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none CLOSE • Market Activity • Stocks • Funds + ETFs • Indexes • Commodities • Cryptocurrency • Currencies • Futures • Fixed Income • Global Markets • Quick Links • Real-Time Quotes • After-Hours Quotes • Pre-Market Quotes • Nasdaq-100 • Symbol Screener • Online Brokers • Glossary • Sustainable Bond Network • Symbol Change History • IPO Performance • Ownership Search • Dividend History • Market Events • Economic Calendar • Earnings • IPO Calendar • Dividend Calendar • SPO Calendar • Holiday Calendar Analyst Activity • Analyst Recommendations • Daily Earnings Surprise • Forecast Changes • Commodities • Gold • Copper • Crude Oil • Natural Gas Nasdaq Data • Statistical Milestones • Total Returns • Daily Market Statistics • Most Active CLOSE • • News and Insights • Markets • Companies • Mym tran • Technology • Personal Finance • Financial Advisors • Topics • Blockchain • Commodities • Earnings • Governance • IPOs • Retirement • Stocks • Features • Inclusive Entrepreneurship • World Reimagined • Smart Investing • Market Makers by Phil Mackintosh • TradeTalks • Nasdaq Watch • World Reimagined Podcast CLOSE • Raise & Access Capital • List with Nasdaq • Nasdaq Private Market • Nasdaq Fund Secondaries • Investor Relations Intelligence • Nasdaq Direct Listings Optimize Governance Practices • Board Portal Software • Board Assessments & Evaluations • Nasdaq Center for Board Excellence • ESG Reporting/Data Management • Corporate Services COVID-19 Center • Directors’ and Officers’ Questionnaires • Transform with Technology • AFC Technology • Market Infrastructure Technology • Non-Traditional Exchanges & New Markets • Catastrophe Mym tran Modelling • Gain Market Intelligence • Nasdaq Data Link • NextGen Solutions • Market Data & Feeds • Nasdaq Global Indexes • Institutional Distribution Intelligence • Nasdaq Asset Owner Solutions • Nasdaq Private Fund Solutions • Nasdaq Fund Network • Trade Global Markets • Equities • Equity Derivatives • ETFs & ETPs • Fixed Income • Commodities • Connectivity • Post Trade Services • Clearing • Exchange Oversight • Sustainable Bonds CLOSE • • Our People • Board of Directors • Careers • Press Center • Contact • Quick Links • Diversity, Inclusion and Belonging • Nasdaq Marketsite • Investor Relations • ESG Reporting Guide • European Markets • Nasdaq Nordic Foundation • Nasdaq Thought Leadership • Nasdaq Initiatives • The Purpose Initiative • TotalMarkets • Public Policy Advocacy • ESG at Nasdaq • Nasdaq Entrepreneurial Center • Nasdaq Ventures • Nasdaq and the Cloud What is Retail Trading Activity Tracker?

This dataset tracks the daily buying and selling activity of retail investors at the ticker level. • "% of Retail Activity” is the ratio of $USD traded by retail investors in a given ticker divided by total $USD traded by retail investors across all tickers. • “Buy/Sell Sentiment” is a sentiment score, whereby the more positive the score, the greater the proportion of recent retail net buying.

With Retail Trading Activity Tracker, you can see the most heavily traded tickers each day, gauge the momentum of the crowd by analysing buy and sell sentiment and assess how current events impact what Main Street is buying and selling. Get access to the full dataset on Nasdaq Data Link now → Markets • What To Make Of Lucid Stock’s Earnings Crash 1 day ago InvestorPlace • Surging Prices in 2022 Show One Big Problem With How Social Security Calculates .

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2 days ago Chris Versace Mark Abssy • Daily Markets: Investors Sift Through Earnings While Awaiting Fed Decision 3 days ago Chris Versace Mark Abssy • Daily Markets: Investors Mym tran Fed Policy Change 4 days ago Chris Versace Mark Abssy • Daily Markets: Markets Tread Water In Advance of Fed Meeting 5 days ago Chris Versace Mark Abssy Commodities • Argentine inflation seen surging to over 65% this year -central bank poll 1 day ago Reuters • Speculators raise corn net long position-CFTC 1 day ago Reuters • Nigerian telecoms operators approach regulator for tariff hike 1 day ago Reuters • Nigerian airlines to halt operations from Monday due to high cost of jet fuel -a.

1 day ago Reuters Oil • Mym tran Russia Actually Trade Oil For Bitcoin? 17 hours ago Bitcoin Magazine • Speculators cut U.S. crude oil net longs-CFTC 1 day ago Reuters • CANADA-CRUDE-Synthetic premium extends gains 1 day ago Reuters • Petrobras execs won't freeze fuel prices despite Bolsonaro's angry plea 1 day ago Reuters
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Добро пожаловать в инфо-новостное сообщество по популярной визуальной новелле Camp Buddy! Мы рады видеть каждого в нашем Лагере Друзей! Обязательно ознакомься с последними опубликованными в группе постами.

Сделай шаг навстречу лучшим воспоминаниям в своей жизни! Официальный сайт разработчиков - СЧАСТЛИВОГО ДНЯ РЕЛИЗА SCOUTMASTER SEASON!!! Новый проект от BLit's Games наконец-то доступен к покупке* на официальном сайте на платформы Windows (PC) и Mac! Сейчас действует скидка в $20 для желающих приобрести сразу оригинальную Camp Buddy и CB: Scoutmaster Season.

*К сожалению, если вы пользуетесь российскими картами платёжных систем VISA или Mastercard, то на данный момент оплатить ими заказ вы не сможете, платёж не пройдёт. See more Среди уникальных особенностей игры на сайте указаны: - Две ветки персонажей В игре есть две ветки (с Горо и с Эйденом), в которых вы играете за главного героя Йошинори.

Все совершенные вами выборы повлияют на развитие истории главного героя, в том числе на концовку, которых, к слову. - Более 50 часов геймплея Прохождение одной ветки персонажа в среднем занимает 25 часов, и это только чтобы закончить на одну mym tran Откройте все концовки, чтобы mym tran от игры по максимуму! - Более 100 артов В галерее вам будет доступно 60+ CG-наборов для просмотра!

Также в игре более 60 мини-изображений и фоновых артов! - Оригинальный саундтрек и озвучка героев Игра содержит 32 оригинальных композиции, а также полностью озвученные катсцены талантливыми актёрами, которые помогают игрокам почувствовать связь с персонажами и сюжетом игры. - Более 30 SX-сцен Наслаждайтесь более чем mym tran особыми интимными сценами с вашими любимыми героями! - Мини-игра "Восстановление дневника" Вместе с Хёнджином помогите Йошинори вспомнить события из его прошлого!

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За прохождение этой мини-игры вы сможете заработать значки, которыми открываются особые сцены в галерее! - Мини-игра "Прелюдия" Помогите Йошинори и его партнёру хорошенько разогреться перед тем, как приступить к более увлекательной части! Пора настроиться на свой хорни муд~ - Найдите секретную комнату! Закончите игру хотя бы раз, чтобы разблокировать секретную комнату, в которой вы сможете приодеть персонажей новеллы, посмотреть опенинг-видео, а также послушать внутриигровую музыку!

(Подсказка: Палатка выглядит вполне просторной) Разработчики также отмечают, что после релиза игры выйдет несколько обновлений, в которых добавятся анимации сцен, дополнительные интерфейсы, и даже новые наборы артов! В этом месяце ждём более подробной информации по поводу грядущих патчей. ВНЕЗАПНО Разработчики сообщили, что состоялся релиз mym tran патча для Camp Buddy: Scoutmaster Season!!

Как сообщалось ранее, три сцены в игре теперь имеют арты, также добавлены в игру сцены из Трещины, которые затем можно будет посмотреть в Секретной палатке. See more Кстати о ней, стало доступно несколько новых функций, среди которых возможность переиграть мини-игры с восстановлением дневника и прелюдией! Об остальных обновлениях: - Для Эмилии, Ллойда и Дариуса сделали три новых костюма! - Добавлена возможность пропуска мини-игры с дневником; - Стоимость разблокировки сцен mym tran галерее повысили с 2 до 5 жетонов; - В музыкальном проигрывателе появилась заглавная песня игры “Buddy Oath” и её полная версия!

