PreciseBank

High-precision bank module for 18-decimal EVM token accounting

The PreciseBank module (x/precisebank) extends the precision of the standard Cosmos SDK bank module from 6 decimals to 18 decimals, enabling full EVM compatibility while maintaining Cosmos coin integrity. This module is required for chains using non-18-decimal native tokens.

Big thanks to the Kava team for their valuable contributions to this module.

Module Overview

Purpose: Bridge the decimal precision gap between Cosmos (typically 6 decimals) and EVM (18 decimals)

Key Functionality:

• Extends token precision without changing the base denomination
• Tracks fractional balances (sub-atomic units) separate from integer balances
• Maintains 1:1 backing between fractional and integer units
• Transparent to users - balances appear as expected in both environments
• Wraps x/bank to provide 18-decimal precision for x/vm

Source Code: x/precisebank Documentation: x/precisebank/README.md

---

When Do You Need PreciseBank?

You NEED PreciseBank if:

Your native token has 6 decimals (or any non-18 decimal count):

• Base denom: ustake, utoken, uatom (micro prefix = 10^6)
• Display denom: stake, token, atom
• Example: 1 STAKE = 1,000,000 ustake = 10^6 smallest units

Why: EVM expects 18 decimals. Without PreciseBank, you lose 12 decimals of precision, causing rounding errors and broken DeFi protocols.

You DON’T NEED PreciseBank if:

Your native token has 18 decimals:

• Base denom: atest, atoken (atto prefix = 10^18)
• Display denom: test, token
• Example: 1 TEST = 1,000,000,000,000,000,000 atest = 10^18 smallest units

Why: Your Cosmos denomination already matches EVM’s 18-decimal expectation. Direct 1:1 mapping with no fractional tracking needed.

---

Mathematical Foundation

The Precision Problem

Cosmos Standard: 6 decimal places

1 ATOM = 1,000,000 uatom (10^6)
Smallest unit: 0.000001 ATOM = 1 uatom

EVM Standard: 18 decimal places

1 ETH = 1,000,000,000,000,000,000 wei (10^18)
Smallest unit: 0.000000000000000001 ETH = 1 wei

Gap: 12 orders of magnitude (10^12)

PreciseBank Solution

PreciseBank subdivides each uatom into 10^12 sub-atomic units called aatom:

1 ATOM = 1,000,000 uatom (Cosmos layer - x/bank)
1 uatom = 1,000,000,000,000 aatom (EVM layer - x/precisebank)
1 ATOM = 1,000,000,000,000,000,000 aatom (10^18 total)

Key Principle: Every aatom is fully backed by uatom in x/bank. You cannot have fractional aatom without corresponding integer uatom reserves.

Balance Representation

For any account n, the total balance in sub-atomic units a(n) is:

$$a(n) = b(n) \cdot C + f(n)$$

Where:

• a(n) = Total aatom balance (18-decimal representation)
• b(n) = Integer uatom balance (stored in x/bank)
• f(n) = Fractional balance (stored in x/precisebank)
• C = Conversion factor = 10^12

Constraints:

0 ≤ f(n) < C
a(n), b(n) ≥ 0

Derivation (quotient-remainder theorem):

b(n) = ⌊a(n) / C⌋  (integer division)
f(n) = a(n) mod C   (remainder)

Example:

User has: 1,500,000,123,456,789,012 aatom
b(n) = ⌊1,500,000,123,456,789,012 / 10^12⌋ = 1,500,000 uatom (in x/bank)
f(n) = 1,500,000,123,456,789,012 mod 10^12 = 123,456,789,012 aatom (in x/precisebank)

Source: README.md Background

---

Module Integration

Adding to Your Chain

PreciseBank requires integration in app/app.go:

1. Import the module:

import (
    precisebankkeeper "github.com/cosmos/evm/x/precisebank/keeper"
    precisebanktypes "github.com/cosmos/evm/x/precisebank/types"
)

2. Add keeper to App struct:

type App struct {
    // ... other keepers ...
    BankKeeper    bankkeeper.Keeper
    PreciseBankKeeper precisebankkeeper.Keeper
    // ... other keepers ...
}

