Thursday, December 5, 2013

The Ultimate/Best cyrptocurrency.

Structure of text --
0) feature -- Analysis/reasoning (importance score of the feature out of 10)
FAQ -
    A) Reason 1
    B) Reason 2
    C) Reason 3
    ...
    Z) ...

1) energy efficiency -- In the long run, it should not waist energy by unlimited mining. (2)
FAQ -
    A) Limited coins will not solve it.
        This's partially true. Miners will compete for TX fee.

2) Stability in prices -- Cryptocurrencies are not commodities. It's designed to be spent and a medium to receive payments from. If a cryptocurrency is not that, it's basically a pump and dump scheme.

Actually if you see for a fair cryptocurrency which is not seen as an investment, there should be a little bit inflation, cause that way people will not hold on to it like investment; instead they'll use it to buy investments like shares and gold -- which is something to be seen as an investment.

In terms of stability of prices, cryptocurrencies are better candidates cause it's something global -- no Forex is involved here. So one of the major strengths of a cryptocurrency compared to the national currency is it's stable non-volatile price. (10)
FAQ -
    A) Limited coins supply is not that good for stability, neither is long term mining of coins.
        The lost coins will increase the prices (instability in prices -- not desired; this'll happen cause Bitcoins are more rare). People will continue to loose they same way they continue to loose data.

        Coins with limited but long term supply by mining (like for 6 years or a above) will have inflation for a long time (till all coins have been mined). Will people wait for this much time for it's prices to stabilize? Or will they quit for their national currency? The prices will increase cause of it's popularity + lost coins, and decrease cause of mining activity; agreed these variables will stabilize it's prices for the decade while it's being mined, but there're just too many variables involved; it's like shares, there's a risk from what news comes about the cryptocurrency, and then there's the mining. If it's good news, mining will help stabilize prices, if it's bad then prices will crash faster than expected.
       
        I may seem like targeting Bitcoins -- but it has an advantage cause, cause it's the first cryptocurrency, people have not yet realized it's disadvantages and it may just happen that by the time it's disadvantage is realized, the inflation will be very low; but still there's the lost coin problem.
       
        The currency should try to reduce the factors it's price depends on in the technical front so as to reduce the variables on which it's price depends on.
       
        External factors will always vary the price of any cryptocurrency regardless of the technicals. External factors also includes initial demands of a new coin.
    B) Unlimited coin supply may also be bad.
        With more coins mined at a high rate, prices will decrease cause of more fresh supply.
       
    C) Proof of stake is not good.
        It encourages holding the coins as investment, avoiding it's circulation, i.e. true use as a cryptocurrency. Then worst -- PoS looks at the coin age; the longer you're holding a large amount of cryptocurrency, the higher the chance of mining a block, which further reduces chance of circulation. However if the profits proof of stake is giving you is negligible, then we may nullify this disadvantage.
       
    D) Limited inflation is ideal.
        Mining should not die; it should be done to recover from lost coins (i.e. there should be inflation). After all coins have been mined (in a relatively short period of time), the coins made per unit time should be enough to maintain the prices. Compared to the same amount of inflation provided by PoS, PoW provides higher increase in difficulty and better return rate than PoW for the reasons stated in 3) B).
       
        Even real currency have an inflation problem and it's usually quiet a lot. Actually it depends on the country. But do people have problem with that?
       
        If they did not like their current currency, they always had options. They whould've chosen fossil fuels anytime (who's value will always increase); the reason why they didn't is cause the inflation of national currency is negligible for them cause they use the national currency to spend, rather than invest and hold.
       
        So inflation rate of the crypto should be at par with inflation rate of real money.
       
        I took into considerations the following data. The average was take out for the last 20 years and excluding outliers and removing inflation about 100%
       
        http://www.coinnews.net/tools/cpi-inflation-calculator/ 3% (USD)
        http://www.measuringworth.com/ukcompare/relativevalue.php?use[]=CPI&use[]=NOMINALEARN&year_late=1994&typeamount=100&amount=100&year_source=1994&year_result=2014 4.535% (British pound)
        http://www.thisismoney.co.uk/money/bills/article-1633409/Historic-inflation-calculator-value-money-changed-1900.html 3.6805% (British pound)
        http://fxtop.com/en/inflation-calculator.php?A=100&C1=GBP&INDICE=UKCPI2005&DD1=10&MM1=06&YYYY1=1994&DD2=10&MM2=06&YYYY2=2014&btnOK=Compute+actual+value 2.65% (British pound)
       