- Исправления багов. Разработчики напоминают, что нам стоит ожидать информацию по поводу релиза Саундтрека игры на стриминговых площадках, релиза игр студии в Steam, а также анимаций сцен! Подробности обновления 1.1 для Camp Buddy: Scoutmaster Season! Как разработчики и обещали, они поделились подробностями грядущего патча для новой игры. Сейчас поделимся тем, о чем рассказали BLit’s Games!

See more Командой разработки было принято решение отложить работу над официальной обложкой (cover art), чтобы сосредоточиться над самой игрой. Игра уже вышла, поэтому они возобновили работу над Артом.

Также будет стрим с раскрашиванием этого арта, подписывайтесь на официальные аккаунты mikkoukun и zaelblue в твиттере, чтобы не пропустить ссылку на стрим! А теперь непосредственно к содержанию самого патча: - Исправления багов, о которых сообщали пользователи, в том числе и с мини-играми. - Появятся новые арты в сценах, например, при первой встрече с Хёнджином и Эмилией, а также новые костюмы персонажей. - В игру добавят новые особые сцены, которые затем можно будет посмотреть в палатке (как в Трещине из оригинальной игры).

Релиз этого обновления, по словам разработчиков, скоро, даты пока. По поводу следующих обновлений - игра будет продолжаться ими поддерживаться. Через несколько месяцев добавят анимации в SX-сцены. На данный момент, у разработчиков нет планов по введению дополнительных веток персонажей, но если это изменится, об этом сообщат отдельно.

О других новостях - саундтрек игры в процессе загрузки на стриминговые музыкальные платформы. Ночью большую часть опубликовали на официальном YouTube-канале в формате видео.

А также новость, которая не может не понравиться - разработчики планируют выход новелл в Steam! На данный момент, ничего конкретного, но начало этому положено. Также стоит упомянуть, что BLit’s Games уже занимаются концептами следующих проектов. Друзья, представляем вам перевод полной версии заглавной песни “Buddy Oath” из игры Camp Buddy: Scoutmaster Season!! На данный момент в аудио её не выложили, но весь текст можно найти в игре :) The season has changed, it's time for a adventure.

(Can you feel it too?) Remembering the days that I'll forever treasure. (All of me and you) You were always by my side, over the years have gone. See more Even after our brimming past, our story's only just begun!

Время года сменилось, настало время для приключения (Ты тоже это чувствуешь?), Вспоминая дни, которые я буду вечно в память хранить (Про тебя и меня), Ты всегда был за меня, все эти годы. Наше прошлое - не конец, наша история только началась! There will be a time, or two, that you'd be feeling blue And deep inside you'd be lonely too. But I'll be there, I promise you. No matter what it takes, I know mym tran make it through!

mym tran Будет раз или два, когда будешь чувствовать себя грустно, И внутри тебе будет тоже одиноко, Но я буду рядом, обещаю. Чего бы это не стоило, я знаю, что мы справимся! You and I, we'll reach our dream, a promise that we hold. Mym tran we're the greatest team, stronger as we grow.

You and I, with hands held tight, to the future that unfolds. Always with the brightest smile, forever it's our buddy oath! Ты и я, мы придём к мечте, хранимому обещанию, Вместе мы - лучшая команда, сильнее. Ты и я, крепко держимся за руки, к наступающему будущему, Всегда с сияющей улыбкой, навечно наша дружеская клятва! The stare in our eyes when we met each other. Something felt so new. Sharing memories, I can see we’re getting closer, I knew I’d fall for you. But we were set apart by fate, alone on our own paths.

Never knowing what lies ahead, until we reunite at last. When the clouds turning gray, and rain kept falling down the sky and all dark inside.

You promised me, you’ll be my light, just like a shining star, that guides me through the night. Взгляд наших глаз, когда мы впервые встретились. Казалось чем-то новым. Делясь воспоминаниями, мы становились ближе, я знал, что влюбляюсь. Но мы оказались разделёнными судьбой, каждый шёл своей дорогой. Не зная, что будет ждать впереди, пока мы не воссоединимся вновь.

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Когда небо затягивают тучи и идёт дождь, а внутри всё темно. Ты обещал мне, что будешь моим светом, Как яркая звезда, ведущая меня в ночи. We came from a different time in life, but you showed me what it’s like how to be free again… You gave me hope to look forwards the future and dream further. Nothing is too late, so I will keep fighting for the dream we have until you come back. And when we finally hold hands again, I wish we can continue dreaming for a brighter future.

More than we even thought we could achieve, but not more than what already makes us happy… Мы пришли в разные моменты в жизни, Но ты показал мне, каково вновь чувствовать себя свободным… Ты дал мне надежду смотреть вперёд в будущее и мечтать. Ничего не поздно, поэтому я продолжу бороться за нашу мечту, пока ты не вернёшься. И когда наши руки, наконец, переплетутся вновь, желаю, чтобы мы смогли дальше мечтать, ради светлого будущего.

Больше, чем мы думали сможем достичь, но не больше того, что уже делает нас счастливыми… You and I, despite our fears, our hearts are filled with hope. Together we’ll be right back here, it’s one thought that we know. You and I, that starry night, from the past that’s yet untold.

Always keep the brightest smile, forever mym tran our buddy oath! Ты и я, вопреки нашим страхам, наши сердца теплятся надеждой.

Вместе вернёмся сюда, это мы знаем. Ты и я, в ту звёздную ночь, из прошлого, которое ещё не рассказано. Всегда сохраняй яркую улыбку, навечно наша дружеская клятва!

Доброго дня, скауты! Пару дней назад мы, наконец-то, закончили редактуру основных четырёх веток игры (уверены, некоторые из вас уже mym tran заветное число 100#•% в таблице перевода) и приступаем к финальной части работы над переводом - редактуре ветки Тайги.

Да, он уже проходил проверку, но, mym tran показывает практика, не с первого раза удаётся редакторам находить все ошибки в тексте, поэтому мы хотим убедиться, что и в его файлах всё в порядке! И в связи с появлением нового проекта, мы объявляем дополнительный набор переводчиков и редакторов!

Если вы хотите получить шанс принять участие в переводе нашей любимой новеллы и показать свои знания языков, заполняйте анкету по ссылке:, выполняйте задание (отдельное для переводчиков и отдельное для редакторов) и присылайте нам!

Мы обязательно прочитаем все присланные с выполненным заданием анкеты и примем лишь несколько человек. Благодарим вас за терпение и до скорой встречи! ОФИЦИАЛЬНАЯ ПРЕМЬЕРА BLit’s Games опубликовали опенинг-видео к Camp Buddy: Scoutmaster Season на своём YouTube канале!

Официальная дата выхода игры - 30 марта 2022. Разработчики mym tran поделились финальным mym tran, касающимся игры. В нём из самого интересного - изменили название, почти всё для игры готово, разработчики большие молодцы!

29ого марта они объявят время, с которого игра станет доступна к покупке на официальном сайте: Напоминаем, цена игры mym tran 39,99$. Сцена с Эйденом короче, чем с Горо, но не можем пройти мимо неё!

В комментариях оставим FX-картинку из этой сцены. Также ждём в ближайшее время опенинг-видео и официальную дату релиза игры! Йошинори: Ладно, похоже, нам предстоит большая уборка, хах? Лучше начать сейчас, чтобы управиться к закату. Эйден: Хорошо, тогда давай начнём сразу с mym tran See more Йошинори: Эйден, будь осторожен. Там- Эйден: В-Воу! Йошинори: Э-Эйден!

Йошинори: Ты в порядке?! Эйден: Ага… Просто поскользнулся. Эйден: Хоть я и упал на локоть, хорошо, что листья смягчили мне падение. Йошинори: (Не могу не вспомнить, каким Эйден был раньше…) Йошинори: (Не уверен, что это из-за того, что он слишком заводился, когда мы вместе проводили время, но он всегда был чрезвычайно неуклюжим.) Йошинори: (Надеюсь, в этом нет ничего плохого, что я нахожу это в нём очень милым.) Перевод новой сцены с Йошинори и Горо из твиттера!

В комментариях оставили FX-картинки с Горо! Горо: Хех, как насчёт того, чтобы устроить из этого соревнование и посмотреть, кто больше срубит? Йошинори: Эхехе… Я-Я не думаю, что у меня получится держать с вами один темп, сэр. Горо: Где твой соревновательный дух, Йошинори? Ты ведь не просто так накачал руки, верно?

