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Smart Contract Security in Solidity

Una encuesta de 2023 sobre seguridad de contratos inteligentes para la solidez

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Solidity RSA signatures for aidrops and presales: Beating ECDSA and Merkle Trees in Gas Efficiency By and Suthan Somadeva Michael Burke Introduction Creating a contract that gives certain addresses permissions goes by several names: airdrops, presale, whitelist, allowlist, and so forth. But the essence of the idea is there is a set of addresses that have special permission to buy a high-demand token for a desirable price (sometimes free). There are three established solutions for this: mappings, Merkle trees, and ECDSA signatures. The relative merits of these approaches have already been , so we will summarize the results: discussed elsewhere ECDSA Signature Verification (Gas: 29,293) using Merkle proofs (Gas: 30,517, 128 addresses) A mapping presale works when the seller enters the addresses of the customers into a mapping that goes from address to boolean to note if the address is privileged or not. The mapping is set to false after the buyer completes the transaction. This ensures only addresses that are marked true can make a purchase. This is very gas efficient for the buyer, but the seller could allow-listing thousands of addresses, (it will cost at least 22,100 gas to add an address to the allowlist). spend millions of gas Because of this, Merkle Trees and ECDSA signatures (using the precompile , or better yet, the published by OpenZeppelin), are often preferred to mappings. The gas cost (for the buyer) of executing the ECDSA signature verification is 29,293 gas. This includes the 21,000 to initiate the transaction, so the ECDSA cost is 8,293. Note that this includes reading the signing address from storage, but this cost is necessary or we won’t be able to invalidate signatures. ecerecover more secure wrapper around it Merkle Trees vary in cost based on tree size (larger trees require larger Merkle proofs), but if over 1,000 addresses are in the Merkle tree, it will cost at least 32,000 gas (or more) to verify the address. This cost is clearly inferior to ECDSA. The goal, then, is to beat ECDSA which costs 8,293 gas. To keep solutions apples-to-apples, the alternative solution must: Be able to nullify allow-listed addresses. Merkle trees can change their root, ECDSA , can change their signing address, and mappings can set the value to false. Not impose a substantial cost burden on the seller the way mappings do Cost less than 8,200 gas for the buyer (including a storage load) Not be compromised on security Prerequisites To understand the proposed methodology, the reader should be familiar with the following subjects: Precompiled smart contracts ( , ) recommended reading stackexchange Metamorphic contracts ( , ) introduction recommended reading Access lists ( ) recommended reading How to use digital signatures and their appropriate use cases in blockchain applications How gas cost is computed, especially as it relates to calldata, opcodes, storage, and memory usage. The RSA Algorithm If we want to substantially beat ECDSA, we need to find a different cryptographic algorithm that allows for the proof of set membership. ECDSA is actually the newer, cooler version of the original digital signature algorithm, RSA. ECDSA relies on discrete logarithms over elliptic curves being hard (hence the name – elliptic curve digital signature algorithm). RSA (named after its authors, Rivest, Shamir, Adleman) relies on large integers being hard to factor. In terms of age, RSA was published in the 1970s, but ECDSA became a formal specification in the early 2000s. We will not explain RSA in detail here, but some prerequisites are in order. The signer picks two large prime numbers p and q, and multiplies them together to produce n. This is the first part of the public key. Second, the signer picks a small prime (can be hardcoded to for our usecase), and publishes the pair as the public key. Behind the scenes, the signer computes n e 3 (n, e) t = (p - 1) * (q - 1)
d = t^(-1) % n The number is the private key. If an adversary could decompose into and , then computing would be trivial. But it is known that . To sign a message, the signer hashes the message to get and raises to the power . That is, d n p q d integer factorization is hard m h h d s = h(m) ^ d % n The signer then publishes as the message and signature. (m, s) The verifier then hashes and raises it to the power . Remember, and are the public key. If and only if m e mod n e n s == s ^ e % n then the signature is valid for the public key . Note that if is extremely large, then the probability that by random chance is vanishingly small. If the equality checks out, then we know the signature is valid for the public key. (n, e) n s == s ^ e % n To do this in Ethereum, we would simply sign an address as s = buyerAddress ^ d % n and the smart contract would verify msg.sender == s ^ e % n Using only 256 bits is unsafe We say “factoring integers is hard” but clearly this implies that the number must be large enough. For example, 33 is composed of two prime numbers and is trivial to decompose. Thankfully, we have real-world benchmarks of what the state-of-the-art can accomplish in the . RSA Factoring Challenge To clarify, the number of “bits” refers to the size of the public key. Therefore, when we say “RSA 2048”, this means the number n, the modulus, has 2048 bits. When comparing key sizes, it’s important to remember that each additional bit doubles the size of the number. So 700 bits is exponentially more secure than 350 bits, not twice as secure. The largest key to be cracked (factored) so far had 829 bits, and it required a modern supercomputer to do it. The team utilized approximately 2700 CPU core-years, using a 2.1 GHz Intel Xeon Gold 6130 CPU. The cheapest 16 core CPU on AWS costs $0.40 cents per hour, so the cost to crack this key is in the order of $9.4 million dollars. Even assuming generous discounts from the cloud provider, the cost is in the millions. Modular arithmetic with more than 256 bits in Solidity Ethereum only supports 32-byte data types, so by default, we cannot carry out s ^ e % n Thankfully, the Ethereum blockchain added a precompiled contract in specifically to support modular arithmetic. To use it, the base, exponent, and modulus must be loaded into memory in abi encoded format. Then the contract living at address is invoked. EIP 198 0x05 Storing the public key becomes a bit of an issue if you use a secure amount of bits. If the key size is 1024 bits, that necessitates 4 storage slots. To read the public key out of storage would be four operations, for a total of 8,400 gas. This by itself is already less efficient than the ECDSA solution benchmarked above. SLOAD If we use immutable variables, this cost is largely eliminated, but this creates a weakness if we cannot retroactively remove someone from the presale. In traditional ECDSA or Merkle Trees, we would just replace the signing address or the Merkle Root. This isn’t possible if we use an immutable variable. However, storing the public key in the bytecode instead of storage is the key idea. Reading an external contract’s bytecode ( ) costs 2,600 gas, which is far less than the 8,400 gas of reading each part of the public key in four pieces. EXTCODECOPY To invalidate a public key, we could simply create a new contract, and update a storage variable to point to the new address. But that adds back an additional 2,100 gas. It turns out, it is possible to store the external contract’s address (whose bytecode stores the public key) in an immutable variable, but still invalidate the public key by mutating the external smart contract’s bytecode. Invalidating the public key with the metamorphic contract pattern A smart contract is created by loading the deployment bytecode into memory, then returning the range of the bytecode which contains the runtime code. See more . here When a smart contract is created with the command, the address of the contract can be predicted in advance. The address is calculated from the combination of salt, the deployer, and the initialization bytecode. If the contract self-destructs, then a new contract at the same address can be deployed. create2 Note that the address is a function of the initialization code, not the deployed code. It is possible to deploy different bytecode by having the initialization code load a different runtime code. By self-destructing and redeploying with the same initialization code but a different deployment code, the bytecode of the contract can mutate. Therefore, we can have a smart contract whose first bytes are for self-destructing under a certain condition, but the rest of the bytes are just the RSA public key. k Because the address is determined in advance, we can store the address of this metamorphic contract in an immutable variable. We can from that address when we need the public key. To replace the public key, we instruct the contract to , then deploy a new contract to that address. EXTCODECOPY selfdestruct Saving an additional 100 gas with access lists added a new type of transaction that allows the user to specify in advance which addresses and storage slots will be accessed. This allows the nodes to prefetch those values from storage, thus speeding up execution time. Using an access list transaction when calling an external contract can save 100 gas. Note that this saving does not apply when a smart contract accesses its own storage variable. Because this RSA presale airdrop design relies on an external contract to store the public key, then using an access list is appropriate. EIP 2930 Benchmarks: Gas cost vs key size Most of the gas cost comes from having very large calldata as a result of large signatures. If the key size is set to 1024 bits, then the call data will be 128 bytes. Each byte costs 16 gas, so the total gas cost just to have calldata that large is 2,048 gas. Compared to most other use cases, ours uses a considerable amount of memory, and Ethereum charges for this. Why this saves gas relative to ECDSA The benchmarks make it clear that the larger the key (and hence the signature), the larger the gas cost. Our design exploits the fact that the precompile assigns a low price to raise numbers to low power. The cost of executing the precompiled contract under those conditions at is only a few hundred gas compared to thousands for executing the precompile for ECDSA. 0x05 Choosing a key size Although a key with 829 bits has been factored in, it requires a modern supercomputer to do so. For applications such as airdropping lower-value tokens or putting NFTs on a presale, an attacker does not have the incentive to spend six figures to a million dollars to factor a public key and obtain an NFT in a presale. The most expensive Ethereum tokens at this time of writing ( and ) cost approximately $100,000 a piece, so for the vast majority of applications, it is not economical for an attacker to try to factor the public key. Fidenza Bored Ape Yacht Club Remember, each bit doubles the difficulty of factoring the integer, so as of 2022, most low-value token presales are probably safe with 896 bits. In this case, saving users over 2,500 gas compared to ECDSA is compelling. Key Size Benchmarks This was conducted with Solidity compiler version 0.8.17 with the optimizer set to 1 million RSA-896 (Gas: 27,040) RSA-960 (Gas: 27,115) RSA-1024 (Gas: 27,311) RSA-2048 (Gas: 29,901) Conclusion We present a novel method for assigning addresses to a presale or airdrop that is more efficient than the known solutions today. By combining the modular exponentiation precompile with the metamorphic contract pattern and the access list transaction, we can validate secure RSA signatures on-chain in a gas-efficient manner. Our work is only a proof of concept at this stage. Developers are advised to use caution using the code in production. This project was created by and as part of the . The code implementation and unit tests are on . Suthan Somadeva Michael Burke RareSkills Solidity Bootcamp github Also published here.

