grin/secp256k1zkp/src/pedersen.rs

// Bitcoin secp256k1 bindings
// Written in 2014 by
//   Dawid Ciężarkiewicz
//   Andrew Poelstra
//
// To the extent possible under law, the author(s) have dedicated all
// copyright and related and neighboring rights to this software to
// the public domain worldwide. This software is distributed without
// any warranty.
//
// You should have received a copy of the CC0 Public Domain Dedication
// along with this software.
// If not, see <http://creativecommons.org/publicdomain/zero/1.0/>.
//

//! # Pedersen commitments and related range proofs

use std::mem;

use ContextFlag;
use Error;
use Secp256k1;

use constants;
use ffi;
use key;
use key::{SecretKey, PublicKey};
use rand::{Rng, OsRng};

/// A Pedersen commitment
pub struct Commitment(pub [u8; constants::PEDERSEN_COMMITMENT_SIZE]);
impl_array_newtype!(Commitment, u8, constants::PEDERSEN_COMMITMENT_SIZE);
impl_pretty_debug!(Commitment);

impl Commitment {
    /// Uninitialized commitment, use with caution
    unsafe fn blank() -> Commitment {
        mem::uninitialized()
    }
    /// Converts a commitment to a public key
    pub fn to_pubkey(&self, secp: &Secp256k1) -> Result<key::PublicKey, Error> {
        key::PublicKey::from_slice(secp, &self.0)
    }
    /// Underlying data
    pub fn bytes(&self) -> &[u8] {
        &self.0
    }
}

/// A range proof. Typically much larger in memory that the above (~5k).
#[derive(Copy)]
pub struct RangeProof {
    /// The proof itself, at most 5134 bytes long
    pub proof: [u8; constants::MAX_PROOF_SIZE],
    /// The length of the proof
    pub plen: usize,
}

impl Clone for RangeProof {
    #[inline]
    fn clone(&self) -> RangeProof {
        unsafe {
            use std::intrinsics::copy_nonoverlapping;
            use std::mem;
            let mut ret: [u8; constants::MAX_PROOF_SIZE] = mem::uninitialized();
            copy_nonoverlapping(self.proof.as_ptr(),
                                ret.as_mut_ptr(),
                                mem::size_of::<RangeProof>());
            RangeProof {
                proof: ret,
                plen: self.plen,
            }
        }
    }
}

impl RangeProof {
    pub fn bytes(&self) -> &[u8] {
        &self.proof[..self.plen as usize]
    }
    pub fn len(&self) -> usize {
        self.plen
    }
}

/// The range that was proven
pub struct ProofRange {
    /// Min value that was proven
    pub min: u64,
    /// Max value that was proven
    pub max: u64,
}

/// Information about a valid proof after rewinding it.
pub struct ProofInfo {
    /// Whether the proof is valid or not
    pub success: bool,
    /// Value that was used by the commitment
    pub value: u64,
    /// Message embedded in the proof
    pub message: [u8; constants::PROOF_MSG_SIZE],
    /// Length of the embedded message
    pub mlen: i32,
    /// Min value that was proven
    pub min: u64,
    /// Max value that was proven
    pub max: u64,
    /// Exponent used by the proof
    pub exp: i32,
    /// Mantissa used by the proof
    pub mantissa: i32,
}

impl ::std::fmt::Debug for RangeProof {
    fn fmt(&self, f: &mut ::std::fmt::Formatter) -> ::std::fmt::Result {
        try!(write!(f, "{}(", stringify!(RangeProof)));
        for i in self.proof[..self.plen].iter().cloned() {
            try!(write!(f, "{:02x}", i));
        }
        write!(f, ")[{}]", self.plen)
    }
}

impl Secp256k1 {
    /// Creates a pedersen commitment from a value and a blinding factor
    pub fn commit(&self, value: u64, blind: SecretKey) -> Result<Commitment, Error> {

        if self.caps != ContextFlag::Commit {
            return Err(Error::IncapableContext);
        }
        let mut commit = [0; 33];
        unsafe {
            ffi::secp256k1_pedersen_commit(self.ctx, commit.as_mut_ptr(), blind.as_ptr(), value)
        };
        Ok(Commitment(commit))
    }

    /// Convenience method to Create a pedersen commitment only from a value, with a zero blinding factor
    pub fn commit_value(&self, value: u64) -> Result<Commitment, Error> {

        if self.caps != ContextFlag::Commit {
            return Err(Error::IncapableContext);
        }
        let mut commit = [0; 33];
        let zblind = [0; 32];
        unsafe {
            ffi::secp256k1_pedersen_commit(self.ctx, commit.as_mut_ptr(), zblind.as_ptr(), value)
        };
        Ok(Commitment(commit))
    }

    /// Taking vectors of positive and negative commitments as well as an
    /// expected excess, verifies that it all sums to zero.
    pub fn verify_commit_sum(&self,
                             positive: Vec<Commitment>,
                             negative: Vec<Commitment>,
                             excess: i64)
                             -> bool {
        let pos = map_vec!(positive, |p| p.0.as_ptr());
        let neg = map_vec!(negative, |n| n.0.as_ptr());
        unsafe {
            ffi::secp256k1_pedersen_verify_tally(self.ctx,
                                                 pos.as_ptr(),
                                                 pos.len() as i32,
                                                 neg.as_ptr(),
                                                 neg.len() as i32,
                                                 excess) == 1
        }
    }