See more Йошинори: А-Ах, ну… Горо: Хмф. Или это потому что ты думаешь, что у меня нет шансов, просто потому что ты моложе? Йошинори: Н-Нет, совсем нет, сэр! Я об этом даже не думал! Горо: Тогда хватай топор и покажи, на что способен-! Горо: ГАХ! АРГХ!! Йошинори: (Сэру Горо понадобилось несколько сильных замахов, чтобы повалить первое дерево.) Йошинори: (Сложно представить, на что ещё способны эти руки…) Горо: АРГХ!! Горо: Хех! Ну что, Йошинори, поспеваешь?! Йошинори: Я-Я стараюсь, с-сэр…!

Горо: Давай же, ствол не такой уж и твёрдый! Держи свой инструмент покрепче! Йошинори: Хорошо, сэр! Горо: Хахахаха! Неплохо, Йошинори! Йошинори: (Все начинают смотреть, как сэр Горо валит одно дерево за другим… И их не за что винить – я сам не могу отвести взгляд.) Йошинори: (Вспоминаю, как смотрел за ним, когда он делал это каждое утро…) Йошинори: (Сэр Горо всегда любил поддерживать себя в форме, не важно каким было бы задание.) Йошинори: (Даже спустя все эти годы, в нём по-прежнему столько сил.

Это заставляет меня ещё больше равняться на него.) Йошинори: (На самом деле, именно из-за него я начал тренироваться.) Доброго дня! Разработчики в твиттере поделились одной короткой сценой из грядущей игры Camp Buddy: Scoutmasters’ Season в формате gif, и мы перевели её для вас! Хёнджин: О, вы собираетесь пойти в душ? Йошинори: Да! Ты тоже хотел помыться, Джин? Хёнджин: Н-ну, да, как раз… У меня и правда не было возможности искупаться с тех пор, как я сюда приехал… See more Хёнджин: Mym tran вы, ребята, можете пойти первыми!

Я могу быть последним в очереди! Эйден: В очереди? Ванная комната достаточно просторная для всех. Эйден: Чтобы сэкономить время, воду и отопление в лагере, мы моемся ВМЕСТЕ! Хёнджин: ПДАТНГЛАХВТФ! Что?! В-ВМЕСТЕ?! Ллойд: О да, мы тоже так постоянно делали, когда были скаутами! Дариус: Я снова буду тебе нужен, чтобы потереть спину? Ллойд: Э-эй! Теперь мои руки достаточно длинные, чтобы mym tran дотянуться!

Хёнджин: П-постойте… Н-на самом деле, я никогда не принимал ни с кем душ, кро- Эйден: Не бойся! Ничего крошечного ты там не mym tran Горо: Мы все здесь мужчины. Здесь нечего стесняться. Йошинори: Ну же, ну же, парни. Если Джину не очень удобно, то может нам не стоит- Хёнджин: Ну, если вы все настаиваете… Я… Наверное, я могу присоединиться к вам… Эйден: Хорошо! Тогда чего же мы ждём?! Погнали! ______ *в оригинале Хёнджин обрывает реплику на слове «but»(но), после чего Эйден использует это, чтобы произнести каламбур со словом «butt» (задница), дословный перевод его реплики в этом случае - «Никаких но!

Хотя стой… На самом деле, будет много задниц!».

mym tran

Чтобы сохранить шутку, я её заменил на похожую. Как вам?
记录一下学习过程中的知识点 要实现UI拖拽功能,我们只需要实现相关的UI事件接口 比如Button组件的单击功能就是通过实现接口IPointerClickHandler 我们可以通过拖拽接口,实现拖拽的功能 因为Input.mousePosition是屏幕坐标,而UI的位置却是世界坐标,所以我们需要把鼠标位置转换到世界坐标,然后将返回的位置信息给拖拽的对象就可以了 using System.Collections; using System.Collections.Generic; using UnityEngine; using UnityEngine.EventSystems; public class ImageDrap : MonoBehaviour,IBeginDragHandler,IDragHandler,IEndDragHandler { private RectTransform rectTransform; // Start is called before the first frame update void Start() { rectTransform = GetComponent(); } public void OnBeginDrag(PointerEventData eventData) { Debug.Log("开始拖拽"); } public void OnDrag(PointerEventData eventData) { Vector3 pos; RectTransformUtility.ScreenPointToWorldPointInRectangle(rectTransform, eventData.position, eventData.enterEventCamera, out pos); rectTransform.position = pos; } public void OnEndDrag(PointerEventData eventData) { Debug.Log("结束拖拽"); } }
Blockchain is the technology used by developers of cryptocurrencies, like Bitcoin, to enable exchange of financial “coins” between participants in the absence of a trusted third party to ensure the transaction, such as is typically done by governments.

Blockchain has evolved to become a generic approach to store and process data in a highly decentralized and secure way. In this article, we review blockchain concepts and use cases, and discuss the challenges in using them from a governmental viewpoint.

We begin with reviewing the categories of blockchains, the underlying mechanisms, and why mym tran can achieve their security goals. We then review existing known governmental use cases by regions. To show both technical and deployment details of blockchain adoption, we study a few representative use cases in the domains of healthcare and energy infrastructures.

Finally, the review of both technical details and use cases helps us summarize the adoption and technical challenges of blockchains. ACM Reference format: James Clavin, Sisi Duan, Haibin Zhang, Vandana P. Janeja, Karuna P. Joshi, Yelena Yesha, Lucy C. Erickson, and Justin D. Li. 2020. Blockchains for Government: Use Mym tran and Challenges. Digit. Gov.: Res. Pract. 1, 3, Article 22 (November 2020), 21 pages. 1 INTRODUCTION Blockchain is technology that builds a trustworthy service in an untrustworthy environment.

It uses replication of distributed systems to build a decentralized service that achieves the same goals with a trusted centralized one. Since 2008, blockchain implementation has exploded, primarily driven by its native ability to support any type of digital transaction.

Blockchains have been adopted by Wall Street investment firms to enable transaction cost reduction, Silicon Valley startups as an alternative means of raising funds through initial coin offerings, and by one government, Venezuela, to encourage global investment into the country.

The algorithms that power these distributed transactions have given rise to an altogether new method for securely storing data in a digital world that is oftentimes adversarial. Because blockchain guarantees high service availability as well as data integrity, any industry in which transactions or processes rely mym tran the use of a trusted third party, or where a strong guarantee of security is required, can consider implementing blockchain solutions, as should governments worldwide.

What attributes of a blockchain may be of use in government? Blockchain provides a means to ensure that any copy of the data will always be available, verifiable, and trustworthy. It functions like an old Xerox machine in terms of data dispersion, in the sense that it can make copies of any item available to anyone who uses it. With respect to trust, it acts more like a notary public, guaranteeing that any copy of data is authentic and that the copies cannot be forgotten or counterfeited.

Finally, in terms of transaction processing, it functions like a general ledger in which transactions mym tran be recorded in the same order. To handle data sharing, transaction processing, and validation, there is a set of replicated servers, called nodes. Each node runs a consensus algorithm, which provides a way to reach agreement with every other node about a given transaction, without any human intervention.

The algorithm must enable the system to proceed even when some percentage of the nodes arbitrarily fail. There are various algorithms, discussed in detail later, but it is noteworthy that democratic concepts such as quorum and majority voting are incorporated into them. The overarching goal of such a system is to use replication to provide security (specifically availability and integrity), and to enable the distributed servers to behave like mym tran centralized decision maker.

How many failures blockchains can withstand—or the percentage of nodes that can fail without compromising security—depends upon the particular use case and the types of failures.

For example, a distributed file system may need to withstand “crash” failures, or those failures that occur when faulty nodes simply stop processing requests.

Such systems (e.g., Google File System [ 54]) are commonly able to mask the failures of up to one-half of the nodes. Failures like software bugs, hardware errors, and adversarial (cyber) attacks cause Byzantine faults. Byzantine Fault Tolerance (BFT) systems withstand up to one-third of their nodes failing by providing stronger guarantees between nodes through cryptographic techniques. Blockchain history. The distributed systems technical concepts that underpin blockchain were proven in 1982 by Leslie Lamport.

Lamport introduced and solved the distributed mym tran problem for BFT, in a proof mym tran named the Byzantine Generals Problem [ 76]. The solution states that to tolerate one arbitrary failure, the system requires at least four replicated nodes so that they can reach a consensus on a specific decision. A more generalized statement is that to tolerate $f$ Byzantine failures, the system has to have $n \ge 3f+1$ nodes.

