- Tokens on blockchains can represent currency, assets, equity, utilities and more. The ERC20 standard defines a common interface for fungible tokens on Ethereum. This allows any ERC20 token to be used interchangeably. - To create an ERC20 token, a smart contract is written following the ERC20 interface which includes functions like totalSupply, balanceOf, transfer, approve and more. This contract manages token ownership and transfers. - Tokens can be used for fundraising through an ICO where a project sells tokens in exchange for ether. The ERC20 standard is commonly used to issue tokens on Ethereum for these types of crowdsales.

1. Tokens
Based on:
Mastering Ethereum: Building Smart Contracts and DApps

2. What Is a Token?
• The term is commonly used to refer to privately issued
special-purpose coin-like items of insignificant intrinsic
value (
‫حقيقيه‬ ‫قيمه‬ ‫بدون‬
) , such as transportation tokens,
laundry tokens, and arcade game tokens.
• Physical versions of tokens are often restricted to specific
businesses, organizations, or locations.
• There are not easily exchangeable and typically have only
one function.

3. Blockchain Token
• The restrictions of physical tokens are not exist any more
• Blockchain tokens serve multiple purposes and can be traded for each
other or for other currencies on global liquid markets
• Generally speaking, a token represents something specific in a given
ecosystem
• This could be could economic value, a dividend, a voting right… really anything.
• Token is not limited to one particular role; it can fulfill various different of roles
in its native ecosystem.
• Tokens are a great fundraising mechanism

4. How Tokens Are Used
• Tokens can be programmed to serve many different functions
• Currency
• A token can serve as a form of currency, with a value determined through private trade.
• Resource
• A token can represent a resource earned or produced in a sharing economy or resource-
sharing environment; for example, a storage or CPU token representing resources that can be
shared over a network
• Asset
• A token can represent ownership of an intrinsic or extrinsic, tangible or intangible asset; for
example, gold, real estate, a car, oil, energy, etc.
• Access
• A token can represent access rights and grant access to a digital or physical property, such as a
discussion forum, an exclusive website, a hotel room, or a rental car.
• Equity (
‫الملكيه‬ ‫حقوق‬
)
• A token can represent shareholder equity in a digital organization
• e.g. Decentralized Autonomous Organization (DAO), placing decision-making power into the
hands of an automated system and a crowdsourced process.

5. How Tokens Are Used
• Voting
• A token can represent voting rights in a digital or legal system.
• Collectible
• A token can represent a digital collectible (e.g., CryptoPunks) or physical collectible (e.g., a
painting).
• Identity
• A token can represent a digital identity (e.g., avatar) or legal identity (e.g. national ID).
• Attestation
• A token can represent a certification or attestation of fact by some authority or by a
decentralized reputation system (e.g., marriage record, birth certificate, college degree).
• Utility
• A token can be used to access or pay for a service.

6. Tokens and Fungibility(‫التبادل‬ ‫)قابليه‬
• Tokens are fungible when we can substitute any single unit of the token for
another without any difference in its value or function.
• If a token’s historical provenance can be tracked, then it is not entirely
fungible
• The ability to track provenance can lead to blacklisting and whitelisting, reducing or
eliminating fungibility.
• Non-fungible tokens are tokens that each represent a unique tangible or
intangible item and therefore are not interchangeable
• E.g . a token that represents ownership of a specific Van Gogh painting is not
equivalent to another token that represents a Picasso, even though they might be
part of the same “art ownership token” system
• Each non-fungible token is associated with a unique identifier, such as a
serial number.