        http://www.inflation.eu/inflation-rates/china/historic-inflation/cpi-inflation-china.aspx 4.5% (Yen)
        http://fxtop.com/en/inflation-calculator.php?A=100&C1=CNY&INDICE=ZHCPI1994&DD1=10&MM1=06&YYYY1=1994&DD2=10&MM2=06&YYYY2=2014&btnOK=Compute+actual+value 4.04% (Yen)
        http://fxtop.com/en/inflation-calculator.php?A=100&C1=CNY&INDICE=ZHCPI1994&DD1=10&MM1=06&YYYY1=1994&DD2=10&MM2=06&YYYY2=2014&btnOK=Compute+actual+value 4% (Yen)
       
        http://www.rba.gov.au/calculator/annualDecimal.html 2.7% AUD
        http://fxtop.com/en/inflation-calculator.php?A=100&C1=AUD&INDICE=AUCPI1990&DD1=10&MM1=06&YYYY1=1994&DD2=10&MM2=06&YYYY2=2014&btnOK=Compute+actual+value 3.9265
        http://meercat9.com/calc/inflation.php 2.56 AUD
       
        http://fxtop.com/en/inflation-calculator.php?A=100&C1=EUR&INDICE=EUCPI2005&DD1=11&MM1=06&YYYY1=1994&DD2=11&MM2=06&YYYY2=2014&btnOK=Compute+actual+value 2.3% (Euro)
        http://www.lawyerdb.de/Inflationrate.aspx 1.7 (euro)
        World bank 3% (Euro)
       
        http://www.inflation.eu/inflation-rates/japan/historic-inflation/cpi-inflation-japan.aspx 0.013% (Yen)
        http://www.measuringworth.com/japancompare/result.php 0% (Yen)
        http://fxtop.com/en/inflation-calculator.php?A=100&C1=JPY&INDICE=JPCPI2010&DD1=11&MM1=06&YYYY1=1994&DD2=11&MM2=06&YYYY2=2014&btnOK=Compute+actual+value 0.235% (Yen)
       
        http://www.inflation.eu/inflation-rates/brazil/historic-inflation/cpi-inflation-brazil.aspx 7.29 (Real)
        http://data.worldbank.org/indicator/NY.GDP.DEFL.KD.ZG?page=3 7.78 (real)
       
        World bank 19.75 (RUS)
        http://www.inflation.eu/inflation-rates/russia/historic-inflation/cpi-inflation-russia.aspx 17.4 (RUS)
       
        http://fxtop.com/en/inflation-calculator.php?A=100&C1=INR&INDICE=INCPI1958&DD1=11&MM1=06&YYYY1=1994&DD2=11&MM2=06&YYYY2=2014&btnOK=Compute+actual+value 15.747 (INR)
        http://www.inflation.eu/inflation-rates/india/historic-inflation/cpi-inflation-india.aspx 7.4 (INR)
        World bank 6.6
       
        World bank inflation rate world since 1992 -- 39%
        Average excluding outliers calculated 5.67
       
        So inflation between 5-6% will not have any impact on stability in prices score, above this value will give almost, if more than 7%, half, more than 9% means partial, more than 10% means no points on this front.
       
3) 51% attack protection -- There should be some means to prevent a 51% attack. (10)
FAQ --
    A) GPU mining is the answer.
        The best 51% protection is provided by a GPU coin, so only full blown computers can compute it, and one doesn't have to buy special purpose hardware. The hardware will be useless otherwise and after all coins have been mined (unless the incentive to mine is still up and maintained in some way like unlimited coins or high transaction fee), the miner will actually throw it away or sell it to someone else (who may be a cracker who's accumulating firepower for bad mining). This is not true for GPU hardware, cause they have a gaming and general computation purpose apart from mining, and probably the miners will themselves use it in gaming, so even after the mining incentive is lost, the hardware wont be sold cheap, and an attacker cannot buy them cheap.
       
        With new ASIC hardware, mining efficiency will increase; the new miners of the new hardware will start accepting transactions of lower transaction fees making the old hardware redundant cause they take more electricity. They will be sold cheap cause of this reason (lower profit margins or even losses).

