Re: [bitcoin-dev] Bitcoin vaults with anti-theft recovery/clawback mechanisms
On Mon, Aug 12, 2019 at 09:09:43PM -0500, Bryan Bishop wrote: > > > Multisig gated by ECDSA pubkey recovery for provably-unknown keys > > > = > > > > > > A group can participate in a multisig scheme with provably-unknown ECDSA > > keys. > > > Instead of deleting the key, the idea is to agree on a blockheight and > > then > > > select the blockhash (or some function of the chosen blockhash like > > > H(H(H(blockhash as the signature. Next, the group agrees on a > > transaction > > > and they recover the public key from the signature using ECDSA pubkey > > recovery. > > > > Could you explain in more detail why you're deriving this from a blockhash? > > > > Well you need to pick an entropy source, and I wouldn't want to tell people > to just trust the first party to tell you a good sequence of bytes. But why does this specifically need to be entropy? If I understand the scheme correctly, the important thing is for the ECDSA private key to be unknown. Under the standard assumption that hash functions are random oracles, hashing anything should be sufficient to create a pubkey whose private key is unknown. Secondly, there's probably better slightly privacy if a random nonce is chosen (perhaps by concatenating a nonce from each party) rather than picking pubkeys unique to this use-case. -- https://petertodd.org 'peter'[:-1]@petertodd.org signature.asc Description: PGP signature ___ bitcoin-dev mailing list bitcoin-dev@lists.linuxfoundation.org https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev
Re: [bitcoin-dev] Bitcoin vaults with anti-theft recovery/clawback mechanisms
Bryan, This is very similar to *CoinVault - Secure Depository and Secure Exchange* technologies that I have shared with you all. ᐧ On Wed, Aug 7, 2019 at 7:23 PM Bryan Bishop via bitcoin-dev < bitcoin-dev@lists.linuxfoundation.org> wrote: > Hi, > > I have a proposal for implementing bitcoin vaults in a way that does not > require any soft-forks or other software upgrades, although it could > benefit > from SIGHASH_NOINPUT which I'll describe later. > > I call them pre-signed vaults. > > Vault definition > > > Here, a vault is defined as a transaction setup scheme that binds both the > user > and the attacker to always using a public observation and delay period > before a > weakly-secured hot key is allowed to arbitrarily spend coins. This is the > same > definition previously used[1]. During the delay period, there is an > opportunity > to initiate recovery/clawback which can either trigger deeper cold storage > parameters or at least reset the delay period to start over again for the > same > keys. > > One of the important components of this is the delete-the-key pre-signed > transaction concept, where only a single transaction is (pre)signed before > deleting the key. This is basically an emulation of a covenant and > enforces a > certain outcome. > > Background and motivation > = > > I was looking at Eyal and Sirer's 2016 vaults paper [1], and I saw this > headscratcher: > > > Vault transactions use a delay mechanism. We note that vault transactions > > cannot be implemented with existing timing mechanisms such as > > CHECKLOCKTIMEVERIFY opcode or transaction locktime. > > This was probably written before the introduction of > OP_CHECKSEQUENCEVERIFY. > Still, a viable construction would have more steps than just using OP_CSV. > They > were probably not thinking about what those steps might be, because in the > context of the paper they were proposing a bitcoin vault implemented using > recursive consensus-enforced covenants via a new opcode, which obviously > cannot > be deployed without an upgrade fork. Covenants have been discussed for > years, > but require new opcodes or other consensus-enforcement changes. > > Relative locktimes are useful here because there is no knowledge as to > when the > transactions might be broadcasted in the future. The delays need to be > relative > to after the transaction is included in the blockchain, not to setup > initialization time. > > Also, from [2]: > > > We show that a [vault transaction] mechanism is currently not possible > in all > > cryptocurrencies [...] Bitcoin's scripting language requires support for > > covenants. > > I haven't seen any previous proposal for how to implement recursive bitcoin > vaults without a fork and without a covenant. After asking around, I am > pretty > sure this is somewhat novel. The closest I guess is [3]. > > Vaults are particularly interesting as a bitcoin cold storage security > mechanism because they enable a publicly observable delay period during > which > time a user could be alerted by a watchtower that a thief might be in the > process of stealing their coins, and then the user may take some actions to > place the coins back into the vault before the relative timelock expires. > There > seems to be no way to get this notification or observation period without a > vault construction. It might have been assumed it required a covenant. > > Having a vault construction might go a long way to discourage would-be > attackers, on principle that the attacker might be incapable of recovering > their cost-of-attack because the recovery mechanism can lock up the coins > indefinitely. Griefing or denial-of-service would still be possible, of > course, > but with multisig there might be some ways to put a halt to that as well. > I am > working under the assumption that the attacker knows that the user is a > vault > user. > > Vaults > == > > The idea is to have a sequence of pre-generated pre-signed transactions > that > are generated in a certain way. The basic components are a vaulting > transaction > that locks coins into a vault, a delayed-spend transaction which is the > only > way to spend from a vault, and a re-vaulting transaction which can > recover/clawback coins from the delayed-spend transaction. The security of > this > scheme is enforced by pre-signing transactions and deleting private keys, > or > with the help of SIGHASH_NOINPUT then there's another scheme where private > keys > are provably never known. This enforces that there's only a specific set of > possible outcomes at every step of the vault. > > Some examples of what the set of broadcasted transactions might look like > in > regular usage: > > coins -> VT -> DST -> exit via hot wallet key > coins -> VT -> DST -> RVT > coins -> VT -> DST -> RVT -> DST -> ... > coins -> VT -> ... -> RVT998 -> nuclear abort > > where: > VT = vault transaction > DST = delayed-spend transaction > RVT =
Re: [bitcoin-dev] Bitcoin vaults with anti-theft recovery/clawback mechanisms
On Mon, Aug 12, 2019 at 10:01 AM Peter Todd wrote: > The key difference being it's not important that this be a *public* > notification: that the public can see just happens to be an (unfortunate) > implementation detail. For example, you could imagine a system where the > "prepare to spend" tx is indistinguishable from any other transaction. > True, I did not intend for everyone to know the meaning of the observed transaction. It turns out to not be too useful to the scheme anyway, unless you're interested in protecting against an adversary dumb enough to tell you he has stolen your key before spending your coins. To reiterate my other follow-up email, the best you can do (... or the best I can do right now) is limit losses to k% where k is selected by the user, e.g. 1 input 100 outputs each with succesively increasing timeouts allowing the rotten non-rotated(pre-inserted) key to spend, and instant spending by a recovery flow. Once the attacker steals any one of the k% outputs, you know to not let the outputs timeout to that key in the future. Unfortunately, without an opcode-style covenant, the only way to know if a stale hot key is stolen is to observe an unexpected spend or, if you're lucky, observe an unexpected signature otherwise unassociated with a transaction. > > * Nuclear abort key: Also unnecessary. This is a key for which only a > single > Obviously normally to provably destroy coins you'd spend to an OP_RETURN > output, or if miner censorship was an issue, a pay-to-script-hash of an > OP_RETURN script. > Oh, right. Well, that works. > > Delete the key (for pre-signed transactions) > > > > > > The delete-the-key trick is simple. The idea is to pre-sign at least one > > transaction and then delete the private key, thus locking in that course > of > > action. > > > > Unfortunately, delete-the-key doesn't really work for multisig scenarios > > because nobody would trust that anyone else in the scheme has actually > deleted > > the secret. If they haven't deleted the secret, then they have full > unilateral > > control to sign anything in that branch of the transaction tree. The > only time > > that delete-the-key might be appropriate would be where the user who > deletes > > the key and controls the key during the setup process is also the sole > > beneficiary of the entire setup with the multisig participants. > > > > Alternative fee rates are easier to deal with using delete-the-key, > compared to > > a technique where the private key never existed which can only be used > to sign > > one fee rate per public key, requiring an entirely new vault subtree for > each > > alternative fee rate. With delete-the-key, the alternative fee rates are > signed > > with the private key before the private key is deleted. > > I think this could use a bit more analysis here: why can't delete the > *keys* > work, with each party deleting a separate private key that's used in an > m-of-n > fashion? So long as at least n-m+1 parties actually deleted their keys > IIUC it > should be secure. > I was thinking about another construction where you pick a key as a group (separate from the