To realize digital cash you need a payment network with accounts, balances, and transaction. That‘s easy to understand. One major problem every payment network has to solve is to prevent the so-called double spending: to prevent that one entity spends the same amount twice. Usually, this is done by a central server who keeps record about the balances.
*Offer valid for one new Individual, Joint or IRA TD Ameritrade account opened by 12/31/2018 and funded within 60 calendar days of account opening with $3,000 or more. To receive $100 bonus, account must be funded with $25,000-$99,999. To receive $300 bonus, account must be funded with $100,000-$249,999. To receive $600 bonus, account must be funded with $250,000 or more. Offer is not valid on tax-exempt trusts, 401k accounts, Keogh plans, profit sharing plan, or money purchase plan. Offer is not transferable and not valid with internal transfers, TD Ameritrade Institutional accounts, accounts managed by TD Ameritrade Investment Management, LLC, current TD Ameritrade accounts or with other offers. Accounts funded with $3,000 or more are eligible for up to 500 commission-free trade internet equity, ETF, or option trades executed within 60 calendar days of account funding. All other trade types are excluded from this offer. Contract, exercise, and assignment fees still apply. No credit will be given for unexecuted trades. Limit one offer per client. Account value of the qualifying account must remain equal to, or greater than, the value after the net deposit was made (minus any losses due to trading or market volatility or margin debit balances) for 12 months, or TD Ameritrade may charge the account for the cost of the offer at its sole discretion. TD Ameritrade reserves the right to restrict or revoke this offer at any time. This is not an offer or solicitation in any jurisdiction where we are not authorized to do business. Please allow 3-5 business days for any cash deposits to post to account. Taxes related to TD Ameritrade offers are your responsibility. All promotional items and cash received during the calendar year will be included on your consolidated Form 1099. Please consult a legal or tax advisor for the most recent changes to the U.S. tax code and for rollover eligibility rules. (Offer Code: 220)
Researchers have pointed out at a "trend towards centralization". Although bitcoin can be sent directly to the bitcoin network, in practice intermediaries are widely used.[31]:220–222 Bitcoin miners join large mining pools to minimize the variance of their income.[31]:215, 219–222[108]:3[109] Because transactions on the network are confirmed by miners, decentralization of the network requires that no single miner or mining pool obtains 51% of the hashing power, which would allow them to double-spend coins, prevent certain transactions from being verified and prevent other miners from earning income.[110] As of 2013 just six mining pools controlled 75% of overall bitcoin hashing power.[110] In 2014 mining pool obtained 51% hashing power which raised significant controversies about the safety of the network. The pool has voluntarily capped their hashing power at 39.99% and requested other pools to act responsibly for the benefit of the whole network.[111]

Cryptocurrencies are a potential tool to evade economic sanctions for example against Russia, Iran, or Venezuela. In April 2018, Russian and Iranian economic representatives met to discuss how to bypass the global SWIFT system through decentralized blockchain technology.[56] Russia also secretly supported Venezuela with the creation of the petro (El Petro), a national cryptocurrency initiated by the Maduro government to obtain valuable oil revenues by circumventing US sanctions.
In January 2009, the bitcoin network was created when Nakamoto mined the first block of the chain, known as the genesis block.[19][20] Embedded in the coinbase of this block was the following text: "The Times 03/Jan/2009 Chancellor on brink of second bailout for banks."[10] This note has been interpreted as both a timestamp and a comment on the instability caused by fractional-reserve banking.[21]:18