`ICRC-3`: Block Log

Status
Accepted

ICRC-3 is a standard for accessing the block log of a Ledger on the Internet Computer.

ICRC-3 specifies:

A way to fetch the archive nodes of a Ledger
A generic format for sharing the block log without information loss. This includes the fields that a block must have
A mechanism to verify the block log on the client side to allow downloading the block log via query calls
A way for new standards to define new transactions types compatible with ICRC-3

Archive Nodes

The Ledger must expose an endpoint icrc3_get_archives listing all the canisters containing its blocks.

Block Log

The block log is a list of blocks where each block contains the hash of its parent (phash). The parent of a block i is block i-1 for i>0 and null for i=0.

   ┌─────────────────────────┐          ┌─────────────────────────┐
   |         Block i         |          |         Block i+1       |
   ├─────────────────────────┤          ├─────────────────────────┤
◄──| phash = hash(Block i-1) |◄─────────| phash = hash(Block i)   |
   | ...                     |          | ...                     |
   └─────────────────────────┘          └─────────────────────────┘

Value

The candid format supports sharing information even when the client and the server involved do not have the same schema (see the Upgrading and subtyping section of the candid spec). While this mechanism allows to evolve services and clients independently without breaking them, it also means that a client may not receive all the information that the server is sending, e.g. in case the client schema lacks some fields that the server schema has.

This loss of information is not an option for ICRC-3. The client must receive the same exact data the server sent in order to verify it. Verification is done by hashing the data and checking that the result is consistent with what has been certified by the server.

For this reason, ICRC-3 introduces the Value type which never changes:

type Value = variant {
    Blob : blob;
    Text : text;
    Nat : nat;
    Int : int;
    Array : vec Value;
    Map : vec record { text; Value };
};

Servers must serve the block log as a list of Value where each Value represent a single block in the block log.

Value Hash

ICRC-3 specifies a standard hash function over Value.

This hash function should be used by Ledgers to calculate the hash of the parent of a block and by clients to verify the downloaded block log.

The hash function is the representation-independent hashing of structured data used by the IC:

the hash of a Blob is the hash of the bytes themselves
the hash of a Text is the hash of the bytes representing the text
the hash of a Nat is the hash of the leb128 encoding of the number
the hash of an Int is the hash of the sleb128 encoding of the number
the hash of an Array is the hash of the concatenation of the hashes of all the elements of the array
the hash of a Map is the hash of the concatenation of all the hashed items of the map sorted lexicographically. A hashed item is the tuple composed by the hash of the key and the hash of the value.

Pseudocode for representation independent hashing of Value, together with test vectors to check compliance with the specification can be found here.

Blocks Verification

The Ledger MUST certify the last block (tip) recorded. The Ledger MUST allow to download the certificate via the icrc3_get_tip_certificate endpoint. The certificate follows the IC Specification for Certificates. The certificate is comprised of a tree containing the certified data and the signature. The tree MUST contain two labelled values (leafs):

last_block_index: the index of the last block in the chain. The values must be expressed as leb128
last_block_hash: the hash of the last block in the chain

Clients SHOULD download the tip certificate first and then download the block backward starting from last_block_index and validate the blocks in the process.

Validation of block i is done by checking the block hash against

if i + 1 < len(chain) then the parent hash phash of the block i+1
otherwise the last_block_hash in the tip certificate.

Generic Block Schema

An ICRC-3 compliant Block

MUST be a Value of variant Map
MUST contain a field phash: Blob which is the hash of its parent if it has a parent block
SHOULD contain a field btype: String which uniquely describes the type of the Block. If this field is not set then the block type falls back to ICRC-1 and ICRC-2 for backward compatibility purposes

Interaction with other standards

Each standard that adheres to ICRC-3 MUST define the list of block schemas that it introduces. Each block schema MUST:

extend the Generic Block Schema
specify the expected value of btype. This MUST be unique accross all the standards. An ICRC-x standard MUST use namespacing for its op identifiers using the following scheme of using the ICRC standard's number as prefix to the name followed by an operation name that must begin with a letter:

op = icrc_number op_name
icrc_number = nonzero_digit *digit
nonzero_digit = "1" / "2" / "3" / "4" / "5" / "6" / "7" / "8" / "9"
digit = "0" / nonzero_digit
op_name = a-z *(a-z / digit / "_" / "-")

For instance, 1xfer is the identifier of the ICRC-1 transfer operation.

Supported Standards

An ICRC-3 compatible Ledger MUST expose an endpoint listing all the supported block types via the endpoint icrc3_supported_block_types. The Ledger MUST return only blocks with btype set to one of the values returned by this endpoint.