multisig setup) and sign with that. But in practice, as you have pointed out, you would do the delete-the-key trick on the multisig construction itself with each party contributing their own pubkey, requiring 1/n honest deletes. > > Multisig gated by ECDSA pubkey recovery for provably-unknown keys > > = > > > > A group can participate in a multisig scheme with provably-unknown ECDSA > keys. > > Instead of deleting the key, the idea is to agree on a blockheight and > then > > select the blockhash (or some function of the chosen blockhash like > > H(H(H(blockhash as the signature. Next, the group agrees on a > transaction > > and they recover the public key from the signature using ECDSA pubkey > recovery. > > Could you explain in more detail why you're deriving this from a blockhash? > Well you need to pick an entropy source, and I wouldn't want to tell people to just trust the first party to tell you a good sequence of bytes. - Bryan http://heybryan.org/ 1 512 203 0507 ___ bitcoin-dev mailing list bitcoin-dev@lists.linuxfoundation.org https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev
Re: [bitcoin-dev] Bitcoin vaults with anti-theft recovery/clawback mechanisms
On Wed, Aug 07, 2019 at 08:48:06AM -0500, Bryan Bishop via bitcoin-dev wrote: > Hi, > > I have a proposal for implementing bitcoin vaults in a way that does not > require any soft-forks or other software upgrades, although it could benefit > from SIGHASH_NOINPUT which I'll describe later. > > I call them pre-signed vaults. > > Vault definition > > > Here, a vault is defined as a transaction setup scheme that binds both the > user > and the attacker to always using a public observation and delay period before > a > weakly-secured hot key is allowed to arbitrarily spend coins. This is the same > definition previously used[1]. During the delay period, there is an > opportunity > to initiate recovery/clawback which can either trigger deeper cold storage > parameters or at least reset the delay period to start over again for the same > keys. So, I'll point out that I'd describe this a little bit differently: The vault is a tx setup scheme that binds coins in such a way that they can only be spent via a proof-of-publication *notification*, followed by a delay period, during which coins can be recovered/clawed back. The key difference being it's not important that this be a *public* notification: that the public can see just happens to be an (unfortunate) implementation detail. For example, you could imagine a system where the "prepare to spend" tx is indistinguishable from any other transaction. > One of the important components of this is the delete-the-key pre-signed > transaction concept, where only a single transaction is (pre)signed before > deleting the key. This is basically an emulation of a covenant and enforces a > certain outcome. It's important to note the reason this is possible is because any coin bound by a convenant simply isn't a coin in the normal sense of the word, and is only acceptable as payment directly if the receiver chooses to accept it. To use an analogy many others have used, if you owe me $100, it's not acceptable for you to pay me that $100 by dumping a time-locked safe on my front lawn containing that $100 unless I've agreed to accept payment that way. > * Nuclear abort key: Also unnecessary. This is a key for which only a single > signed transaction will ever exist, and that single transaction will spend to > a > proof-of-burn key like 0x00. This key must be extremely secure, and if there So to be clear, you're spending to a proof-of-burn _key_ because of the use of adapter signatures for multisig? I'm not sure where the 0x00 is coming from here. Obviously normally to provably destroy coins you'd spend to an OP_RETURN output, or if miner censorship was an issue, a pay-to-script-hash of an OP_RETURN script. > Delete the key (for pre-signed transactions) > > > The delete-the-key trick is simple. The idea is to pre-sign at least one > transaction and then delete the private key, thus locking in that course of > action. > > Unfortunately, delete-the-key doesn't really work for multisig scenarios > because nobody would trust that anyone else in the scheme has actually deleted > the secret. If they haven't deleted the secret, then they have full unilateral > control to sign anything in that branch of the transaction tree. The only time > that delete-the-key might be appropriate would be where the user who deletes > the key and controls the key during the setup process is also the sole > beneficiary of the entire setup with the multisig participants. > > Alternative fee rates are easier to deal with using delete-the-key, compared > to > a technique where the private key never existed which can only be used to sign > one fee rate per public key, requiring an entirely new vault subtree for each > alternative fee rate. With