    /// Computes the sum of multiple positive and negative pedersen commitments.
    pub fn commit_sum(&self,
                      positive: Vec<Commitment>,
                      negative: Vec<Commitment>)
                      -> Result<Commitment, Error> {
        let pos = map_vec!(positive, |p| p.0.as_ptr());
        let neg = map_vec!(negative, |n| n.0.as_ptr());
        let mut ret = unsafe { Commitment::blank() };
        let err = unsafe {
            ffi::secp256k1_pedersen_commit_sum(self.ctx,
                                               ret.as_mut_ptr(),
                                               pos.as_ptr(),
                                               pos.len() as i32,
                                               neg.as_ptr(),
                                               neg.len() as i32)
        };
        if err == 1 {
            Ok(ret)
        } else {
            Err(Error::IncorrectCommitSum)
        }
    }

    /// Computes the sum of multiple positive and negative blinding factors.
    pub fn blind_sum(&self,
                     positive: Vec<SecretKey>,
                     negative: Vec<SecretKey>)
                     -> Result<SecretKey, Error> {
        let mut neg = map_vec!(negative, |n| n.as_ptr());
        let mut all = map_vec!(positive, |p| p.as_ptr());
        all.append(&mut neg);
        let mut ret: [u8; 32] = unsafe { mem::uninitialized() };
        unsafe {
            assert_eq!(ffi::secp256k1_pedersen_blind_sum(self.ctx,
                                                         ret.as_mut_ptr(),
                                                         all.as_ptr(),
                                                         all.len() as i32,
                                                         positive.len() as i32),
                       1)
        }
        // secp256k1 should never return an invalid private
        SecretKey::from_slice(self, &ret)
    }

    /// Produces a range proof for the provided value, using min and max bounds, relying
    /// on the blinding factor and commitment.
    pub fn range_proof(&self,
                       min: u64,
                       value: u64,
                       blind: SecretKey,
                       commit: Commitment)
                       -> RangeProof {

        let mut rng = OsRng::new().unwrap();
        let mut nonce = [0u8; 32];
        rng.fill_bytes(&mut nonce);

        let mut retried = false;
        let mut proof = [0; constants::MAX_PROOF_SIZE];
        let mut plen = constants::MAX_PROOF_SIZE as i32;
        loop {
            let err = unsafe {
                // because: "This can randomly fail with probability around one in 2^100.
                // If this happens, buy a lottery ticket and retry."
                ffi::secp256k1_rangeproof_sign(self.ctx,
                                               proof.as_mut_ptr(),
                                               &mut plen,
                                               min,
                                               commit.as_ptr(),
                                               blind.as_ptr(),
                                               nonce.as_ptr(),
                                               0,
                                               64,
                                               value)
            };
            if retried {
                break;
            }
            if err == 1 {
                retried = true;
            }
        }
        RangeProof {
            proof: proof,
            plen: plen as usize,
        }
    }

    /// Verify a proof that a committed value is within a range.
    pub fn verify_range_proof(&self,
                              commit: Commitment,
                              proof: RangeProof)
                              -> Result<ProofRange, Error> {
        let mut min: u64 = 0;
        let mut max: u64 = 0;

        let success = unsafe {
            ffi::secp256k1_rangeproof_verify(self.ctx,
                                             &mut min,
                                             &mut max,
                                             commit.as_ptr(),
                                             proof.proof.as_ptr(),
                                             proof.plen as i32) == 1
        };
        if success {
            Ok(ProofRange {
                min: min,
                max: max,
            })
        } else {
            Err(Error::InvalidRangeProof)
        }
    }

    /// Verify a range proof proof and rewind the proof to recover information sent by its author.
    pub fn rewind_range_proof(&self,
                              commit: Commitment,
                              proof: RangeProof,
                              nonce: [u8; 32])
                              -> ProofInfo {
        let mut value: u64 = 0;
        let mut blind: [u8; 32] = unsafe { mem::uninitialized() };
        let mut message = [0u8; constants::PROOF_MSG_SIZE];
        let mut mlen: i32 = 0;
        let mut min: u64 = 0;
        let mut max: u64 = 0;
        let success = unsafe {
            ffi::secp256k1_rangeproof_rewind(self.ctx,
                                             blind.as_mut_ptr(),
                                             &mut value,
                                             message.as_mut_ptr(),
                                             &mut mlen,
                                             nonce.as_ptr(),
                                             &mut min,
                                             &mut max,
                                             commit.as_ptr(),
                                             proof.proof.as_ptr(),
                                             proof.plen as i32) == 1
        };
        ProofInfo {
            success: success,
            value: value,
            message: message,
            mlen: mlen,
            min: min,
            max: max,
            exp: 0,
            mantissa: 0,
        }
    }

    pub fn range_proof_info(&self, proof: RangeProof) -> ProofInfo {
        let mut exp: i32 = 0;
        let mut mantissa: i32 = 0;
        let mut min: u64 = 0;
        let mut max: u64 = 0;
        let success = unsafe {
            ffi::secp256k1_rangeproof_info(self.ctx,
                                           &mut exp,
                                           &mut mantissa,
                                           &mut min,
                                           &mut max,
                                           proof.proof.as_ptr(),
                                           proof.plen as i32) == 1
        };
        ProofInfo {
            success: success,
            value: 0,
            message: [0; 4096],
            mlen: 0,
            min: min,
            max: max,
            exp: exp,
            mantissa: mantissa,
        }
    }
}