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:

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.

Core Pattern: Three Functions

Every computation needs three functions in your Solana program:

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 for the full size table.

Encryption Context

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.
• 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.

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