7. Tokens and Intrinsicality
• Tokens that represent intrinsic assets do not carry additional counterparty risk
• If you hold the keys for a CryptoKitty, there is no other party holding that CryptoKitty for you—you own
it directly
• The blockchain consensus rules apply and your ownership (i.e., control) of the private keys is equivalent
to ownership of the asset, without any intermediary
• Conversely, many tokens are used to represent extrinsic things, such as real estate, and gold
bars
• The ownership of these items, which are not “within” the blockchain, is governed by law, and policy,
separate from the consensus rules that govern the token
• Token issuers and owners may still depend on real-world non-smart contracts
• These extrinsic assets carry additional counterparty risk because they are held by custodians, recorded in
external registries, or controlled by laws and policies outside the blockchain environment.
• Blockchain tokens may be used to convert extrinsic assets into intrinsic assets and thereby
remove counterparty risk
• E.g. A good example is moving from equity in a corporation (extrinsic) to an equity or voting token in a
DAO or similar (intrinsic) organization

8. Using Tokens: Utility or Equity
• Almost all projects in Ethereum today launch with some kind of token.
• Tokens are used one of two ways: either as “utility tokens” or as “equity tokens.”
• Utility tokens are those where the use of the token is required to gain
access to a service, application, or resource
• E.g. tokens that represent resources such as shared storage, or access to services
such as social media networks.
• Equity tokens are those that represent shares in the control or ownership
of something, such as a startup
• E.g nonvoting shares for distribution of dividends and profits, or voting shares in a
decentralized autonomous organization, where complex governance system based
on votes by the token holders.

9. Tokens on Ethereum
• Tokens are different from ether because the Ethereum protocol does not
know anything about them
• Sending ether is an intrinsic action of the Ethereum platform, but sending
or even owning tokens is not
• The ether balance of Ethereum accounts is handled at the protocol level,
whereas the token balance of Ethereum accounts is handled at the smart
contract level
• In order to create a new token on Ethereum, you must create a new smart
contract
• Smart contract handles everything, including ownership, transfers, and access rights
• You can write your smart contract to perform all the necessary actions any way you
want, but it is probably wisest to follow an existing standard.

10. An Initial Coin Offering (ICO)
• Is a crowdfunding mechanism used by companies and organizations
to raise money by selling tokens.
• The term is derived from Initial Public Offering (IPO), which is the
process by which a public company offers shares for sale to investors
on a stock exchange
• Unlike the highly regulated IPO markets, ICOs are open, global, and
messy

11. The ERC20 Token Standard
• Ethereum Request for Comments (ERC)
• The vast majority of tokens are currently based on the ERC20 standard
• ERC20 is a standard for fungible tokens
• ERC20 token are interchangeable and have no unique properties.
• It defines a common interface for contracts implementing a token, such
that any compatible token can be accessed and used in the same way.
• The interface consists of a number of functions that must be present in every
implementation of the standard

12. ERC20 required functions and events (1/2)
• totalSupply
• Returns the total units of this token that currently exist
• ERC20 tokens can have a fixed or a variable supply.
• balanceOf
• Given an address, returns the token balance of that address.
• transfer
• Given an address and amount, transfers that amount of tokens to that address,
from the balance of the address that executed the transfer.
• transferFrom
• Given a sender, recipient, and amount, transfers tokens from one account to
another
• Used in combination with approve.

13. ERC20 required functions and events (2/2)
• approve
• Given a recipient address and amount, authorizes that address to execute several
transfers up to that amount, from the account that issued the approval.
• allowance
• Given an owner address and a spender address, returns the remaining amount
that the spender is approved to withdraw from the owner.
• Transfer
• Event triggered upon a successful transfer (call to transfer or transferFrom) (even
for zero-value transfers).
• Approval
• Event logged upon a successful call to approve.

14. ERC20 optional functions
• name
• Returns the human-readable name (e.g., “US Dollars”) of the token.
• symbol
• Returns a human-readable symbol (e.g., “USD”) for the token.
• decimals
• Returns the number of decimals used to divide token amounts
• E.g, if decimals is 2, then the token amount is divided by 100 to get its user
representation.

