Adding implicit account creation (#71)

* Adding implicit account creation

* Update docs

* Update comment

Co-authored-by: Evgeny Kuzyakov <ek@nearprotocol.com>

specs/DataStructures/Account.md

@@ -12,12 +12,15 @@ NEAR Protocol has an account names system. Account ID is similar to a username.
 - maximum length is 64
 - **Account ID** consists of **Account ID parts** separated by `.`
 - **Account ID part** consists of lowercase alphanumeric symbols separated by either `_` or `-`.
+- **Account ID** that is 64 characters long consists of lowercase hex characters is a specific **implicit account ID**.
 
 Account names are similar to a domain names.
-Anyone can create a top level account (TLA) without separators, e.g. `near`.
+Top level account (TLA) like `near`, `com`, `eth` can only be created by `registrar` account (see next section for more details).
 Only `near` can create `alice.near`. And only `alice.near` can create `app.alice.near` and so on.
 Note, `near` can NOT create `app.alice.near` directly.
 
+Additionally, there is an implicit account creation path. Account ids, that are 64 character long, can only be created with `AccessKey` that matches account id via `hex` derivation. Allowing to create new key pair - and the sender of funds to this account to actually create an account.
+
 Regex for a full account ID, without checking for length:
 
 ```regex
@@ -86,6 +89,29 @@ me@google.com // @ is not allowed (it was allowed in the past)
 abcdefghijklmnopqrstuvwxyz.abcdefghijklmnopqrstuvwxyz.abcdefghijklmnopqrstuvwxyz
 ```
 
+
+## Implicit account IDs
+
+Implicit accounts work similarly to Bitcoin/Ethereum accounts.
+It allows you to reserve an account ID before it's created by generating a ED25519 key-pair locally.
+This key-pair has a public key that maps to the account ID. The account ID is a lowercase hex representation of the public key.
+ED25519 Public key is 32 bytes that maps to 64 characters account ID.
+
+Example: public key in base58 `BGCCDDHfysuuVnaNVtEhhqeT4k9Muyem3Kpgq2U1m9HX` will map to an account ID `98793cd91a3f870fb126f66285808c7e094afcfc4eda8a970f6648cdf0dbd6de`.
+
+The corresponding secret key allows you to sign transactions on behalf of this account once it's created on chain.
+
+### Implicit account creation
+
+An account with implicit account ID can only be created by sending a transaction/receipt with a single `Transfer` action to the implicit account ID receiver:
+- The account will be created with the account ID.
+- The account will have a new full access key with the ED25519-curve public key of `decode_hex(account_id)` and nonce `0`.
+- The account balance will have a transfer balance deposited to it.
+
+This account can not be created using `CreateAccount` action to avoid being able to hijack the account without having the corresponding private key.
+
+Once an implicit account is created it acts as a regular account until it's deleted.
+
 ## Account
 
 [account]: #account

specs/GenesisConfig/RuntimeFeeConfig/ActionCreationConfig.md

@@ -38,6 +38,10 @@ _type: Fee_
 
 Base cost of making a transfer.
 
+NOTE: If the account ID is an implicit account ID (64-length hex account ID), then the cost of the transfer fee
+will be `transfer_cost + create_account_cost + add_key_cost.full_access_cost`.
+This is needed to account for the implicit account creation costs.
+
 ## stake_cost
 
 _type: Fee_

specs/RuntimeSpec/Actions.md

@@ -35,6 +35,8 @@ for the rest of the actions until `DeleteAccount`. This gives permission by anot
 pub struct CreateAccountAction {}
 ```
 
+If `receiver_id` has length == 64, this account id is considered to be `hex(public_key)`, meaning creation of account only succeeds if followed up with `AddKey(public_key)` action.
+
 **Outcome**:
 - creates an account with `id` = `receiver_id`
 - sets Account `storage_usage` to `account_cost` (genesis config)