delete-the-key, the alternative fee rates are > signed > with the private key before the private key is deleted. I think this could use a bit more analysis here: why can't delete the *keys* work, with each party deleting a separate private key that's used in an m-of-n fashion? So long as at least n-m+1 parties actually deleted their keys IIUC it should be secure. > Multisig gated by ECDSA pubkey recovery for provably-unknown keys > = > > A group can participate in a multisig scheme with provably-unknown ECDSA keys. > Instead of deleting the key, the idea is to agree on a blockheight and then > select the blockhash (or some function of the chosen blockhash like > H(H(H(blockhash as the signature. Next, the group agrees on a transaction > and they recover the public key from the signature using ECDSA pubkey > recovery. Could you explain in more detail why you're deriving this from a blockhash? > Deploying exceedingly large scripts > === > > A brief interlude to share a somewhat obvious construction. I haven't seen > this > written down yet. > > Suppose there is a
Re: [bitcoin-dev] Bitcoin vaults with anti-theft recovery/clawback mechanisms
Good morning Sergio, Sent with ProtonMail Secure Email. ‐‐‐ Original Message ‐‐‐ On Thursday, August 8, 2019 10:09 AM, Sergio Demian Lerner via bitcoin-dev wrote: > Seems to be comparable to the proposed "Tick Method" from 2013: > https://bitcointalk.org/index.php?topic=307211.msg3308565#msg3308565 > > However I remember that someone told me the tick method had a flaw.. Maybe the use of `SIGHASH_NONE` for both inputs of the TxOut transactions? Also txid malleability. The first can be fixed by not using `SIGHASH_NONE` for one of the inputs and requiring a hot privkey to sign with that. The second can be fixed by using SegWit outputs. Regards, ZmnSCPxj > > > On Wed, Aug 7, 2019 at 6:28 PM Dustin Dettmer via bitcoin-dev > wrote: > > > Does revaulting vault up with the same keys, or new ones? > > > > Are they new derivation paths on the same key? > > > > Would love some expanded explanation on how you’re proposing this would > > work. > > > > Thanks, > > Dustin > > > > On Wed, Aug 7, 2019 at 1:35 PM Bryan Bishop via bitcoin-dev > > wrote: > > > > > Hi, > > > > > > One of the biggest problems with the vault scheme (besides all of the > > > setup data that has to be stored for a long time) is an attacker that > > > silently steals the hot wallet private key and waits for the vault's > > > owner to make a delayed-spend transaction to initiate a withdrawal > > > from the vault. If the user was unaware of the theft of the key, then > > > the attacker could steal the funds after the delay period. > > > > > > To mitigate this, it is important to choose a stipend or withdrawal > > > amount per withdrawal period like x% of the funds. This limits the > > > total stolen funds to x% because once the funds are stolen the user > > > would know their hot key is compromised, and the user would know to > > > instead use one of the other clawback paths during all of the future > > > withdrawal delay periods instead of letting the delay timeout all the > > > way to the (stolen) default/hot key. > > > > > > The reason why a loss limiter is the way to go is because there's > > > currently no way (that I am aware of, without an upgrade) to force an > > > attacker to reveal his key on the blockchain while also forcing the > > > attacker to use a timelock before the key can spend the coins. I am > > > curious about what the smallest least invasive soft-fork would be for > > > enabling this kind of timelock. There are so many covenant proposals > > > at this point (CHECKSIGFROMSTACK, SECURETHEBAG, CHECKOUTPUTVERIFY, > > > ). Or there's crazy things like a fork that enables a transaction > > > mode where the (timelock...) script of the first output is > > > automatically prefixed to any of the other scripts on any of the other > > > outputs when an input tries to spend in the future. A thief could add > > > his key to a new output on the transaction and try to spend (just like > > > a user would with a fresh/rotated key), but the OP_CSV would be > > > automatically added to his script to implement the public observation > > > delay window. > > > > > > Also, there was other previous work that I was only informed about > > > today after posting my proposal, so I should mention these as related > > > work: > > > https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2018-February/015793.html > > > https://blog.oleganza.com/post/163955782228/how-segwit-makes-security-better > > > https://www.youtube.com/watch?v=diNxp3ZTquo > > > https://bitcointalk.org/index.php?topic=5111656 > > > > > > - Bryan > > > http://heybryan.org/ > > > ___ > > > bitcoin-dev mailing list > > > bitcoin-dev@lists.linuxfoundation.org > > > https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev > > > > ___ > > bitcoin-dev mailing list > > bitcoin-dev@lists.linuxfoundation.org > > https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev ___ bitcoin-dev mailing list bitcoin-dev@lists.linuxfoundation.org https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev
Re: [bitcoin-dev] Bitcoin vaults with anti-theft recovery/clawback mechanisms
Seems to be comparable to the proposed "Tick Method" from 2013: https://bitcointalk.org/index.php?topic=307211.msg3308565#msg3308565 However I remember that someone told me the tick method had a flaw.. On Wed, Aug 7, 2019 at 6:28 PM Dustin Dettmer via bitcoin-dev < bitcoin-dev@lists.linuxfoundation.org> wrote: > Does revaulting vault up with the same keys, or new ones? > > Are they new derivation paths on the same key? > > Would love some expanded explanation on how you’re proposing this would > work. > > Thanks, > Dustin > > On Wed, Aug 7, 2019 at 1:35 PM Bryan Bishop via bitcoin-dev < > bitcoin-dev@lists.linuxfoundation.org> wrote: > >> Hi, >> >> One of the biggest problems with the vault scheme (besides all of the >> setup data that has to be stored for a long time) is an attacker that >> silently steals the hot wallet private key and waits for the vault's >> owner to make a delayed-spend transaction to initiate a withdrawal >> from the vault. If the user was unaware of the theft of the key, then >> the attacker could steal the funds after the delay period. >> >> To mitigate this, it is important to choose a stipend or withdrawal >> amount per withdrawal period like x% of the funds. This limits the >> total stolen funds to x% because once the funds are stolen the user >> would know their hot key is compromised, and the user would know to >> instead use one of the other clawback paths during all of the future >> withdrawal delay periods instead of letting the delay timeout all the >> way to the (stolen) default/hot key. >> >> The reason why a loss limiter is the way to go is because there's >> currently no way (that I am aware of, without an upgrade) to force an >> attacker to reveal his key on the blockchain while also forcing the >> attacker to use a timelock before the key can spend the coins. I am >> curious about what the smallest least invasive soft-fork would be for >> enabling this kind of timelock. There are so many covenant proposals >> at this point (CHECKSIGFROMSTACK, SECURETHEBAG, CHECKOUTPUTVERIFY, >> ). Or there's crazy things like a fork that enables a transaction >> mode where the (timelock...) script of the first output is >> automatically prefixed to any of the other scripts on any of the other >> outputs when an input tries to spend in the future. A thief could add >> his key to a new output on the transaction and try to spend (just like >> a user would with a fresh/rotated key), but the OP_CSV would be >> automatically added to his script to implement the public observation >> delay window. >> >> Also, there was other previous work that I was only informed about >> today after posting my proposal, so I should mention these as related >> work: >> >> https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2018-February/015793.html >> >> https://blog.oleganza.com/post/163955782228/how-segwit-makes-security-better >> https://www.youtube.com/watch?v=diNxp3ZTquo >> https://bitcointalk.org/index.php?topic=5111656 >> >> - Bryan >> http://heybryan.org/ >> ___ >> bitcoin-dev mailing list >> bitcoin-dev@lists.linuxfoundation.org >> https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev >> > ___ > bitcoin-dev mailing list > bitcoin-dev@lists.linuxfoundation.org > https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev > ___ bitcoin-dev mailing list bitcoin-dev@lists.linuxfoundation.org https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev
Re: [bitcoin-dev] Bitcoin vaults with anti-theft recovery/clawback mechanisms
Replying to two emails below. On Wed, Aug 7, 2019 at 7:27 PM ZmnSCPxj wrote: > > - Re-vaulting transaction. This is where the magic happens. The > re-vaulting > > transaction is signed during transaction tree setup, before > constructing the > > delayed-spend transaction for the parent vault. The re-vaulting > transaction is > > broadcasted when someone wants to prevent a coin withdrawal during > the public > > observation delay period. The re-vaulting transaction spends the > delayed-spend > > transaction outputs. It has a single output with a script created by > running > > the entire vault setup function again. Hence, when the re-vaulting > transaction > > is confirmed, all of the coins go back into a new > identically-configured vault > > instead of being relinquished through the delayed-spend transaction > timeout for > > hot wallet key signing. > > As transactions need to be signed in reverse order, it seems to me that > there is a practical limit in the number of times a vault can be used. > Basically, the number of times we run the vault setup function is the > limit