Consensus Layer - Bitcoin Core

Overview

The consensus layer defines the rules that determine which blocks and transactions are valid in Bitcoin. These rules must be followed by all nodes to maintain network consensus. Bitcoin Core implements these rules in a modular fashion, with consensus-critical code isolated in specific libraries. Primary locations:

src/consensus/ - Core consensus validation
src/script/interpreter.cpp - Script execution
src/validation.cpp - Block and transaction validation
libbitcoin_consensus - Consensus library

Consensus Architecture

Consensus Parameters

Chain Parameters

Defined in src/consensus/params.h:

struct Params {
    uint256 hashGenesisBlock;
    int nSubsidyHalvingInterval;        // 210,000 blocks
    
    // Soft fork activation heights
    int BIP34Height;                     // Coinbase height
    int BIP65Height;                     // OP_CHECKLOCKTIMEVERIFY
    int BIP66Height;                     // Strict DER signatures
    int CSVHeight;                       // OP_CHECKSEQUENCEVERIFY
    int SegwitHeight;                    // Segregated Witness
    
    // Proof-of-work parameters
    uint256 powLimit;                    // Maximum target
    int64_t nPowTargetSpacing;          // 10 minutes
    int64_t nPowTargetTimespan;         // 2 weeks
    
    // Deployment tracking
    std::array<BIP9Deployment, MAX_VERSION_BITS_DEPLOYMENTS> vDeployments;
};

Network-Specific Values

Mainnet:

Target block time: 600 seconds (10 minutes)
Difficulty adjustment: Every 2016 blocks
Subsidy halving: Every 210,000 blocks
Initial subsidy: 50 BTC
Maximum supply: ~21 million BTC

Testnet:

Same rules as mainnet
Minimum difficulty blocks allowed
Different genesis block

Regtest:

Instant mining (difficulty = 1)
No peers required
Customizable parameters

Block Consensus Rules

Block Header Validation

bool CheckProofOfWork(uint256 hash, unsigned int nBits, const Consensus::Params& params)

Rules:

Hash < Target: Block hash must be below difficulty target
Timestamp: Not more than 2 hours in the future
Version: Recognized version number
Previous block: Must reference known block

Block Structure Rules

Size and weight limits:

static const unsigned int MAX_BLOCK_SERIALIZED_SIZE = 4'000'000; // 4MB
static const unsigned int MAX_BLOCK_WEIGHT = 4'000'000;          // 4M weight units

Weight calculation:

weight = base_size * 3 + total_size

Base size: Transaction size without witness data
Total size: Complete transaction size including witness

Witness scale factor:

static const unsigned int WITNESS_SCALE_FACTOR = 4;

Coinbase Transaction

Requirements:

First transaction in block must be coinbase
Exactly one coinbase per block
Coinbase input must have:
- Previous output: null hash (0x00…00)
- Output index: 0xFFFFFFFF
- Script sig: 2-100 bytes (includes height per BIP34)

Coinbase output value:

CAmount GetBlockSubsidy(int nHeight, const Consensus::Params& consensusParams) {
    int halvings = nHeight / consensusParams.nSubsidyHalvingInterval;
    if (halvings >= 64) return 0;
    
    CAmount nSubsidy = 50 * COIN;
    nSubsidy >>= halvings;  // Divide by 2^halvings
    return nSubsidy;
}

Total value constraint:

coinbase_output_value ≤ subsidy + transaction_fees

Transaction Consensus Rules

Transaction Structure

class CTransaction {
    int32_t nVersion;
    std::vector<CTxIn> vin;
    std::vector<CTxOut> vout;
    uint32_t nLockTime;
    // Witness data (not in txid hash)
};

Basic Transaction Rules

Version: Currently 1 or 2 (version 3 for TRUC)
Inputs: At least 1 input (except coinbase)
Outputs: At least 1 output
Output values: Each output ≥ 0, total < 21M BTC
Size: ≤ 400,000 weight units

Input Validation

class CTxIn {
    COutPoint prevout;    // (txid, vout) of output being spent
    CScript scriptSig;    // Spending script
    uint32_t nSequence;   // Sequence number (for timelock)
    CScriptWitness scriptWitness;  // Witness data (SegWit)
};

Rules:

Referenced output must exist in UTXO set
Not already spent (no double-spends)
Script validation passes
Sequence/locktime constraints satisfied

