Designing “honest ransomware” with Ethereum smart contracts: postscript (part III)

[continued from part II]

Postscript: Shadow Brokers auction done right

Incidentally this protocol also works for the type of auction Shadow Brokers attempted in 2016— if they intended it in earnest, which does not appear to have been the case for the stash of NSA exploits they planned to dump. To recap: this threat-actor had gotten its hand on an exploit kit associated with the NSA and initially offered to sell it to the highest bidder. This “auction” however was curiously designed to mirror the dollar auction from game theory, featuring a winner-takes-all property. Bids are placed by sending funds to a Bitcoin address, highest-bidder gets the stash of exploits while everyone else gets nothing— they forfeit any Bitcoin sent. Not surprisingly there were few takers for this model, given the odds that even the winner may end up with nothing.

While the auction setting introduces additional complexity, the protocols sketched earlier can help with a simpler version of the problem: a shady group claims to have a lucrative stash of documents up for sale in exchange for cryptocurrency. Given the nature of such underground transactions, both sides are concerned about the risk of being defrauded. The seller worries about delivering the stash without getting paid and the buyer is concerned about paying for worthless information. A variant of the fair-exchange payment protocol can be built on Ethereum smart contracts:

  1. Seller encrypts all documents using a hybrid-scheme with a fixed public-key and makes all ciphertexts available to the buyer. (The granularity of encryption need not be at the level of individual documents. Each page or 10K chunk of source code could be individually encrypted, as long as each fragment contains enough information for the buyer to make a judgment on its veracity.)
  2. Buyer and seller jointly select a random subset of ciphertexts to be opened by the seller, to verify that they conform to the uniform encryption format expected for the entire batch. This assumes the seller has some way to validate the authenticity of individual fragments.
  3. Buyer launches an Ethereum smart-contract, designed to release payment on delivery of the private-key. He funds the contract with an amount of ether corresponding to the sale price.
  4. Seller invokes the contract method disclosing the private-key and collects the proceeds.

There is of course one last optional step for the buyer: call up the Ethereum Foundation and demand a hard-fork to reverse the payment. After all, the fairness expressed in a smart contract is only as reliable as the immutability of the blockchain that contract executes on.

CP

PS: Similar ideas are explored in a blog post on “The future of ransomware”, which in turns references the notion of zero-knowledge of contingent payments first demonstrated in Bitcoin. That approach front-loads the work into developing cryptographic proofs systems to verify that the encrypted data has the right structure, such as being the solution to a particular puzzle which can be verified by operating on encrypted data. It relies on disclosing the preimage for a hash (as opposed to a private-key) which can be expressed even with the limited scripting capabilities of Bitcoin. But that approach runs into the same problem as verifiable encryption when applied to ransomware: the plaintext has no particular structure, and we only assume the user has access to an oracle that can answer thumbs up/down on whether the result of a decryption corresponds to an authentic file which had been hijacked by malware.

 

Designing “honest ransomware” with Ethereum smart contracts (part II)

[continued from part I]

There are special-case solutions for common public-key algorithms to prove that a given ciphertext decrypts to an alleged plaintext. For example, with RSA the key-holder can return the plaintext without removing its padding. Recall that RSA is typically used in conjunction with a padding scheme such as PKCS1.5 or OAEP. These schemes apply randomized transformation to plaintext prior to encryption and undo that transformation after decryption. Without knowing the padding used during encryption, it is not possible to check that a given unpadded plaintext corresponds to some known ciphertext. But once the fully padded version is revealed, the user can both check that it encrypts to the original ciphertext and that removing the padding results in expected plaintext, because both of those steps are fully deterministic.

This straightforward approach does not carry over to other algorithms. For example ElGamal is a randomized public-key encryption scheme—each encryption operation uses a randomly chosen nonce, so reencrypting the exact same message can yield different ciphertexts. Yet it is not possible to recover the random nonce used after the fact, even with knowledge of the private key.  Unless the private-key owner takes additional steps to stash that nonce someplace, they are stuck in a strange position: they can decrypt any ciphertext but they can not simply prove the decryption is correct by sharing the result. That is not the case for RSA: if you can decrypt, you can also recover the transformed input complete with randomized padding. ECDSA calls for something more complex, such as a zero-knowledge proof of discrete logarithm equality to show that the multiplication of the random point (first part of the ciphertext) by the private-key was done correctly.

For a more generic solution, we can leverage blind decryption: ask the private-key holder to decrypt a ciphertext without that party realizing which ciphertext is being decrypted. This is typically achieved by exploiting homomorphic properties of cryptosystems. For instance raw RSA—without any padding— has a simple multiplicative relationship: encryption of a product is the product of encryptions.

RSA-ENC(a·b) = RSA-ENC(a) * RSA-ENC(b)

where all multiplication is done modulo N associated with the key. A similar property also holds true for decryption where X and Y are ciphertexts:

RSA-DEC(X·Y) = RSA-DEC(X) * RSA-DEC(Y)

Looked another way, we can decrypt a ciphertext indirectly by decrypting a different ciphertext with known relationship to the original:

RSA-DEC(X) = RSA-DEC(X·Y) * RSA-DEC⁻¹(Y)

To decrypt X, we ask the ransomware operator to instead decrypt X·Y as long we know the decryption of Y. (Typically because we obtained Y by encrypting a random plaintext m of our choice to begin with.) Note the original encryption can still be using a strict padding mode such as OAEP, as long as the decryption side is willing to handle arbitrary ciphertext that results in plaintext with no specific padding format.

This additional step prevents the ransomware author from cheating: X·Y looks like a random ciphertext, completely unrelated to the original encryption X of the symmetric key. Even if there is a cheat sheet listing every such X and its associated plaintext to answer chalenges, there is no way to correlate the particular challenge presented to one of these entries. Each challenge is equally likely to be derived from any ciphertext in the collection. The intuition is that if files were not encrypted according to a consistent scheme with the same RSA public-key, it would not be possible to produce the correct answer when presented with such a random-looking ciphertext.

