Skip to main content

Key Terminology

Assumption

An assumption is receipt claim upon which a receipt is conditional. When the guest uses composition to verify a receipt in the zkVM, an assumption is added to the receipt. This assumption can be proven and resolved later, either through recursion or by providing proof of the assumption to the verifier.

Circuit

A mathematical construct that acts as the "CPU" of the zkVM in a manner that enables creating proofs.
See also: Arithmetic Circuits

Clock cycles

The smallest unit of compute in the zkVM circuit, analogous to a clock cycle on a physical CPU. The complexity of a guest program's execution is measured in clock cycles as they directly affect the memory, proof size, and time performance of the zkVM.

Generally, a single cycle corresponds to a single RISC-V operation. However, some operations require two cycles.

See the Optimization Guide for more information about the zkVM cycles and performance.

Commit

Append data to the journal.
See also: env::commit()

Composition

As of our 0.20 release, the zkVM supports verification of RISC Zero receipts inside the RISC Zero guest through a feature known as composition. With this, multiple zkVM programs can be composed and produce a single receipt that verifies all computation done to reach the final result.

Some use cases for composition include:

  • Splitting a program into multiple parts, proven by different parties, to preserve privacy and data ownership of each party.
    • E.g. Produce a proof that a ciphertext is a correct encryption of some value to a valid public key.
    • E.g. Produce a proof for a database query by joining receipts from the query over each privately held shard.
  • Aggregating many proofs into one for efficient batch verification.
    • E.g. Produce a proof for a block of transactions, where each transaction is itself verified by a receipt.
  • Creating a single receipt for a workflow that might be split into many different operations.
    • E.g. Produce a single receipt for the result of an image processing pipeline, where different filters are in their own guests.

Conditional

A receipt can be described as "conditional" if it is only true when one or more unproven assumptions are also true. When the assumptions are not proven to be true, a conditional receipt does not provide any guarantees, and so verifiers will reject conditional receipts. A conditional receipt can be made unconditional by providing receipts proving all assumptions. Conditional receipts are a part of the composition feature.

Continuations

As of our 0.15 release, the zkVM uses continuations to enable proof generation for very large programs without blowing up memory requirements. Large proof generation is made possible by splitting the execution trace for a single session into a number of segments, and proving those segments independently.
See also: Continuations study club, Continuations blog

Control ID

The control ID is the Merkle hash of the contents of the control columns, which are assumed to be known to the verifier as part of the circuit definition.

The control ID is the first entry in the seal and plays a key role in defining the operations of the circuit. Verifying a receipt involves:

  • Checking the control ID on the receipt against the control ID for the expected version of the zkVM.
  • Verifying the validity of the Merkle branches for various entries of the Control columns.

For a closer look at how the control columns are used to manage constraint enforcement, check out our STARK by Hand explainer.

Control Root

The control root is the Merkle hash of a set of control IDs.

The control root plays a key role in enforcing zkVM version control. Each control ID identifies one recursion program; the control root identifies the list of allowed recursion programs. When the RISC-V circuit or recursion programs change, the control root is updated to reflect the new set of allowed functions.

This design allows for updates to the RISC-V circuit without necessitating a new trusted setup for the Groth16 circuit.
See also: Code reference for control root

Deterministic Builds

A compilation process is called "deterministic" (or "reproducible") if it reliably produces the same binary file, on a byte-by-byte level.

In the context of RISC Zero application development, deterministic builds are necessary to ensure a clear linkage between the source code for the guest program and the resulting Image ID.

To access deterministic builds for your zkVM application, use cargo risczero build. Deterministic builds are made possible by running the rustc compiler inside a Docker container.

ELF Binary

The executable format for the RISC-V instruction set. Each method in a guest program is written in Rust and compiled into an ELF binary before execution begins. Then, the ELF binary is executed and proven.

Execute

Run a method inside the zkVM and produce an execution trace which can be used by the prover to create a proof of correct execution.

Executor

The portion of the zkVM responsible for generating the execution trace.
See also: Executor documentation, Prover

Execution Trace

A complete record of a computation. The execution trace is organized as a rectangular array, where each row describes a complete snapshot of the state of the zkVM at a given moment in time. The width of the execution trace relates to the number of registers/components in the machine, and the length of the execution trace relates to the number of clock cycles of the program's execution.

The execution trace is generated by the executor and checked for validity by the prover. A valid trace means that the ELF binary was faithfully executed according to the rules of the RISC-V instruction set architecture.

Groth16

A proof system that offers a very small proof size, suitable for on-chain verification.

Groth16 Circuit

RISC Zero's Groth16 Circuit is used to generate receipts that are small enough to support on-chain verification. The Groth16 Circuit is a circom implementation of the verifier for the Recursion Circuit.