Output Validation

class CTxOut {
    CAmount nValue;       // Amount in satoshis
    CScript scriptPubKey; // Locking script
};

Rules:

Value ≥ 0
Value ≤ 21M BTC (consensus limit)
Sum of outputs ≤ sum of inputs (for non-coinbase)

Script Validation

Script Types

Bitcoin supports multiple script types:

1. Pay to Public Key Hash (P2PKH)

scriptPubKey: OP_DUP OP_HASH160 <pubKeyHash> OP_EQUALVERIFY OP_CHECKSIG
scriptSig:    <signature> <pubKey>

2. Pay to Script Hash (P2SH - BIP16)

scriptPubKey: OP_HASH160 <scriptHash> OP_EQUAL
scriptSig:    <data> ... <redeemScript>

Activation: Block 173,805 (April 1, 2012) Validation:

Verify scriptSig + scriptPubKey
Extract redeemScript from scriptSig
Verify redeemScript execution

3. Pay to Witness Public Key Hash (P2WPKH - BIP141)

scriptPubKey: OP_0 <20-byte-hash>
witness:      <signature> <pubkey>

Benefits:

Fixes transaction malleability
More efficient (lower fees)
Enables Lightning Network

4. Pay to Witness Script Hash (P2WSH - BIP141)

scriptPubKey: OP_0 <32-byte-hash>
witness:      <data> ... <witnessScript>

5. Pay to Taproot (P2TR - BIP341)

scriptPubKey: OP_1 <32-byte-pubkey>
witness:      <signature>  (key path)
         OR:  <data> ... <script> <control_block>  (script path)

Features:

Schnorr signatures (BIP340)
MAST (Merkelized Alternative Script Trees)
Tapscript (BIP342)
Better privacy and efficiency

Script Interpreter

Script execution in src/script/interpreter.cpp:

bool EvalScript(std::vector<std::vector<unsigned char>>& stack,
                const CScript& script,
                unsigned int flags,
                const BaseSignatureChecker& checker,
                SigVersion sigversion,
                ScriptError* serror)

Process:

Execute scriptSig → produces stack
Execute scriptPubKey with that stack
Top stack element must be true
For P2SH/P2WSH: Additional script execution

Signature Validation

ECDSA (Legacy and SegWit v0)

bool CheckSignatureEncoding(const std::vector<unsigned char>& vchSig,
                           unsigned int flags,
                           ScriptError* serror)

Requirements:

DER encoding (BIP66)
Low-S values (BIP146)
Proper SIGHASH type
Valid secp256k1 signature

Schnorr (Taproot/SegWit v1)

bool VerifySchnorrSignature(Span<const unsigned char> sig,
                           const XOnlyPubKey& pubkey,
                           const uint256& sighash)

Benefits:

64-byte signatures (vs ~72 for ECDSA)
Batch verification
Key aggregation (MuSig2)
Provable security

Soft Fork Mechanisms

Buried Deployments (BIP90)

Soft forks activated at specific heights:

enum BuriedDeployment : int16_t {
    DEPLOYMENT_HEIGHTINCB,  // BIP34 - Coinbase height
    DEPLOYMENT_CLTV,        // BIP65 - CHECKLOCKTIMEVERIFY  
    DEPLOYMENT_DERSIG,      // BIP66 - Strict DER signatures
    DEPLOYMENT_CSV,         // BIP68/112/113 - CHECKSEQUENCEVERIFY
    DEPLOYMENT_SEGWIT,      // BIP141/143/147 - Segregated Witness
};

These are “buried” because they activated long ago and are now enforced unconditionally.

BIP9 Version Bits

Flexible soft fork deployment:

struct BIP9Deployment {
    int bit;                    // Version bit (0-28)
    int64_t nStartTime;        // Earliest activation
    int64_t nTimeout;          // Latest activation
    int min_activation_height; // Minimum height
    uint32_t period;           // Signal period (2016 blocks)
    uint32_t threshold;        // Activation threshold (1916/2016 = 95%)
};

States:

enum class ThresholdState {
    DEFINED,    // Before start time
    STARTED,    // Signaling period active
    LOCKED_IN,  // Threshold met, waiting for activation
    ACTIVE,     // Rules enforced
    FAILED,     // Timeout without activation
};

Taproot activation (BIP341/342):

Bit: 2
Start time: April 24, 2021
Timeout: August 11, 2021
Activation height: 709,632 (November 2021)
Status: ACTIVE

