A blockchain has three key elements: 1) A contract that specifies how individuals can interact with the blockchain and their obligations, 2) An immutable history of all valid transactions within the contract, and 3) Cryptographic encoding of the contract and proofs of compliance that can be verified but keep details private.

1. A blockchain has three elements
1. A contract . Every blockchain is based on one or more
contracts. If more than one, the contracts should be
logically connected.
2. An immutable history of valid transactions within the
contract.
3. A cryptographic encoding of the contract and how one
can prove an assertion about compliance with the
contract and how one can verify that proof.
What is a Blockchain?
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2. A blockchain contract
• A contract in a blockchain is a specification of how
individuals or entities can interact with the blockchain
and their obligations and rewards from this interaction.
• Contracts can be
• explicit or implicit
• hard coded into the platform or programmable
• Examples
• Bitcoin is hard coded and implicit
• Zero cash is hard coded and explicit
• Ethereum is programmable and some portions are implicit and
the rest are defined by the Ethereum program
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3. An immutable history
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• Immutable public ledger
• Time stamped transactions
• Audit trail of what happened
• Distributed and replicated

4. Cryptography
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contract Key
Generator
Prover key
Verifier key
Comments:
• Everything is public to
users of the blockchain
except the internals of
the key generator
• Figure might be slightly
different when a
different theoretical
basis is used. This
figure is based on
Quadratic Span
Programs
• All details of the provers
assertion such as
who/what/when can be
kept secret.
• There may be elements
of the contract outside
of the prover’s control.
E.g. Bitcoin consensus
process
Prover – I have
complied with all
elements of the
contract under my
control
Compliance
Token
Compliance
Token
Verifier – Prover
has (has not)
complied

5. Other Blockchain attributes
• Public/private. Public blockchains are available to
everyone on the internet. Private blockchains are
available to stakeholders of the blockchain.
• On chain/off chain. In some blockchains, all computations
are on chain and are a portion of the history. On others,
some computations are moved off chain and are not a
portion of the history.
• Permissions/no permissions. Are permissions required to
access the blockchain?
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