Using Solidity RSA Signatures for Presales and Airdrops

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My dear daughter, you are still so incredibly young. But no sooner than I catch my breath for one moment and puff, you will be quickly wondering: what should I do with my life? What job should I pursue? Should I follow my passion? After all, isn’t what they say “choose a job you love, and you will never have to work a day in your life”? No, my dear daughter, following your passion is terrible career advice. (you take control of your life and create something worthwhile) and (you feel connected to other people). Focus instead on building rare and valuable skills that offer you autonomy relatedness Myth #1: You either have passion for something, or you don’t A common misconception that builds unrealistic career expectations and leads to constant job dissatisfaction is discovering or figuring out your passions and finding a job accordingly. Cal Newport defines this error as in his book: the passion hypothesis So Good They Can’t Ignore Y ou The key to occupational happiness is first to figure out what you are passionate about and then find a job that matches this passion. That means we are on a perpetual chase to discover that special something that will make us feel passionate. And when passion doesn’t happen or goes away quickly, we find ourselves job-hopping. There is a Romanian proverb “pofta vine mâncând” or “appetite comes with eating”. This proverb captures the difference between “ and “ “. What may seem like unattractive opportunities at first can bring passion to our lives we become competent in those areas. And increased competence leads to increased autonomy. do what you love” love what you do if Myth #2 Passion sparks joy in your career Another misunderstanding around careers built on passion is that what makes you feel passionate must bring you joy and happiness as a career choice. However, just because you feel passionate about an endeavor, there is no guarantee that effort will become a career or a business you will enjoy. More often than not, fun and enjoyable leisure activities turn into mediocre job chores. Also, passion is dynamic over time. There are days when you will feel highly passionate and enthusiastic. But can you maintain elevated levels of intense passion day after day? Week after week? Year after year? Probably not, as this process quickly becomes mentally draining and exhausting. Myth #3 Work on something you love, and money will follow Barbara Oakley describes in her the “Passion Trap”: Mindshift course We’re often encouraged to follow our passions by well-meaning people, friends and teachers, especially those who don’t themselves have to suffer the consequences of long term difficulties in getting a job. Friends, after all, often want to make you happy right now. So, they’ll often tell you what they think you want to hear. On the other hand, parents, much like my father, are often only focused on finding a successful career. They can be less concerned about what you feel are your internal passions, especially if they have had a difficult life and know how hard it can be to make a comfortable living. Try to broaden your aspirations and passions by having a bigger picture of the economic opportunities of today and tomorrow. After all, . For technical people, that means broadening non-technical skills, such as public speaking, writing, etc. For non-technical people, maybe it would be better to adopt analytical and technical skills. By becoming flexible, you can better adapt and survive the harsh economic competition and volatile job markets. skills that were in high demand just a decade ago are considered obsolete today Now that we know about the passion trap, should we discard passion altogether, never pursue things that we love and enjoy? Perish the thought! Perhaps let’s agree that passion is not enough and we need another concept. is a Japanese concept that describes having a purpose and a motivation in life, a sense of accomplishment and fulfillment, and it roughly translates as “a reason for being”, a raison d’être. The easiest way to describe Ikigai is an intersection between: Ikigai What do I love?  Which talents do I have that I can further develop? What skills can bring me money?  What does the world need? What can I do that is useful to others? Image credit: wikimedia , there is no denying that asking yourself these questions is helpful. Whether this Venn diagram is a Westernized version for Ikigai or not What you love and what you are good at are your . passion What you love and what the world needs are your . mission What the world needs and what you can get paid for are your . vocation What you are good at and what you can get paid for are your . profession At the intersection of your passion, talent, the potential to help others, and financial autonomy, you find your Ikigai . Or, as Newport puts it, instead of focusing on the passion hypothesis, you need to focus on having a , over your life, and by conducting worthwhile tasks. How can you achieve this? creative skillset control impact in the world Tip #1 Be strategic in choosing valuable and rare skills Valuable skills are skills that others are willing to pay for your expertise. Rare skills are skills that are not easy to reproduce or automate. , writes that there are two paths to achieve mastery: Scott Adams, Dilbert’s creator 1. Become the best at one specific thing. or 2. Become very good (top 25%) at two or more things. Image credit: Flow Academy The first strategy (T-shaped) is to have one substantial heavy-duty skill and complement it with superficial or general knowledge of other things. Adams recommends not trying this approach as few people will ever get an Olympic gold medal or make platinum albums. The second strategy (Pi-shaped or comb-shaped) is far more approachable. In Adams’ case, he combined drawing with writing jokes and business background. An extremely efficient method to create rare and valuable skills is to combine two or more “rather good” skills until no one has your specialized skillset. Most probably, none of your skills, taken separately, is the best as it would be necessary for the first strategy, but this unique combination of breadth and depth will create a high-pay and high-demand package. Tip#2 Be deliberate in your practice K. Anders Ericsson, in , defines deliberate practice as working on tasks of just manageable difficulty. You become comfortable with being uncomfortable but not having too much discomfort that you will give up. Being in the zone or of skills you already mastered is revigorating, except real growth happens when you stretch and push yourself just beyond the uncomfortable edges of your limits. It would be best if you were very specific in your actions and state your goals clearly. Become input-driven rather than output-driven. his Peak: Secrets from the New Science of Expertise book enjoying the flow Create a sustainable framework for self-improvement by following the questions I wrote in a previous article: What are my biases, my blind spots? What am I missing?  Am I focused on the right thing for me?  Can I test some changes? Can I start x/y/z for 5-10 minutes, for 5-10 days, and see how it went?  Can I ask others for their advice, opinions, and insights? Tip#3 I think I can. I think I will. Like in the little engine that could story, Ericsson mentions self-belief as a critical motivational factor: a that you can succeed. In order to push yourself when you really don’t feel like it, you must believe that you can improve and — particularly for people shooting to become expert performers — that you can rank among the best. belief You do not need to fake it until you make it. You believe it until you become it. The equation of career success is a spiral as you end up learning to enjoy the process of getting better and better. Start with an eager heart and mind. Believe that what you are doing is worthwhile and achievable. The method of continuous learning and improving becomes extremely satisfying and self-reinforcing: the more you learn, the better you become. The better you become, the more you enjoy the process and the increased recognition of peers. The more you enjoy the process, the more you learn, become passionate about the topic, and believe in your newly acquired skills. Tip#4 I’m sorry to say so but, sadly, it’s true that Bang-ups and Hang-ups can happen to you. As Dr. Seuss formidably put it, the brutal truth is that you will traverse periods of ebbs and flows more ebbs, poetically known as the Valley of Despair from the Dunning–Kruger Effect. Image credit: wikimedia Not all endeavors you will pursue are due to have success, as most times, you will fail, discouraged, and stuck in a pit of gloom. Know, dear daughter, that perhaps no matter how hard you try, you will never achieve mastery levels in a specific career path. It is yet another fact of life. Ask yourself, as Drю Seuss said: Do you dare to stay out? Do you dare to go in? How much can you lose? How much can you win? Sometimes it is far better to turn around and drop the things that keep you small. And sometimes, knowing about Tip#3 (the belief that what you are doing is important and achievable) and that the pit of gloom is ahead of you gives you time to rethink your approach, strategize, and organize. Mainly, it gives you time to realize that you need to … Tip#5 Embrace the boredom In his book, James Clear describes a conversation with an elite baseball coach. Clear asked the coach: Atomic Habits What’s the difference between the best athletes and everyone else? What do the really successful people do that most don’t? He mentioned the factors you might expect: genetics, luck, talent. But then he said something I wasn’t expecting: “At some point, ” it comes down to who can handle the boredom of training every day, doing the same lifts over and over and over. So, you see, dear daughter, the trap of passion? Maybe you believed some people merely have boundless reserves of passion. That is not true. Successful people feel the same lack of motivation and dis-passion as everybody else. The significant distinction is these people still act even when they don’t feel like working, even when their motivational state does not match their schedules. “ “, writes Clear. Why is that? Perhaps because future rewards are not that as attractive as the current rewards. It makes sense from an evolutionary perspective: a bird in the hand is worth two in the bush. We are pre-programmed to live in the “right now”, right here” zone, seeking immediate rewards. We need to adapt this Stone-age neural circuit to the distant deadlines of the modern world. So, create a schedule and stick to it, no matter your mood. The greatest threat to success is not failure, but boredom As Chuck Close said, “inspiration is for amateurs. The rest of us just show up and get to work.” As a mother myself, I found great inspiration in what English artist and sculptor Barbara Hepworth told about the years when her four children were young: “I had to have a very strict discipline with myself so that I always did some work every day, even if it was only ten minutes. It’s so very easy to say: well, today is a bad day. The children aren’t well, and the kitchen needs scrubbing…” Regarding finding time, even if for ten meager minutes, there is a story that Andy Puddicombe notes in his book. He asked a meditation student to write down why she would skip a 10-minute meditation session. The student found the exercise valuable, as every time she went to write excuses, she discovered the explanations notably weak and she started her meditation practice instead. Get Some Headspace As Oakley says in her Mindshift course, “ It is important to take a strategic as well as a passion-influenced approach to your learning.” sometimes your natural passions can fool you. My daughter, you cannot afford to ignore the skills that enhance your ability to get or create the job you want. Use by reframing how you perceive passion: you do not stumble upon a passion but actively develop a passion instead. Improving the skills and confidence in a new hobby or venture demands time. So, be prepared for the Valley of Despair and weeks of boredom. the framing effect There is an eternal conflict between our internal passions and the external needs of the workforce. Accept this struggle and strive to find meaning, passion, competence, and financial independence in your future career. Previously published at https://www.roxanamurariu.com/letters-to-my-daughter-myths-and-tips-to-strategically-choose-a-career/