ICRC-1 and ICRC-2 Block Schema

ICRC-1 and ICRC-2 use the tx field to store input from the user and use the external block to store data set by the Ledger. For instance, the amount of a transaction is stored in the field tx.amt because it has been specified by the user, while the time when the block was added to the Ledger is stored in the field ts because it is set by the Ledger.

A generic ICRC-1 or ICRC-2 Block:

it MUST contain a field ts: Nat which is the timestamp of when the block was added to the Ledger
if the operation requires a fee and if the tx field doesn't specify the fee then it MUST contain a field fee: Nat which specifies the fee payed to add this block to the Ledger
its field tx
1. CAN contain a field op: String that uniquely defines the type of operation
2. MUST contain a field amt: Nat that represents the amount
3. MUST contain the fee: Nat field for operations that require a fee if the user specifies the fee in the request. If the user does not specify the fee in the request, then this field is not set and the top-level fee is set.
4. CAN contain the memo: Blob field if specified by the user
5. CAN contain the ts: Nat field if the user sets the created_at_time field in the request.

Operations that require paying a fee: Transfer, and Approve.

The type of a generic ICRC-1 or ICRC-2 Block is defined by either the field btype or the field tx.op. The first approach is preferred, the second one exists for backward compatibility. If both are specified then btype defines the type of the block regardless of tx.op.

icrc3_supported_block_types should always return all the btypes supported by the Ledger even if the Ledger doesn't support the btype field yet. For example, if the Ledger supports mint blocks using the backward compatibility schema, i.e. without btype, then the endpoint icrc3_supported_block_types will have to return "1mint" among the supported block types.

Account Type

ICRC-1 Account is represented as an Array containing the owner bytes and optionally the subaccount bytes.

Burn Block Schema

the btype field MUST be "1burn" or tx.op field MUST be "burn"
it MUST contain a field tx.from: Account

Example with btype:

variant { Map = vec {
    record { "btype"; "variant" { Text = "1burn" }};
    record { "phash"; variant {
        Blob = blob "\a1\a9p\f5\17\e5\e2\92\87\96(\c8\f1\88iM\0d(tN\f4-~u\19\88\83\d8_\b2\01\ec"
    }};
    record { "ts"; variant { Nat = 1_701_108_969_851_098_255 : nat }};
    record { "tx"; variant { Map = vec {
        record { "amt"; variant { Nat = 1_228_990 : nat } };
        record { "from"; variant { Array = vec {
                variant { Blob = blob "\00\00\00\00\020\00\07\01\01" };
                variant { Blob = blob "&\99\c0H\7f\a4\a5Q\af\c7\f4;\d9\e9\ca\e5 \e3\94\84\b5c\b6\97/\00\e6\a0\e9\d3p\1a" };
        }}};
        record { "memo"; variant { Blob = blob "\82\00\83x\223K7Bg3LUkiXZ5hatPT1b9h3XxJ89DYSU2e\19\07\d0\00"
        }};
    }}};
}};

Example without btype:

variant { Map = vec {
    record { "phash"; variant {
        Blob = blob "\a1\a9p\f5\17\e5\e2\92\87\96(\c8\f1\88iM\0d(tN\f4-~u\19\88\83\d8_\b2\01\ec"
    }};
    record { "ts"; variant { Nat = 1_701_108_969_851_098_255 : nat }};
    record { "tx"; variant { Map = vec {
        record { "op"; variant { Text = "burn" } };
        record { "amt"; variant { Nat = 1_228_990 : nat } };
        record { "from"; variant { Array = vec {
                variant { Blob = blob "\00\00\00\00\020\00\07\01\01" };
                variant { Blob = blob "&\99\c0H\7f\a4\a5Q\af\c7\f4;\d9\e9\ca\e5 \e3\94\84\b5c\b6\97/\00\e6\a0\e9\d3p\1a" };
        }}};
        record { "memo"; variant { Blob = blob "\82\00\83x\223K7Bg3LUkiXZ5hatPT1b9h3XxJ89DYSU2e\19\07\d0\00"
        }};
    }}};
}};

Mint Block Schema

the btype field MUST be "1mint" or the tx.op field MUST be "mint"
it MUST contain a field tx.to: Account

Example with btype:

variant { Map = vec {
    record { "btype"; "variant" { Text = "1mint" }};
    record { "ts"; variant { Nat = 1_675_241_149_669_614_928 : nat } };
    record { "tx"; variant { Map = vec {
        record { "amt"; variant { Nat = 100_000 : nat } };
        record { "to"; variant { Array = vec {
                variant { Blob = blob "Z\d0\ea\e8;\04*\c2CY\8b\delN\ea>]\ff\12^. WGj0\10\e4\02" };
        }}};
    }}};
}};