This step can be repeated for a small subset of files to achieve high-confidence that most files have been encrypted according to the expected scheme. As long as the challenges are selected randomly, even a small number of successful tests provides high assurance against cheating. For example, suppose ransomware encrypted a million files but replaced 2% of them with random data instead of following the claimed hybrid-encryption process. If 100 files out of that collection are randomly selected for spot-checking, odds are 87% that this departure from protocol will be caught. The downside is diminishing returns with increased coverage. While a handful of challenges can rule out cheating on a wide scale— for example every other file being corrupted— it is not feasible to prove that 100% of data is recoverable. If ransomware deliberately mangled exactly 1 file in the collection, it would take a great deal of luck to discover that; one must pick that specific file as a challenge. That also suggests a strategy of picking the files you care about most for the challenge phase, given that for every other files there is a small but non-zero probability of failure to recover. (Incidentally the ransomware author has a diametrically opposed interest in making sure users do not get to choose the challenge subset unilaterally. Otherwise they could pick the 100 files they care about most and abandon the rest.)

There is one more subtlety here: the decryption challenge returns a symmetric key, which is then used to undo the bulk symmetric-encryption applied to the file. But there is still the problem of deciding whether the outcome of that decryption is “correct” in the sense that the plaintext was a file originally belonging to this user. Neither authenticated encryption or integrity checks help with that. After all ransomware could have replaced an MP3 music file with the “correct” AES-GCM encryption of random noise. Given the right AES key, that file will decrypt successfully, including validation of the GCM tag. It will not result in recovery of the original data, which is the only outcome the user cares about. To work around this we have to posit that users have access to an oracle that can examine a file (complete with all meta-data such as directory path and timestamps) and determine whether it is one of the documents originally belonging to their collection. In practice this could be based on a catalog of hashes or digitally signatures over documents, or in the worst-case scenario, manual inspection of files.

Once the user is convinced all files are indeed encrypted correctly, the next step is crafting a smart-contract to release payment conditional on the private-key being revealed. This contract will have a function intended to be invoked by the key-holder. After checking that the parameters supplied reveal enough information to recover the private-key, the function sends funds to an address agreed upon in advance. There is one subtlety here: specifying the destination address as part of the function call or inferring it from the message sender results in a race condition. Since Ethereum contract calls are broadcast on the network before they are mind in, anyone could copy the disclosed private-key and craft an alternative method invocation to shuttle funds some place else, hoping to get mined in first. Fixing the destination during contract creation solves this problem. Even if someone else succeeds in preempting the original method invocation with a different one sending the exact same information, the funds are still delivered to the intended address.

Just in case the private-key holder never shows up, the contract also needs an “escape hatch.” That is a second, time-locked method that can be invoked by the user after a mandatory delay to recover unclaimed funds. It is critical that these are the only ways of withdrawing any funds from the contract. If there was some other avenue for getting funds out, it would result in a race condition. When the private-key holder invokes the contract to collect payment, the contract owner can observe that transaction (including the disclosed private-key) and try to race them with a different transaction that siphons funds from the contract before it can pay out.

In terms of disclosing a private-key in a manner that can be verified by Ethereum smart-contracts, there are two natural solutions:

  1. Staying in the RSA setting, the smart-contract method that receives two large integers p and q, and verifies that their product is equal to the modulus N. While doable in principle, this requires implementing arbitrary precision integer multiplication in Solidity, which does not natively support such operations. The underlying Ethereum Virtual Machine (EVM) has 256-bit integer primitives and supports multiplication of words into 256-bit product, discarding more significant bits. One could write a big-number library to multiple large numbers by splitting them into 128-bit chunks. But this runs into another practical issue around gas costs: Ethereum smart-contract execution costs money proportional to the complexity of operations. Trying to check whether the product of two 1024-bit factors is equal to a known RSA modulus can become an “expensive” operation.
  2. Switch to an elliptic-curve setting and leverage the fragility of ECDSA for deliberately disclosing private keys. Because Ethereum natively supports verification of ECDSA signatures over the secp256k1 curve, this makes for a straightforward implementation. So instead of trying to make RSA operations work in Ethereum, we alter file-encryption model. ECIES or ElGamal can both be adapted to work over secp256k1. ElGamal in particular has a simple homomorphism that can be used to mask ciphertexts: multiplying both components of an ElGamal ciphertext by a scalar produces the encryption of the original message multiplied by that scalar. As with ECDSA, the public-key is a point on the curve and the private-key is the discrete logarithm of that with respect to the generator point. Since that key-pair is equally usable for ECDSA signatures, built-in EVM operations are sufficient to check for private key disclosure. As before, the function expects two valid ECDSA signatures over fixed messages such as “hello” and “world.” But in addition verifying these signatures— which is a primitive operation built into EVM— the contract also confirms these signatures share an identical nonce.

Summary

To recap: an honest ransomware scheme— or “third-party backup encryption service”— can be implemented using smart-contracts to make data recovery contingent on payment. We encrypt all files of interest using a hybrid encryption scheme with a single public-key. When it is time to recover the data, the user and TBES execute an interactive challenge protocol to decrypt randomly selected files, verifying that the encryption followed the expected format. (This step is redundant if encryption was done by the user, unless the integrity of encrypted backups is itself in question.) Assuming the proofs check out, the next step is for the user to create a smart-contract on the Ethereum blockchain. This contract is parametrized by an amount of Ether agreed upon, public-key used for encryption and address chosen by the TBES. Once the contract is setup and funded, TBES can invoke one of its methods to disclose the private-key associated with that public-key and collect the funds.

The logic of Ethereum smart-contract execution guarantees this exchange will be fair to both sides: payment only in exchange for valid private-key and no way to get out of payment once private-key is delivered.

[continued]

CP

Updated: 6/19, to describe alternative solution for ElGamal.

Designing “honest ransomware” with Ethereum smart contracts (part I)

“But to live outside the law, you must be honest” – Bob Dylan

By all accounts Wannacry ransomware made quite the splash, bringing thousands of systems to a standstill and forcing victims to shell out Bitcoin with little hope of recovery. In fact the ransomware aspect may well have been a diversion. Attribution for such attacks is tricky but both Kaspersky Labs and Symantec research groups have linked Wannacry to the Lazarus Group, a threat actor associated with the DPRK. (WaPo recently reported that the NSA concurs.) Their previous claim to fame: massive theft of funds from the Bangladesh central bank by exploiting the SWIFT network. That heist netted somewhere in the neighborhood of $80M, but the take would have been much higher were it not for the attackers’ rookie mistakes that resulted in even larger transfers being stopped or reversed. By comparison Wannacry earned a pittance, less than $100K at the time of writing.

