arcium

Arcium

Encrypted computation on Solana via MPC. Data stays encrypted during computation. The arcium CLI (wraps Anchor) handles init, build, test, and deploy — use MCP for current flags and options.

MCP Tools: search_arcium_docs for discovery (returns page path), then query_docs_filesystem_arcium_docs with cat <path>.mdx for full-page reads (e.g., cat /developers/arcis/mental-model.mdx).

When to Use

Use when:

• You need trustless computation -- cryptographically guaranteed, no single party sees the data
• Multiple parties compute on combined data without revealing inputs
• On-chain state must remain encrypted but computable
• Privacy: sealed-bid auctions, voting, hidden game state, dark pools, confidential DeFi

Constraints:

• Fixed loop bounds required (no variable-length iteration)

Mental Model

Arcium apps have three coupled surfaces. Most bugs are mismatches across their boundaries:

| Surface | Responsibility | Common Boundary Bugs | |---------|---------------|----------------------| | Circuit (Arcis/Rust) | Pure fixed-shape MPC logic | Variable loops, dynamic collections, .reveal() inside conditionals | | Program (Anchor/Rust) | Orchestration: init + queue + callback | Macro name mismatch, callback accounts not writable, wrong ArgBuilder order | | Client (TypeScript) | Key exchange, encryption, submission, decryption | Nonce reuse, missing .x25519_pubkey() for Shared, param order ≠ circuit order |

MPC constraints (from how secret sharing works):

• Both branches of if/else execute unless the condition is a compile-time constant — cost = sum of both branches, not max
• Loops must have fixed bounds — no while, break, continue
• Comparisons are expensive; arithmetic (add/multiply) is nearly free
• .reveal() and .from_arcis() cannot be called inside conditionals (exception: compile-time constant conditions)
• All data must be fixed-size — no Vec, String, HashMap; use [T; N]

Intent Router

Identify what you're building, then read the linked reference before coding. For API details, CLI flags, deployment, and versions, use MCP directly.

| Intent | Read | MCP Query | |--------|------|-----------| | First Arcium app | [minimal-circuit.md](examples/minimal-circuit.md) | "hello world tutorial" | | Choose a pattern (stateless, stateful, multi-party) | [patterns.md](examples/patterns.md) | "arcium examples" | | Circuit syntax (#[encrypted], #[instruction]) | [patterns.md](examples/patterns.md) | "arcis encrypted instruction" | | Shared vs Mxe encryption | See [Encryption Context](#encryption-context) below | "Shared vs Mxe encryption" | | ArgBuilder ordering / ciphertext errors | [troubleshooting.md -- ArgBuilder Ordering Errors](references/troubleshooting.md#argbuilder-ordering-errors) | "ArgBuilder encrypted plaintext" | | Callback not firing / computation stuck | [troubleshooting.md -- Computation Never Finalizes](references/troubleshooting.md#computation-never-finalizes) | "arcium_callback queue_computation" | | Nonce / decryption errors | [troubleshooting.md -- Nonce Errors](references/troubleshooting.md#nonce-errors) | "RescueCipher encrypt nonce" | | Client-side encryption (RescueCipher, x25519) | [minimal-circuit.md](examples/minimal-circuit.md) -- Test section | "RescueCipher encrypt nonce" | | Threshold signing / secure randomness | — | "MXESigningKey sign" or "ArcisRNG" | | Deployment (devnet/mainnet) | — | "arcium deploy cluster-offset" | | Version / installation requirements | — | "arcium installation anchor solana" |

Core Pattern: Three Functions

Every computation needs three functions in your Solana program:

| Function | Purpose | When Called | |----------|---------|-------------| | init_<name>_comp_def | Initialize computation definition | Once per instruction | | <name> | Build args + queue computation | Each request | | <name>_callback | Handle result from Arx nodes | After MPC completes |

const COMP_DEF_OFFSET_FLIP: u32 = comp_def_offset("flip");