        For an attacker, electricity cost is not of a concern, cause the attack time will be limited; he's going to earn money by making false transactions in blocks he mined.
       
        Yes -- the old hardware is old generation, but that doesn't mean it'll be slower.
       
        New hardware will not focus on higher hash rate, but power consumed per hash rate, thus, hash rate of new hardware may not increase exponentially making the network more vulnerable to attacks with older hardware.
       
        I'm not sure about the architecture of ASIC miners to comment on that. It's not an ARM processor which can be made faster by simply by increasing the clock speed. Modern AMD and Intel processors are not 4xPentium/athlon 64 @6GHZ. So an improvement in architecture does not necessarily mean faster speeds, it may have been done to reduce electricity consumption by half. Look at mobile processors for instance; they are more efficient than their Desktop (per Whetstone and dhrystone score) counterpart but slower.

        We just may have a similar trend for ASIC miners. Same consumption in electricity@little less hashing power.
       
        This's still worst for small alternative cryptocurrency using the same hashing algorithm. They're easy to exploit cause of low difficulty.
       
        If the algorithm is complex and unique, it'll need a LOT of R&D, the hardware will be expensive and insanely hard to make. It'll require a well funded team which only large organizations can afford and that too cost recovery will be from mining + selling of hardware not just by mining or attacks (where the price of cryptocurrency will drop resulting in a loss of the cracker cause he has to exchange a lot of coins in a small amount of time in order for it to be profitable). It'll take years to make such special purpose hardware; buying GPUs is cheaper. In contrast sha256 is way too simple and that's why making special hardware for was comparatively easy.
       
        CPU mining has a major disadvantage -- there's a lot of CPU power (not GPU power or control over special purpose hardware) in the hands of Botnets; under control of a cracker, he may have 51% hashing power for a small amount of time. But people are moving away from their virus-infested Microsoft Windows Desktop/Laptops to Android tablets and phones. So feasibility of such an attack is questionable, but still as of the current time botnets are big. Another disadvantage is that if an attacker uses CPU power of a system all the time, the owner will grow suspicious and reformat his Windows computer, but that wont be a problem for a short term to do a 51% attack; but one thing's for sure, the power of botnets to mine on a regular basis will be very limited cause most people own and work on laptops. Apart from this, if someone is wanting to do a 51% attack on a CPU only crypto, his best shot is to buy a lot of VPS for a short period of time; and then it's to be noticed that CPU is something owned by everyone, graphics card and ASICs are not. So expect the difficult to be high cause a lot of people will be mining it. People easily buy a more expensive CPU cause it's generally useful, not only for mining which's the case with ASIC and partially with Graphics chips (in case the miner is not a gamer or not using GPGPU for other purposes). So the crypto mining power will be more in the hands of the common people. It's also to be seen that the cost of VPS is mostly for the bandwidth available, not the CPU. The CPU model is not give in the most of the cases, giving a high risk factor. Also VPS cannot be sold off. So the attackers profitability relies completely by selling the coin who's price may decrease rapidly after the attack. Then check pointing makes things more difficult for him. But compared to GPU -- as of the current time, there's no one who lends GPUs. So the only way to attack a GPU coin is to buy GPUs which's the most expensive.
       
        GPU only mining will increase rating by 1. Anything below this (even CPU/GPU hybrid) will not add any points.
        B) Unlimited mining.
            Unlimited mining will sure shot prevent a 51% attack; however it's not known if mining profitability will increase or decrease in the future. This combined with CPU only mining will make an artillery against 51% attacks, however in this case (i.e in the long run), the algorithm being CPU only is highly questionable.
           
            If almost all coins have already been mined and if there's limited mining of coins forever (like with 2) D)), then the incentive to mine will neither be high or low. If the coin is CPU only (i.e. complex hashing algorithm), then there'll be less initiative to make exclusive mining hardware, cause all coins have already been mined on the CPU. This's going to prevent a 51% attack further. Then people pool mine, they'll use their free CPU cycles to earn a little bit of money driving the difficulty up, further reducing the chance of an attack. This is very likely to happen cause everyone known they got a CPU, everyone has and there's no harm in using it to earn a little bit of money for almost no electricity cost (65W). So difficulty will be up, and still there'll be no incentive to make new mining hardware. 5-6% inflation will be ideal for the purpose. Anything above this value will not improve this rating, below 5% will reduce ratings.
           