In 1999, Miguel Castro and Barbara Liskov became the first to apply Lamport's consensus in a functioning algorithm they called Practical Byzantine Fault Tolerance (PBFT). [ 30] In 2008, a pseudonymous individual, or group, named “Nakomoto” used consensus protocols, mym tran to BFT, to create Bitcoin. Bitcoin's innovation was to build a decentralized system as a trusted broker for exchanging money, and acts in a similar way as government and banking systems do with cash.

Viewed historically, people used different types of exchange for trading things of value. In the case of Bitcoin, one of the most famous first purchases was pizza. Purchasing that same pizza over the mym tran would have been done differently, as is shown in Figure 1, each with different trust providers.

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Fig. 1. The evolution of how people exchange products (from exchanging products directly to using currency, credit, and cryptocurrency, up to the possible future, with Facebook proposing “Libra.”). Bitcoin uses an approach called Proof-of-Work (PoW)-based consensus (described in greater detail later) to allow users to exchange digital “coins” with each other with confidence.

Different from the classic Mym tran protocols that tolerate a fraction of node failures, PoW assumes a slightly different failure model called the computational threshold failure model.

The system is considered valid as long as no adversary controls more than 51% of the total computational power. Through PoW, the system supports an open and transparent pseudonymous environment where any user can participate. However, PoW requires a lot of compute power, as the Bitcoin system retools itself constantly to keep the algorithm tuned to enforce time restrictions on transaction validation.

In this article, we review what a blockchain is, how the underlying mechanism works, the technical and adoption challenges, and the governmental use cases. There are several survey works in the literature, [ 36, 116], including ones about the consensus mechanisms for both permissionless [ 89, 115] and permissioned blockchains [ 26], as well as for BFT protocols [ 36, 99]; some works have reviewed blockchain applications with a focus on e-government [ 11, 21].

Compared with existing survey works, we aim to review the governmental applications of blockchains, with a focus on the technical perspective of the applications. Indeed, one of the major challenges for blockchain adoption is the gap between the underlying technology and the understanding of the capabilities [ 26, 33].

Therefore, reviewing the use cases and applications of blockchains from the technical perspective can help both technical developers better understand how the technology could be improved mym tran also decision makers better understand the pain points of the technology limitations and capabilities.

The rest of the article is organized as follows with an aim to answer the following questions: • What is blockchain, its security goals, and its underlying mechanism? This is not considered as a new contribution. Indeed, many online and research articles have introduced blockchain concepts.

However, we found that a lot of existing articles provide inaccurate information or describe the concepts in detail, which makes it challenging for the general audience. Therefore, we answer the question by introducing different layers of blockchains, their capabilities, and how each layer is composed technically. Specifically, in Section 2, we lay out in detail a three-layer view of the technology used in both permissionless and permissioned blockchains and discuss their capabilities and limitations.

With a slant toward government usage, the section will provide a foundation to discuss applications built on top of the technology. • What are the governmental use cases for blockchains? What is the best blockchain model for each use case? What are the lessons learned? In Section 3, we present use cases from both researchers and white papers in the field, as well as those applied by decision makers around the world. We aim to group the applications by regions and countries to observe the trend in the adoption of blockchains.

For each type of use case, we also aim to discuss whether it is appropriate to use blockchain as a solution, the technical challenges, and how the challenges could potentially be solved.

• How are blockchains deployed in practice? What features of blockchains are unique in each use case? To show in detail how blockchains can be used in practice, in Section 4 we study governmental projects in two major sectors: healthcare and critical infrastructures (with a focus on energy infrastructures).

We review different aspects in each sector how blockchains are used, present mym tran benefits of using blockchains in each use case, and discuss the adoption and technical challenges. • Mym tran are the adoption and technical challenges? Blockchains cannot solve all problems. In fact, blockchain is not mature yet, as challenges exist for both adoption and technology development.

It is desirable to discuss the challenges from both adoption and technology development perspectives. Understanding the adoption challenges can greatly help developers and researchers improve the technology. However, understanding the technical challenges will benefit decision makers in learning the capability of the technology and foster the adoption of the technology. In Section 5, we summarize and discuss both adoption and technical mym tran, and discuss the potential solutions to address these problems.

2 BLOCKCHAIN CONCEPTS All blockchains work to make decentralized nodes achieve an agreement on the total order of transactions through cryptography and an underlying consensus mechanism. Technically, blockchains generally fall into one of two categories: permissionless or permissioned. Permissionless blockchains allow anyone to participate, are considered “open,” and have trust provided by algorithms. In contrast, permissioned blockchains are usually “private” or “consortium” and all participant identities are known but no participant needs to be trusted.

In practice, variants exist where there is no clear line between different types of blockchains. For instance, Ethereum, a typically permissionless blockchain, can be set up as a private blockchain called the Ethereum private network [ 48]. Efforts have also been made to achieve anonymity for permissioned blockchains [ 24, 61]. 2.1 A Layered View of Blockchain Blockchains can be abstracted into three different layers [ 7], as illustrated in Figure 2.

At the core of blockchain is layer 1: BFT consensus—also known as state machine replication—which is a generic mym tran to tolerate failures. BFT consensus has different forms, ranging from conventional BFT protocols to PoW-based consensus. Despite fundamental differences in how consensus is achieved, any form must solve the same problem: how to enable nodes to reach consensus on the total order (i.e., consistency) of transactions submitted by clients in the form of requests.

After nodes reach a consensus about the order, the data/operations of the transactions are then processed according to the order of the transactions. As a result, distributed nodes functionally behave as if there were one centralized node.

This ensures that there is only one sequence of client transactions, known as “the longest chain.” Layer 2 of blockchain is the smart contract, which is essentially software code. A smart contract provides an interface for blockchain developers to implement new functions. Smart contracts can then facilitate, verify, or enforce the execution of business transactions.

A smart contract can be viewed as a program that connects the underlying consensus protocols with layer 3, applications and use cases. Fig. 2. Overview of blockchains: categories, underlying techniques, and use cases. 2.2 Building the Hash Chain The cryptographic concepts of “hashing” and “digital signatures” provide tamper proofing and validation.

One way hash functions generate a unique output of alphanumeric text given an input of a list of transactions. Change a single thing about the list of transactions and the resulting hash is significantly different. Digital signatures like Rivest–Shamir–Adleman or Elliptic Curve Digital Signature Algorithm are used to “sign” transactions. The hashes are mym tran linked together in a chain of blocks, with any mym tran except the first one, called the genesis block, pointing to prior hashes and signatures.

Such a hash chain ensures that no one can manipulate the contents of any block or reverse the mym tran order. 2.3 Permissioned Blockchains Permissioned blockchains provide consensus and security using provably secure distributed consensus protocols.

The consensus protocols do not involve expensive procedures such as in PoW. Therefore, permissioned systems have low latency (the time between the client sending a transaction until the client receives a reply), they are also scalable (both in the number of clients and transactions as well as the number of servers) [ 112], and they consume less energy than permissionless blockchains (described in detail later).

Most permissioned blockchains, especially those widely employed or piloted by government, use provably secure BFT protocols. Among these BFT solutions, the leader-based protocols are widely used, such as PBFT [ 30] and its variants [ 106, 108]. In these types of protocols, there is a specific leader, which proposes the order of transactions. The nodes then communicate with each other in several steps to reach agreement on the order.

In most leader-based protocols, each node sends messages to all other nodes in each step and collects matching messages from a fraction of nodes before moving to the next step. If the leader is potentially faulty or malicious, other nodes will run a leader change protocol until a mym tran leader is elected. On top of the consensus protocols, blockchains have different approaches to store the transactions. Figure 3 illustrates a typical system architecture used by permissioned blockchains.

Specifically, after receiving requests from the clients, a number of nodes run a BFT protocol to assign order to the transactions. The transactions and their order are then forwarded to all other nodes in the system. Finally, the transactions are stored and processed according to that order. In this architecture, the nodes that store the transactions act as learners that passively learn the order from the consensus nodes.

Fig. 3. The normal operation for a permissioned blockchain running PBFT [ 30]. Control messages refer to the mym tran for nodes to reach a consensus. Numerous BFT protocols have been proposed in the literature [ 34, 37, 41, 43, 59, 106]. Chain-based approaches organize nodes in a logical chain where a node only needs to communicate with its previous node and its subsequent node, if any [ 42], avoiding the all-to-all communication described previously, resulting in performance improvements.

