# [−][src]Crate one_of_many_proofs

Zero knowledge membership proofs based on the One-out-of-Many proof scheme.

This membership proofs allow you to prove knowledge of the opening of a pedersen commitment, within a set of pedersen commitments, without revealing anything about the commitment or its position within the set.

# Examples

Prove you know a commitment to zero, `C_l`, within a set of commitments:

```// Set up proof generators
let gens = ProofGens::new(5).unwrap();

// Create the prover's commitment to zero
let l: usize = 3; // The prover's commitment will be third in the set
let v = Scalar::zero();
let r = Scalar::random(&mut OsRng); // You should use a more secure RNG
let C_l = gens.commit(&v, &r).unwrap();

// Build a random set containing the prover's commitment at index `l`
let mut set = (1..gens.max_set_size())
.map(|_| RistrettoPoint::random(&mut OsRng))
.collect::<Vec<RistrettoPoint>>();
set.insert(l, C_l);

// Compute a `OneOfMany` membership proof for this commitment
let mut t = Transcript::new(b"OneOfMany-Test");
let proof = set.iter().prove(&gens, &mut t.clone(), l, &r).unwrap();

// Verify this membership proof, without any knowledge of `l` or `r`.
assert!(set
.iter()
.verify(&gens, &mut t.clone(), &proof)
.is_ok());```

Prove you know a commitment that opens to any value (possibly non-zero), within a set of commitments:

```// Set up proof generators
let gens = ProofGens::new(5).unwrap();

// Create the prover's commitment to zero
let l: usize = 3; // The prover's commitment will be third in the set
let v = Scalar::random(&mut OsRng); // Commit to any random value
let r = Scalar::random(&mut OsRng); // You should use a more secure RNG
let C_l = gens.commit(&v, &r).unwrap();

// Build a random set containing the prover's commitment at index `l`
let mut set = (1..gens.max_set_size())
.map(|_| RistrettoPoint::random(&mut OsRng))
.collect::<Vec<RistrettoPoint>>();
set.insert(l, C_l);

// Create a new commitment to the same value as `C_l`
let r_new = Scalar::random(&mut OsRng); // You should use a more secure RNG
let C_new = gens.commit(&v, &r_new).unwrap();

// Compute a `OneOfMany` membership proof for this commitment
let mut t = Transcript::new(b"OneOfMany-Test");
let proof = set.iter().prove_with_offset(&gens, &mut t.clone(), l, &(r - r_new), Some(&C_new)).unwrap();

// Verify this membership proof, without any knowledge of `l` or `r`.
assert!(set
.iter()
.verify_with_offset(&gens, &mut t.clone(), &proof, Some(&C_new))
.is_ok());```

# Ring Signatures

One particularly useful application of membership proofs is ring signatures. This can easily be accomplished by committing to some message before computing or verifying a proof. Consider the below example, signing and verifying a message from an anonymous member of the set:

```// Set up proof generators
let gens = ProofGens::new(5).unwrap();

// Create the prover's commitment to zero
let l: usize = 3; // The signer's commitment will be third in the set
let v = Scalar::random(&mut OsRng); // Commit to any random value
let r = Scalar::random(&mut OsRng); // You should use a more secure RNG
let C_l = gens.commit(&v, &r).unwrap();

// Build a random set containing the prover's commitment at index `l`
let mut set = (1..gens.max_set_size())
.map(|_| RistrettoPoint::random(&mut OsRng))
.collect::<Vec<RistrettoPoint>>();
set.insert(l, C_l);

// Create a new commitment to the same value as `C_l`
let r_new = Scalar::random(&mut OsRng); // You should use a more secure RNG
let C_new = gens.commit(&v, &r_new).unwrap();

// Compute a `OneOfMany` membership proof for this commitment
let mut t = Transcript::new(b"OneOfMany-Test");

// Commit to a message to be signed
t.append_message(b"msg", b"Hello, World!");

// Sign the message anonymously
let proof = set.iter().prove_with_offset(&gens, &mut t.clone(), l, &(r - r_new), Some(&C_new)).unwrap();

// Compute a `OneOfMany` membership proof for this commitment
let mut t = Transcript::new(b"OneOfMany-Test");

// Verification will fail, because this transcript doesn't commit to the same message
assert!(set
.iter()
.verify_with_offset(&gens, &mut t.clone(), &proof, Some(&C_new))
.is_err());

// Commit to a message to be signed
t.append_message(b"msg", b"Hello, World!");

// Verification will now succeed, because this transcript commits to the signed message
assert!(set
.iter()
.verify_with_offset(&gens, &mut t.clone(), &proof, Some(&C_new))
.is_ok());```

# Perfomance

The proof(s) provided by this crate depend heavily on the curve25519-dalek for elliptic curve operations on the ristretto255 curve group. These operations can be optimized by compiling to use the SIMD backend. To do set this compile option, set the following environment variable:

``````export RUSTFLAGS="-C target_cpu=native"
``````

Benchmarks are run using criterion.rs:

``````cargo bench
``````