on number of re-vaultings possible. > > Is my understanding correct? > Yes, that is correct. When setting up the vault, plan it "all the way to the end" like next 100+ years. With exponential backoff on the relative timelock values, the total number of pre-signed transactions isn't really that high. With a few thousand pre-signed transactions (more than enough), you can have high resolution timelocks well into the future. On Wed, Aug 7, 2019 at 4:19 PM Dustin Dettmer wrote: > Does revaulting vault up with the same keys, or new ones? > Are they new derivation paths on the same key? > Honestly, no idea. The answer to that might depend on each individual vault user. If the user doesn't want to deal with the expense of managing a bunch of unique keys and other data, then it might make more sense to use the same values and have a small blob that has to be stored for a long time, rather than many different blobs stored in different places to deal with. - Bryan http://heybryan.org/ ___ bitcoin-dev mailing list bitcoin-dev@lists.linuxfoundation.org https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev
Re: [bitcoin-dev] Bitcoin vaults with anti-theft recovery/clawback mechanisms
Good morning Bryan, > - Re-vaulting transaction. This is where the magic happens. The re-vaulting > transaction is signed during transaction tree setup, before constructing > the > delayed-spend transaction for the parent vault. The re-vaulting > transaction is > broadcasted when someone wants to prevent a coin withdrawal during the > public > observation delay period. The re-vaulting transaction spends the > delayed-spend > transaction outputs. It has a single output with a script created by > running > the entire vault setup function again. Hence, when the re-vaulting > transaction > is confirmed, all of the coins go back into a new identically-configured > vault > instead of being relinquished through the delayed-spend transaction > timeout for > hot wallet key signing. As transactions need to be signed in reverse order, it seems to me that there is a practical limit in the number of times a vault can be used. Basically, the number of times we run the vault setup function is the limit on number of re-vaultings possible. Is my understanding correct? Regards, ZmnSCPxj ___ bitcoin-dev mailing list bitcoin-dev@lists.linuxfoundation.org https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev
Re: [bitcoin-dev] Bitcoin vaults with anti-theft recovery/clawback mechanisms
Hi, One of the biggest problems with the vault scheme (besides all of the setup data that has to be stored for a long time) is an attacker that silently steals the hot wallet private key and waits for the vault's owner to make a delayed-spend transaction to initiate a withdrawal from the vault. If the user was unaware of the theft of the key, then the attacker could steal the funds after the delay period. To mitigate this, it is important to choose a stipend or withdrawal amount per withdrawal period like x% of the funds. This limits the total stolen funds to x% because once the funds are stolen the user would know their hot key is compromised, and the user would know to instead use one of the other clawback paths during all of the future withdrawal delay periods instead of letting the delay timeout all the way to the (stolen) default/hot key. The reason why a loss limiter is the way to go is because there's currently no way (that I am aware of, without an upgrade) to force an attacker to reveal his key on the blockchain while also forcing the attacker to use a timelock before the key can spend the coins. I am curious about what the smallest least invasive soft-fork would be for enabling this kind of timelock. There are so many covenant proposals at this point (CHECKSIGFROMSTACK, SECURETHEBAG, CHECKOUTPUTVERIFY, ). Or there's crazy things like a fork that enables a transaction mode where the (timelock...) script of the first output is automatically prefixed to any of the other scripts on any of the other outputs when an input tries to spend in the future. A thief could add his key to a new output on the transaction and try to spend (just like a user would with a fresh/rotated key), but the OP_CSV would be automatically added to his script to implement the public observation delay window. Also, there was other previous work that I was only informed about today after posting my proposal, so I should mention these as related work: https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2018-February/015793.html https://blog.oleganza.com/post/163955782228/how-segwit-makes-security-better https://www.youtube.com/watch?v=diNxp3ZTquo https://bitcointalk.org/index.php?topic=5111656 - Bryan http://heybryan.org/ ___ bitcoin-dev mailing list bitcoin-dev@lists.linuxfoundation.org https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev
[bitcoin-dev] Bitcoin vaults with anti-theft recovery/clawback mechanisms