Another approach is a hybrid that combines BFT protocols, such as Aliph [ 59]. The reason Aliph takes a hybrid approach is to combine the best features from more than one BFT protocol is because there is no one-size-fits-all consensus protocol. In Aliph, the protocol can use one cheap protocol to achieve great performance with fewer failures. When failures occur or become more frequent, the system switches to another more expensive one to guarantee system security.

2.4 Permissionless Blockchains Most permissionless blockchains adopt a “Proof-of-Something” strategy. In the case of Bitcoin, this is PoW, a mathematical challenge offered to all nodes in the system to try to overcome (or work through) by an activity called mining. Once mined, a node can propose a block of transactions and get rewarded in Bitcoin if the proposal is accepted. The drawback to this approach is that throughput (the number of mym tran processed per second) is limited, and the energy consumption is high.

Furthermore, collusion occurs—nodes form cartel-like entities called mining pools—concentrating mining activity under the control of one group.

With mining pools, the blockchain becomes less decentralized and therefore less secure, and more susceptible to attack and manipulation. Compared with BFT-based consensus, PoW-based consensus does not have a fixed leader and can be viewed as a system where the leader changes after mym tran block of transactions.

To propose a new transaction, a node needs to first solve PoW from the previous transaction. When a node proposes a transaction $n$, it also generates a pseudorandom mym tran that is called a cryptographic nonce. As illustrated in Figure 4, the nonce is broadcast to all other nodes. Nodes compete to become the next leader by selecting random pending transactions and generating a hash of the selected transactions. The node that first generates a hash smaller than the nonce value is the winner and becomes the next leader.

Compared with BFT consensus, PoW-based consensus involves fewer messages for nodes to reach a consensus on the transactions. The blockchains based on it can easily scale to thousands of nodes. The challenge is that more than one node might solve the puzzle at the same time, creating a fork of the hash chain. Nodes in the PoW consensus will detect the fork, eventually agree on the longest hash chain, and use it.

It takes time for each transaction to be finalized after it mym tran been proposed, usually after six blocks, each taking about 10 minutes, in the case of Bitcoin—about an hour. This finalization time can be reduced using different approaches. Fig. 4. The message flow for PoW-based blockchains. Control messages are the messages for nodes to compete for PoW. Multiple Proof-of-Something approaches have been proposed to enhance the performance of PoW-based consensus, some of which are shown in Table 1.

The workflow usually remains the same, but the protocols use other strategies. For instance, Proof-of-Elapsed-Time replaces PoW with trusted hardware, using Intel Software Guard Extension (SGX), a trusted execution environment. Specifically, computers running an Intel SGX processor have a set of security-related instruction codes built into them that makes the piece of hardware protected.

Instead of generating a hash to solve PoW, every node utilizes SGX to wait for a random amount of time. The node that finishes waiting earlier than all other nodes “wins” and can propose new transactions. Proof-of-Elapsed Time is in use as a consensus option in the Hyperledger Sawtooth platform [ 95]. The major benefit is a mym tran improved system performance. The drawback is that each trusted execution environment has its own vulnerability, and one has to trust a single vendor to use the blockchain.

Other examples include Proof-of-Stake (PoS) and Proof-of-Authority (PoA). PoS and PoA are each designed to improve the performance of Ethereum, and in both a small group of nodes is selected as representatives. PoA selects the representatives based on their reputation, whereas PoS selects representatives using one of several approaches. In Delegated PoS, nodes can vote for certain replicas to select them as representatives.

After the group of representatives is selected, the nodes have the authority to propose new transactions and notify others of the results. The major challenge with representative-based systems is that the selected representatives must behave correctly to ensure system mym tran.

For instance, in PoA, the reputation system must be trusted, and one has to assume that malicious nodes do not have motivation to build up their reputation and then corrupt the entire system. Table 1. Permissionless Systems/Cryptocurrency and the Proof They Use to Come to a Consensus System/Cryptocurrency Proof-of-Something Strategy Bitcoin [ 87], Ethereum [ 117] Proof-of-Work Computing a nonce Ethereum-PoS, Hybrid Consensus [ 98], Elastico [ 80] Proof-of-Stake PoW with weighted value Hyperledger Sawtooth [ 95] Proof-of-Elapsed-Time PoW done by computer processors PoA Network [ 100] Proof-of-Authority PoS with weighted reputation 2.5 Smart Contracts Smart contracts are programs that automatically fire when nodes come to consensus, without any human intervention.

Smart contracts are not the normal contracts people use.

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Instead, the nodes in a blockchain are configured to check a series of conditions to see whether the triggering criteria has been met. If the requirements are met, then the nodes execute an agreed upon contract, a program that executes business-defined functions. Smart mym tran allow users to deploy new capabilities and functions while the blockchains are running; services do not have to be stopped. Specifically, developers could write a new smart contract that includes a set of functions.

After the contract is deployed on the blockchain, authorized users could call the contract to use those functions. Other running services on the blockchain do not have to be interrupted at all to support these new functions. The most popular smart contract platforms include the Ethereum Virtual Machine (written in a language called Solidity) and Hyperledger Fabric's Chaincode (written using a combination of the languages Go, node.js, and Java).

Since all blockchain transactions are included in the hash chain, and therefore unchangeable, having a bug in the contract, or a flaw that can be exploited, introduces risk into the system. It is also worth noting that the use of smart contracts will likely degrade the performance of the system, as observed by several research works [ 15, 60]. 2.6 Blockchain vs. Databases Modern databases are frequently designed to be replicated and distributed to achieve high reliability. The most typical method is primary-backup replication, in which the data are replicated as copies across multiple servers or virtual machines.

When one copy is lost, additional copies are available to continue the service. This shares certain similarities with blockchain systems, with three major differences. First, distributed databases focus mym tran the management of data. In contrast, blockchains aim to ensure data security. Second, blockchain systems aim to tolerate Byzantine/arbitrary failures, whereas distributed databases usually handle only crash failures. Third, blockchain systems aim to achieve the strongest guarantee of data consistency across multiple machines, whereas distributed databases usually only achieve weaker guarantees of data consistency, such as causal consistency [ 25].

In causal consistency, data can be written concurrently by different nodes, introducing potential conflicts to be resolved later. In comparison, blockchain systems guarantee linearizability, the strongest consistency guarantee in distributed systems [ 62].

Informally, linearizability ensures that the data are always consistent across all nodes, so the distributed nodes behave like a centralized one. 3 GOVERNMENT ADOPTION OF BLOCKCHAIN We have done a review of the known projects and use cases supported by governments across the world. Our goal is to provide a comprehensive and representative, mym tran not exhaustive, list. Our purpose is to discuss several applications that are both representative and meaningful. Indeed, with the increasing interest in blockchains, applications can be discovered in potentially all industries.

A lot of them, however, are far away from being practical or useful. Therefore, we select the representative use cases and group them by countries and regions. In this way, we will be able to better see the trend in government use cases. Government adoption of blockchain can be viewed from regulatory, consumer, and developer perspectives. As a governing body, a state may wish to monitor how blockchains are used, as in the case of cryptocurrencies.

As a user of applications, governments may use blockchains to improve processes. And in some instances, a government may develop its own blockchain-based application to address an internal need.

In this section, we review the governmental efforts made by countries worldwide in piloting blockchain solutions, the setup, and lessons learned. Since blockchains are widely used by cryptocurrencies, most of the applications reviewed were financial. In Table 2, we include other domains such as medical, infrastructure, city governance, asset and data management, and education. Table 2. Blockchain Use Cases Adopted by Governments and the Focus of Blockchain Applications Use Cases Representative Countries Focus Medical and healthcare Mym tran, United States, Switzerland, Phillippines, Japan, Brazil, etc.

Supply chain, Internet-of-Things, etc. Financial applications (Almost) All Cryptocurrencies, asset management, etc. Critical infrastructures South Korea Mym tran management, optimization, etc. Blockchain city Malaysia Cryptocurrency, data management Asset management Georgia, Sweden, Switzerland Land registry, property transactions, etc. Education Japan, Malta Certificate management Data management Phillipines, Australia Cloud data management 3.1 U.S.

Government The U.S. Health and Human Services (HHS) department has developed an application called Accelerate for management of contract billing that utilizes blockchain, AI, ML, and process automation. Accelerate is designed to better manage the HHS portfolio of 100,000 contracts worth around $25B across about 50 systems.