3. Initialize keeper (before VM keeper):

// Create precisebank keeper wrapping bank keeper
app.PreciseBankKeeper = precisebankkeeper.NewKeeper(
    appCodec,
    keys[precisebanktypes.StoreKey],
    app.BankKeeper,        // Wrapped bank keeper
    app.AccountKeeper,
)

4. Pass PreciseBankKeeper to VM module:

// VM keeper needs precisebank for 18-decimal operations
app.VMKeeper = vmkeeper.NewKeeper(
    appCodec,
    keys[vmtypes.StoreKey],
    app.PreciseBankKeeper,  // Use precisebank instead of bank
    app.StakingKeeper,
    // ... other keepers ...
)

5. Add to module manager:

app.ModuleManager = module.NewManager(
    // ... other modules ...
    precisebank.NewAppModule(app.PreciseBankKeeper),
    // ... other modules ...
)

Critical: PreciseBank must wrap BankKeeper and be passed to VMKeeper, not BankKeeper directly.

---

Configuration

Genesis Configuration

PreciseBank has minimal genesis configuration - it primarily tracks state, not parameters.

File Location: ~/.evmd/config/genesis.json under app_state.precisebank

Structure:

{
  "app_state": {
    "precisebank": {
      "fractional_balances": [],
      "remainder": "0"
    }
  }
}

Required VM Module Configuration

When using PreciseBank, you MUST configure extended_denom_options in the VM module:

{
  "app_state": {
    "vm": {
      "params": {
        "evm_denom": "ustake",
        "extended_denom_options": [
          {
            "native_denom": "ustake",
            "extended_denom": "astake"
          }
        ]
      }
    }
  }
}

Explanation:

• native_denom: 6-decimal Cosmos denom (ustake)
• extended_denom: 18-decimal EVM denom (astake)
• Conversion: 1 ustake = 10^12 astake

Naming Pattern:

• u prefix (micro, 10^6) → a prefix (atto, 10^18): ustake → astake
• Other prefixes → add evm prefix: stake → evmstake

---

State

fractional_balances

What It Stores: The fractional (sub-atomic) portion of each account’s balance that cannot be represented as whole integer units.

Type: Array of FractionalBalance objects

Structure (fractional_balance.go:43-48):

message FractionalBalance {
  string address = 1;  // Bech32 account address
  string amount = 2;   // Fractional amount (0 < amount < 10^12)
}

Validation (fractional_balance.go:64-78):

• Amount must be positive (amount > 0)
• Amount must be less than conversion factor (amount < 10^12)
• Address must be valid Bech32

Example:

{
  "fractional_balances": [
    {
      "address": "cosmos1abc...",
      "amount": "123456789012"
    },
    {
      "address": "cosmos1def...",
      "amount": "999999999999"
    }
  ]
}

Storage Key: keys.go:17

FractionalBalancePrefix = []byte{0x01}
FractionalBalanceKey(address) = address.Bytes()

---

remainder

What It Stores: A module-level reserve balance that backs all fractional units in circulation.

Type: Integer (sdkmath.Int)

Purpose: Maintains invariant that total fractional balances equal the module reserve

Invariant:

$$\text{remainder} = \sum_{n \in \mathcal{A}} f(n)$$

Where:

• remainder = Module reserve in fractional units
• $\sum f(n)$ = Sum of all account fractional balances

Why Needed: Since fractional units aren’t tracked in x/bank’s total supply, this reserve account holds integer units to back them. When fractional balances sum to 10^12, one integer unit is held in reserve.

Example:

Account 1 fractional: 400,000,000,000 aatom
Account 2 fractional: 600,000,000,000 aatom
Total fractional: 1,000,000,000,000 aatom = 1 ustake

Module reserve: 1 ustake held in x/bank to back these fractional units
Remainder in precisebank: 1,000,000,000,000 aatom

Storage Key: keys.go:22

RemainderBalanceKey = []byte{0x02}

Source: remainder_amount.go

---

Operations

Transfer

When transferring fractional amounts, PreciseBank handles the complexity automatically:

Example Transfer: Alice sends 1.5 ustake + 500 billion aatom to Bob

Alice initial:
  x/bank: 10 ustake
  x/precisebank: 500,000,000,000 aatom
  Total: 10,500,000,000,000 aatom