Example without btype:

variant { Map = vec {
    record { "ts"; variant { Nat = 1_675_241_149_669_614_928 : nat } };
    record { "tx"; variant { Map = vec {
        record { "op"; variant { Text = "mint" } };
        record { "amt"; variant { Nat = 100_000 : nat } };
        record { "to"; variant { Array = vec {
                variant { Blob = blob "Z\d0\ea\e8;\04*\c2CY\8b\delN\ea>]\ff\12^. WGj0\10\e4\02" };
        }}};
    }}};
}};

Transfer and Transfer From Block Schema

the btype field MUST be
1. "2xfer" for icrc2_transfer_from blocks
2. "1xfer" for icrc1_transfer blocks
if btype is not set then tx.op field MUST be "xfer"
it MUST contain a field tx.from: Account
it MUST contain a field tx.to: Account
it CAN contain a field tx.spender: Account

Example with btype:

variant { Map = vec {
    record { "btype"; "variant" { Text = "1xfer" }};
    record { "fee"; variant { Nat = 10 : nat } };
    record { "phash"; variant { Blob =
        blob "h,,\97\82\ff.\9cx&l\a2e\e7KFVv\d1\89\beJ\c5\c5\ad,h\5c<\ca\ce\be"
    }};
    record { "ts"; variant { Nat = 1_701_109_006_692_276_133 : nat } };
    record { "tx"; variant { Map = vec {
        record { "amt"; variant { Nat = 609_618 : nat } };
        record { "from"; variant { Array = vec {
                variant { Blob = blob "\00\00\00\00\00\f0\13x\01\01" };
                variant { Blob = blob "\00\00\00\00\00\00\00\00\00\00\00\00\00\00\00\00\00\00\00\00\00\00\00\00\00\00\00\00\00\00\00\00" };
        }}};
        record { "to"; variant { Array = vec {
            variant { Blob = blob " \ef\1f\83Zs\0a?\dc\d5y\e7\ccS\9f\0b\14a\ac\9f\fb\f0bf\f3\a9\c7D\02" };
            variant { Blob = blob "\00\00\00\00\00\00\00\00\00\00\00\00\00\00\00\00\00\00\00\00\00\00\00\00\00\00\00\00\00\00\00\00" };
        }}};
    }}};
}};

Example without btype:

variant { Map = vec {
    record { "fee"; variant { Nat = 10 : nat } };
    record { "phash"; variant { Blob =
        blob "h,,\97\82\ff.\9cx&l\a2e\e7KFVv\d1\89\beJ\c5\c5\ad,h\5c<\ca\ce\be"
    }};
    record { "ts"; variant { Nat = 1_701_109_006_692_276_133 : nat } };
    record { "tx"; variant { Map = vec {
        record { "op"; variant { Text = "xfer" } };
        record { "amt"; variant { Nat = 609_618 : nat } };
        record { "from"; variant { Array = vec {
                variant { Blob = blob "\00\00\00\00\00\f0\13x\01\01" };
                variant { Blob = blob "\00\00\00\00\00\00\00\00\00\00\00\00\00\00\00\00\00\00\00\00\00\00\00\00\00\00\00\00\00\00\00\00" };
        }}};
        record { "to"; variant { Array = vec {
            variant { Blob = blob " \ef\1f\83Zs\0a?\dc\d5y\e7\ccS\9f\0b\14a\ac\9f\fb\f0bf\f3\a9\c7D\02" };
            variant { Blob = blob "\00\00\00\00\00\00\00\00\00\00\00\00\00\00\00\00\00\00\00\00\00\00\00\00\00\00\00\00\00\00\00\00" };
        }}};
    }}};
}};

Approve Block Schema

the btype field MUST be "2approve" or tx.op field MUST be "approve"
it MUST contain a field tx.from: Account
it MUST contain a field tx.spender: Account
it CAN contain a field tx.expected_allowance: Nat if set by the user
it CAN contain a field tx.expires_at: Nat if set by the user