// 1. INIT (once per instruction type)
pub fn init_flip_comp_def(ctx: Context<InitFlipCompDef>) -> Result<()> {
    init_comp_def(ctx.accounts, None, None)
}

// 2. QUEUE (each computation)
pub fn flip(ctx: Context<Flip>, offset: u64, ...) -> Result<()> {
    let args = ArgBuilder::new()...build();
    queue_computation(ctx.accounts, offset, args,
        vec![FlipCallback::callback_ix(offset, &ctx.accounts.mxe_account, &[])?],
        1, 0,
    )?;
    Ok(())
}

// 3. CALLBACK (after MPC completes)
#[arcium_callback(encrypted_ix = "flip")]
pub fn flip_callback(ctx: Context<FlipCallback>,
    output: SignedComputationOutputs<FlipOutput>) -> Result<()> {
    let result = output.verify_output(...)?;
    // Use result...
}

Encryption size: RescueCipher encrypts any scalar to 32 bytes regardless of type. Formula: ciphertext_size = 32 * number_of_scalar_values. See [troubleshooting.md](references/troubleshooting.md) for the full size table.

Encryption Context

| Scenario | Use | |----------|-----| | User inputs, results returned to user | Enc<Shared, T> | | Internal state users shouldn't access | Enc<Mxe, T> | | State persisted across computations | Enc<Mxe, T> | | Final reveal to all parties | .reveal() |

Gotchas

Reference during development to avoid common mistakes.

NEVER:

• NEVER reuse a nonce — every cipher.encrypt() call needs a fresh randomBytes(16)
• NEVER combine multiple ciphertexts into one ArgBuilder call — each encrypted scalar is its own [u8; 32] call
• NEVER omit .x25519_pubkey() for Enc<Shared, T> (silent failure); Enc<Mxe, T> skips it

Critical (silent failures)

• Macro string matching: All macro strings must exactly match #[instruction] fn NAME across #[arcium_callback], comp_def_offset(), #[init_computation_definition_accounts], #[queue_computation_accounts], #[callback_accounts]
• ArgBuilder ordering: Calls must match circuit parameter order left-to-right. For Enc<Shared, T>: .x25519_pubkey() then .plaintext_u128(nonce) then ciphertexts. For Enc<Mxe, T>: .plaintext_u128(nonce) then ciphertexts. Missing .x25519_pubkey() for Shared = silent failure.
• Division by secret zero: Guard divisors with the safe divisor pattern -- both branches execute in MPC, so the division always runs. See [patterns.md — Safe Division](examples/patterns.md).
• Combined ciphertext arrays: Each encrypted scalar needs a separate [u8; 32] ArgBuilder call — do NOT pass [u8; 64] for a two-scalar type. See [troubleshooting.md — Ciphertext Size Mismatch](references/troubleshooting.md#ciphertext-size-mismatch).

Warning (wrong results)

• Nonce reuse: Same nonce for multiple encryptions = garbled output. Use unique randomBytes(16) per encryption.
• Callback account writability: Pass extra accounts via CallbackAccount { pubkey, is_writable: true } in callback_ix(..., &[...]). Also mark #[account(mut)] in callback struct. Accounts cannot be created or resized during callbacks.
• Output struct naming: Circuit fn add_together generates AddTogetherOutput. Single returns use field_0 (a SharedEncryptedStruct<1> or MXEEncryptedStruct<1> with .ciphertexts and .nonce). Tuple returns nest field_0, field_1, etc.

Tips

• Prefer arithmetic over comparisons (cheaper in MPC)
• Comparisons/divisions are cheaper with narrower types (u64 vs u128); storage cost is identical

Debug Triage Order

Start here when a computation fails or returns wrong results.

When a computation fails, returns wrong results, or never finalizes — check in this order:

1. Names match exactly — #[instruction] fn NAME must match across #[arcium_callback(encrypted_ix = "NAME")], comp_def_offset("NAME"), and all account macros
2. Comp def initialized — init_*_comp_def must be called once before any computation
3. ArgBuilder param order — calls must match circuit fn parameters left-to-right