This document summarizes and discusses different types of blockchain technologies. It begins by explaining Bitcoin's blockchain and then discusses alternative cryptocurrency blockchains using various consensus algorithms like proof-of-work, proof-of-stake, etc. It also covers private and permissioned blockchains being developed by companies for financial applications. Key debates discussed include permissioned vs permissionless blockchains, and whether banks should build on top of Bitcoin or develop private alternatives. The conclusion is that cyberpunk supporters favor scaling Bitcoin, while banks favor private blockchains, so both approaches will likely continue.

1. ブロックチェーンの整理
と展望
10/1/2015
日本デジタルマネー協会
本間善実

2. 概観
・ビットコインのブロックチェーン（ザ・ブロックチェーン）
・アルトコインのブロックチェーン / レジャー
・コインなしのブロックチェーン / レジャー

3. ビットコインのザ・ブロックチェーン
・最も旧く強固なブロックチェーン
・2009年〜2015年 ダウンタイムゼロ ストレステストに耐え続け、現在に至る
・パブリックレジャー（公開台帳）とも言われる
・分散型データベース
・レジャーとチェーンの違い（チェーンは階層構造で、レジャーは最新レジャーに全ての情報を記載？）
・PoW
・Counterparty on top of Bitcoin Blockchain = permissionless on permissionless
・sidechains = permissioned or premissionless on permissionless

4. アルトコインのブロックチェーン
・独自コインを持つ
・PoW: ライトコイン、ドッジコイン、モナコイン
・PoS: NXT
・DPoS: Bitshares
・Hybrid of PoW and PoS?: Ethereum
・PoI: NEM
・Orb: smartcoin（=JPY）

5. Bitcoin & Ethereum
・特徴は、センサーシップレジスタント
・つまり、検閲に耐える
・完全な分散型

6. コインなしのプライベートチェーン/レジャー
・Tim Swansonがリード
・報酬としてのコインがないので、パブリックは困難
・Eris（Ethereumのフォーク）
・Hyperledger（Digital Asset Holdings）
・R3DEV（Tim Swanson, IBM, etc.）
・peernova（これはchain同様にビットコイン→プライベートという文脈で要研究）
・chain
・mijin（NEMのプライベート版）

7. コインありのプライベートレジャー
・Ripple：時々止まったり、フォークしたり
・Stellar：同様

8. プライベート 対 パブリック
・プライベート（VPN接続、暗号処理）にすると、Proof of 〜 が不要
・よってグローバルコンセンサスが速い
・ノードはプライベートで、アクセスもプライベート（DAH等の選択肢）
・ノードはプライベートだが、アクセスはパブリック（セミプライベート？）

9. クラサバとプライベートチェーンの違いは？
・クライアント・サーバーの場合、サーバー冗長化で攻撃耐性だが高価
・ビットコインのブロックチェーンはグローバルに分散化、かつマイナー同士で競争のた
め、外からの攻撃に強い
・ビットコインのマイナーの動機は報酬としてのビットコイン
・プライベートチェーンのインセンティブは、システムの構築・利用価値
・プライベートチェーンはクライアントサーバーよりも有利？
・プライベートチェーンは外部からの攻撃に耐える？

10. プライベートチェーン側の主張
・クラサバと比較すると格安でグローバル対応可能
・ダウンタイムなし
・24/7/365を格安でやれる
・安くなるだけではなく、ビジネス機会も増える
・スマートコントラクト
・プログラマブルマネー

11. 用語に関して
・レジャー ＝ チェーン（微妙に異なる模様）
・パーミションド ＝ プライベート
・パーミッションレス ＝ パブリック
・トラストレス ＝ 分散型

12. MIT Technology Review
http://www.technologyreview.
com/news/541686/banks-embrace-bitcoins-heart-
but-not-its-soul/?
utm_content=bufferc4430&utm_medium=social&utm
_source=twitter.com&utm_campaign=buffer

13. Chain ビットコイン → プライベート
Visa, Nasdaq, Citi, CapitalOne, fiserv., orangeから$30M出資
Chain was founded in early 2014 to provide software for the nascent Bitcoin industry. But the company
decided to switch to developing private blockchain technology after meeting with executives from Wall
Street companies who made it clear that Bitcoin was not suited to large-scale financial services, says
Ludwin.
For one, Bitcoin was designed to primarily support transactions in Bitcoin over the Internet, but Ludwin
found that financial companies didn’t care much for the currency. They were interested in better ways
to move their existing assets around. Another problem was the volume and speed of transactions.
Bitcoin’s design currently supports only roughly seven transactions per second. On average, it
takes a new transaction 10 minutes to be added to the blockchain. And the total value of all
Bitcoins today, underpinned by its blockchain, is $3.4 billion, a small figure to large banks.

