this - very likely irrelevant - visual property.

Also, I think a recovery based on a sole xpriv (or + limited amount of meta-data as described in this proposal) is a disaster recovery (or forensic recovery).

Long term, I would wish, if wallet-metadata including transaction based user metadata would be backed up - after encrypted with a key that can be derived from the seed - in a way, where you need the seed to recover that backup thus it can be stored in cheap, insecure spaces.

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

The proposal I made can be seen as a replacement for WIF (it can replace WIF and xpriv/xpub) since it can encode a single private key into 275bits (still pretty short Bech32 string).

/jonas

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

[Gregory Maxwell]

On Wed, May 30, 2018 at 6:30 AM, shiva sitamraju via bitcoin-dev
<bitcoin-dev@lists.linuxfoundation.org> wrote:
> 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)

Can you explain exactly what you mean there? I can think of to
plausible meanings (that two valid keys could differ by only a single
symbol, which wouldn't be true due to the checksum and could be made
even stronger if we thought that would be useful or I think you could
also be complaining that the same "key material" could be encoded two
ways which I think is both harmless and unavoidable for anything
versioned).

> - 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

Personally, I think it's a mistake to believe that any key format can
really make private keying material strongly compatible between
wallets. At best you can hope for a mostly compatible kind of recovery
handling.

But the lookahead amount may be pretty integral to the design of the
software, so signaling it may not mean the other side can obey the
signal... but that wouldn't make the signal completely useless.

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

[shiva sitamraju]

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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@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@jonasschnelli.ch>
> To: Bitcoin Protocol Discussion
>         <bitcoin-dev@lists.linuxfoundation.org>
> Subject: [bitcoin-dev] New serialization/encoding format for key
>         material
> Message-ID: <03557E21-8CFC-4822-8494-F4A78E23860B@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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> End of bitcoin-dev Digest, Vol 36, Issue 46
> *******************************************
>



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