mediocre-blog/srv/pow/pow.go

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2021-08-01 23:54:53 +00:00
// Package pow creates proof-of-work challenges and validates their solutions.
package pow
import (
"bytes"
"crypto/hmac"
"crypto/md5"
"crypto/rand"
"crypto/sha512"
"encoding/binary"
"errors"
"fmt"
"hash"
"time"
"github.com/tilinna/clock"
)
type challengeParams struct {
Target uint32
ExpiresAt int64
Random []byte
}
func (c challengeParams) MarshalBinary() ([]byte, error) {
buf := new(bytes.Buffer)
var err error
write := func(v interface{}) {
if err != nil {
return
}
err = binary.Write(buf, binary.BigEndian, v)
}
write(c.Target)
write(c.ExpiresAt)
if err != nil {
return nil, err
}
if _, err := buf.Write(c.Random); err != nil {
panic(err)
}
return buf.Bytes(), nil
}
func (c *challengeParams) UnmarshalBinary(b []byte) error {
buf := bytes.NewBuffer(b)
var err error
read := func(into interface{}) {
if err != nil {
return
}
err = binary.Read(buf, binary.BigEndian, into)
}
read(&c.Target)
read(&c.ExpiresAt)
if buf.Len() > 0 {
c.Random = buf.Bytes() // whatever is left
}
return err
}
// The seed takes the form:
//
// (version)+(signature of challengeParams)+(challengeParams)
//
// Version is currently always 0.
func newSeed(c challengeParams, secret []byte) ([]byte, error) {
buf := new(bytes.Buffer)
buf.WriteByte(0) // version
cb, err := c.MarshalBinary()
if err != nil {
return nil, err
}
h := hmac.New(md5.New, secret)
h.Write(cb)
buf.Write(h.Sum(nil))
buf.Write(cb)
return buf.Bytes(), nil
}
var errMalformedSeed = errors.New("malformed seed")
func challengeParamsFromSeed(seed, secret []byte) (challengeParams, error) {
h := hmac.New(md5.New, secret)
hSize := h.Size()
if len(seed) < hSize+1 || seed[0] != 0 {
return challengeParams{}, errMalformedSeed
}
seed = seed[1:]
sig, cb := seed[:hSize], seed[hSize:]
// check signature
h.Write(cb)
if !hmac.Equal(sig, h.Sum(nil)) {
return challengeParams{}, errMalformedSeed
}
var c challengeParams
if err := c.UnmarshalBinary(cb); err != nil {
return challengeParams{}, fmt.Errorf("unmarshaling challenge parameters: %w", err)
}
return c, nil
}
// Challenge is a set of fields presented to a client, with which they must
// generate a solution.
//
// Generating a solution is done by:
//
// - Collect up to len(Seed) random bytes. These will be the potential
// solution.
//
// - Calculate the sha512 of the concatenation of Seed and PotentialSolution.
//
// - Parse the first 4 bytes of the sha512 result as a big-endian uint32.
//
// - If the resulting number is _less_ than Target, the solution has been
// found. Otherwise go back to step 1 and try again.
//
type Challenge struct {
Seed []byte
Target uint32
}
// Errors which may be produced by a Manager.
var (
ErrInvalidSolution = errors.New("invalid solution")
ErrExpiredSeed = errors.New("expired seed")
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)
// Manager is used to both produce proof-of-work challenges and check their
// solutions.
type Manager interface {
NewChallenge() Challenge
// Will produce ErrInvalidSolution if the solution is invalid, or
// ErrExpiredSeed if the seed has expired.
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CheckSolution(seed, solution []byte) error
}
// ManagerParams are used to initialize a new Manager instance. All fields are
// required unless otherwise noted.
type ManagerParams struct {
Clock clock.Clock
Store Store
// Secret is used to sign each Challenge's Seed, it should _not_ be shared
// with clients.
Secret []byte
// The Target which Challenges should hit. Lower is more difficult.
//
// Defaults to 0x00FFFFFF
Target uint32
// ChallengeTimeout indicates how long before Challenges are considered
// expired and cannot be solved.
//
// Defaults to 1 minute.
ChallengeTimeout time.Duration
}
func (p ManagerParams) withDefaults() ManagerParams {
if p.Target == 0 {
p.Target = 0x00FFFFFF
}
if p.ChallengeTimeout == 0 {
p.ChallengeTimeout = 1 * time.Minute
}
return p
}
type manager struct {
params ManagerParams
}
// NewManager initializes and returns a Manager instance using the given
// parameters.
func NewManager(params ManagerParams) Manager {
params = params.withDefaults()
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return &manager{
params: params,
}
}
func (m *manager) NewChallenge() Challenge {
target := m.params.Target
c := challengeParams{
Target: target,
ExpiresAt: m.params.Clock.Now().Add(m.params.ChallengeTimeout).Unix(),
Random: make([]byte, 8),
}
if _, err := rand.Read(c.Random); err != nil {
panic(err)
}
seed, err := newSeed(c, m.params.Secret)
if err != nil {
panic(err)
}
return Challenge{
Seed: seed,
Target: target,
}
}
// SolutionChecker can be used to check possible Challenge solutions. It will
// cache certain values internally to save on allocations when used in a loop
// (e.g. when generating a solution).
//
// SolutionChecker is not thread-safe.
type SolutionChecker struct {
h hash.Hash // sha512
sum []byte
}
// Check returns true if the given bytes are a solution to the given Challenge.
func (s SolutionChecker) Check(challenge Challenge, solution []byte) bool {
if s.h == nil {
s.h = sha512.New()
}
s.h.Reset()
s.h.Write(challenge.Seed)
s.h.Write(solution)
s.sum = s.h.Sum(s.sum[:0])
i := binary.BigEndian.Uint32(s.sum[:4])
return i < challenge.Target
}
func (m *manager) CheckSolution(seed, solution []byte) error {
c, err := challengeParamsFromSeed(seed, m.params.Secret)
if err != nil {
return fmt.Errorf("parsing challenge parameters from seed: %w", err)
} else if now := m.params.Clock.Now().Unix(); c.ExpiresAt <= now {
return ErrExpiredSeed
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}
ok := (SolutionChecker{}).Check(
Challenge{Seed: seed, Target: c.Target}, solution,
)
if !ok {
return ErrInvalidSolution
}
expiresAt := time.Unix(c.ExpiresAt, 0)
if err := m.params.Store.MarkSolved(seed, expiresAt.Add(1*time.Minute)); err != nil {
return fmt.Errorf("marking solution as solved: %w", err)
}
return nil
}
// Solve returns a solution for the given Challenge. This may take a while.
func Solve(challenge Challenge) []byte {
chk := SolutionChecker{}
b := make([]byte, len(challenge.Seed))
for {
if _, err := rand.Read(b); err != nil {
panic(err)
} else if chk.Check(challenge, b) {
return b
}
}
}