Starknet Architecture: Cairo, STARK Prover, and AA

Frequently Asked Questions

01.What programming language does Starknet use for smart contracts?

Starknet uses Cairo 2.x, a Rust-inspired language that compiles to Sierra (Safe Intermediate Representation) and then to Cairo Assembly (Casm). The Sierra layer guarantees every valid Cairo program produces a provable execution trace, preventing unprovable contract states from reaching L1. Cairo 2.x introduced a linear type system, generics, and traits that substantially improve developer ergonomics over Cairo 0.x.

02.How does account abstraction work on Starknet?

Every Starknet account is a smart contract. There are no externally owned accounts (EOAs). Every account contract must implement two entry points: `__validate__` and `__execute__`. The sequencer calls `__validate__` first to check signature validity, then `__execute__` to run the transaction logic. This allows arbitrary signature schemes, including multisig, WebAuthn/passkeys, and session keys, at the protocol level without any EIP process or protocol upgrade.

03.What is the SHARP prover in Starknet?

SHARP (Shared Prover) is StarkWare's production STARK proof generation service. It aggregates execution traces from multiple Starknet blocks into a single recursive STARK proof and submits that proof to the Ethereum L1 verifier contract. This amortizes L1 proof-verification gas across many transactions. SHARP is also shared across StarkEx-based applications (dYdX v3, Sorare, ImmutableX), allowing non-Starknet traces to contribute to the same proof batch.

04.How are fees calculated on Starknet?

Starknet fees have two components: L2 execution gas, billed in STRK or ETH based on Cairo VM step count, builtins used, and storage writes; and L1 data availability cost. From Starknet v0.13.0 onward, with EIP-4844 blob support introduced in March 2024, L1 DA cost uses blob space at blob-gas price rather than calldata gas, reducing data fees by 5 to 10 times under normal market conditions.

05.What is the difference between Sierra and Casm in Cairo 2.x?

Sierra (Safe Intermediate Representation) is a typed intermediate language between Cairo 2.x source and Cairo VM bytecode. Its defining property is that every Sierra program is guaranteed to compile to a valid, provable execution trace. Casm (Cairo Assembly) is the final bytecode that the Cairo VM executes. The Sierra-to-Casm compilation step is performed by the sequencer at declare time, not by the contract deployer, which is why the DECLARE transaction carries Sierra rather than Casm.