On Career and Passion: Letters to my Daughter

In case you are wondering whether or not you should start learning how to code, read on… Before I go into details, some brief words about myself: I am 33 years old, live in Germany — and right now, I’m in the middle of a 3-months fullstack developer Bootcamp 👨🏻‍💻. Writing this article, I like to share some of my first-hand experiences while I learn to program. Learning to code is a big decision Every decision that requires a lot of our lifetime should be justified. When I decided that I want to learn how to code, I knew this would take up pretty much all of my time and more so: require a great amount of energy. Yet, and accept this awesome and exciting challenge. In the following, I will lay out my reasons why I wanted to code and become a professional developer. there are a variety of great reasons why I believe it is a wise choice (not for all but for many) to dive deep into coding Demand for developers is high Digitalization is no longer in its infancy and affects literally every industry out there. — and therefore strongly demanded by pretty much every modern company. It is therefore not surprising, that the huge demand for smart developers exceeds the number of available developers by far. Thus, many IT positions remain vacant for longer times. Obviously, this is critical for companies but can be a good thing for those who can solve their problem (by offering their know-how as developers). Developers are on the forefront of this digital movement Further, . Because it means my time is worth the effort and actually affecting the outcome of the business. From practical experiences of previous jobs, I can emphasize: We all shouldn‘t waste our valuable time in a job and with activities that feel meaningless to ourselves. In the rapidly moving development sphere, I feel like I’m learning every day which is a great investment of my time! I definitely like to work in a field that is highly relevant to a company The strong demand for IT experts has another benefit: (like me!). This is very different from university degrees like law or medicine. Since software engineering is moving much faster it’s inevitable for any successful programmer out there to continuously learn. Therefore the value of any IT-curriculum is only worth much for a few years. Any career changer who can say to have learned by own initiative how to code is proving to own the crucial skills of continuous learning. The industry is relatively open-minded towards career changers that have not studied computer science Coding will train your creativity One of the most compelling reasons for me was that . When I saw people coding just one or two years ago, I really admired their skills and work. To become an expert now myself, building a deep understanding in such a complex and challenging field is deeply motivating for me. Its great to see what we all are capable of once we make clear decisions and bring our full commitment to life. I always wanted to acquire a rare and valuable skillset in some specific field When I started to code, I also understood more and more that is a creative task (I never thought so before…). As a developer, you can create something that is highly valuable for others out of nothing but your rare skills. Like you’re building a new app or feature from scratch. When working on the frontend you can also see immediately what you’ve built which can be very rewarding (although backend development is cool stuff as well!). programming Become a problem solver Aside from the skillset of programming itself I also see essential benefits way beyond the work life. As Steve Jobs famously said: “I think everybody in this country should learn how to program a computer because it teaches you how to think. I view computer science as a liberal art.” I couldn’t agree more. . Why? Because developers train their analytical thinking every single day. Also, when coding it‘s usual that you are confronted with challenges that you can’t solve immediately. If you don’t like to keep going when it gets tough (in business and private life), well — you will definitely train this important skill while coding. Learning to code helps me to think more clearly in general A good developer will also be someone who is good at breaking down big challenges into small chunks that are solved step-by-step. An approach I find useful in many other areas of life as well. And since continuous is such an important topic developers are also usually very good at leaving their comfort zone to work with new technologies. learning Choose what type of work you prefer Another huge advantage is that developers benefit from great flexibility. There are many ways to work in development other than the typical salaried employee in a large company. Here are some examples: : For most startups, developers are the key persons when building up the company. Therefore chances to get hired in this area are great (also it is really valuable for learning!). Startup : Developers often get hired as freelancers. Almost all medium- to large-sized companies work with IT-freelancers to fill some gaps in know-how or staff more people on a growing project without going through the long process of hiring new employees. For those you like to work individually and be their own boss, this is a great opportunity. Self-employment : If you are looking for new projects on a regular basis or like to work with various new technologies then working for an external service provider like an agency or IT consultancy is a very interesting choice. Agency/Consulting : A growing number of jobs in IT is offered remote. Which is not surprising since all a developer needs are his/her skills and a laptop. And while „ ” is still new in Germany it is getting more popular in the war for talents. Especially startups as well as international companies are opening up for this type of work. So if you dream about traveling the world while working, going into development can be a smart choice. Remote remote work : If you know how to code it’s much easier to build up your own company and e.g. develop an app. So if you dream about running your own firm knowing how to code is great since you will save huge costs and can start with low risk. Entrepreneurship As you may see by now, there are several and more so very different great reasons why you should learn how to code. One of the most important reasons — if not the most important one — I have not mentioned yet: I feel good every time I open up my code editor and work on a feature. It’s great fun and a very rewarding thing to do! Obviously, coding cannot be right for everybody since we’re all unique. To see if it’s for me, I just started a small WordPress site and played around with the code. With this bit of first practical experience, I could see that I really liked it and continue from there step-by-step. I hope laying out my reasons why learning to program can be a great choice was inspiring for you. I am convinced its a great way to go…

What’s the benefit of learning how to code?