            An advantage as compared to PoS for the same inflation rate is that, cause less no. of people will be mining via PoW as compared to PoS (cause PoW requires investment in electricity cost, and hardware), the distribution of the inflation rate will be limited to less no. of people making the amount profit via the limited inflation rate more significant. This can be seen practically. For popular alts, the hashing power is more distributed cause the difficulty is low and profitability is high. Overtime, more people come up to mine the coins many of which are large professional miners and as the difficulty rises profitability falls reducing the no. of small time miners (cause profit is not significant anymore -- low profit margin), but the large professional miners persist (low profit margin, but higher hashing rate means more $$$s for a small % margin), increasing profitability for them. Of course the no. of miners also depends on the popularity of the crypto; but given the same popularity, we see this trend.
           
           
        C ) Proof of stake adds vulnerabilities.
       
            Following this article --
           
            https://bitcointalk.org/index.php?topic=604716
           
            A higher degree of PoS means more vulnerabilities. PoS coins will not get any score in 51% protection.
           
            Apart from this, you need to run a full node in order for the wallet to mine blocks based on PoS and it should be up always; how many people will do that? (especially when the interest rate is low)? When these coins will be made popular, 90% people will run light weight wallets giving power to the hands of these 10% (PoS difficulty will be low in this case cause the no. of people mining is less). Yes it is true, like with PoW the distribution of mining power will shift towards more professional miners who'll hold large amount of coins, but if the interest rate is low, they rather sell their coins to invest in real world schemes which provides more profits than this crypto, as an advantage they'll have lower risk cause the crypto market is very volatile.
           
            In case of PoW, power will may be in the hands of 5% (lower no. of people than PoS) but they'll have a lot of hashing power which if compared to the amount of coins held by these 10% is a much higher value increasing the difficulty in comparison to PoS, which results in higher comparative difficulty for PoW.  As compared to PoW, acquiring enough coins to do a 51% is easier (at these low difficulty and especially when interest rate is low.).
           
            If you try to fix 2) C), this disadvantage will be more apparent.
           
        D) Quick difficulty re targets adds protection.
       
                If a crypto has fast difficulty re-targets, it's difficulty to do a 51% attack, cause in the forked chain the difficulty will increase rapidly and will soon reach the target block times, the block time of the main chain will be the same, making a 51% attack impossible.
               
                If the main chain's difficulty was high cause of the attacker's majority hashing power, it'll drop to sustain a block interval equal to the attacker's fork chain.
               
                As of KGW, it's 'smooth' difficulty retarget means, while the attacker creates a long forked chain, the difficult will remain the same initially. But we want sudden difficulty increase to protect the network. I'm not sure about KGW, so if a coin does have KGW, it wont be taken as a plus.
               
                It's not known now DGW works either.
               
                Aggressive difficulty retarget each block will increase this rating by 1. The definition of 'aggression'' can be seen in 5) A). Apart form that the last 3 blocks or lower should be considered to get a +1 point off this rating. More no. of blocks considered means slow adoption to difficulty spikes.
               
        E) More confirmations add protection
                For a forked chain, it'll need a high hashing power for a longer amount of time to overcome the main chain, on top of that, the difficulty re-target algorithm will increase the difficulty making it yet more difficult to overcome the main chain.
               
                Since the amount of confirmation blocks depends on the receiver, this factor does not have any affect on the ratings.
               
        F) Changing algos to maintain ASIC resistance is ideal and will add 1 point.
       
4) Fast confirmation times -- like 5 minutes (50 seconds block). Less than a minute if a the coin wants to enter physical point of sales (10)
FAQ --
    A) This can kill the cryptocurrency.
        People will prefer using their cards for payment instead of waiting for hours for a confirmation. If confirmations take that much long, people will only use cyrptocurrencies for illegal payments, international payments where using other means to pay is practically impossible and transfer of funds among individuals where an hour or so is not that long. However, in these applications also, shorter confirmation times is desired. For e.g. for currency transfer among merchants, it may have to be done frequently. Even an hour is too much for that.
       
        To avoid long confirmation times, people may be willing to go through the formalities and technicals of cards.
    B) Disadvantage of short block times -- Chain will be forked way too often.
        This will turn off miners cause their confirmed mined blocks will no longer be valid. Receivers of coins who have broadcasted their transactions may have to retry attempts to broadcast.
       