Worse, this malware is not even  capable of living up to its raison d’etre: decrypting files after the attackers are paid off. For starters, only a handful of Bitcoin addresses were hard-coded in the binary, as opposed to unique deposit addresses for each victim. That makes it difficult to distinguish between different victims paying the bounty, which in turn violates the cardinal rule of ransomware: only users who pay the ransom get their data back. This is not so much a principle of fairness— there is no honor among thieves— as it is one of economic competitiveness: ransomware can only scale if victims are convinced that by paying the ransom they can recover their files. This is why successful ransomware campaigns in the past went so far as to feature helpful instructions to educate users about Bitcoin and staff customer-support operations to help “customers” recovery their data. Wannacry seems to have taken little interest in living up to such lofty standards of customer service.

But this incident does raise a question: are users at the mercy of ransomware authors when it comes to recovering their data? Is there a way to guarantee that payment will result in disclosure of the decryption key? After all, the crooks are demanding payment in cryptocurrency. Even Bitcoin with its relatively modest scripting language can express complex conditions for payment. Is it possible to design a fair-exchange protocol where decryption key is released if and only if corresponding payment is made?

This scenario is admittedly contrived and unlikely to be implemented. If there is no honor among thieves, certainly there is no desire to adapt more transparent and fair payment mechanisms to protect consumers from getting ripped-off by unscrupulous ransomware operators. But one can imagine more legitimate use-cases such as backup/escrow services that assist users encrypt their data for long-term storage. To avoid a single point of failure, the encryption key would be split using a threshold secret-sharing scheme. Specifically it is shared into N shares such that any quorum of M can reconstitute the original secret. Each share is in turn encrypted to the public-key of one trustee. When the time comes to decrypt this data, the consumer asks some subset of trustees to decrypt their share. This interaction calls for a fair-exchange protocol where the consumer receives the decryption result if and only if the trustee gets paid for its assistance.

Ethereum can solve this problem using the same idea behind fair-exchange of cryptocurrency across different blockchains, with a few caveats. The smart contract sketched out in previous blog-posts is designed to send funds when a caller discloses a specific private-key. But there are is a deeper problem around knowing which private-key to look for. In theoretical cryptography this falls under the rubric of “verifiable encryption” where it is possible to prove that some ciphertext is the encryption of an unspecified plaintext value that meets certain properties. Typically these constructs operate on abstract mathematical properties of plaintext, such as proving that it is an even number. In the more concrete setting of ransomware, plaintext under consideration are not mathematical structures but large complex data formats such as PDF documents. This model lends itself better to less-efficient statistical approach for verifying that the encryption process has been followed according to specification.

Let’s assume all files are encrypted using a hybrid-encryption scheme:

  • For each file/object to be encrypted, a random symmetric key is generated and bulk data is encrypted using a symmetric block-cipher such as AES.
  • The symmetric key is in turn encrypted using a fixed public-key cryptosystem such as RSA using the public-key of the trustee. This “wrapped” key is saved with the output of the bulk encryption.

This is similar to the format used by email encryption standards such as PGP and S/MIME. It is also how Wannacry operates, with an additional level of indirection. It generates a unique 2048-bit RSA keypair on each infected target and then encrypts that private-key using a different, fixed RSA public-key, presumably held by the ransomware crooks.  That means revealing the private-key used in step #2 is sufficient to decrypt all ciphertexts created according to this recipe.

There is one major difference between ransomware and the more legitimate, voluntary backup scenarios sketched out earlier: in the latter case the user can be certain of the public-key used for the encryption—because they performed the encryption themselves. In the former situation, they have just stumbled open a collection of ciphertext along with a ransom note asserting that all data has been encrypted using the process above with a private-key held by the author. Some proof is required that this claim is legitimate and the author is in possession of private-key required to recover their data. (Similar to fake DDoS threats, one can imagine fake ransomware authors reaching out to users to offer assistance, with no real capability to decrypt anything.)

A naive solution is to challenge the private-key holder to decrypt a handful of ciphertexts, effectively asking for free samples. But such a protocol can be cheated if it works by sending the full ciphertext or even the wrapped symmetric-key produced in step #2. For all we know, the ransomware encrypts each file using random symmetric keys and then stores those keys in a database. (In other words, it does not use a single public-key to wrap each of the symmetric keys; that part of the ciphertext is a decoy.) This operation could still respond to every challenge query successfully with the correct symmetric keys, by doing database lookups. But the encryption does not conform to the expected pattern above; there is no single private-key to unlock all files. In effect the user would be paying for a bogus key that has no bearing on the ability to decrypt ciphertexts of interest.

[continued]

CP

Two-factor authentication: a matter of time

How policy change in Russia locked Google users out of their account

Two-factor authentication is increasingly becoming common for online services to improve the protection afforded to customer accounts. Google started down this road in 2009, and its efforts were greatly accelerated when the company got 0wned by China in the Aurora attacks. [Full disclosure: this blogger worked on Google security team 2007-2013] At the time, and to a large extent today, the most popular way of augmenting an existing password-based system with an additional factor involved one-time passcodes or “OTP” for short. Unlike passwords, OTPs changed each time and could not be captured once for indefinite access going forward. (Note this is not equivalent to saying they can not be phished—nothing prevents a crook from creating fake login pages which ask for both password and OTP, and in fact such attacks have been observed in the wild.)

Counters and ticking seconds

There are many ways to generate OTPs but they all follow a similar pattern: there is a secret key, often referred to as a “seed” and this secret is combined with a variable factor such as current time or an incrementing sequence number using a one-way function. This one-way property is critical: it is a security requirement that even if an adversary can observe multiple OTPs and know the conditions they were generated in (such as exact timing) they can not recover the secret-seed required to generate future codes.

Perhaps the most well-known 2FA solution and one of the oldest is a proprietary offering from RSA called SecurID. It  was initially implemented only on hardware tokens sold by the company. SecurID is a decidedly closed ecosystem, at least in its original incarnation: not only did customers have to buy the hardware from RSA Inc but you also had to integrate with a service operated by the same company in order to verify if OTP codes submitted by users. That is because only RSA knew the secret-seed embedded in each token; customers did not receive these secrets or have any way to reprogram the token with their own secrets.

