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// Copyright (C) 2019-2021 Algorand, Inc.
// This file is part of go-algorand
//
// go-algorand is free software: you can redistribute it and/or modify
// it under the terms of the GNU Affero General Public License as
// published by the Free Software Foundation, either version 3 of the
// License, or (at your option) any later version.
//
// go-algorand is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
// GNU Affero General Public License for more details.
//
// You should have received a copy of the GNU Affero General Public License
// along with go-algorand. If not, see <https://www.gnu.org/licenses/>.
package compactcert
import (
"fmt"
"math/big"
"github.com/algorand/go-algorand/crypto"
"github.com/algorand/go-algorand/protocol"
)
// The coinChoice type defines the fields that go into the hash for choosing
// the index of the coin to reveal as part of the compact certificate.
type coinChoice struct {
_struct struct{} `codec:",omitempty,omitemptyarray"`
J uint64 `codec:"j"`
SignedWeight uint64 `codec:"sigweight"`
ProvenWeight uint64 `codec:"provenweight"`
Sigcom crypto.Digest `codec:"sigcom"`
Partcom crypto.Digest `codec:"partcom"`
MsgHash crypto.Digest `codec:"msghash"`
}
// ToBeHashed implements the crypto.Hashable interface.
func (cc coinChoice) ToBeHashed() (protocol.HashID, []byte) {
return protocol.CompactCertCoin, protocol.Encode(&cc)
}
// hashCoin returns a number in [0, choice.SignedWeight) with a nearly uniform
// distribution, "randomized" by all of the fields in choice.
func hashCoin(choice coinChoice) uint64 {
h := crypto.HashObj(choice)
i := &big.Int{}
i.SetBytes(h[:])
w := &big.Int{}
w.SetUint64(choice.SignedWeight)
res := &big.Int{}
res.Mod(i, w)
return res.Uint64()
}
// numReveals computes the number of reveals necessary to achieve the desired
// security parameters. See section 8 of the ``Compact Certificates''
// document for the analysis.
//
// numReveals is the smallest number that satisfies
//
// 2^-k >= 2^q * (provenWeight / signedWeight) ^ numReveals
//
// which is equivalent to the following:
//
// signedWeight ^ numReveals >= 2^(k+q) * provenWeight ^ numReveals
//
// To ensure that rounding errors do not reduce the security parameter,
// we compute the left-hand side with rounding-down, and compute the
// right-hand side with rounding-up.
func numReveals(signedWeight uint64, provenWeight uint64, secKQ uint64, bound uint64) (uint64, error) {
n := uint64(0)
sw := &bigFloatDn{}
err := sw.setu64(signedWeight)
if err != nil {
return 0, err
}
pw := &bigFloatUp{}
err = pw.setu64(provenWeight)
if err != nil {
return 0, err
}
lhs := &bigFloatDn{}
err = lhs.setu64(1)
if err != nil {
return 0, err
}
rhs := &bigFloatUp{}
rhs.setpow2(int32(secKQ))
for {
if lhs.ge(rhs) {
return n, nil
}
if n >= bound {
return 0, fmt.Errorf("numReveals(%d, %d, %d) > %d", signedWeight, provenWeight, secKQ, bound)
}
lhs.mul(sw)
rhs.mul(pw)
n++
}
}
func (p Params) numReveals(signedWeight uint64) (uint64, error) {
return numReveals(signedWeight, p.ProvenWeight, p.SecKQ, MaxReveals)
}
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