The blockchain within Accelerate captures a pointer to unstructured data (e.g., documents) rather than storing the data itself. Accelerate was able to get contract information dispersed across the entire bureaucracy through replication of data and became the first federal blockchain-based application to be certified by a designated approving authority, an internal senior management official, as having the authorization to operate [ 49], indicating that the system had an acceptable level of risk and may be used in government applications.

Accelerate was expanded to acquisition management—getting contract information to researchers more readily so they could find suitable materials for their research. HHS has projected savings at the point of purchase of up to $720M over time and may expand Accelerate into clinical data—HHS leadership discussed using blockchain for tracking sepsis data [ 104].

Research is being done by the U.S. Centers for Disease Control and Prevention (CDC) to use blockchain to help track public health outbreaks such as hepatitis A [ 96]. In 2017, the chief software architect for the CDC's Center for Surveillance Epidemiology mym tran Laboratory Services began building proofs of concept for improving surveillance across state lines.

Since then, the CDC and IBM have come together to work on a blockchain-backed solution for tracking the ongoing opioid disease crisis [ 83]. We assume that using blockchain to track COVID-19 is a consideration. Figure 5 shows that interest in blockchain for use in biomedical applications mym tran growing rapidly after many years of no published research. Most of these publications are mym tran theoretical research, with few discussing deployment of blockchain at the point of care.

Several discuss blockchain's tamper resistant property, as well as its distributed nature—attributes relevant for health data interoperability.

These blockchains tend to be private permissioned ones; Ethereum is studied because of its smart contract capability, and Hyperledger Fabric because it is open source and has some support from large companies such as IBM. Fig. 5. keyword “blockchain” search results January 2008 through December 2019, as of August 2020. 3.2 Asian Governments In 2019, the Filipino government approved the adoption of an Ethereum-based solution for approximately 80 rural banks to get mym tran to financial services.

Motivating the effort is the fact that only 42% of Filipinos aged 15 or older have a bank account due to a combination of factors [ 38, 120]. The concept of blockchain city has been used and made live at Malaysia's Melaka Straits city, a tourist city funded by the Chinese government. The project aims to use blockchain to track tourist visas, passengers, luggage, and booking services [ 102].

The city will also manage its own token, the DMI coin, for tourists to exchange their money into digital currencies for payment in the city via their mobile phones. South Korea's government announced a 4B Korean won (about $3.5 million) award to set up a blockchain-enabled virtual power plant in the city of Busan, the country's second-most populous city [ 97].

The power plant is to be cloud based and should integrate multiple energy resources to optimize power generation. 3.3 European Governments The European Horizon program supports blockchain projects across the European Union [ 111].

Luxembourg launched a digital Luxembourg initiative in 2017, with a focus of building a blockchain governance framework. The purpose is to build a blockchain competence community and develop blockchain governance standards; the project is ongoing. The e-Estonia program [ 45] supports mym tran features such mym tran e-identity, e-healthcare, and e-governance. Most are already operational, with 98% of Estonians filing tax declarations completed online, and 99% of their health data is digitized and stored on blockchain.

Although issues and concerns remain [ 93, 107], blockchains have indeed revolutionized the way this government stores and processes data. Countries such as Georgia and Sweden (and non-European Union countries like Switzerland) use blockchains to manage assets [ 13]. Georgia (at the juncture of Asia and Europe) has implemented blockchain for land title registry and related property transactions; the technology has helped make the process more efficient [ 105].

Sweden has also created a blockchain-based application for land registration and real estate transactions [ 78]. Blockchain in education has been applied as well [ 56, 57]. The Maltese government recently completed the first national pilot of a blockchain to manage academic credentials such as diplomas, school certificates, and transcripts.

This has been shown to improve the safety of personal information, minimize bureaucracy, and allow students to access their credentials more easily. 3.4 Others Several major Australian government departments use cloud-based blockchain solutions, or Blockchain-as-a-Service [ 88]. The Canadian government launched a pilot recently to use blockchain for digital credentials management, allowing employees to maintain a permanent, self-owned, and secure record of their digital credentials [ 77].

Anti-money laundering is another major initiative for several governments [ 69, 71, 75, 101]. Mym tran instance, the Financial Action Task Force, an intergovernmental entity, issued guidelines on virtual asset, anti-money laundering and counter-terrorist financing regulations [ 69]. It has been shown that existing approaches are effective in balancing between the threats and opportunities. Continuous monitoring and investigation are desirable as the technology rapidly changes [ 27].

4 USE CASE STUDIES Mym tran our review, we found that the majority of the announced government-supported blockchain projects do not provide enough technical details about the setup or system architecture, among others. This is in part because a lot of the projects are ongoing or in their initial phases.

In this section, we review two use cases in two sectors in detail: healthcare, and critical infrastructure. We review the use cases, present how blockchains are used in each use case, and discuss potential challenges. Note that although we focus on healthcare and critical infrastructure sectors, the use cases involve domains beyond these two, such as finance and Internet-of-Things (IoT).

Therefore, we consider only these two representative sectors and discuss the applications in detail. 4.1 Healthcare As measured by the number of articles published on PubMed, the National Institute of Health's search engine of medical references, interest in blockchain for use in biomedical applications has almost doubled year over year from 2015 to 2019. Blockchain usage in Electronic Health Records (EHRs) holds promise, with five characteristics of EHRs that must be addressed by any blockchain solution: governance, mym tran, privacy, scalability, and security [ 29, 82].

The blockchain characteristics to meet those needs include immutability, cryptography, distribution, decentralization, transparency, auditability, and nonrepudiation [ 82].

Technical solutions for guaranteeing privacy and security in biomedical blockchain applications using blockchain have been proposed, including using the cryptocurrency Ethereum along with the Onion Router for remote health monitoring [ 10], as well as a novel blockchain called Enigma, designed for exchanging EHR data [ 121]. The use of blockchain as an overarching health information exchange for protected health information to be exchanged nationally in the United States was proposed, with a theoretical blockchain-based technology to be used as a record locator service pointing to demographic copies of medical records stored in a shared set of servers called Patient Identity Brokers [ 51].

Such a design lends itself well mym tran recent regulatory changes in the United States. The 21st Cures Act not only gave patients a right to their health data but also said that healthcare systems cannot information block that data.

The legislation was made in 2016, and it was not until March 2020 that a final rule was issued by the Office of the National Coordinator detailing how the information exchange from the provider to the patient was to work. That office created a technical framework and compliance structure for enabling protected health information to flow securely as the patient directs it.

The technical framework is essentially a distributed system that lends itself to the possibility of blockchain usage, with interoperability between IT systems more possible than ever through the creation of a standardized core dataset that all participants must use.

Most of the projects in the healthcare sector focus on utilizing blockchain as a reliable platform for data sharing. Indeed, the critical nature of healthcare data makes blockchains unique in facilitating secure data sharing. However, implementing a large-scale EHR data exchange system is not easy, with or without using blockchains.

Effort from multiple disciplines and across stakeholders are mym tran to make it possible, and realizing that vision in a final system, while also maintaining security, will be challenging. Perhaps, though, now more than ever such a mym tran is possible, as the global pandemic has made it a necessity that health data flow more freely than ever before.

New technology is needed to address both public health needs and the need for the individual's medical chart to be readily accessible to the patient or provider. 4.2 Critical Infrastructures (Energy Sector) The U.S.

Department of Homeland Security defines 16 critical infrastructures including energy, food and agriculture, and transportation, among others. It is stated that “the security and mym tran advances a national policy to strengthen and maintain secure, functioning, and resilient critical infrastructure” [ 91].

Due to the nature of critical infrastructures, blockchain application in critical infrastructures has been widely explored. Besides the financial and healthcare sectors (also considered critical infrastructures), other domains such as the energy sector are under investigation by governments around the world. As an example, the U.S. Department of Energy awarded several projects to both industry and academia to create the “Energy Internet” [ 66].

The purpose is to build an advanced management framework for distributed energy resources to support fast, mym tran, and secure peer-to-peer communications. Blockchain in the energy sector can involve multiple aspects, ranging from energy trading to management of IoT devices and energy resources management [ 14, 85].