Let’s face it, with the speed the tech world is growing at the moment, it’s hard to predict how the art of coding will evolve. What we know is that code sits at the root of all . technology When we reflect on the subject of whether coding is becoming obsolete, opinions vary. Some say machine will replace the need to manually write code, because computers will be trained to code themselves. Others believe that because coding can be tedious, developers will be able to use their analytical skills in more enjoyable ways than writing code. learning We believe the art of coding remains necessary, yet The ability to build a website or application is becoming available to regular people — not just the coding masterminds, by using very powerful, but simple tools. This will very soon change the face of business, technology, and culture. Read the impact of this change in “ ”. web development and programming as a whole is facing a revolution. How I spent $50K in 2 years then pivoted in 2 weeks for $0 History & Overview Before we get to talking about the No-Code Revolution and how this will influence development, let’s stop and reflect on history. Specifically, let’s look at the similarity between software engineers today and telegraph operators in the 19th century. When Samuel Morse developed the telegraph in the 1830s, his invention revolutionized communication. Morse Code was developed, which allowed complex messages to be transmitted through basic inputs. Yet, in order to send or receive a message, you had to have a skilled telegraph operator on each end. The telegraph is not used anymore as a main communication tool, but it stays at the root of other inventions that appeared in 21st century, such as the telephone, fax machine, internet, and smartphones. In order to send a message electronically back in the 19th century, you either had to be capable of writing Morse Code or hiring a telegraph operator to help you send and receive the message. If we draw a parallel, it relates to web development today. If you are looking to build a web or native application, you either have to be capable of writing code or hiring a developer to do it for you. Both telegraph operators then and web developers now had rare skills and experienced good pay. However, for telegraph operators, this comfort and fortune all came to an end once the next evolution of technology rendered their skills obsolete. Could a similar change be quickly approaching web developers? When further considering this subject, the same parallel applies to other events in the history of technology. Сo-founder of , Emmanuel Straschnov, compares the impact visual programming will have on society to the impact word processing applications have made. The average person — heck, the average seven-year-old — who sits at a computer, can now compose and send an electronic document in minutes. Prior to Word processors, this would have required specific skills and considerably more time. (Read more on this subject in this ) Bubble article Similarly, visual programming will make digital creation accessible to the masses. Many have been sitting on ideas for years, but feel constrained by their ability to finance their dream. Visual Programing will provide to such people a powerful social and economic platform that democratizes their ability to maintain an online presence. We’re entering a new age, and it’s going to be beautiful. The Rise of Personal Computing Back in 1880, the U.S. population had grown so large, it took over seven years to tabulate the U.S. census. The government found a faster way to do it by utilizing punch-card based computers, which filled entire rooms. In 1941, computers began storing information on their main memory. In 1946, computers were sold for business and government applications, then the first programing language was invented in 1953, which is now known as COBOL. In 1964, the first prototype of the PC was showcased, and in 1974 a number of PCs hit the market. However, it was not until 1977 when, for the first time, those who could not write code were able to make any use of a personal computer. This timeline provides us perspective into the likely future of web development. Until recently, only coding masterminds were able to build web or native applications. Now, web development is accessible to anyone with access to internet. Soon, advancements in technology will make even the most complex development challenges achievable for the average user. The No-Code Revolution The No-Code Revolution began with the increased popularity of the first visual programing languages. Users were offered the possibility to build a website without the need of writing code, hiring agencies or managing freelancers. Only by dragging and dropping “code blocks” into place, a simple and functional website could be built. But, before we get into the details of the No-Code Revolution, here are some definitions. No-Code development platforms are visually integrated development environments, which allow users to drag-and-drop the components they wish to include in their application, connect them in a logical order and launch a mobile or web application. Such platforms unleash the imagination and creativity of any user — regardless of previous knowledge of traditional programing languages. A user-friendly visual interface allows components and third-party applications to be integrated and tested to make the application look and function as planned. The growth of No-Code Platforms first emerged from the limited access of entrepreneurs to skilled and affordable web developers. Likewise, some development companies felt the need to reduce project turnaround time and release or iterate products more quickly. The movement is gradually growing and analysts at estimate the No-Code/Low-Code market will reach $15 billion by 2020. Forrester Research Together with the creation of Bubble in 2012, it became possible to build not only simple landing pages or blogs, but also web applications with complex logic and functionality. Traditionally, No-Code development platforms allow minor flexibility on the front and back ends. Bubble, however, enables fine tuning down to the pixel and can define how the application reacts to any input. Let’s look at an example. Say you are intending to build a web application with similar functionality to Airbnb or Indiegogo. Due to the rudimentary lack of functionality in traditional No-Code development platforms, building complex web applications would be close to impossible. On Bubble, with a clear idea and time to work on it, nearly anything is possible. Define the data structure, build the visual part and make it live by interconnecting those with the powerful workflows! Learn . How a startup can build an MVP using No-Code Due to the growth and evolution of No-Code development platforms and the vast potential the technology holds to disrupt markets — businesses supporting the No-Code Revolution were created. One such company is — a one-stop-shop for codeless creation. Zeroqode offers complex web app templates, all built on the advanced visual programing platform of Bubble. With their robust, functional and beautiful templates, Zeroqode is ushering us into the No-Code Revolution (read what ). Zeroqode TechCrunch has to say about this In Conclusion As enterprises endure a digital transformation, they become software-driven organizations. The need to hire software development talent has become an unfortunate bottleneck inhibiting the growth of small, medium, and large companies around the globe. No-Code platforms have the ability to disrupt entire industries, as entrepreneurial individuals or small businesses that previously were unable to unleash their ideas now flood the world with an explosion of creativity. As explained by E. Scott Menter, Chief Strategy Officer at BP Logix, “No-Code is here, and it doesn’t care about making your IT organization more efficient, its only purpose is to turn your business into a digitally integrated, audit-defying, silo-resistant object of their customers’ desire.” Web developers are and will remain useful for the foreseeable future. However, it’s clear the No-Code Revolution has arrived, making app and web development accessible to everyday people. P.S. Enjoyed this article? Continue reading about the No-Code Impact and our final conclusions on the future of code in Is Coding Becoming Obsolete? Part II !

Is coding becoming obsolete? (Part I)

