# Data Model¶

The BTC-Relay, as opposed to Bitcoin SPV clients, only stores a subset of information contained in block headers and does not store transactions. Specifically, only data that is absolutely necessary to perform correct verification of block headers and transaction inclusion is stored.

## Types¶

An 80 bytes long Bitcoin blockchain header.

## Constants¶

The interval in number of blocks at which Bitcoin adjusts its difficulty (approx. every 2 weeks = 2016 blocks).

### TARGET_TIMESPAN¶

Expected duration of the different adjustment interval in seconds, 1209600 seconds (two weeks) in the case of Bitcoin.

### TARGET_TIMESPAN_DIVISOR¶

Auxiliary constant used in Bitcoin’s difficulty re-target mechanism.

### UNROUNDED_MAX_TARGET¶

The maximum difficulty target, $$2^{224}-1$$ in the case of Bitcoin. For more information, see the Bitcoin Wiki.

### MAIN_CHAIN_ID¶

Identifier of the Bitcoin main chain tracked in the ChainsIndex mapping. At any point in time, the BlockChain with this identifier is considered to be the main chain and will be used to transaction inclusion verification.

### STABLE_BITCOIN_CONFIRMATIONS¶

Global security parameter (typically referred to as k in scientific literature), determining the umber of confirmations (in blocks) necessary for a transaction to be considered “stable” in Bitcoin. Stable thereby means that the probability of the transaction being excluded from the blockchain due to a fork is negligible.

### STABLE_PARACHAIN_CONFIRMATIONS¶

Global security parameter (typically referred to as k in scientific literature), determining the umber of confirmations (in blocks) necessary for a transaction to be considered “stable” in the BTC Bridge. Stable thereby means that the probability of the transaction being excluded from the blockchain due to a fork is negligible.

Note

We use this to enforce a minimum delay on Bitcoin block header acceptance in the BTC-Bridge in cases where a (large) number of block headers are submitted as a batch.

## Structs¶

Representation of a Bitcoin block header, as stored in the 80 byte byte representation in the Bitcoin block chain (contains no additional metadata - see RichBlockHeader). This struct is only used for parsing the 80 byte block header - not for storage!

Note

Fields marked as [Optional] are not critical for the secure operation of BTC-Relay, but can be stored anyway, at the developers discretion. We omit these fields in the rest of this specification.

Parameter Type Description
merkleRoot byte32 Root of the Merkle tree referencing transactions included in the block.
target u256 Difficulty target of this block (converted from nBits, see Bitcoin documentation.).
timestamp timestamp UNIX timestamp indicating when this block was mined in Bitcoin.
hashPrevBlock byte32 Block hash of the predecessor of this block.
. . .
version i32 [Optional] Version of the submitted block.
nonce u32 [Optional] Nonce used to solve the PoW of this block.

Note

Fields marked as [Optional] are not critical for the secure operation of BTC-Relay, but can be stored anyway, at the developers discretion. We omit these fields in the rest of this specification.

Parameter Type Description
blockhash bytes32 Bitcoin’s double SHA256 PoW block hash
blockHeight u32 Height of this block in the Bitcoin main chain.
chainRef u32 Pointer to the BlockChain struct in which this block header is contained.
blockHeader BlockHeader Associated parsed BlockHeader struct

### BlockChain¶

Representation of a Bitcoin blockchain / fork.

Parameter Type Description
chainId U256 Unique identifier for faster lookup in ChainsIndex
chain Map<U256,H256> Mapping of blockHeight to blockHash, which points to a RichBlockHeader entry in BlockHeaders.
startHeight U256 Starting/lowest block height in the chain mapping. Used to determine the forking point during chain reorganizations.
maxHeight U256 Max. block height in the chain mapping. Used for ordering in the Chains priority queue.
noData Vec<U256> List of block heights in chain referencing block hashes of RichBlockHeader entries in BlockHeaders which have been flagged as noData by Staked Relayers.
invalid Vec<U256> List of block heights in chain referencing block hashes of RichBlockHeader entries in BlockHeaders which have been flagged as invalid by Staked Relayers.

## Data Structures¶

Mapping of <blockHash, RichBlockHeader>, storing all verified Bitcoin block headers (fork and main chain) submitted to BTC-Relay.

### Chains¶

Priority queue of BlockChain elements, ordered by maxHeight (descending). The BlockChain entry with the most significant maxHeight value (i.e., topmost element) in this mapping is considered to be the Bitcoin main chain.

The exact choice of data structure is left to the developer. We recommend to use a heap, which allows re-balancing (changing the priority/order of items while in the heap). Specifically, we require the following operations to be available:

• max … returns the item with the maximum value (as used for sorting).
• insert … inserts a new item, maintaining ordering in relation to other items.
• delete … removes an item.
• find … returns an item with a given index (by sorting key and stored value).
• update … [Optional] modifies the sorting key of an item and updates ordering if necessary (incrementing maxHeight of a BlockChain entry). Can be implemented using delete and insert.

Attention

If two BlockChain entries have the same maxHeight, do not change ordering!

Note

The assumption for Chains is that, in the majority of cases, block headers will be appended to the main chain (longest chain), i.e., the BlockChain entry at the most significant position in the queue/heap. Similarly, transaction inclusion proofs (verifyTransactionInclusion) are only checked against the main chain. This means, in the average case lookup complexity will be O(1). Furthermore, block headers can only be appended if they (i) have a valid PoW and (ii) do not yet exist in BlockHeaders - hence, spamming is very costly and unlikely. Finally, blockchain forks and re-organizations occur infrequently, especially in Bitcoin. In principle, optimizing lookup costs should be prioritized, ideally O(1), while inserting of new items and re-balancing can even be O(n).

### ChainsIndex¶

Auxiliary mapping of BlockChain structs to unique identifiers, for faster read access / lookup <U256, BlockChain>,

### BestBlock¶

32 byte Bitcoin block hash (double SHA256) identifying the current blockchain tip, i.e., the RichBlockHeader with the highest blockHeight in the BlockChain entry, which has the most significant height in the Chains priority queue (topmost position).

Note

Bitcoin uses SHA256 (32 bytes) for its block hashes, transaction identifiers and Merkle trees. In Substrate, we hence use H256 to represent these hashes.

### BestBlockHeight¶

Integer representing the maximum block height (height) in the Chains priority queue. This is also the blockHeight of the RichBlockHeader entry pointed to by BestBlock.

### ChainCounter¶

Integer increment-only counter used to track existing BlockChain entries. Initialized with 1 (0 is reserved for MAIN_CHAIN_ID).