14. Barry Silvert ザ・ブロックチェーン
“We’re likely going to see years of exploration and talking and some preliminary
products with companies like Chain, but it’ll be a very narrow use case,” says
Silbert. He predicts that in coming years Bitcoin will rise to become a recognized
store of value of something like gold, and that innovation in its design and services
built on top will see the original, public blockchain become an underpinning for
financial services of all kinds. “Eventually Wall Street will come to appreciate
that the Bitcoin blockchain is the most secure and most flexible and can
solve a lot of the issues that they have,” he says.

15. Chain
Ludwin also thinks Bitcoin will survive, but he says it will only represent a fraction
of the value of less-radically open blockchains. Nakamoto’s invention will persist
as a kind of backstop that offers a way for people unable to use or trust more
conventional financial systems to move money around, he says. “The
encroachment of other asset classes is actually bad for that; we need Bitcoin to
thrive at that level,” he says. “The long-term vision is where we have many
blockchains that are interoperable, as opposed to a single chain where
everything is wedged in.”

16. Tim Swanson
・アンチビットコインの最右翼で、R3のmarketing director
http://www.ofnumbers.com/2015/06/27/the-distributed-ledger-landscape-who-is-
developing-shared-replicated-ledgers-and-why/

17. Nick Szabo
“[Bank] bureaucracies are so heavily invested in the expertise and importance of local regulations and standards that it’s
extremely difficult for them to cut the Gordian knot and implement seamless global systems,” said Szabo. “So they keep
trying to re-inject points of control, and thus points of vulnerability, into blockchains, e.g. through ‘permissioning’; but this
nullifies their main benefits, which come from removing points of vulnerability.”
On the contrary, according to Szabo, the banks should embrace the crowd-sourced power and resiliency of
permissionless blockchains like Bitcoin.
https://bitcoinmagazine.com/21883/nick-szabo-permissioned-blockchains-block-
size/

18. Nick Szabo
If banks were to embrace permissionless blockchains they could participate as well or better than the newcomers, Szabo told IBTimes UK.
"But their bureaucracies are so heavily invested in the expertise and importance of local regulations and standards that it's extremely difficult for them to cut the Gordian knot and
implement seamless global systems.
"So they keep trying to re-inject points of control, and thus points of vulnerability, into blockchains, e.g. through 'permissioning'; but this nullifies their main benefits, which come from
removing points of vulnerability."
Szabo reiterated the point: to remove vulnerability banks also have to remove individual human control and the individuals in charge or with root access. Banks naturally hate that loss to
their power. But they don't have any choice if they want to gain the benefits of having an army of independent computers that rigorously, constantly and securely check each others' work,
he said.
Szabo pointed out that proper financial controls are already somewhat decentralised, thanks to a "human blockchain" of accountants, auditors, etc. checking each others' work.
"There are half a dozen or so different entities involved when you do a stock trade on Wall Street, again checking each others' work. You typically see full vulnerability to unaccountable
third parties only in pathological situations where naive newcomers try to do money on the centralised web, as with Mt.Gox.
"But the traditional 'human blockchain' is very labor-intensive and very local — each is based on local regulations and customs and thus splits the world up into mutually untrusting
national silos. Permissionless blockchains cut through that like Alexander cutting through the Gordian knot."
http://www.ibtimes.co.uk/nick-szabo-if-banks-want-benefits-blockchains-they-must-go-permissionless-1518874

19. Nick Szabo
Nick Szabo: If banks want benefits of blockchains they must
go permissionless
ビットコインは失敗する点ないのが、すごい＝内部監査、外部監査不要
失敗する可能性ある点があると攻撃され得る
http://www.ibtimes.co.uk/nick-szabo-if-banks-want-benefits-blockchains-they-
must-go-permissionless-1518874

20. Barry Silvert
"I'm surprised they honed in on that first. A private
federated blockchain doesn't solve any major problems
and ultimately I don't have a high level of optimism it's
going to succeed. I'm surprised they haven't figured that
out yet and I'm surprised they are doing that first."
http://www.coindesk.com/barry-silbert-private-
blockchains-will-capitulate-to-bitcoin/

21. 結論
・サイファーパンク 対 巨大投資銀行
・サイファーパンク（Nick Szabo, Adam Back, Greg Maxwell and etc.）はビットコインを
スケールさせる方向
・金融業界はプライベートチェーンを開発する方向
・よって、ビットコインを育てる方向と、
・mijinをプロモートの両貼りで、進めます
・イノヴェーションのジレンマで、ビットコイン有利と予想中