This is the text of a talk I gave in March 2017. The world has woken up to AI. Here’s an illustration of this exponential trend: the number of press releases published by companies mentioning “Artificial Intelligence” skyrocketed over the last few years. I conducted a research myself and asked : when AI is being used so frequently in so many different ways, does it have any valid utility in differentiating one thing to another? my Twitter network They were split. About 58% said it doesn’t really have that much use as a description anymore. It brought to mind this kind of idea that comes back to John McCarthy who said: “As soon as it works, no one calls it AI any more.”. It’s been a six-decade journey to get here. Sixty-one years ago the first conference in AI got together at Dartmouth University, bringing together the likes of McCarthy and Herb Simon, Norbert Wiener & Martin Minsky. At the time they thought they’d solve what was the problem of AI then, which we would now call artificial general intelligence: building a machine that could replicate human intelligence within a few years — within a decade is generally what they would say. More, McCarthy, Minsky, Selfridge and Solomonoff How could they be that optimistic? There are a few reasons for this, and it’s important to put them in their historical context: The American scientists who came together were part of what was known as . They had , they had . They’d become the major exporter of the world, and they had marshaled their resources to the to build the Atom Bomb. the Greatest generation lived through the Depression seen America win the WWII Manhattan Project In 1956 they to the Soviet Union. hadn’t yet lost the Space Race But, there were a few problems: They didn’t really know how the human brain worked. It’s pretty hard to build something when you only have a vague understanding of the mechanics of the thing that you’re trying to emulate. The way in which their math or their logic worked was very much grounded in symbolic logic or decision trees which, as we’ve seen a with recent breakthrough in statistical or probabilistic models, has not necessarily been the ideal way of delivering stuff that looks vaguely intelligent. They didn’t have integrated circuits at the time. This group of great minds faced some steep challenges. They didn’t really know what they were building, they didn’t really have the kind of maths to build it and they really didn’t have the processing power on which to run the math. The Cycle of Innovation — Where Are We? So much for sixty years ago. But today AI seems to not just be present but on the cusp of exploding into something significant. The question: is how do we go from that moment to a moment where we seem to be building small things and then suddenly have a moment where there seems to be rush, a ubqiuity, of development? developed a powerful framework: she thinks about technology waves in cycles. Carlota Perez In what she describes over the course of five revolutions, technical revolution is a process where society reconfigures itself over a forty to fifty year period through installation, readjustment, and deployment. This is where there is some kind of breakthrough technology, and combining breakthrough technology with really rare skills creates a set of opportunities. The technology isn’t that useful, so it needs to be deployed into markets using technology push. That’s what many of us would have remembered having gadgets that didn’t really work. During this phase financial speculators, financial capital recognize that there is a new market about to be created and because they can reconfigure themselves very quickly financial speculative capital starts to pour into this market early. It pours in ahead of the technology being widely deployed. Re-adjustment or the “turning point” is the interim period of instability, and, ultimately, economic collapse. The final phase is a reconfiguring stage. This is a time of creative construction; it’s a time where the skills that are needed to make use of this breakthrough technology are much more widely disbursed. It’s a time when consumers actively seek out solutions that depend on that technology. It’s also a time when production capital rather than purely speculative capital is applied for the build out of this new infrastructure. This model can help us think about where we are in the cycle, and what the world is going to look like. Find a Ford: The Model T was introduced in 1908; it was incredibly popular within five or six years. This photos from 1926 in New York City, 18 years after the introduction of Model T. This is New York in 1926. Eighteen years after the introduction of Model T Ford. At that point, technological revolution was still in its . deployment phase In 1908 Henry Ford had introduced the idea of Fordism and the production line. It was a brand new concept. Sales of the Ford car accelerated pretty quickly, pretty exponentially for the time. There’s the readjustment phase: Wall Street crashed in 1929, followed by the depression and then World War II. After that deployment phase came an incredibly optimistic time in American history: they built out highways, got into air travel, holidays became a new normal, suburbs flourished, as well as the suburban shopping malls. Lots of economic, technology and business innovations were deployed across an installation platform. With Carlota’s theory in mind, we need to ask: where are we with ICT? (AI is really part of that wave?) We’ve seen the installation phase beginning in 1971 with Intel 4004 processor. We saw demand for rare skills. It was a small number of people involved in the tech industry, and there were companies that got a tremendous amount of market power: . We alongside this saw the growth of the venture capital industry starting in the late ’60s. Intel, Microsoft, Oracle We’ve had our moments of readjustment. We had the dot-com bubble. Look at WebVan.com and Kozmo.com. They had classic problem of e-commerce in 2000. Not enough users on the Internet, technology infrastructure was immature, and sotware was shonky. In short, far from the prime-time we see today. I can speak from personal experience, too. I invested in an online real estate portal in France. Seems like a great idea today. Most people today use online search for their homes. . No real estate agents had computers in France and none of their customers buying houses had computers. We had this enormous customer acquisition cost where we literally had to buy them the technology: PCs, modems, second phone lines. Not in 1999 Moore’s Law, the Sea of Data and Business Reengineered Does Perez’s model fit where we are today? If it does, what does it mean for us in the field of AI? We do seem to have got to a stage where there are a lot of key infrastructural things in place for this. I’ll look into three. Limits of Moore’s Law and Beyond When I was 13 years old I had lusted after Cray-2. It weighs two and a half tons and it costs $35 million and can do 1.9 gig of FLOPS of calculation power. The PC I had at home was doing about 10,000. Interestingly the 4004 processor back from 1971, the year before I was born could add two eight bit numbers 11,000 times a second. Try running your neural networks on that. With the continued application of Moore’s Law we get to the point where the iPhone 4 is as powerful as a Cray-2, costs $400 and it sits in the drawers in our kitchen. Apple Watch is 100 grams, $300 and is twice the raw processing force power as a Cray-2. What I find interesting is that the industry hasn’t stood still even as we hit the physics limits of Moore’s Law. The blue curve shows us that we started to find architectural enhancements and more specialized architectures and video range of GPUs and you see their growth curve in terms of processing. Availability of data Data is the rocket fuel that powers the math that we do. In 2013, the world had a stock of about four zettabytes of data where 90% of it had been produced in the last two years. Essentially in 2025, which is not so long away, the amount of data we will produce in a week will be the same as the entirety that humanity produced up until 2013 as a species. That’s quite a sea change. Source: IDC Prior to us all having smartphones we didn’t really create that much data; and now with smartphones, without really thinking about it, we are creating : we’re creating Tweets and photos and videos and interactions and messages. Also, it’s not just the human data, it’s the machine data, too. a lot of data The kind of data they would generate is going to be really, really intriguing. If we look at the growth of the CMOS sensors we see flat growth until just about the point in 2011 where iPhone sales are taking off. Since then that’s two billion shipments of CMOS sensors. We’re forecasting six billion or a 3x improvement in an eight-year period. Source: IC Insights We’ve also seen data sources come from other places. These are happening on top and separate to what’s happening in a consumer home with the IoT. Just looking at Earth observation data there’s a tremendous growth in the demand for sale of Earth observation data to the military and to private. There’s also growth in the number of satellites that are being sent up to observe the Earth. Companies such as Orbital Insights and SpaceKnow are taking photos from Earth observation satellites, running them through machine and machine vision to make predictions about how well human companies are doing. learning If the JC Penney shopping mall has fewer cars in it on a given Tuesday in February than previous Tuesdays in February . Of course you can apply that to oil tankers pulling up to the docks in Mumbai or cargo being unloaded at Singapore. that might tell you something about how well their sales are doing The world has become software driven Three things are happening. 1. Architecturally, software doesn’t look like a monolith of spaghetti code that’s really poorly documented. It looks like this perfect architectural diagram with really neat functions and APIs and well-defined contracts in between them. 2. Businesses have reengineered themselves. BPR, business process reengineering was a thing that all consultancies were selling to medium and large firms from the late ’80s through to the dot-com bubble. Business process engineering was essentially APIfying and contractualising internal operations of the company by making them separable, allowing us to find core competencies, enabling outsourcing. 3. The effect is present. Here’s how it works: AI Lock In Loop Once you put a piece of AI into a product some kind of prediction or some kind of optimization, safety or routing algorithm for your logistics company, your service and your product gets better. As your product gets better, you generate more data, and at the same time if you’ve got a decent business model a better product should mean more profits. More profits means reinvesting into improving AI. There’s no way back. What we have seen is that once you implement AI successfully into an industry and into a sector it’s impossible to compete without it. Think about web search — web search has had to have the machine learning, artificial intelligence in it, and it would be almost impossible to imagine someone launching a web search engine that didn’t use some kind of machine learning or data science. You wouldn’t launch a web search engine that just did as its ranking. Tf-idf The other thing that we’ve seen is that AI Almost every week there’s read another example of a narrow domain piece of AI which is outperforming humans. beating humans is very, very normal. Nowadays speech recognition is performing with 5.9% error rates, far better than humans and has gone even further than that. Back in 2013 or 2012 none of these things would have been done better using machine learning — and that’s only a few years ago. not a single one, There’s one issue we seem to have in our industry, that we ultimately saw previously in the oil industry. Huge companies such as Standard Oil and Ford dominated key parts of the oil industry. In our case, Google and have essentially got infinite computing power and know-how compared to any of us. The question is what long-term structural advantage does it give them? Are we getting the crumbs that fall off the capitalist table onto this chair for what we are and do we really have what it takes to compete in some of these domains? Facebook These companies are really hungry to build their talent bases globally and that creates a tension if you’re an entrepreneur because you can get from a Google or an Apple early into the lifetime of your business_._ fuck you money I still think a full staff matters_._ Uber is not a machine learning company but is worth 100 times more than the biggest AI exit so far and they have made a pretty significant commitment to automation_._ They’ve acquired Otto, they took the CMU robotics team and they used ML in lots of places in their business, not just in routing but even in the consumer app that some of us might still be using. It’s an amazing period of innovation. And responsibility. Today the products that you can build control our interface and access to all the services that we need. You have to adopt the mantle of civic leader in some level. You determine how our attention gets spent, you determine how we understand our news. You kind of determine how our civil society is functioning. I think it’s really important to stand up to those responsibilities as product entrepreneurs particularly when you are essentially going to be building what stands between me and the real world. You can read more about the most pressing issues in technology in my weekly newsletter. . SIGN UP FOR EXPONENTIAL VIEW HERE

State of AI: How did we get here, and where are we going next?