HOTP

Such closed models may have been great for customer lock-in but would clearly not fly in a world accustomed to open standards, interoperability and transparency. (Not to mention the wisdom of relying on a third-party for your authentication, a lesson that many companies including Lockheed-Martin would learn the hard way when RSA was breached by threat-actors linked to China in 2010.) Other industry players began pushing for an open standard, eventually resulting in a new design called HOTP being published as an RFC. The letter “H” stands for HMAC, a modern cryptographic primitive with a sound security model, compared to the home-brew Rube-Goldberg contraption used in SecurID. HOTP uses an incrementing counter as the internal “state” of the token. Each time an OTP is generated, this counter is incremented by one.

That model relies on synchronization beween the side generating OTP codes and the side responsible for verifying them. Both must use the same sequence number in order to arrive at the same OTP value. The sides can get out of sync due to any number of problems: imagine that you generated an OTP (bumping up your own sequence number) but your attempt to login to a website failed because of a network timeout. The server never received the OTP and therefore its sequence number is one behind yours. In practice this is solved by checking a submitted OTP not just against the current sequence number N but a range of values {N, N+1, N+2, …, N + t} for some tolerance value t. If a given OTP checks out against one of the later values in this sequence, the server updates its own copy of the counter, on the assumption that the client skipped past a few values. Even then things can go awry since collisions are possible: a user can accidentally mistype an OTP which then happens to match one of these later numbers, incorrectly advancing the counter.

TOTP

An alternative to maintaining a counter is using implicit state both sides independently have access to without having to synchronize. Time is the most obvious example: as long as both the client generating OTP value and server verifying it have access to an accurate clock, they can agree on the state of the OTP generator. Again in practice there is some clock drift permitted; instead of using a very accurate time down the millisecond, it is instead quantized into intervals of say 30 or 60 seconds. OTP codes are then generated by applying the HMAC function to the secret seed and this time-interval count. This was standardized in an open standard called TOTP, or Time-Based One-Time Password Algorithm.

There is one catch with TOTP: both sides must have an accurate source of time. This is easier on the server-side verifying OTP codes, but more difficult client-side where OTP generation takes place. The reason HOTP and sequence-numbers historically came first is that they could be implemented offline, using compact hardware without network connectivity. While embedded devices can have a clock, the challenge is those clocks require a battery to remain powered 24/7 and more importantly they eventually start drifting, running slower/faster than true time. A token that runs one second too fast every day will be a full 6 minutes ahead after a year.

Fast forward to 2009 with the smartphone revolution already underway, our model envisioned mobile apps handling OTP generation. Unlike stand-alone tokens, apps running on a smartphone have access to a system clock that is constantly being synchronized as long as the device is online. That take cares of time drift—even when the phone is only sporadically connected to the internet— making TOTP more appealing.

Which time?

One of the first questions that comes up about TOTP is the effect of time-zones. What happens when a user sitting in New York computes a TOTP that is submitted to a service in California for verification? In this case client and server separated by three time-zones. At first glance it looks like since they disagree on the current time, time-based OTP would break down in this model. Luckily that is not the case: as with most protocols relying on an accurate clock, TOTP calls on both participants to use an absolute frame of reference, namely the UNIX epoch time. Defined as the number of seconds elapsed since midnight January 1st, 1970 on Greenwich timezone, it does not depend on current location or daylight saving adjustments. That means TOTP generators only need to worry about having an approximately correct clock, a problem that is easily solved when the 2FA app runs on a mobile device with internet connectivity periodically checking some server in the cloud for authoritative time.

Daylight saving time considered harmful

Never underestimate the ability of the real world to throw a wrench in the plans. In 2011 Russia announced that it would not adjust back from daylight saving in the fall:

“President Medvedev has announced that Russia will not come off daylight saving time starting autumn 2011. Medvedev argued that switching clocks twice a year is harmful for people’s health and triggers stress. “

While the medical profession may continue to debate the effects of switching clocks on the general population, this change caused a good deal of frustration for software engineers. Typically “local time” displayed to users is determined by starting from a reference time such as GMT, making adjustments for local timezone and seasonal factors such as daylight saving. In the case of the Android operating system, those adjustments were hard-coded. If Russia did not going switch back to DST as scheduled, those devices would end up displaying the “wrong” time, even when they have perfectly accurate internal clocks.

In principle, this is only a matter of updating the operating system to follow the new, health-conscious Russian regime. In reality of course updating Android devices in the field has been a public quagmire of indifference and mutual hostility amongst device manufacturers, wireless carriers and Google. While the picture has improved drastically with Google moving to assert greater control over the update pipeline, in 2011 the situation was dire. Except for the handful of users on “pure” Google-experience devices such as Nexus S, everyone else was at the mercy of their wireless carrier for receiving software updates and those carriers were far more interested in locking users into 2-3 year contracts by selling another subsidized device than supporting existing units in the field. Critical security updates? Maybe, if you are lucky. Bug fixes and feature improvements? Forget about it.

Given that abject negligence from carriers and handset manufacturers, what is the average user to do when their phone displays 3’o clock when every one else is convinced it is 4’o clock? This user will take matters into their own hands and fix it somehow. The “correct” way to do that is shifting the timezone over by one, effectively going from Kaliningrad to Moscow. But this is far from obvious: the more intuitive fix given this predicament is to manually adjust the system clock forward by an hour. (One soon discovers that automatic time adjustments must also be disabled, or the next check-in against an authoritative time-server on the Internet will promptly restore the “correct” time.) Problem solved, the phone now reports that the local-time is 4’o clock as expected.

Off-by-one (hour)

Except for the unintended interaction with two-factor authentication, specifically TOTP which uses the current time to generate temporary codes. Overriding the system clock will shift the UNIX epoch time too. Now the TOTP generator is being fed from a clock with full one-hour skew. Garbage-in, garbage out. Most TOTP implementations will try to correct for slight clock drifts by checking a few adjacent intervals around current time, where each “interval” is typically 30 or 60 seconds. But no sane deployment is going to look back/forward as far as one hour, on the assumption that if your local clock is that far off, you are going to have many other problems.

Sure enough, reports started trickling in that users in Russia were getting locked out of their Google accounts because the 2FA codes generated by Google Authenticator on Android were not working. (In this blogger’s recollection, our response was adding a special-case check for a handful intervals around the +1 hour mark measured from current time— not all intervals between now and +1 hour mark. This has the effect of slightly lowering security, by increasing the number of “valid” OTP codes accepted, since each interval typically corresponds to a different OTP code modulo collisions.)