Groth16 Receipt

A Groth16 Receipt is a very small validity proof, used primarily for on-chain verification. The Groth16 Receipt is constructed using the Groth16 Circuit.

Guest

The system running inside the zkVM.
See also: Rust crate for zkVM guest

Guest Program

The portion of a zkVM application that gets proven. The guest program receives inputs from an (untrusted) host. Depending on the application, the guest program can commit results to the journal and/or send them to the host.
See also: Rust crate for zkVM guest

Host

The system the zkVM runs on.

Host program

The host-native, untrusted portion of a zkVM application. The host program loads the guest program and provides inputs to the guest program as necessary.

Image ID

A small cryptographic identifier that indicates the method or boot image for zkVM execution.
See also: ImageID documentation, ImageID excerpt from Study Club, segment

Journal

The portion of the receipt that contains the public outputs of a zkVM application.
See also: commit

Method

A single 'main' entry point for code that runs inside the zkVM.

Prove

Generate a receipt that serves as proof of correct execution of a guest program.

Prover

The portion of the zkVM that executes and proves a guest program, thereby constructing a receipt.
See also: Prover documentation, Executor

Receipt

A receipt attests to valid execution of a guest program. Verifying the receipt provides cryptographic assurance that the journal was indeed constructed using the expected circuit and the expected imageID. The receipt consists of the receipt claim and the seal. The receipt claim contains the journal along with other important details, and constitutes to the public outputs of the program. The seal is the opaque blob that cryptographically attests to the validity of the receipt claim.

Receipt Claim

Every receipt has an associated "receipt claim" or "claim", also known as the "public output". The claim is the statement that the receipt provides proof of. It contains the journal, and it additionally includes information about the imageID, exit status (e.g. if the program exits successfully or encountered an error), and the memory state at the end of execution. A simple example of a claim is "I ran the Fibonacci sequence calculator with input "21" and got output "10946". RISC Zero provides a formal system for defining and proving claims.

Recursion

When a zero-knowledge proof is verified within a zero-knowledge proof, this is referred to as "recursion". RISC Zero uses recursion to underpin features such as continuations and composition. Through recursion, RISC Zero can take two or more receipts and compress them into one receipt. By repeating this compression, an arbitrary number of related receipts (e.g. all the Segments of a Session) can be compressed into a single receipt.

Recursion Circuit

The recursion circuit is used to aggregate and compose proofs.

Recursion Program

The recursion circuit is capable of efficiently evaluating polynomial constraints, and was specifically designed to verify STARK proofs. Programs written for this circuit are loaded into the control columns. Each recursion program is identified by a Control ID.

For a list of all supported recursion programs, see the documentation for the zkVM API Client.

RISC-V

The 5th edition of UC Berkeley's reduced instruction set computer architecture. RISC Zero uses RISC-V, specifically the rv32im instruction set, as the basis for the RISC Zero zkVM.

RISC-V Circuit

The RISC-V circuit is the core of the zkVM.

Seal

The portion of the receipt that cryptographically attests to the receipt claim and the correct execution of the guest program. Concretely, the seal is a zk-STARK and is generated by the prover.
See also: validity proof

Segment

The execution trace of a portion of a guest program. The execution of a segment begins at some initial memory image (identified by the ImageID) and proceeds until terminated by the system or user. This represents a chunk of execution work that will be proven in a single call to the zero-knowledge proof system.
See also: Session

Session

The execution trace of a guest program. The session's execution starts from an initial memory image (which includes the starting program counter) and proceeds until either a sys_halt or a sys_pause syscall is encountered. This record is stored as a vector of Segments.

STARK

A STARK (scalable, transparent argument of knowledge) serves as proof of validity for a zkVM program.
See also: About STARKs, Sequence Diagram for RISC Zero's STARK, RISC Zero's ZKP Whitepaper

SNARK

A SNARK (succinct non-interactive argument of knowledge) serves as proof of validity for Blockchain applications due to their small size compared to STARK proofs.
See also: About SNARKs

Validity Proof

A validity proof is a cryptographic argument that attests to the validity of an execution trace. The seal on the receipt serves as a validity proof for the RISC-V instruction set. A validity proof is often referred to as a "zero-knowledge proof", or simply a "proof".

Verifier

A program on the host that verifies a receipt.

Verify

Check that the receipt is valid. Verifying segment receipts requires checking that the seal is valid and was generated by the expected ImageID. Verifying session receipts requires checking the validity of each segment and checking that the initial/final memory snapshots for each segment align appropriately.

Zero-Knowledge Virtual Machine (zkVM)

A virtual machine that runs trusted code and generates proofs. RISC Zero's zkVM implements the RISC-V instruction set architecture and uses a STARK-based proof system.