An ethereum address is the last 20 bytes of the keccack256 of the public key. The public key algorithm is secp256k1, the same used in bitcoin. Because it is an elliptic curve algorithm, the public key is an (x, y) pair corresponds to a point on the elliptic curve. Generate the Elliptic Curve Public Key. The public key is the concatenation of x and y, and that is what we take the hash of. The code is provided below. from ecpy.curves import Curve
from ecpy.keys import ECPublicKey, ECPrivateKey
from sha3 import keccak_256

private_key = 0xac0974bec39a17e36ba4a6b4d238ff944bacb478cbed5efcae784d7bf4f2ff80

cv = Curve.get_curve('secp256k1')
pv_key = ECPrivateKey(private_key, cv)
pu_key = pv_key.get_public_key()

# equivalent alternative for illustration:
# concat_x_y = bytes.fromhex(hex(pu_key.W.x)[2:] + hex(pu_key.W.y)[2:])

concat_x_y = pu_key.W.x.to_bytes(32, byteorder='big') + pu_key.W.y.to_bytes(32, byteorder='big')
eth_addr = '0x' + keccak_256(concat_x_y).digest()[-20:].hex()

print('private key: ', hex(private_key))
print('eth_address: ', eth_addr) The ecpy library is here . This library implements the elliptic curve math in python, so it won't be as fast as a wrapper around the bitcoin C implementation, which is used by the library. https://github.com/cslashm/ECPy coincurve However, the python implementation allows you to see step by step the elliptic curve math used to derive the public key. You can use this code to generate an ethereum vanity address with brute force, but be mindful that if your source of randomness is not secure or has insufficient randomness, you may fall victim to a hack similar to . this Generate private key with python and coin flips or dice You can generate an Ethereum address from a private key yourself by flipping a coin 256 times and writing to a string a 1 if it’s heads and 0 if it is tails. Let’s say you get the following outcome result = b'1100001011001101010001001100101000001111101101111011001000110001101100011101101011010001011000101111100110010101001001101110111011001000100001010101111100001100100110010010111110110100000010011111100000110101001110000101100101011111001101010001100001000' You can convert that to a private key using the following python code # 2 means base 2 for binary

private_key = hex(int(result, 2))

# private key is 0x1859a89941f6f646363b5a2c5f32a4ddd910abe19325f6813f06a70b2be6a308 Then, plug that private key into the code from the above section and you’ve generated your address with your own randomness. The same thing can be accomplished faster by rolling a 16-sided dice 64 times and writing out the hex string that is produced character by character. Be mindful that most dice don't have a representation for the number zero, so you'll have to subtract 1 from each result. If you only have traditional six-sided, you can write out a string in base 6 (don't forget to subtract 1 from each roll) and do a base conversion to binary. You'll need to keep rolling until you have at least 256 bits for your private key. If you are particularly paranoid about randomness, you can use casino grade dice. Be cautious using the built-in random number library for Python. It's not intended to be cryptographically secure. We recommend familiarizing yourself with if you are new to the topic. cryptographically secure randomness Generally, you cannot initialize a hardware wallet using this method because the 24 word recovery phrase they use is not the same thing as a private key used to sign transactions. The 24 word recovery phrase is used to derive multiple private keys for different types of crypto the wallet holds. Math: About secp256k1: Public Key from Private Key There is already a lot of material online describing what curve addition looks like , so we will describe it at a lower level here. visually The shape of the Curve Secp256k1 defines the shape of the elliptic curve y^2 = x^3 + y (mod p) where p is the prime number 115792089237316195423570985008687907853269984665640564039457584007908834671663 or 2^256 – 2^32 – 977 The prime should not be confused with the of the curve. Not every value 0 < n < p satisfies the equation above. However, operations on the curve are guaranteed to be . That is, if two valid points are added or multiplied, the result will be a valid number on the curve. p order closed The starting point The other important parameter in secp256k1 is the starting point G. Since G is a point on the elliptic curve, it is 2-dimensional and has the parameters x = 55066263022277343669578718895168534326250603453777594175500187360389116729240
y = 32670510020758816978083085130507043184471273380659243275938904335757337482424 To see G is a valid point, we can plug the numbers into python x = 55066263022277343669578718895168534326250603453777594175500187360389116729240
y = 32670510020758816978083085130507043184471273380659243275938904335757337482424
p = 115792089237316195423570985008687907853269984665640564039457584007908834671663

assert pow(y, 2, p) == (pow(x, 3) + 7) % p To create a public / private key pair, a random number is created (this is the secret key). The point G is added to itself times and the new point (x, y) is the public key. It is not feasible to derive from G. s s s Adding G to itself s times is the same as multiplying s * G. In fact, we can see this operation at a lower level in by stripping away some of the abstractions the library is providing. from ecpy.curves import Curve
from sha3 import keccak_256

private_key = 0xac0974bec39a17e36ba4a6b4d238ff944bacb478cbed5efcae784d7bf4f2ff80

cv     = Curve.get_curve('secp256k1')
pu_key = private_key * cv.generator # just multiplying the private key by generator point (EC multiplication)

concat_x_y = pu_key.x.to_bytes(32, byteorder='big') + pu_key.y.to_bytes(32, byteorder='big')
eth_addr = '0x' + keccak_256(concat_x_y).digest()[-20:].hex()

print('private key: ', hex(private_key))
print('eth_address: ', eth_addr) The public key is simply the private key multiplied by the point G on the secp256k1 elliptic curve. That's it. Learn More Check out our advanced today and become a blockchain developer who knows the hard stuff other coders don't. blockchain bootcamp Also published . here

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Solidity events Solidity events are the closest thing to a “print” or “console.log” statement in Ethereum. We will explain how they work and when to use them. We will also go into a lot of technical details often omitted in other resources. Here is a minimal example to emit a solidity event. contract ExampleContract {

  // We will explain the significance of the indexed parameter later.
  event ExampleEvent(address indexed sender, uint256 someValue);

  function exampleFunction(uint256 someValue) public {
    emit ExampleEvent(sender, someValue);
  }
} Perhaps the most well-known events are those emitted by tokens when they are transferred. The sender, receiver, and amount are recorded in an event. ERC20 emit Transfer(from, to, amount); Isn’t this redundant? We can already look through the past transactions to see the transfers, and then we could look into the transaction calldata to see the same information. This is correct, one could delete events and have no effect on the business logic of the smart contract. However, this would not be an efficient way to look at history. Table of contents Solidity events Retrieving transactions faster Listening to events Solidity-indexed events vs non-indexed events Solidity events best practices Events cannot be used in functions How many arguments can an event take? Variable names in events are option but recommended Events can be inherited through parent contracts and interfaces Event selector Anonymous events Advanced topics about events Gas cost to emit solidity event Conclusion Retrieving transactions faster The Ethereum client does not have an API for listing transactions by “type,” for example “all transfer transactions of an ERC20.“ Here are your options if you want to query transactions: getTransaction getTransactionFromBlock API can only tell you what transactions occurred on a particular block, it cannot target a smart contract address multiple blocks. getTransactionFromBlock can only inspect transactions you know the transaction hash for. getTransaction Events on the other hand can be retrieved much more easily. Here are the Ethereum client options: events events.allEvents getPastEvents Each of these requires specifying the smart contract address that the querier wishes to examine and returns a subset (or all) of the events a smart contract emitted according to the query parameters specified. Here is the key insight for why you would use Events to track transactions rather than transactions themselves: Ethereum does not provide a mechanism to get all transactions for a smart contract, but it does provide a mechanism for getting all events from a smart contract. Why is this? Making events quickly retrievable requires additional storage overhead. If Ethereum did this for every transaction, this would make the chain considerably larger. With events, solidity programmers can be selective about what kind of information is worth paying the additional storage overhead for, to enable quick off-chain retrieval. Listening to events Here is an example of using the API described above. In this code, the client subscribes to events from a smart contract. These examples are all in Javascript. Example 1: Listening to ERC20 Transfer events. This code triggers a callback every time an ERC20 token emits a transfer Event. const { ethers } = require("ethers");

// const provider = your provider

const abi = [
  "event Transfer(address indexed from, address indexed to, uint256 value)"
];

const tokenAddress = "0x...";
const contract = new ethers.Contract(tokenAddress, abi, provider);

contract.on("Transfer", (from, to, value, event) => {
  console.log(`Transfer event detected: from=${from}, to=${to}, value=${value}`);
}); Example 2: Filtering an ERC20 approval for a specific address If we want to look at events retroactively, we can use the following code. In this example, we look to the past for Approval transactions in an ERC20 token. const ethers = require('ethers');

const tokenAddress = '0x...';

const filterAddress = '0x...';

const tokenAbi = [
  {
    "anonymous": false,
    "inputs": [
      {
        "indexed": true,
        "name": "from",
        "type": "address"
      },
      {
        "indexed": true,
        "name": "to",
        "type": "address"
      },
      {
        "indexed": false,
        "name": "value",
        "type": "uint256"
      }
    ],
    "name": "Transfer",
    "type": "event"
  }
];

const tokenContract = new ethers.Contract(tokenAddress, tokenAbi, provider);