Bob initial:
  x/bank: 5 ustake
  x/precisebank: 300,000,000,000 aatom
  Total: 5,300,000,000,000 aatom

Transfer amount: 2,000,000,000,000 aatom
  = 2 ustake + 0 aatom fractional

After transfer:
Alice:
  x/bank: 8 ustake (10 - 2)
  x/precisebank: 500,000,000,000 aatom (unchanged - no fractional change)
  Total: 8,500,000,000,000 aatom

Bob:
  x/bank: 7 ustake (5 + 2)
  x/precisebank: 300,000,000,000 aatom (unchanged)
  Total: 7,300,000,000,000 aatom

Complex Transfer: Alice sends 1,234,567,890,123 aatom to Bob

Transfer: 1,234,567,890,123 aatom
  = 1 ustake + 234,567,890,123 aatom fractional

Alice:
  x/bank: 10 - 1 = 9 ustake
  x/precisebank: 500,000,000,000 - 234,567,890,123 = 265,432,109,877 aatom
  Total: 9,265,432,109,877 aatom

Bob:
  x/bank: 5 + 1 = 6 ustake
  x/precisebank: 300,000,000,000 + 234,567,890,123 = 534,567,890,123 aatom
  Total: 6,534,567,890,123 aatom

Source: send.go

---

Mint

Operation: Create new fractional units

// Mint 1.5 ustake worth of fractional units (1,500,000,000,000 aatom)
preciseBankKeeper.MintCoins(ctx, moduleName, coins)

Process:

1. Split amount into integer and fractional parts
2. Mint integer part via x/bank
3. Update fractional balance in x/precisebank
4. Update remainder to maintain backing invariant

Source: mint.go

---

Burn

Operation: Destroy fractional units

// Burn 2.3 ustake worth of fractional units (2,300,000,000,000 aatom)
preciseBankKeeper.BurnCoins(ctx, moduleName, coins)

Process:

1. Split amount into integer and fractional parts
2. Burn integer part via x/bank
3. Update fractional balance in x/precisebank
4. Update remainder to maintain backing invariant

Source: burn.go

---

Keeper Interface

PreciseBank implements the full BankKeeper interface, making it a drop-in replacement:

Source: keeper.go:16

var _ evmtypes.BankKeeper = Keeper{}

Key Methods:

• SendCoins(ctx, from, to, coins) - Transfer with fractional precision
• MintCoins(ctx, module, coins) - Create new fractional units
• BurnCoins(ctx, module, coins) - Destroy fractional units
• GetBalance(ctx, addr, denom) - Get extended balance (integer + fractional)
• SpendableCoins(ctx, addr) - Get spendable balances with fractional precision

Passthrough Methods: Methods not requiring fractional logic delegate directly to x/bank (keeper.go:44-50):

• GetSupply() - Total supply
• IterateTotalSupply() - Supply iteration

---

Queries

gRPC Queries

Query Fractional Balance:

# Query fractional balance for specific address
evmd query precisebank fractional-balance cosmos1abc... --chain-id mychain-1

Query Total Fractional Balances:

# Sum of all fractional balances in the system
evmd query precisebank total-fractional-balances --chain-id mychain-1

Query Remainder:

# Query module reserve backing fractional units
evmd query precisebank remainder --chain-id mychain-1

Source: grpc_query.go

---

EVM Integration

In Solidity Contracts

From the EVM perspective, users interact with the extended denomination:

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

contract Example {
    // Native token precompile (astake with 18 decimals)
    IERC20 constant NATIVE = IERC20(0xEeeeeEeeeEeEeeEeEeEeeEEEeeeeEeeeeeeeEEeE);

    function deposit() external payable {
        // User sends astake (18 decimals)
        // PreciseBank automatically handles conversion to/from ustake
        require(msg.value >= 1e18, "Minimum 1 STAKE");

        // Transfer uses full 18-decimal precision
        // 1.5 STAKE = 1,500,000,000,000,000,000 astake
        NATIVE.transfer(address(this), 1.5e18);
    }

    function getBalance(address user) external view returns (uint256) {
        // Returns balance in astake (18 decimals)
        // PreciseBank computes: (b(n) * 10^12) + f(n)
        return NATIVE.balanceOf(user);
    }
}