In Table 3, we present the representative areas of blockchain adoption in the energy sector. Table 3. Blockchain Use Cases in the Energy Sector Areas Blockchain Usage Benefits Energy trading Transaction management Real-time and peer-to-peer exchange Smart energy IoT management, resource management Secure asset management System protection (SCADA) Data and service protection Intrusion tolerance Energy trading, especially renewable energy trading and management, is one of the main areas found in current blockchain projects in several countries [ 35, 67, 70].

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For instance, a project supported by the Japanese Ministry of the Environment aims at building a system for measuring and managing self-consumed renewable energy mym tran 35]. The project utilizes the cryptocurrency side of the blockchain technology to build the trading system. Specifically, the self-consumed renewable energy is first converted into tradable values and sent to the blockchain network. The real-time trading prices are then calculated according to the exchange cost and demand.

When the transactions are finalized, energy can be exchanged locally without having to be transmitted to a central location. Similar methods have been used in projects from other countries such as the United States and Australia [ 67, 70] that have been shown to greatly reduce the management and trading cost.

Besides other components such as energy resource management, such a use case shares a lot of similarity with blockchains in the financial domain. In other words, the adoption of blockchain could potentially reduce the cost for any financial transaction in the energy sector and remove the need for a trusted single party. Smart energy involves the management of IoT devices and energy assets. Research projects, industrial projects, and governmental pilots have been found [ 46, 68, 92]. Such projects usually involve efforts from both industry and academia to be successful.

For instance, in the past few years, the Department of Energy announced funding for both university-led and industry-led research projects to integrate IoT technologies with the energy infrastructure, several of which focus on blockchain and IoT integration [ 47, 66].

The purpose is to provide robust and scalable infrastructure in the energy sector. This use case is similar with managing IoT assets using blockchain as used in other application mym tran, such as supply chain and healthcare. There are still many challenges, because the integration of energy-related infrastructure and blockchain software is not easy. Energy-related devices suffer from numerous physical threats since the deployment environments are heterogeneous.

Despite the challenges, such an integration can greatly benefit energy infrastructure by enhancing the security and efficiency of resource management, potentially disrupting different aspects in the energy sector such as energy sharing [ 114] and electric vehicle charging [ 72].

Another noteworthy aspect in the energy sector is the protection of mission critical systems, although the available approaches do not directly utilize the “blockchain” technology. For instance, several works focus on building intrusion-tolerant Supervisory Control and Data Acquisition (SCADA) systems [ 18, mym tran, 90], where SCADA is the core control system for the power grid.

Such approaches use BFT to build an intrusion-tolerant SCADA system. Since BFT is the core mechanism for permissioned blockchains and some hybrid blockchains, BFT-based SCADA can provide the same security guarantee with blockchains (i.e., high availability and integrity).

The benefits of using only BFT instead of blockchains include better performance and more flexible system design. 5 CHALLENGES AND CONSIDERATIONS In a 2018 report on cryptocurrencies and blockchain in Europe and Central Asia, the World Bank states that “policy makers should strike a balance between curbing the hype surrounding these new technologies and unleashing potentially transformational new opportunities. While encouraging and facilitating these innovations, they should prepare to craft new regulations to create a level playing field for new and old industries, by adjusting financial oversight, consumer protection, and tax administration” [ 58].

The following year, the European Commission summarized to the World Trade Organization's Global Trade and Blockchain Forum what the technical and legal challenges for government use of blockchain are: integration with existing systems, scalability, blockchain-to-blockchain interoperability, lack of a policy framework for cryptocurrencies, and the enforceability of smart contracts [ 2].

That lack of a framework was also mentioned at the 2019 Organisation for Economic Co-operation and Development Global Blockchain Policy Forum [ 3].

Without such a framework, the economic implications of central government usage of so-called stablecoins (e.g., Facebook's Libra) would be unpredictable and wide ranging [ 3]. The lessons learned from government adoption of blockchain range from addressing the security implications of ledger transparency through cryptography to planning for the increased costs of implementing blockchain relative to more mature technology [ 5, 63].

In this section, we review adoption challenges and how governments have faced them to encourage innovation, provide information about the technical challenges and close with mym tran brief discussion. 5.1 Adoption Challenges Industry and government adoption. Grasping the different implementations of blockchain and their capabilities pose challenges for decision makers when it comes to data governance, privacy and security regulations, and standards [ 110, 113].

To address such issues, policy makers should take time to assess the technology, look for standards to be developed, and gather experience with the technology [ 40, 113]. Academic and industrial efforts have been made to discuss and create mym tran for blockchains to be used in different domains [ 12, 40, 65]. The International Standards Organization Technical Committee 307 has published three standards, including a vocabulary, a privacy and personally identifiable protection consideration, and a smart contract overview [ 86].

Another concern is interoperability mym tran blockchains, which includes both exchanging data among blockchains and transferring assets between different blockchain systems. Both mym tran and industry efforts have been made to create such a service [ 22, 79].

Yet due to the rapidity with which blockchains types are being developed and adapted, it remains to be seen whether interoperability will create new development opportunities for developers, and—if so—whether governmental decision makers will wish to fund the work. Governmental role in adoption. Governments worldwide have started to develop policies or government strategies for the adoption of solutions [ 1, 109].

The experience that they have gained with cryptocurrencies has given them insight into how they could use mym tran to reduce transaction costs [ 52].

They may determine that it is more efficient to execute cross-border trade transactions via blockchain through cryptocurrency. The realization of such transactions, however, requires complex integration work and a conducive regulatory environment. Governments that utilize blockchain technology should partner with private firms to both encourage innovation and develop a flexible regulatory framework [ 3].

An example is the United Arab Emirates’ support of blockchain, in which they created a regulatory sandbox for technology companies to test blockchain solutions for FinTech and for streamlining data interoperability across government services [ 4].

In the United States, the Boldline Accelerator program is similar in its support of public-private collaboration and has discussed how to use blockchain for identity management, tracking human trafficking, Visas, and shipping fraud [ 39]. Public-private collaborations should focus on developing blockchain interoperability and standardization, as applied to a mym tran selected set of inefficient bureaucratic use cases [ 16, 94]. Cost of blockchain. The potential for blockchain to reduce transaction costs is appealing to government, as noted earlier [ 52].

For digital platforms, blockchain can reduce the cost to start up new marketplaces, as well as audit the validity of mym tran [ 31]. However, their decentralized nature can introduce new inefficiencies and data governance issues [ 31]. Blockchains’ strength in guaranteeing data integrity through immutability may come at a premium, relative mym tran having the same guarantee in a centralized application [ 64].

Transaction costs mym tran been found to be higher for permissionless blockchains when compared to centralized mym tran [ 13], and blockchain applications can cost significantly more to operate than a cloud-based centralized equivalent, even after controlling for cloud service utilization fees [ 103]. Data quality. Blockchain does not protect against data from untrustworthy sources, such as authorized but potentially tainted parties. It cannot prevent well-formatted but incorrect or inaccurate data from being sent and stored in the system [ 119].

As a result, blockchain may be used as an illegal content distribution channel. The system may also consist of data with low quality or high inaccuracy.

Such data quality issues might be harmful in applications where transparency of the data is desirable, especially in government applications [ 5]. Although blockchain can be used as an auditing system for validating these data, the data are already distributed and cannot be retrieved from all parties with certainty. A decentralized system that allows any two parties to anonymously exchange assets may provide a haven for those wishing to perform illicit activities without fear of reprisal.

As a result, existing solutions usually involve additional mym tran to detect or ensure data quality [ 28, 119]. It is not clear whether such a layer will mym tran the desirable analysis and become generic enough to ensure data quality. Correctness and security of the system. Several blockchain systems intentionally make their consensus protocols proprietary, making it difficult to trust in the correctness and security of the platforms [ 26].

Consensus protocols are complicated and mym tran implementation in a complex real system requires extensive development, which may introduce unintended consequences, as has been observed [ 32]. Before adopting a blockchain solution, the underlying mechanism and the system implementation should be carefully reviewed. Even though most peer-reviewed works have been carefully reviewed by experts, errors in some solutions are still found later [ 6]. Therefore, it is important to evaluate whether the implementation matches the theory and design of the blockchain.

Some efforts have been made for e-government applications [ 53]. It is yet to be seen whether the solutions are generic and useful enough to fully evaluate the systems. 5.2 Technical Challenges The performance trade-offs and blockchain standards. There is no one-size-fits-all blockchain system [ 20, 37, 59, 112].