        Since the difficulty will be such that a new block will be found by the network at around the same time as the block interval; a block interval large enough to broadcast a mined block throughout the whole network is desirable to avoid forking chains.
    C) Proof of stake may be problematic.
            What guarantees do you take that the person holding a large amount of coins and running a full node, keeping his wallet up and who has just generated a block has a good Internet connection so as to receive all the broadcasted transactions and include it in the block? These people are not experts, they've common stuff like firewall, anti-virus which may be up blocking the incoming transactions. This's unlike miners who usually have a good Internet connection and technical understanding.
5) Prevent intamine. (6)
    FAQ --
        A) Quick difficulty change may be the answer.
            Quick difficulty retargets ensures a power miner does not take advantage of the low difficulty coupled with laggy difficulty retargets.
           
            This accommodates for a sudden increase in network hash rate and to a provide smooth confirmation times. A sudden powerful miner may increase the difficulty to sky high; then the miner may be down skyrocketing the block times cause the difficulty is so high increasing the confirmation times to an unbearable amount.
           
            This may also serve as a protection against downtime of large networks which will lower the difficulty cause miners will be disconnected. Otherwise the confirmation times will suffer for the same reason.
           
            Aggressive difficulty retarget each block is recommended. The network should fix itself within 1 hour (and lower) and the block times should be back on track; that means a difficulty retarget should be 15 minutes; that way even if the block times reduce to 1/4th (1/4th the hashing power goes missing), the network can fix itself in an hour.
           
            As of instamine, allowing an instaminer to take away 3 blocks at a time is a reasonable. That's negligible advantage, no one would bother with that. 3 blocks means making a forked chain will be harder (since the average confirmation times of most coins is 6 blocks).
           
            So the block retargets should be 3 or lower and difficult retargets should be 15 minutes or lower for a coin to take full advantage of this rating.
            Block retarget of < 6 will give almost rating, less than <= 9 means half, <=12 means partial.
           
            KWG is characterized by smooth difficulty increase on rapid increase in network hashrate, which helps intaminers as compared to sudden difficulty retargets of a simple algo.

            Cause the difficulty increase slowly, the intaminer will take advantage by quickly generating blocks and stop mining once the difficulty is high enough.

            But when the instaminer is gone, the high difficulty is left for regular miners to take care off, and the block chain speed slows to a crawl. What makes matters worst is that the difficulty goes down slowly over time instead of suddenly going down like we have with regular algorithms. It takes the same amount of time to recover from the high difficulty set by the instaminer.

            So as compared to simple algorithms which re-target every block, KGW has a disadvantage -- the regular algo will increase the difficulty suddenly preventing an instamine, KGW does it slowly helping in the instamine.

            KGW has only one minor advantage -- it'll prevent the blockchain from halting for a longer period of time, but will not prevent the instamine.

            Digishield is worst -- it helps the instaminer further by slowly increasing the difficulty, while the difficulty goes up, the total no. of blocks mined is over the target. But Digishield brings the difficulty down aggressively which again means more blocks mined per unit time as compared to the target block interval. So basically that means in these situations the target block interval will be horribly off target and there will be over inflation.
           
            DGW is also similar. It's 'smooth' difficulty retarget helps the instaminer.
           
            So coins with KGW and Digishield will get half rating cause it does help in reducing instamine as compared to very simple slow difficulty retarget algos.
           
            Essentially, with the above algos, you're going to have similar behavior when a large no. of blocks are considered

6) Compressed protocol -- For quick setup of a full node. However this's only an advantage to miners who probably have a fast Internet connection. (2)
    FAQ
        B) how will this help?
            Notice the size of you block chain vs the amount you download while synchronizing? Thus there's a lot of scope of improvement. For starters, the older blocks can be delivered in compressed groups (of like 1000) as per the request of the full node.

7) Transaction comments -- both public (recorded in the block chain) and private (only between the sender and receiver) (4)
FAQ --
    A) What real use?
        For an online store, they may ask their customers to copy paste some one time code to prove that they have paid the bill.
       
        Then in general it helps in management in a human readable way.
    B) Size of transactions.
        This may increase by quiet a lot cause of this feature. This may be a disadvantage.