Implemented BIPs

Bitcoin Core implements numerous BIPs (Bitcoin Improvement Proposals):

Consensus Changes

BIP	Description	Version	Status
BIP16	Pay to Script Hash (P2SH)	v0.6.0	Active since 2012
BIP30	Duplicate txid prevention	v0.6.0	Active since 2012
BIP34	Block height in coinbase	v0.7.0	Buried
BIP65	OP_CHECKLOCKTIMEVERIFY	v0.12.0	Buried
BIP66	Strict DER signatures	v0.10.0	Buried
BIP68	Relative lock-time	v0.12.1	Buried
BIP112	OP_CHECKSEQUENCEVERIFY	v0.12.1	Buried
BIP113	Median time-past	v0.12.1	Buried
BIP141	Segregated Witness (consensus)	v0.13.0	Buried
BIP143	Transaction signature verification (SegWit)	v0.13.0	Buried
BIP147	NULLDUMMY	v0.13.1	Buried
BIP340	Schnorr signatures	v0.21.0	Active
BIP341	Taproot	v0.21.0	Active
BIP342	Tapscript	v0.21.0	Active
BIP431	TRUC (v3 transactions)	v28.0	Active

Network Protocol

BIP	Description	Version
BIP14	Protocol version and user agent	v0.6.0
BIP31	Pong message	v0.6.1
BIP35	Mempool message	v0.7.0
BIP130	Direct headers announcement	v0.12.0
BIP133	feefilter message	v0.13.0
BIP152	Compact block relay	v0.13.0
BIP155	addrv2 message	v0.21.0
BIP324	Version 2 P2P transport	v26.0
BIP339	Wtxid relay	v0.21.0

Wallet and Addresses

BIP	Description	Version
BIP32	Hierarchical Deterministic Wallets	v0.13.0
BIP39	Mnemonic code (external implementation)	-
BIP44	Multi-account hierarchy	v0.21.0
BIP49	Derivation for P2WPKH-nested-in-P2SH	v0.21.0
BIP84	Derivation for P2WPKH	v0.21.0
BIP86	Derivation for P2TR	v23.0
BIP173	Bech32 address format	v0.16.0
BIP174	Partially Signed Bitcoin Transactions (PSBT)	v0.17.0
BIP350	Bech32m address format	v22.0
BIP371	Taproot fields for PSBT	v24.0
BIP380-385	Output script descriptors	v0.17.0
BIP386	tr() descriptors	v22.0

Consensus-Critical Code

Isolation Strategy

Consensus code is carefully isolated:

// src/consensus/
validation.h      // Validation state classes
tx_check.cpp      // Transaction structure checks
tx_verify.cpp     // Transaction input verification  
merkle.cpp        // Merkle tree computation
params.h          // Consensus parameters

Design principles:

No external dependencies (except crypto)
Deterministic behavior
No floating-point arithmetic
Careful integer overflow handling
Identical behavior across platforms

libbitcoin_consensus

Extractable consensus library:

// Public API
bitcoinconsensus_verify_script(
    const unsigned char *scriptPubKey,
    unsigned int scriptPubKeyLen,
    const unsigned char *txTo,
    unsigned int txToLen,
    unsigned int nIn,
    unsigned int flags,
    bitcoinconsensus_error* err
);

Use cases:

Alternative implementations can validate using Bitcoin Core’s consensus
Third-party verification
Cross-platform consensus checking

Validation Caching

Script Execution Cache

Caches script execution results:

class CachingTransactionSignatureChecker : public TransactionSignatureChecker {
    bool VerifySignature(...);
    // Caches (txid, script, flags) -> valid/invalid
};

Cache types:

Signature cache: Individual signature verifications
Script cache: Full script execution results

Benefits:

Avoid re-executing scripts during reorgs
Faster block validation
Reduced CPU usage

Assumevalid

Skip signature validation for old blocks:

uint256 defaultAssumeValid = uint256S("0x...");  // Updated each release

Security model:

Relies on proof-of-work accumulation
All other consensus rules still enforced
Significantly faster initial sync
Configurable via -assumevalid=<hash>

Time Locks

Absolute Time Locks

nLockTime (transaction-level):

uint32_t nLockTime;  // Block height or Unix timestamp

OP_CHECKLOCKTIMEVERIFY (BIP65):