Real world deployments have a way of rudely bringing about the “impossible” condition. Until this incident, it was commonplace to assert that the reliability of TOTP based two-factor authentication is not affected by timezones or quirks of daylight saving time. In a narrow sense that statement is still true but that would have been no consolation to the customers in Russia locked out of their own accounts. Looking for a pace to pointer fingers, this is decidedly not a case of PEBKAC. Confronted with an obvious bug in their software, those Android users picked the most intuitive way of solving it. Surely they are not responsible for understanding the intricacies of local-time computation or the repercussions of shifting epoch time. Two other culprits emerge. Is the entire Android ecosystem to blame for not being able to deliver software upgrades to users, a problem the platform is still struggling with today? After all the announcement in Russia came months ahead of the actual change and iOS devices were not affected to the same extent. Or is the root-cause a design flaw in Android, having hard-coded rules about when daylight saving time kick-in? This information could have been retrieved from the cloud periodically, allowing the platform to respond gracefully when the powers-that-be declare DST a threat to the well-being of their citizenry.

Either way this incident is a great example of a security feature going awry because of decisions made in a completely different policy sphere affecting factors originally considered irrelevant to the system.

CP

 

Bitcoin and the ship of Theseus

Change and identity in a decentralized system

The Ship of Theseus is a philosophical conundrum about the continuity of identity in the face of change. Theseus and his ship sail the wide-open seas. Natural wear-and-tear takes its toll on the vessel, requiring its components to be replaced gradually over time. One day it is a few planks in the hull, the next season one of the masts are swapped out, followed by the sails. Eventually there comes a point where not a single nail or piece of fabric is left from the original build, and some parts have been replaced several times over. But unbeknownst to Theseus, a mysterious collector of maritime souvenirs has carefully preserved every component from the original ship taken out during repairs. (This is a variation on the original paradox, due to Hobbes.) In what may be the first case of retro-design, this person meticulously reassembles the original components into their original configuration. The riddle on which much ink has been spilled: which one is the true ship of Theseus? The one that has been sailing the seas all this time or the carefully restored one in the docks, which contains every last nut, bolt and rope from the original?

Bitcoin has been confronting a version of this riddle, most acutely during a few days in March when a hard-fork of the network appeared imminent. To recap: Bitcoin is a distributed ledger recording transactions and ownership of funds. This ledger is organized into “blocks,” with miners competing to tack on new blocks to the ledger, one block on average every 10 minutes. The catch is there is a limit on the size of blocks, which constrains how many transactions can be processed. Currently that stands at 1 megabyte—a gratuitous and arbitrary limit which may have seemed generous back in 2011 when it was first introduced, with plenty of spare room left in blocks to solve the problem. But kicking the can down the road predictably ends exactly as one would expect: increasing popularity of the network all but guaranteed that ceiling would be hit. Results of scarcity follow: transactions both became slower and more expensive. The time expected for a transaction to appear in a block increased and the fees paid to miners for that privilege sky-rocketed.

It is clear the situation calls for some form of scaling improvement. But there is no governance framework for Bitcoin. A system marvelously effective at bringing about distributed consensus at the technology level—everyone agrees on who owns what and which payments were sent—turns out to be terrible at producing consensus at the political level among its participants. Even the existence of a scaling crisis has been disputed. Some argue that Bitcoin excels as a settlement layer or store of value, and there is no reason to increase its on-chain capacity to handle everyday payment scenarios.

After much internecine fighting and several false-starts, the community coalesced around two opposing camps. One side mobilized under the Bitcoin Unlimited (BU) banner seeks to increase block size with a disruptive change, ratcheting up the arbitrary 1MB cap to some other, equally arbitrary but higher limit. On the other side is a group pushing for segregated-witness, a more complex proposal that solves multiple problems (including transaction malleability) but conveniently has the side-effect of providing an effective capacity increase. At the time of writing, this controversy remains in a deadlock. Segregated witness must reach roughly 75% miner support to activate. It has stalled at 30%, prompting supporters to give up on miners and seek an alternative approach called user-activated soft-fork or UASF. Meanwhile BU has been plagued by code-quality problems and DDoS attacks.

At some point in March, the miner support for BU was hovering dangerously close to the magic 50% mark. If that threshold is crossed, those miners could realistically start producing large blocks. While anyone can mine a large block any time, such blocks would be ignored by miners following the 1MB limit. It makes no sense to start producing them when they are only recognized by a minority. Such additions to the ledger would be quickly crowded-out and discarded in favor of alternative blocks obeying the 1MB restriction. But suppose BU exceeds 50%. (With some safety margin thrown in; otherwise there is the risk of block reorganization, where the original chain catches up and results in the entire history of big-blocks getting overwritten.) What happens if BU miners in the majority start producing those large blocks?
This was the question on everyone’s mind in March. It would result in a spit or “forking” of the Bitcoin ledger. Instead of one ledger there would be two parallel ledgers, maintained according to different rules. All blocks up to the point of the fork would be identical. If you owned 1 bitcoin, you still have 1 bitcoin according to both ledgers. But new transactions after the fork point can result in divergence, appearing on only one ledger. In effect two parallel universes emerge, where the same funds are owned by different people.

That brings us full-circle to the philosophical riddle of Theseus: which one of these is “Bitcoin”? Major cryptocurrency exchanges opted for a pragmatic answer: the original chain is Bitcoin-proper. According to this interpretation, the alternate ledger with large blocks will be considered an alternative cryptocurrency traded under its own ticker symbol BTU. (Reuse of the acronym for “British Thermal Unit,” a measure of heat, provides unintended irony for those who consider BU to be a dumpster-fire.)