15. The ERC20 interface defined in Solidity
contract ERC20 {
function totalSupply() constant returns (uint theTotalSupply);
function balanceOf(address _owner) constant returns (uint balance);
function transfer(address _to, uint _value) returns (bool success);
function transferFrom(address _from, address _to, uint _value) returns (bool success);
function approve(address _spender, uint _value) returns (bool success);
function allowance(address _owner, address _spender) constant returns (uint remaining);
event Transfer(address indexed _from, address indexed _to, uint _value);
event Approval(address indexed _owner, address indexed _spender, uint _value);
}

16. ERC20 data structures
• It uses two data structures that is implemented with a data mapping
• One to track balances
• This allows the token contract to keep track of who owns the tokens
• Each transfer is adeduction from one balance and an addition to another balance
mapping(address => uint256) balances;
• And the other to track allowances
• An owner of a token can delegate authority to a spender, allowing them to spend a
specific amount (allowance) from the owner’s balance
mapping (address => mapping (address => uint256)) public allowed;

17. ERC20 two different workflows
“transfer” and “approve & transferFrom”
• transfer
• Is used by wallets to send tokens to other wallets.
• If Alice wants to send 10 tokens to Bob, her wallet sends a transaction to the token
contract’s address, calling the transfer function with Bob’s address and 10 as the
arguments.
• The token contract adjusts Alice’s balance (–10) and Bob’s balance (+10) and issues a
Transfer event.
• approve followed , transferFrom
• Are used to delegate control to another address for distribution of tokens
• E.g, if a company is selling tokens for an ICO, they can approve a crowdsale contract
address to distribute a certain amount of tokens. The crowdsale contract can then
transferFrom the token contract owner’s balance to each buyer of the token

18. “approve & transferFrom” workflows

19. “approve & transferFrom” workflows
• If Alice wants to allow the AliceICO contract to sell 50% of all the AliceCoin
tokens to buyers like Bob and Charlie.
• First, Alice launches the AliceCoin ERC20 contract, issuing all the AliceCoin to her own
address.
• Then, Alice launches the AliceICO contract that can sell tokens for ether.
• Next, Alice initiates the approve & transferFrom workflow.(How?)
• She sends a transaction to the AliceCoin contract, calling approve with the address of the AliceICO contract and
50% of the totalSupply as arguments.
• This will trigger the Approval event. Now, the AliceICO contract can sell AliceCoin.
• When the AliceICO contract receives ether from Bob, it needs to send some AliceCoin to Bob in return.
• AliceICO contract calls the AliceCoin transfer From function, it sets Alice’s address as the sender and Bob’s
address as the recipient
• The AliceCoin contract transfers the balance from Alice’s address to Bob’s address and triggers a Transfer event.
• The AliceICO contract can call transferFrom an unlimited number of times, as long as it doesn’t exceed the
approval limit Alice set.
• The AliceICO contract can keep track of how many AliceCoin tokens it can sell by calling the allowance function.

20. 4) Bob send ether and request tokens
(100 token for example)
5) AliceICO Call transfereFrom where,
address of AliceCoinERC20 as _from
address of Bob as _to
100 token as _value
contract AliceICO{
// This contract will be delegated by Alice to distribute tokens …..
function RequestTokens() public payable {}…..
}
2) Deploy AliceICO contract
contract AliceCoinERC20{
constructor(){
//Assigne all tokens to Alice }
…….
…….
function totalSupply() constant
returns (uint theTotalSupply);
function balanceOf(address _owner) constant
returns (uint balance);
function transfer(address _to, uint _value)
returns (bool success);
function approve(address _spender, uint _value) returns
(bool success);
function allowance(address _owner, address _spender) constant
returns (uint remaining);
function transferFrom(address _from, address _to, uint _value)
returns (bool success);
}
1) Deploy the contract and
Assign all tokens to Alice

21. https://github.com/ConsenSys/Tok
ens/blob/master/contracts/eip20/
EIP20.sol
ERC20 Implementation example