Script-level absolute time lock
Compares against nLockTime
Enables refund transactions

Relative Time Locks

nSequence (input-level - BIP68):

uint32_t nSequence;
// Bit 31: Disable flag
// Bit 22: Type flag (0=block height, 1=time)
// Bits 0-15: Value

OP_CHECKSEQUENCEVERIFY (BIP112):

Script-level relative time lock
Enables payment channels
Used in Lightning Network

Median Time Past (BIP113)

Time locks compare against median of last 11 blocks:

int64_t GetMedianTimePast(const CBlockIndex* pindex);

Benefits:

Prevents miner time manipulation
More reliable than single block timestamp

Consensus Upgrades in Practice

Activation Process

Proposal: BIP drafted and discussed
Implementation: Code merged (inactive)
Signaling: Miners signal readiness via version bits
Lock-in: Threshold reached (95% of 2016 blocks)
Activation: Rules enforced after lock-in period

Recent Example: Taproot

Future Considerations

Potential Soft Forks

Under discussion:

ANYPREVOUT (BIP118) - Signature flexibility for eltoo
CHECKSIGFROMSTACK - Covenant functionality
OP_CAT - Concatenation operation
Cross-input signature aggregation

Consensus Evolution

Challenges:

Backward compatibility
Decentralized coordination
Testing and verification
Long-term maintenance

Process improvements:

Better soft fork deployment mechanisms
More comprehensive testing (fuzzing, formal verification)
Clearer activation paths

Validation Engine - Validation implementation details
Architecture Overview - System architecture
BIPs Documentation - Bitcoin Improvement Proposals

​Overview

​Consensus Architecture

​Consensus Parameters

​Chain Parameters

​Network-Specific Values

​Block Consensus Rules

​Block Header Validation

​Block Structure Rules

​Coinbase Transaction

​Transaction Consensus Rules

​Transaction Structure

​Basic Transaction Rules

​Input Validation

​Output Validation

​Script Validation

​Script Types

​1. Pay to Public Key Hash (P2PKH)

​2. Pay to Script Hash (P2SH - BIP16)

​3. Pay to Witness Public Key Hash (P2WPKH - BIP141)

​4. Pay to Witness Script Hash (P2WSH - BIP141)

​5. Pay to Taproot (P2TR - BIP341)

​Script Interpreter

​Signature Validation

​ECDSA (Legacy and SegWit v0)

​Schnorr (Taproot/SegWit v1)

​Soft Fork Mechanisms

​Buried Deployments (BIP90)

​BIP9 Version Bits

​Implemented BIPs

​Consensus Changes

​Network Protocol

​Wallet and Addresses

​Consensus-Critical Code

​Isolation Strategy

​libbitcoin_consensus

​Validation Caching

​Script Execution Cache

​Assumevalid

​Time Locks

​Absolute Time Locks

​Relative Time Locks

​Median Time Past (BIP113)

​Consensus Upgrades in Practice

​Activation Process

​Recent Example: Taproot

​Future Considerations

​Potential Soft Forks

​Consensus Evolution

​Related Documentation

Overview

Consensus Architecture

Consensus Parameters

Chain Parameters

Network-Specific Values

Block Consensus Rules

Block Header Validation

Block Structure Rules

Coinbase Transaction

Transaction Consensus Rules

Transaction Structure

Basic Transaction Rules

Input Validation

Output Validation

Script Validation

Script Types

1. Pay to Public Key Hash (P2PKH)

2. Pay to Script Hash (P2SH - BIP16)

3. Pay to Witness Public Key Hash (P2WPKH - BIP141)

4. Pay to Witness Script Hash (P2WSH - BIP141)

5. Pay to Taproot (P2TR - BIP341)

Script Interpreter

Signature Validation

ECDSA (Legacy and SegWit v0)

Schnorr (Taproot/SegWit v1)

Soft Fork Mechanisms

Buried Deployments (BIP90)

BIP9 Version Bits

Implemented BIPs

Consensus Changes

Network Protocol

Wallet and Addresses

Consensus-Critical Code

Isolation Strategy

libbitcoin_consensus

Validation Caching

Script Execution Cache

Assumevalid

Time Locks

Absolute Time Locks

Relative Time Locks

Median Time Past (BIP113)

Consensus Upgrades in Practice

Activation Process

Recent Example: Taproot

Future Considerations

Potential Soft Forks

Consensus Evolution

Related Documentation