While that tactical response addressed the uncertainty in markets, it did not provide a coherent definition of what exactly counts as Bitcoin. In effects the signatories were declaring that the chain with the large blocks would be relegated to “alternate coin” status regardless of hash power. For a system where security is derived from miners’ hash power to issue a blanket declaration of the irrelevance of hash power is extraordinary. Arguably the harshest criticism came from left field: the Ethereum community. Ethereum itself had taken flak in the past for taking exactly the same stance of ignoring miner choices during the 2016 DAO bailout. Orchestrated by the Ethereum Foundation and widely panned as crony-capitalism, this intervention resulted in a permanent, with a minority chain “Ethereum Classic” continuing at ~10% of hash power. In a case Orwellian terminology, this alternative chain is in reality the true continuation of the original Ethereum blockchain. What is now referred to as “Ethereum” incorporates the deus ex machina of the DAO intervention. But from a governance perspective, the most troubling aspect of the DAO debacle concerns how the legitimacy of the fork was ordained. When faced with a faction of the community expressing doubts about the wisdom of intervention, the Foundation insisted that regardless of what miners do, the branch reversing the DAO theft would become the official Ethereum branch. Such a priori declarations of the “correct chain” go against the design principle of miners providing integrity of the blockchain through costly, energy-intensive proof-of-work. Why waste all that electricity if you can just ask the Ethereum Foundation what the correct ledger is? In anointing a winner of the hard-fork by fiat without regard for hash power, the Bitcoin community had exhibited precisely the same disregard for market preferences.

Once unmoored from the economics of hash power, arguments about which chain is “legitimate” quickly devolves into philosophical questions about identity. If you hold that 1MB block-size is the sine quo non of Bitcoin, then any hard-fork modifying that property is by definition not Bitcoin. Yet some of the same individuals arguing that changing to 2MB blocks would results in complete loss of identity have also proposed  changing the proof-of-work function, after evidence emerged suggesting that mining hardware from a particular vendor may be exploiting a quirk of the existing PoW function to optimize their hardware. Changing the PoW is arguably a far more disruptive change than tweaking block size.

So it remains an open question what exactly defines Bitcoin and to what extent the system can evolve over time while unambiguously retaining its identity as Bitcoin. Is it still Bitcoin if block sizes are allowed to increase based on demand? If the proof-of-work function is replaced by a different one? Or if the environmentally wasteful proof-of-work model is abandoned entirely in favor of proof-of-stake approach? What if the distribution of coinbase rewards is altered to decrease continuously instead of having abrupt “halving” moments? Or to take a more extreme example, if the deflationary model with money supply capped at 21 million bitcoin is lifted, allowing the money supply to continue expanding indefinitely? Is it still “Bitcoin” or does that system deserve to be relegated to alt-coin status with an adjective attached to its name? A related question is who gets to make the branding determination? When Ethereum went through its hard-fork to bail to the DAO, it was the altered chain bestowed with the privilege of carrying the Ethereum name; the original, unmodified chain got relegated to second-class citizen as “Ethereum Classic.” Would the situation have been reversed if the Ethereum Foundation was instead opposed to intervention and the hard-fork was instead driven by a grassroots community effort to rescue the DAO at all costs?

Having been pronounced for dead multiple times, Bitcoin continues to defy the odds. It is already up more than 50% against the USD for 2017 at the time of writing, having survived a crack-down on capital controls in China. Yet the contentious scaling debate shows no signs of slowing down. BU proponents continue to lobby for a disruptive hard-fork, while segregated-witness adherents play a a game-of-chicken with user-activated soft forks.  Will the resulting system—or one of the resulting systems, in case the contentious fork results in a proliferation of incompatible blockchains— still qualify as “Bitcoin”? Beyond the crisis du jour, it remains unclear if Bitcoin is capable of improving by incorporating new ideas, especially when these ideas call for a disruptive change  breaking backwards compatibility. If the community interprets every hard-fork as an identity crisis that calls into question the meaning of “Bitcoin,” the resulting stasis will  place Bitcoin at a disadvantage compared to alternative cryptocurrencies which are more responsive to market demand. (To wit, the so-called “Bitcoin dominance index” which measures the market capitalization of BTC as a fraction of all cryptocurrencies is now at an all-time low, having dipped below 50% mark.) There is something to be said about stability and consistency. Whimsical changes and excessive interventionism of the type demonstrated during the Ethereum DAO hard-fork do not inspire confidence in the long-term reliability of a currency either. Bitcoin so far has stubbornly occupied the opposite end of the spectrum, clinging to a literal, originalist interpretation of its identity defined by Satoshi.

That is one way of dodging the paradox of Theseus: this ship may be taking on water, but at least every single one of its planks is original.

CP

 

Trading cryptocurrency without trusted third-parties (part III)

[continued from part II]

Reliable execution matters

The preceding discussions suggest it is possible in principle to exchange cryptocurrency across different blockchains, without calling on a trusted third-party to hold funds in escrow. (Or viewed another way, the blockchain itself is the trusted third-party equivalent, its immutable rules guaranteeing all-or-nothing fair exchange where neither side can cheat the other.) That however is not enough to achieve feature parity with existing marketplaces. It provides only one piece of the puzzle: arranging for settlement of funds after a trade is agreed upon. The problem statement made a leap of faith in assuming that Alice and Bob already found each other, and somehow came to an agreement on price/quantity. But that arguably is the raison d’être of markets: helping buyers and sellers locate each other while facilitating price discovery. In realistic equity markets,  such arrangements of crossing buy/sell orders can take far more complex forms than  pairwise arrangements. For example it is very common for an order to be executed piece-meal: when Alice places an order to sell 10BTC, some fraction of that order is paired with Bob while the remainder goes to Carol. These can even take place at different times; Alice has a partially-executed order in the interim before Carol shows up, and she could even cancel the remainder.

Meanwhile accurate price discovery depends on reliable trade execution. Suppose the exchange stopped at matching Alice and Bob, delegating the actual trading for the individual parties to work out. Imagine Alice and Bob each receiving an email: “Congratulations, we found a counter-party for your trade. Here is their contact information.” At this point there is no guarantee that settlement will take place. If Alice or Bob back-out—which does not violate the fair-exchange property as long as neither side delivered anything—this trade did not occur as far as the market is concerned. That means the bid/ask quotes come with a prominent disclaimer: you can buy/sell at this price as long as your counter-party is in the mood for executing the settlement. This is very different from the expectation of trading in equity markets: if there is an order on the book to sell 10 shares of Google stock at a specified price, and a buyer shows up offering that exact price/quantity, there is very high confidence that this trade will execute. (In fact one of the main objections to high-frequency trading popularized by accounts like Flash Boys involve edge-cases where those guarantees are weakened due to order- spoofing and phantom liquidity: seemingly available trades disappearing when somebody attempts to take advantage of it by posting the corresponding buy/sell order.)