Different approaches have been proposed to meet different needs, such mym tran improved latency, throughput, scalability, and bandwidth [ 34, 37, 42, 59]. Indeed, each protocol has mym tran trade-offs—for example, to reduce the number of messages nodes needed to exchange in the protocol, the consensus usually involves more steps to complete. In other words, such a protocol has longer latency to achieve higher throughput. Before widespread development mym tran adoption, some innovative first movers must implement solutions that consider the trade-offs among security, efficiency, and robustness.

Although significant effort has been put into developing new blockchain platforms, it is not easy to develop both correct and efficient systems. In fact, developing consensus protocols is like engineering cryptographic systems, which require expertise in cryptography, security, and the theory of distributed systems [ 26]. Therefore, expert review, validation of both the theory and implementation of new blockchain platforms, and standards recommendation [ 65] (e.g., cryptocurrency exchanges, running blockchains in applications like clinical trials, etc.) are desirable if the full potential of blockchain is to be realized.

Scalability. Scalability can be interpreted as the number of nodes and the number of clients. The number of nodes is a concern during blockchain deployment—how many nodes should one use to start the service? The number of clients is a concern for the workload—how many requests should one expect and what are the sizes of the requests? Both permissionless and permissioned blockchains have scalability limits [ 112].

The open nature of the consensus mechanisms of permissionless blockchains allows anyone to join and therefore usually involves thousands of nodes. The problem for such blockchains is that they usually suffer from long transaction latency (where it takes longer for the transaction to be available) and have not scaled to many client transactions in real-world applications. However, permissioned blockchains can scale to a large number of clients with less latency, but they rely upon a small number of blockchain servers.

Hybrid blockchains address the scalability problem [ 8, 50, 73, 74, 80, 98, 118], but each has its own challenges and most have a sufficiently large number of representatives to guarantee correctness of the system (safety and liveness)—for example, greater than 600 [ 80].

A BFT protocol of such a size, however, can be impractical. Other BFT algorithms, such as the cryptocurrency Algorand [ 55], remove the need to run PoW by applying proven cryptographic techniques along with verifiable random functions and committees, but, again, have a limitation.

Algorand relies upon the number of coins, which might limit its practicality in real-world deployment. The optimal blockchain that balances scalability for both clients and servers has yet to be found. Privacy and compliance. Privacy and compliance are always major concerns in governmental applications. Although conventional blockchains provide availability and integrity, the data are essentially transparent—all participants may freely review transactions.

This means that an architect mym tran be careful in selecting the type of blockchain and perform a use case analysis that includes privacy and security guarantees relative to performance needs. With the current regulatory climate of governments focused on protecting user data, blockchains become especially problematic given their open and immutable nature.

At the same time, laws designed to safeguard the privacy and security of individuals’ information do provide a roadmap for designers. Generalized examples include the California Consumer Privacy Act of 2018 and the European Union's General Data Protection Regulation of 2016. In the healthcare space, the Health Insurance Portability and Accountability Act and the Health Information Technology for Economic and Clinical Health Act are the basis for interoperability rule changes proposed by the Office of the National Coordinator, as well as the Centers for Medicare and Medicaid Services.

With the preceding two acts in mind, researchers at MIT built a PoW consensus protocol called Medrec for mining patient information. This type of clinical data is becoming standardized through the implementation of EHR systems that leverage mym tran protocols, such as Health Level 7 [ 17].

Timing assumptions. Most permissionless blockchains assume a synchronous network (i.e., all replicas know the message transmission time), which is not a practical assumption. For the system to be correct (safety and liveness), there must be a large number of nodes that actively participate.

Therefore, the correctness of such a system in a small-scale or private setting can be questionable. However, most permissioned blockchain protocols assume something called partial synchrony [ 44], in which the network delay and processing delay by the nodes are bounded by an upper limit unknown to all nodes. It is assumed that each node in the network will eventually respond, and if a given node does not respond, other nodes will handle it according to the protocol, providing an answer and ensuring that the network will not get stuck waiting indefinitely.

The shortcoming of this approach is that it introduces performance and security issues—what if an adversary can somehow manipulate this network delay in such a way that causes nodes to misbehave or to give up information? In this type of network, the system may simply stop processing any requests just like a crashed service, even if all nodes in the system are correct.

A potential solution to this may be the use of what is known as a purely asynchronous BFT consensus protocol, in which nodes have no upper bound in response time so the protocols are resilient to all kinds of attacks. Research into this area is ongoing and includes several possible solutions [ 9, 43, 81, 84]. 5.3 Discussion Permissioned vs. permissionless. Most of the government blockchain implementations are permissioned.

Although some use permissionless blockchain, in these cases the blockchain is still deployed in a closed, private setting. Many of the countries we studied for blockchain adoption have either banned or regulated cryptocurrencies, which are fully permissionless.

We conclude that the development of cryptocurrencies by governments is unlikely unless the adoption is a specialized use case such as critical infrastructure.

Even in such a case, the blockchain would likely be tethered to the currency of the given nation-state. The quest for high performance. We have not found any published results that have measured performance, or assessed the performance needs, in government blockchain implementations. Many applications are new, and the long-term feasibility will depend upon a cost-benefit analysis.

Many use cases involve large volumes of data, however, so we expect scalability and throughput needs for these systems to drive changes to their blockchain implementation. The quest for technology improvement. We have found little information regarding the feedback or lessons learned based upon government blockchain implementations.

mym tran

We believe this to be in part because most projects are still in their early stages. We advocate for research and industry to continue to collaborate and improve systems based upon our observations of past successes [ 26, 32]. Cryptocurrencies regulation. Many countries have developed regulations for cryptocurrencies, and yet no country has fully determined how to implement the regulations. Part of the challenge is how to classify cryptocurrencies using existing financial constructs.

Taxing or regulating a cryptocurrency as a currency, a security, or an mym tran is difficult, as a cryptocurrency can be any one or all three. 6 FUTURE OF BLOCKCHAIN AND CONCLUSION Blockchains have evolved beyond cryptocurrencies to general purpose and can be used across an array of applications, particularly those that need high service availability and data integrity. If their adoption increases, then blockchain-based solutions may reintroduce a trusted broker: the data center, whether in the cloud or on premise.

A cloud-based blockchain system makes the cloud mym tran into a new type of trusted broker. If instead nodes are on premise but are used by the public, then whatever entity is hosting them mym tran the trusted broker, and the system becomes vulnerable to any failures that may render the entire system unreliable. Therefore, replacing a fallible human or bureaucracy with a blockchain may shift risk rather than eliminate it [ 23].

The technical challenges for blockchains, such as being fully privacy preserving, ensuring compliance when necessary, and being scalable, have yet to be fully solved, and more work is needed to address them.

Yet despite these challenges, blockchains can make applications better and will begin to be the solution for use case–specific distributed systems problems. Most blockchain applications were financial at first, just as many good and proven technologies have been. Blockchains are now being used in other spaces, such as government. They may be the best technology to deploy when a need to distribute data through a system that needs to guarantee data integrity and service availability exists, but the ability to make it happen is limited.

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2015. Enigma: Decentralized computation platform with guaranteed privacy. arXiv:1506.03471 Footnote This research was supported by an NSF Workshop supplement to NSF award 1747724, Phase I IUCRC UMBC: Center for Accelerated Real-Time Analytics (CARTA). For J. Li, this activity was supported by a U.S. Department of Homeland Security (DHS) American Association for the Advancement of Science (AAAS) Fellowship, sponsored by DHS and administered by the Oak Ridge Institute for Science and Education (ORISE) for the DOE under contract number DE-SC0014664.

All opinions expressed in this article are those of the authors and do not necessarily reflect the policies and views of DHS, ORAU, or ORISE. Authors’ addresses: J. Clavin, S. Duan, H. Zhang, V. P. Janeja, K. P. Joshi, mym tran Y. Yesha, University of Maryland, Baltimore County, 1000 Hilltop Circle, Baltimore, MD 21250; emails:,,,,,; L.

C. Erickson, American Association for the Advancement of Science, 1200 New York Ave, NW Washington, DC 20005; email:; J.

D. Li, Department of Homeland Security, Science and Technology Directorate, 245 Murray Lane, SW Washington, DC 20528; email: Permission to make digital or hard copies of all or part of this work for personal or classroom use is granted without fee provided that copies are not made or distributed for profit or commercial advantage and that copies bear this notice and the full citation on the first page.

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