Hi, I have a proposal for implementing bitcoin vaults in a way that does not require any soft-forks or other software upgrades, although it could benefit from SIGHASH_NOINPUT which I'll describe later. I call them pre-signed vaults. Vault definition Here, a vault is defined as a transaction setup scheme that binds both the user and the attacker to always using a public observation and delay period before a weakly-secured hot key is allowed to arbitrarily spend coins. This is the same definition previously used[1]. During the delay period, there is an opportunity to initiate recovery/clawback which can either trigger deeper cold storage parameters or at least reset the delay period to start over again for the same keys. One of the important components of this is the delete-the-key pre-signed transaction concept, where only a single transaction is (pre)signed before deleting the key. This is basically an emulation of a covenant and enforces a certain outcome. Background and motivation = I was looking at Eyal and Sirer's 2016 vaults paper [1], and I saw this headscratcher: > Vault transactions use a delay mechanism. We note that vault transactions > cannot be implemented with existing timing mechanisms such as > CHECKLOCKTIMEVERIFY opcode or transaction locktime. This was probably written before the introduction of OP_CHECKSEQUENCEVERIFY. Still, a viable construction would have more steps than just using OP_CSV. They were probably not thinking about what those steps might be, because in the context of the paper they were proposing a bitcoin vault implemented using recursive consensus-enforced covenants via a new opcode, which obviously cannot be deployed without an upgrade fork. Covenants have been discussed for years, but require new opcodes or other consensus-enforcement changes. Relative locktimes are useful here because there is no knowledge as to when the transactions might be broadcasted in the future. The delays need to be relative to after the transaction is included in the blockchain, not to setup initialization time. Also, from [2]: > We show that a [vault transaction] mechanism is currently not possible in all > cryptocurrencies [...] Bitcoin's scripting language requires support for > covenants. I haven't seen any previous proposal for how to implement recursive bitcoin vaults without a fork and without a covenant. After asking around, I am pretty sure this is somewhat novel. The closest I guess is [3]. Vaults are particularly interesting as a bitcoin cold storage security mechanism because they enable a publicly observable delay period during which time a user could be alerted by a watchtower that a thief might be in the process of stealing their coins, and then the user may take some actions to place the coins back into the vault before the relative timelock expires. There seems to be no way to get this notification or observation period without a vault construction. It might have been assumed it required a covenant. Having a vault construction might go a long way to discourage would-be attackers, on principle that the attacker might be incapable of recovering their cost-of-attack because the recovery mechanism can lock up the coins indefinitely. Griefing or denial-of-service would still be possible, of course, but with multisig there might be some ways to put a halt to that as well. I am working under the assumption that the attacker knows that the user is a vault user. Vaults == The idea is to have a sequence of pre-generated pre-signed transactions that are generated in a certain way. The basic components are a vaulting transaction that locks coins into a vault, a delayed-spend transaction which is the only way to spend from a vault, and a re-vaulting transaction which can recover/clawback coins from the delayed-spend transaction. The security of this scheme is enforced by pre-signing transactions and deleting private keys, or with the help of SIGHASH_NOINPUT then there's another scheme where private keys are provably never known. This enforces that there's only a specific set of possible outcomes at every step of the vault. Some examples of what the set of broadcasted transactions might look like in regular usage: coins -> VT -> DST -> exit via hot wallet key coins -> VT -> DST -> RVT coins -> VT -> DST -> RVT -> DST -> ... coins -> VT -> ... -> RVT998 -> nuclear abort where: VT = vault transaction DST = delayed-spend transaction RVT = re-vaulting transaction The delayed-spending transaction would have a single output with a script like: ( 30 days AND hot wallet key OR 10 days AND re-vaulting public key OR 1 day AND 4-of-7 multisig OR 0 days and super-secure nuclear abort ragequit key ) Another diagram: VT_100 -> DST -> (optionally) RVT -> coins are now in VT_99 VT_99 -> DST -> (optionally) RVT -> coins are now in VT_98 ... VT_1 -> burn-all-coins nuclear abort ragequit