In principle, trade execution can be incentivized in a P2P exchange by creating an economic structure of rewards and fines. Customers who bail out on the settlement can be forced to pay additional restitution to the exchange or their counter-party. In some cases the guilty party is easy to determine. Fair-exchange protocols that leverage the blockchain can be audited publicly. Anyone can observe its progress and determine who backed out at which stage. But turning this into a fee/reward structure already requires creating a financial dependency between the exchange and its customers. For example, customers may have to post bond as insurance against abandoned trades.

Second there is still the problem of friction and delays introduced by forcing every trade to hit the blockchain. In a traditional exchange, when Alice and Bob swap BTC for ETH that trade is not reflected on any external blockchain. Only an internal ledger reflecting their balances is updated. Requiring all such transactions to execute on-chain both introduces delays and aggravates the scaling challenge, particularly in the case of Bitcoin which is already facing acute congestion while proposed solutions are mired in political gridlock. (Ethereum is relatively fast with block times measured in seconds and plenty of room in blocks to accommodate expanded usage.) At its current capacity, the network has an estimated total capacity around ~7 transactions per second. By comparison roughly 1 trade per second occurred for USD/BTC alone on major exchanges over the last thirty days. If all of that activity were reflected on the blockchain, it would account a significant fraction of overall capacity and further strain the overloaded network. And that is just one trading pair among many currencies for which BTC markets exist. Not to mention variable fees that must be paid to miners for moving bitcoin on chain, compared to the efficiency of updating an internal ledger.

Unless settlement is immediate and guaranteed, at best Alice and Bob have something akin to a futures contract in place. Each is promising to deliver some asset (BTC or ETH) at a future time for a price agreed upon today. That price is not an accurate reflection of present BTC/ETH price: neither party is guaranteed to receive their BTC or ETH immediately at that price. Especially for volatile assets such as cryptocurrency where price can fluctuate widely in a short span of time, this is an important consideration. Paradoxically such high volatility can encourage parties to back out of settlement if they have the chance. Suppose Alice agreed to sell 1 BTC for 20ETH but while she is working through the peer-to-peer settlement process with Bob, a spike in BTC price makes her assets now worth 21ETH. She has every incentive at this point to walk away from the trade and seek an alternative buyer at the improved price. Meanwhile Bob who assumed that he had a deal to exchange his ETH for BTC discovers that the offer was a mirage. Without a forcing mechanism to guarantee timely (ideally, real-time) settlement of trades, prices quoted on the order-book become an unreliable indicator of supply/demand.

Granted none of the preceding implies that a fully decentralized, trust-free exchange with real-time settlement can not exist. It simply points to the chasm that exists between current attempts to replace the traditional exchange model, and places the problem of settlement—which may well turn out to be the easy piece—in context with the full spectrum of functionality that full-fledged market places are expected to provide. There are many challenges and open problems involved in designing a solution that can reasonably compete with the existing paradigm.

CP

Trading cryptocurrency without trusted third-parties (part II)

[continued from part I]

To recap the scenario: Alice and Bob are interested in trading bitcoin (BTC) for ether (ETH.) Alice owns BTC, Bob has ETH, and they have agreed on pricing and quantity. (Note we are fast-forwarding past the scene where Alice and Bob miraculously located each other and organized this trade. That is one of the most valuable functions of a market, a point that we will return to.) Now they want to set up a fair-exchange where Alice only receives her ETH if Bob receives the corresponding amount of BTC.

Fragility of ECDSA as a feature

One way to do this involves turning what could be considered a “bug” in the ECDSA signature algorithm—used by both Bitcoin and Ethereum— into a feature. ECDSA is a randomized signature algorithm. Signing a message involves picking a random nonce each time. The random choice of nonce for each operations means even signing the same message multiple times can yield a different result each time. This is in contrast to RSA for example, where the most common padding mode is deterministic. Processing the same message again will yield the exact same signature.** It is critical for this nonce to be unpredictable and unique, otherwise the security of ECDSA completely breaks down:

  • If you know the nonce, you can recover the private key.
  • If the same unknown nonce is reused across different messages you can recover the private key. (Just ask Sony about their PlayStation code-signing debacle.)
  • It gets worse: if multiple messages are signed with different nonces with known relationship (such as, linear combination of some nonces equals another one) you can still recover the private key.

That makes ECDSA highly fragile, dependent critically on a robust source of randomness. It also means implementations susceptible to backdoors: a malicious version can leak private-keys by cooking the nonce while appearing to operate correctly by producing valid signatures. Variants have been introduced to improve this state of affairs. For example deterministic ECDSA schemes compute the nonce as a  one-way function of secret-key and message, without relying on any source of randomness from the environment.

But this same fragility can prove useful as a primitive for exchanging funds across different blockchains, by deliberately forcing disclosure of a private key. Specifically, it’s possible to craft an Ethereum smart-contract that releases funds conditionally on observing two valid signatures for different messages with the same nonce.

Setup

  • Alice has her public-key A, which can be used to create corresponding addresses on both Bitcoin & Ethereum blockchains.
  • Bob likewise has public-key B.
  • Alice generates a temporary ECDSA key, the “transfer-key” T.

Before starting execution, Alice rearranges her funds and moves the agreed-upon quantity of bitcoin into a UTXO with a specific redeem script. The script is designed to allow spending if either one of these two conditions are satisfied:

  • One signature using Alice’s own public key A but only after some time Δ has elapsed. This is a time-lock enabled by the  check-locktime-verify instruction.
  • 2-of-2 multi-signature using Bob’s public key B and the transfer key T.

Once this UTXO is confirmed, Alice sends Bob a pointer to the UTXO on the blockchain. In practice she would also have to send the redeem script for Bob to verify that it has been constructed. (Since the P2SH address is based on a one-way hash of the script, it is not possible in general to infer the original script from an address alone.)

Once Bob is satisfied that Alice has put forward the expected Bitcoin amount subject to the right spending conditions, he sets up an Ethereum contract. This contract has two methods:

  • Refund(): Can only be called by Bob using B and only after some future date. Sends all funds back to Bob’s address. This is used by Bob to reclaim funds tied up in the contract in case Alice abandons the protocol.
  • Exchange(signature1, signature2): This method is called by Alice and implements the fair-exchange logic. It expects two signatures using the transfer-key T over predefined messages, which can be fixed ahead of time such as “foo” and “bar”. The method verifies that both signatures are valid and more importantly they are reusing the ECDSA nonce. (In other words, the private key for T has been disclosed.) If these conditions are met, the contract sends all of its available balance to Alice’s address.