Example with btype:

variant { Map = vec {
    record { "btype"; "variant" { Text = "2approve" }};
    record { "fee"; variant { Nat = 10 : nat } };
    record { "phash"; variant {
        Blob = blob ";\f7\bet\b6\90\b7\ea2\f4\98\a5\b0\60\a5li3\dcXN\1f##2\b5\db\de\b1\b3\02\f5"
    }};
    record { "ts"; variant { Nat = 1_701_167_840_950_358_788 : nat } };
    record { "tx"; variant { Map = vec {
        record { "amt"; variant { Nat = 18_446_744_073_709_551_615 : nat } };
        record { "from"; variant { Array = vec {
                variant { Blob = blob "\16c\e1\91v\eb\e5)\84:\b2\80\13\cc\09\02\01\a8\03[X\a5\a0\d3\1f\e4\c3{\02" };
        }}};
        record { "spender"; variant { Array = vec {
            variant { Blob = blob "\00\00\00\00\00\e0\1dI\01\01" };
        }}};
    }}};
}}};

Example without btype:

variant { Map = vec {
    record { "fee"; variant { Nat = 10 : nat } };
    record { "phash"; variant {
        Blob = blob ";\f7\bet\b6\90\b7\ea2\f4\98\a5\b0\60\a5li3\dcXN\1f##2\b5\db\de\b1\b3\02\f5"
    }};
    record { "ts"; variant { Nat = 1_701_167_840_950_358_788 : nat } };
    record { "tx"; variant { Map = vec {
        record { "op"; variant { Text = "approve" } };
        record { "amt"; variant { Nat = 18_446_744_073_709_551_615 : nat } };
        record { "from"; variant { Array = vec {
                variant { Blob = blob "\16c\e1\91v\eb\e5)\84:\b2\80\13\cc\09\02\01\a8\03[X\a5\a0\d3\1f\e4\c3{\02" };
        }}};
        record { "spender"; variant { Array = vec {
            variant { Blob = blob "\00\00\00\00\00\e0\1dI\01\01" };
        }}};
    }}};
}}};

Specification

`icrc3_get_blocks`

type Value = variant {
    Blob : blob;
    Text : text;
    Nat : nat; // do we need this or can we just use Int?
    Int : int;
    Array : vec Value;
    Map : vec record { text; Value };
};

type GetArchivesArgs = record {
    // The last archive seen by the client.
    // The Ledger will return archives coming
    // after this one if set, otherwise it
    // will return the first archives.
    from : opt principal;
};

type GetArchivesResult = vec record {
    // The id of the archive
    canister_id : principal;

    // The first block in the archive
    start : nat;

    // The last block in the archive
    end : nat;
};

type GetBlocksArgs = vec record { start : nat; length : nat };

type GetBlocksResult = record {
    // Total number of blocks in the block log
    log_length : nat;

    // Blocks found locally to the Ledger
    blocks : vec record { id : nat; block: Value };

    // List of callbacks to fetch the blocks that are not local
    // to the Ledger, i.e. archived blocks
    archived_blocks : vec record {
        args : GetBlocksArgs;
        callback : func (GetBlocksArgs) -> (GetBlocksResult) query;
    };
};

service : {
    icrc3_get_archives : (GetArchivesArgs) -> (GetArchivesResult) query;
    icrc3_get_blocks : (GetBlocksArgs) -> (GetBlocksResult) query;
};

`icrc3_get_tip_certificate`

// See https://internetcomputer.org/docs/current/references/ic-interface-spec#certification
type DataCertificate = record {

  // Signature of the root of the hash_tree
  certificate : blob;

  // CBOR encoded hash_tree
  hash_tree : blob;
};

service : {
  icrc3_get_tip_certificate : () -> (opt DataCertificate) query;
};

`icrc3_supported_block_types`

service : {
    icrc3_supported_block_types : () -> (vec record { block_type : text; url : text }) query;
};

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`ICRC-3`: Block Log

Archive Nodes

Block Log

Value

Value Hash

Blocks Verification

Generic Block Schema

Interaction with other standards

Supported Standards

ICRC-1 and ICRC-2 Block Schema

Account Type

Burn Block Schema

Mint Block Schema

Transfer and Transfer From Block Schema

Approve Block Schema

Specification

`icrc3_get_blocks`

`icrc3_get_tip_certificate`

`icrc3_supported_block_types`

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ICRC-3: Block Log

Archive Nodes

Block Log

Value

Value Hash

Blocks Verification

Generic Block Schema

Interaction with other standards

Supported Standards

ICRC-1 and ICRC-2 Block Schema

Account Type

Burn Block Schema

Mint Block Schema

Transfer and Transfer From Block Schema

Approve Block Schema

Specification

icrc3_get_blocks

icrc3_get_tip_certificate

icrc3_supported_block_types

`ICRC-3`: Block Log

`icrc3_get_blocks`

`icrc3_get_tip_certificate`

`icrc3_supported_block_types`