Behind the Scenes:

• transfer(recipient, 1.5e18 astake)
• PreciseBank: Transfers 1 ustake via x/bank + 500,000,000,000 aatom fractional
• User sees seamless 18-decimal precision

---

Events

PreciseBank emits events for fractional balance changes:

SendCoins Event

{
  "type": "precisebank_send",
  "attributes": [
    {"key": "from", "value": "cosmos1abc..."},
    {"key": "to", "value": "cosmos1def..."},
    {"key": "amount", "value": "1234567890123astake"}
  ]
}

MintCoins Event

{
  "type": "precisebank_mint",
  "attributes": [
    {"key": "minter", "value": "evm"},
    {"key": "amount", "value": "1000000000000astake"}
  ]
}

BurnCoins Event

{
  "type": "precisebank_burn",
  "attributes": [
    {"key": "burner", "value": "evm"},
    {"key": "amount", "value": "500000000000astake"}
  ]
}

Source: events.go

---

Common Issues and Solutions

Issue: “Fractional amount exceeds conversion factor”

Symptom: Transaction fails with fractional validation error

Cause: Fractional balance >= 10^12 (should have been converted to integer unit)

Solution: This indicates a bug in the keeper logic. Report to Cosmos EVM team.

Issue: Balances Don’t Match Between Cosmos/EVM

Symptom: User sees different balance in Cosmos vs MetaMask

Cause:

• PreciseBank not integrated correctly in app.go
• VM module not using PreciseBankKeeper
• Missing extended_denom_options configuration

Solution:

// In app.go - WRONG:
app.VMKeeper = vmkeeper.NewKeeper(..., app.BankKeeper, ...)

// In app.go - CORRECT:
app.VMKeeper = vmkeeper.NewKeeper(..., app.PreciseBankKeeper, ...)

Issue: Chain Won’t Start After Adding PreciseBank

Symptom: Genesis validation fails

Cause: Missing extended_denom_options in VM params

Solution: Add to genesis.json:

{
  "vm": {
    "params": {
      "extended_denom_options": [{
        "native_denom": "ustake",
        "extended_denom": "astake"
      }]
    }
  }
}

Issue: Total Supply Mismatch

Symptom: Sum of balances doesn’t equal total supply

Cause: Remainder not properly maintained

Solution: Query remainder and verify:

# Remainder should equal sum of all fractional balances
evmd query precisebank remainder
evmd query precisebank total-fractional-balances

---

Testing and Verification

Verify Integration

1. Check module is loaded:

evmd query precisebank params

2. Query remainder (should be 0 at genesis):

evmd query precisebank remainder

3. Send fractional amount via EVM:

# Use MetaMask or web3 to send 1.5 STAKE
# Then check fractional balance:
evmd query precisebank fractional-balance cosmos1abc...

4. Verify invariant:

# Total fractional balances should equal remainder
TOTAL=$(evmd query precisebank total-fractional-balances -o json | jq -r '.total')
REMAINDER=$(evmd query precisebank remainder -o json | jq -r '.remainder')
[ "$TOTAL" == "$REMAINDER" ] && echo "Invariant maintained" || echo "ERROR: Invariant broken"

---

Performance Considerations

Storage: Fractional balances add one storage entry per account with non-zero fractional amount

Gas Cost: Fractional operations add minimal gas overhead (~5-10% more than standard bank operations)

Scaling: Module has been tested with millions of accounts, no performance degradation

Optimization: Fractional balances are only created when needed. Transfers of exact integer amounts don’t create fractional entries.

---

Related Documentation

• Building Your Chain Guide - Main configuration walkthrough
• VM Module - extended_denom_options setup
• ERC20 Module - Token pair configuration

---

Source Code References

• Module Implementation: x/precisebank
• README (Math Background): x/precisebank/README.md
• Keeper: keeper/keeper.go
• Fractional Balance Logic: types/fractional_balance.go
• Send Operations: keeper/send.go
• Mint Operations: keeper/mint.go
• Burn Operations: keeper/burn.go
• Remainder Management: keeper/remainder_amount.go
• Storage Keys: types/keys.go