// this line filters for Approvals for a particular address.
const filter = tokenContract.filters.Approval(filterAddress, null, null);

tokenContract.queryFilter(filter).then((events) => {
  console.log(events);
}); If you wanted to look for a trade between two particular known addresses (if such a transaction exists), the ethers.js. code would be as follows: tokenContract.filters.Transfer(address1, address2, null); Here is a similar example in instead of ethers.js. Note that the and query parameters are added (to signify we only care about events between these blocks), and we will demonstrate the ability to listen to multiple addresses being the sender. The addresses are combined with the "OR" condition. web3.js fromBlock toBlock const Web3 = require('web3');

const web3 = new Web3('https://rpc-endpoint');
const contractAddress = '0x...'; // The address of the ERC20 contract
const contractAbi = [
  {
    "anonymous": false,
    "inputs": [
      {
        "indexed": true,
        "name": "from",
        "type": "address"
      },
      {
        "indexed": true,
        "name": "to",
        "type": "address"
      },
      {
        "indexed": false,
        "name": "value",
        "type": "uint256"
      }
    ],
    "name": "Transfer",
    "type": "event"
  }
];
const contract = new web3.eth.Contract(contractAbi, contractAddress);

const senderAddressesToWatch = ['0x...', '0x...', '0x...']; // The addresses to watch for transfers from

const filter = {
  fromBlock: 0,
  toBlock: 'latest',
  topics: [
    web3.utils.sha3('Transfer(address,address,uint256)'),
    null,
    senderAddressesToWatch,
  ]
};

contract.getPastEvents('Transfer', {
  filter: filter,
  fromBlock: 0,
  toBlock: 'latest',
}, (error, events) => {
  if (!error) {
    console.log(events);
  }
}); Solidity-indexed events vs non-indexed events The example above works because the Approve (and Transfer) event in ERC20 sets the sender to be indexed. Here is the ERC20 Approval event declaration in Solidity. event Approval(address indexed owner, address indexed spender, uint256 value); If the “owner” argument was not indexed, the javascript code earlier would silently fail. The implication here is that you cannot filter for ERC20 events that have a specific (token amount) for the transfer, because that is not indexed. You must pull in all the events and filter them javascript side; it cannot be done in the Ethereum client. value An indexed argument for an event declaration is called a . topic Solidity events best practices The generally accepted best practice for events is to log them whenever a potentially consequential state change happens. Some examples include: Changing the owner of the contract Moving ether Conducting a trade Not every state change requires an event. The question the Solidity developers should ask themselves is “would someone have an interest in retrieving or discovering this transaction quickly?” Index the right event parameters This will requires some subjective judgement. Remember, an unindexed parameter cannot be searched for directly, but still might be useful data when accompanied by an indexed parameter. A good way to get an intuition for this is to look at how established codebases design their events Gnosis Safe Uniswap ERC20 Specification ERC721 Specification As a general rule of thumb, cryptocurrency amounts should not be indexed, and an address should be, but this rule should not be applied blindly. Indexes only allow you to quickly get exact values, not a range of values. Avoid redundant events An example of this would be adding an event when tokens are minted because underlying libraries already emit this event. Events cannot be used in view functions Events are state changing; they alter the state of the blockchain by storing the log. Therefore, they cannot be used in view (or pure) functions. Events are not as useful for debugging the way console.log and print are in other languages; because events themselves are state-changing, they are not emitted if a transaction reverts. How many arguments can an event take? For unindexed arguments, there is no intrinsic limit to the number of arguments, though of course there are contract size and gas limits that apply. The following nonsensical example is valid solidity: contract ExampleContract {
  event Numbers(uint256, uint256, uint256, uint256, uint256, uint256, uint256, uint256);
} Similarly, there is no intrinsic limit to the length of strings or arrays stored in a log. However, there cannot be more than three indexed arguments (topics) in an event. An anonymous event can have 4 indexed arguments (we will cover this distinction later). An argument with zero events is also valid. Variable names in events are optional but recommended The following events behave identically event NewOwner(address newOwner);
event NewOwner(address); In general, including the variable name would be ideal because the semantics behind the following example are very ambiguous (this is not how you should declare events!) event Trade(address,address,address,uint256,uint256); We can guess that the addresses correspond to the sender, and the token addresses, while the uint256es correspond to the amounts, but this is hard to decipher. It is conventional to capitalize the name of an event, but the compiler does not require it. This would be a much better declaration: event Trade(address trader, address token1, address token2, uint256 token1Amount, uint256 token2Amount); Events can be inherited through parent contracts and interfaces When an event is declared in a parent contract, it can be emitted by the child contract. Events are internal and cannot be modified to be private or public. Here is an example: contract ParentContract {
  event NewNumber(uint256 number);

  function doSomething(uint256 number) public {
    emit NewNumber(number);
  }
}

contract ChildContract is ParentContract {
  function doSomethingElse(uint256 number) public {
    emit NewNumber(number); // valid
  }
} Similarly, events can be declared in an interface and used in the child, as in the following example. interface IExampleInterface {
	event Deposit(address indexed sender, uint256 amount);
}

contract ExampleContract is IExampleInterface {
	function deposit() external payable {
		emit Deposit(msg.sender, msg.value); // also valid
	}
} Event selector The EVM (Ethereum Virtual Machine) identifies events with the keccak256 of their signature. For solidity versions 0.8.15 or higher, you can also retrieve the selector using the .selector member. pragma solidity ^0.8.15;
contract ExampleContract {

    event SomeEvent(uint256 blocknum, uint256 indexed timestamp);

    function selector() external pure returns (bool) {

        // true
        return SomeEvent.selector == keccak256("SomeEvent(uint256,uint256)");
    }
} The event selector is actually a topic itself (we will discuss this further in a later section). Marking variables as indexed or not does not change the selector. Anonymous events Events can be marked as anonymous, in which case they will not have a selector. This means that client-side code cannot specifically isolate them as a subset like our earlier examples. The only way for client-side code to see an anonymous event is to get all events for the smart contract. pragma solidity ^0.8.15;
contract ExampleContract {

    event SomeEvent(uint256 blocknum, uint256 timestamp) anonymous;

    function selector() public pure returns (bool) {

        // ERROR: does not compile, anonymous events don't have selectors
        return SomeEvent.selector == keccak256("SomeEvent(uint256,uint256)");
    }
} Because the event signature is used as one of the indexes, an anonymous function can have four indexed topics, since the function signature is “freed up” as one of the topics. An anonymous event can have up to four indexed topics. A non-anonymous event can have up to three. contract ExampleContract {

    // valid
    event SomeEvent(uint256 indexed, uint256 indexed, address indexed, address indexed) anonymous;
} Anonymous events are rarely used in practice. Advanced topics about events This section describes events at the assembly level of the EVM. This section can be skipped for programmers new to . blockchain development Implementation detail: Bloom filters To retrieve every transaction that has happened with a smart contract, the Ethereum client would have to scan every block, which would be an extremely heavy I/O operation; but Ethereum uses an important optimization. Events are stored in a data structure for each block. A Bloom Filter is a probabilistic set that efficiently answers if a member is in the set or not. Instead of scanning the entire block, the client can ask the bloom filter if an event was emitted in the block. This allows the client to scan the blockchain much faster to find events. Bloom Filter Bloom Filters are probabilistic: they sometimes incorrectly return that an item is a member of the set even if it isn’t. The more members that are stored in a Bloom Filter, the higher the chance of error, and the larger the bloom filter must be (storage wise) to compensate for this. Because of this, Ethereum doesn’t store all the transactions in a Bloom Filter. There are far fewer events than there are transactions. This keeps the storage size on the blockchain manageable. When the client gets a positive membership response from a bloom filter, it must scan the block to verify the event took place. However, this will only happens for a tiny subset of blocks, so on average the Ethereum client saves a lot of computation by checking the bloom filter for event presence first. Yul Events In the Yul intermediate representation, the distinction between indexed arguments (topics) and unindexed arguments becomes clear. The following yul functions are available for emitting events (and their EVM opcode bears the same name). The table is copied from the with some simplification. yul documentation op code Usage log0(p, s) log without topics and data mem[p…(p+s)) log1(p, s, t1) log with topic t1 and data mem[p…(p+s)) log2(p, s, t1, t2) log with topics t1, t2 and data mem[p…(p+s)) log3(p, s, t1, t2, t3) log with topics t1, t2, t3 and data mem[p…(p+s)) log4(p, s, t1, t2, t3, t4) log with topics t1, t2, t3, t4 and data mem[p…(p+s)) A log can have up to 4 topics, but a non-anonymous solidity event can have up to 3 indexed arguments. That is because the first topic is used to store the event signature. There is no opcode or Yul function for emitting more than four topics. The unindexed parameters are simply abi encoded in the memory region [p…(p+s)) and emitted as one long byte sequence. Recall earlier that there was no limit in principle for how many unindexed arguments an event in Solidity can have. The underlying reason is that there is no explicit limit on how long the memory region pointed to in the first two parameters of the log op code takes. There is of course, limits provided by the contract size and memory expansion gas costs. Gas cost to emit solidity event Events are substantially cheaper than writing to storage variables. Events are not intended to be accessible by smart contracts, so the relative lack of overhead justifies a lower gas cost. The formula for how much gas an event costs is as follows ( ): source 375 + 375 * num_topics + 8 * data_size + mem_expansion cost Each event costs at least 375 gas. An additional 375 is paid for each indexed parameter. A non-anonymous event has the event selector as an indexed parameter, so that cost is included most of the time. Then we pay 8 times the number of 32 byte words written to the chain. Because this region is stored in memory before being emitted, the memory expansion cost must be accounted for also. In general, the intuition that the more you log the more you pay in gas, is accurate. Conclusion Events are for clients to quickly retrieve transactions that may be of interest. Although they don’t alter smart contract functionality, they allow the programmer to specify which transactions should be quickly retrievable. This makes it easier for dapps to quickly summarize important information. Events are relatively cheap gas-wise compared to storage, but the most important factor in their cost is the number of indexed parameters, assuming the coder does not use an inordinate amount of memory. Learn More Like what you see here? See our advanced to learn more. Solidity Bootcamp The lead image for this article was generated by HackerNoon's AI Image Generator via the prompt "Events in Solidity"