Alice in turn needs to verify that this contract has been setup correctly. As a practical matter, all instances of the contract can share the same source-code, differentiated only by parameters they receive during the contract creation. These constructor parameters are the Ethereum addresses for Alice and Bob, along with the public-key for T to check signatures against. That way there is no need to reverse-engineer the contract logic from EVM byte-code. A single reference implementation can be used for all invocations of the protocol. Only the constructor arguments need to be compared against expected values, along with the current contract balance.

Assuming this smart-contract is setup correctly, Alice can proceed with taking delivery of the ETH from Bob. She signs two messages with her private-key, reusing the same nonce for both. Then she invokes the Exchange method on the contract with these signatures. Immutability of smart-contract logic dictates that upon receiving two signatures with the right properties, the contract has no choice but to send all its funds to Alice.

At this point Alice has her ETH but Bob has not claimed his BTC. This is where the fair-exchange logic comes into play: Alice staked her claim to the ETH by deliberately disclosing the private-key T. Looking back at the redeem script for Alice’s UTXO on the blockchain, possession of T and Bob’s key B allows taking control of those funds. Bob can now sign a transaction using both private-keys to move that BTC to a new address he controls exclusively. Meanwhile Alice herself is prevented from taking those funds back herself because of the timelock.

The fine-print: caveats and improvements

A few subtleties about this protocol. Invoking Exchange() on the contract means the entire world learns the private key for T, not just Bob; blockchain messages are broadcast so all nodes can verify correct execution. Why not have Alice send one of the signatures to Bob out-of-band, in private? A related question is why not allow the Bitcoin funds to be moved using the transfer-key T only, instead of requiring a multi-signature? The answer to both of these is that Bob can not count on his knowledge of T being exclusive. Even if the Ethereum smart-contract only expected a single signature (having the expected nonce hard-coded) Alice can still publish the private-key for T to the entire world after she receives her ETH. If her funds only depended on a single key T for control, it would become a race-condition between Bob and everyone else in the world to claim them. Alice does not care; once she discloses T someone will take her BTC. But Bob cares very much that he is the only recipient and not have to race against others to get their TX mined first. Including an additional key B only known to Bob guarantees this, while also making it moot whether other people come in possession of the private-key for T.

Speaking of race conditions, there is still one case of Bob racing against the clock: he must claim the bitcoin before the time-lock on the alternative spending path expires. Recall that Alice can claw-back her bitcoin after some time/block-height  is reached. That path is reserved for the case when the protocol does not run to completion, for example Bob never publishes the ethereum smart-contract. But even after Bob has published the contract and Alice invoked it to claim her ETH, the alternative redemption path remains. So there is an obligation for Bob to act in a timely manner. The deadline is driven by how the time-locks are chosen. Recall that the Ethereum smart-contract also has a deadline after which Bob can claw the funds back if Alice fails to deliver T. If this is set to say midnight on a given day while the Bitcoin UTXO is time-locked to midnight the next day (these are approximate, especially when specified as block-height since mining times are randomly distributed) then Bob has 24 hours to broadcast the transaction. That time window can be adjusted based on the preferences of two sides, but only at the risk of increasing recovery time after protocol is abandoned. In that situation Alice is stuck waiting out the expiration of this lock before she can regain control of her funds.

Another limitation in the basic protocol as described is lack of privacy. The transaction is linkable across blockchains: the keys A, B and T are reused on both sides, allowing observes to trace funds from Bitcoin into Ethereum. This situation can be improved. There is no reason for Alice to reuse the same key A for reclaiming her Bitcoin as the key she uses to receive Ethereum from Bob. (In fact Bob only cares about the second one since that is given as a parameter to the contract.) Similarly Bob can split B into two different keys. Dealing with T is a little more tricky. At first it looks like this must be identical on both chains to allow private-key disclosure to work. But there is another trick Alice and Bob can use. After Alice gives the public-key for T to Bob, Bob can craft his Ethereum contract to expect the related key T* = m·T for a random scalar m used to mask the original key. He in turn shares this masking factor with Alice. Since Alice has the private key for T, she can also compute the private key for T* by simply multiplying with m. When she discloses that private-key, Bob can now recover the original key for T by using the inverse of m. Meanwhile to outside observers the keys T and T* appear unrelated. This provides a form of plausible deniability. If many people were engaging in transactions of this exact format with identical parameters, it would not be possible to link the Bitcoin side of the exchange to the Ethereum side. But “identical parameters” is the operative qualification. If Alice and Bob are trading 1BTC while Carol and David are trading 1000BTC, the transactions are easily separated. Similarly if the time-locks on ETH and BTC side are not overlapping, it becomes possible to rule out an ETH contract as being the counterpart of another BTC transaction posted around the same time.

Finally an implementation detail: why use the repeated-nonce trick for disclosing private key instead of simply sending private-key bits to the contract? Because the Solidity language used for writing smart-contract has a convenient primitive for verifying ECDSA signatures given a public-key. It does not have a similar primitive to check if a given private-key corresponds to a public-key. In fact it makes sense for Solidity to have no facilities for working with private-keys. Since all smart-contract execution is public, the assumption is only publicly available information would ever be processed by the contract and never secret material. For this reason we resort to the nonce reuse trick. Ethereum virtual machine also has the additional primitives required to compare two signatures for nonce equality. Interestingly Bitcoin script-language is exactly one instruction shy of being able to accomplish that. The instruction OP_CAT is already defined in the scripting language but currently disabled and for good reason: without other limits, it can be used as a denial-of-service vector. But if OP_CAT were enabled, it could be used to construct a redeem script that receives ECDSA signatures in suitably encoded form (nonce and second component as individual stack-operands) and checks them for nonce reuse. Other “splicing” opcodes such as OP_SUBSTR can also achieve the same effect by parsing the full ASN1 encoded ECDSA signature to extract the nonce piece into an individual stack operand where it can be compared for equality against another nonce. Either way, it would allow inverting the protocol sequence: Bob posts a smart-contract on the Ethereum blockchain first, Alice sets up the corresponding Bitcoin UTXO, which Bob proceeds to claim by disclosing the transfer key.

[continued]

CP

** RSA does have a randomized padding mode as well called PSS.