8) Expansion scope. In case small & new feature has to be added, it can be done in a backward compatible way. This requires void space allocation in the block and updating the clients to use that space. For e.g. transaction comments may be added this way to existing coins. Then one may add support for the name of the client to which transaction has been received which may unlock features exclusive to that client. (5)

9) Anonymous payment -- It should be impossible to intercept information about the owner of the address in any way. (10)
FAQ
    A) Bitcoin is not anonymous, the following vulnerabilities exist --
        A) Owner to address mapping.
            Usually a single person holds a Bitcoin receiving address.
           
            All transactions to that Bitcoin address is known, based on the value of the transactions and other evidences like third parity transaction records, receipts, Bills etc... which contain the amount of the Bitcoins sent, it can be proved that the address belongs to a particular person.
           
            Similarly along with 3rd party evidences of a rough transaction amount, and time of the transaction, the owner of an address can be traced with a certain degree of accuracy even when the sender does not corporate.
           
            Solution is to create new address for each transaction, but that inconvenient and sometimes not possible when there is no communication between the sender and the receiver.
           
            All these disadvantage can be applied to the sender also.
        B) Address balance.
            Continuing from A, after it has been proven that the address belongs to a particular person, his balance can be found out.
           
            It's to be noted that if there's 100% 3rd party evidence of the transaction, the balance the receiver holds cannot be hidden by the amount the 3rd party evidence proves. So what can be done is hiding the rest of the balance.
           
            Same thing can be said about the sender also. The balance of the sender can be determined after a transaction that has proved the owner of the address.

10) No or less transaction fees -- i.e. you may try to reduce the transaction fee by other means like PoS or inflation. Higher transaction fee may make people avoid Bitcoin transactions and instead use their Visa/Mastercard cards. Small transactions will be of no use cause the transaction fee will be too high. Transaction fee adds more complexities to the picture like adoption of the coin in poor countries and rejection of transaction based on low/no transaction fee.

No one wants to pay taxes or Bitcoin transaction fees.

Good amount of transaction fee may ensure quick confirmation (cause miners may reject based on low transaction fee); the delay will vary in multiples of the block times.

 What happens when you're using cryptocurrencies in low economy places like somewhere in Africa, South Asia & America or many of the islands. The transaction fee will simply be too much for them to pay; they'll avoid using it. There's no guarantee that people will pay transaction fee in the first place putting the miners at risk. Right now you see good transaction fee cause most of Bitcoin economy is in developed nations where they can afford to pay the fee in the first place.
 Then there's competition involved -- If the developed nations realize that their developing/under developed counterparts are not paying transaction fee, they too may stop paying. It's human behavior, it's uncertain.(10)
    FAQ
        A) Fast block times will help.
            As said before -- "the delay will vary in multiples of the block times"; faster block times may reduce this delay for the same reason.
           
        B) Inflation via mining will help.
            Certain amount of inflation per year will ensure that the transaction fee is distributed among users (cause in the long run, and without inflation the users will mine for transaction fee, but if there's inflation, they'll mine for coins removing the uncertainty of transaction fee), thus the effective transaction fee will be negligible per user cause it's distributed among the large user base and there will be no transaction fee wars.
           
            Following point 2) D), 6% or above inflation will give this full points. Below 5% will give it almost. Below 3% half, below 1% partial and less than equal 0.1% no points.
           
        C) PoS inflation will be given no weightage.
            The reason why TX fee exist is to strength the network. But PoS is full of security flaws. No matter how much inflation is provided via PoS, this point will remain 0 for a 100% PoS coin.
           
In my blog, coins will be rated based on these criteria. Each coin will the following ratings for each criteria --
No -- 0/(importance score)
Partial -- (importance score)/3
Half -- (importance score)/2
Almost -- (importance score)/1.5
Full -- (importance score)

For each coin the total will be calculated.
           
If you're making a new cryptocurrency based on this, please DO notify. Thank you!

QRK -- QMYbuBKzLTsrpDTqvoW5a6u6pN8149UMSY
BTC -- 1CrNnuoCXpggwajwasDTnvpxNpVNJPCTuc

1 comment:

  1. Very nice blog. Thanks for sharing most important FAQ on cryptocurrency. I found this information very helpful.
    Coinage crypto currency

    ReplyDelete