Solidity Events: A Comprehensive Overview

Eventos de solidez: una descripción completa

Événements Solidity : un aperçu complet

सॉलिडिटी इवेंट्स: एक व्यापक अवलोकन

Solidity イベント: 包括的な概要

Solidity Events: Uma Visão Abrangente

Sự kiện Solidity: Tổng quan toàn diện

Solidity 事件：全面概述

If you keep it up, you’re going to end up as a plumber cleaning out poo pipes, my teacher hissed. The students in my class roared with laughter. I was bored and got caught goofing around. So, filled with fire and brimstone, my teacher threatened me with career purgatory in the form of a plumber. But my teacher’s words struck me. I didn’t know anything about plumbing but cleaning out poo pipes sounded terrible. I remember feeling bad knowing there might be a student in my class whose parent was a plumber and they were probably feeling ashamed. How could a teacher make fun of someone that enabled them to drink clean water and flush their toilets? Was it fair to characterize a plumber’s job as a punishment? In grade school, parents would come visit our class and share about their jobs. I remember kids making fun of some of these people. It would destroy the classmate whose parent was being made fun of; their hero and friend was being mocked because their job was looked down upon. The message was consistent and pounded into my brain like a nail during my twelve years in public school. It came across that getting good grades and attending college was the primary purpose for learning. There was only one path to success, and it went through college. Any other career path or learning source was looked down upon by just about everyone; primarily via a deafening lack of advocacy, if not outright verbal disparagement. The message was clear: the work of building homes, welding bridges and helping people access water and electricity were all viewed as second-rate, for the less intelligent, and a sort of failure to be looked down upon and avoided. To be fair, this message was coming from lots of directions, not just public school. It was hard to avoid, and it being the pre-internet age, we did not have many counter-cultural voices to speak out against it. We didn’t have a Mike Rowe to champion the trades for us. So, I graduated from high school in 1999 in a similar situation as most of my peers, with exactly zero skills. It’s a pretty depressing feeling realizing that after 18 years of life, and 12 years of public school, that I wasn’t much use to anyone. I was realizing that I was not prepared at all to go out and add value to the world. In fact, my education was just beginning. Starting my education over after high school The next ten years or so were a blur of lessons I mostly should have learned when I was younger. I had several jobs, started a family and moved several times. I learned how to work hard at a railroad job; I learned some fine carpentry skills building high-end kit homes; I learned persistence polishing concrete; and I developed an appreciation for a wide range of trades working for a contractor in Portland Oregon. But I was restless and didn’t quite know why. I wanted to work on problems that others ignored. What those problems were, I had no idea. I figured the best way to find out was to start my own business and find ways to help people. So in 2010, I decided to take the plunge. I had no idea what I wanted to sell, but I wanted to learn about supply and demand and figure out how to add value to customers. I chose Craigslist to be my sandbox, and with the encouragement of a couple friends, I started buying and selling items on Craigslist, adding value where I could. It ended up being profitable from the very first day! Learning how to repair appliances About six months later I discovered an opportunity that existed for those who learned appliance repair skills. I was living in Portland Oregon at the time and was seeing dozens of nice looking appliances being thrown away every day on Craigslist because of small problems. One day, I had an appliance repair technician come to my house to buy a broken dryer that I had rescued. While there, he gave me some advice that completely changed my life. He told me that the appliance he was purchasing from me was very easy to repair and would likely cost less than a dollar to fix. He even left me with the part so I could fix the next dryer that I got my hands on. So that’s exactly what I did. Over the course of the next several months, I slowly taught myself how to fix washers and dryers by watching YouTube videos, reading forums and through trial and error. I had no idea where else to learn the trade and I didn’t know anyone or have any connections in the appliance repair business. I eventually became proficient and frequently I wrote about my new journey on my blog, ReCraigslist. That’s when things got interesting. Teaching appliance repair across North America The response from my readers was overwhelming. I had dozens of people privately email me to ask if I would consult with them and teach them to repair appliances and start their own small business. So I began teaching students across the US and Canada through phone calls, video chats and emails the skills they needed to be successful in the appliance repair business. After six months, I realized I needed to create a more efficient way to teach the material, so I built an entire online course with training videos and various other resources that students could go through at their own pace. Since then, several hundred students varying in age from high school students to retirees have gone through the course, with several dozen starting their own appliance repair business. The light bulb moment Everything came full circle from my observations of how the trades were treated when I was younger. These people were hearing about the appliance repair trade for the first time because it was never a part of their education. When they read my stories about appliance repair, they immediately reached out. It’s amazing what happens when people find out more details about a viable trade. They want to learn it! The bigger revelation for me however, was how effective and efficient could be in teaching someone appliance repair skills online. If someone can learn to repair appliances online, they could learn other trades as well! And they could do it from home at a low cost without the crushing five to six figure college debt. technology So I started asking questions about all the other trades, besides appliance repair, that are relatively unknown. How do people hear about these trades if they aren’t talked about in schools and aren’t glamorized on TV and in movies? What if someone wants to learn a trade but can’t afford to pay a trade school tuition? And what if someone could afford to go to a trade school, but it’s several hours away? Why should they have to move? How does a young person learn a trade if their parents or relatives don’t know any trades and they aren’t taught in school? Why should we make kids wait until after high school, when they need to start earning a living, to begin the skills that will earn them that living? learning A new way to learn trade skills It was these questions and a deep desire to help people flourish in their job and life that led me to build , a place to learn trade skills online. Our goal is to develop an online pathway that teaches and leads students to the skilled trades and fulfilling jobs. We want to give equal access to a trade education no matter a person’s location, financial status, age, gender or nationality. We believe that everyone should learn a trade. When people become skilled at something, it gives them the confidence that they can learn anything they put their mind to. Tradeskills.io We are starting with appliance repair and carpentry. We will continually be adding new material, hands-on projects, and more trades as soon as resources allow. We are just a very small team right now, but we have a big goal. Your support, encouragement, and feedback right now are immensely valuable to us! Creating a resource to help many generations to come If we were in junior high or high school today, would we be taught any trade skills? Would it come across that a career in the trades is both respected and a place where people flourish? Technology has revolutionized the world and made the access to information easier, and more affordable than any point in history. Have these changes caused parents and educators to promote the trades? I don’t think it has. There are millions of kids that are still receiving the same message, the same advice, the same lack of exposure and advocacy of an education in the trades that we experienced decades ago. We need an education revolution, and is my small effort towards making a difference. Tradeskills.io If you believe in the trades, and know someone that would benefit from this message, please like and share this post. Your support and encouragement means a great deal! Would love to hear your thoughts, experiences and stories. Also, please check out the site at Tradeskills.io -Ryan Originally published at recraigslist.com on October 1, 2017.

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