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// Package pow implements a self-contained hashcash-style proof of work for
// the /submit endpoint. The client computes "<unix-ts>:<nonce>" locally
// (no challenge round-trip) such that SHA-256 of the stamp has a required
// number of leading zero bits. The server only has to verify, not compute.
package pow
import (
"crypto/sha256"
"errors"
"math/bits"
"strconv"
"strings"
"sync"
"time"
)
// MaxAge bounds how long a stamp remains acceptable, limiting replay window
// and precomputation of stamps far in advance.
const MaxAge = 5 * time.Minute
// maxClockSkew tolerates a stamp timestamped slightly ahead of the server's clock.
const maxClockSkew = time.Minute
// Verifier checks proof-of-work stamps against a fixed difficulty and keeps
// a bounded in-memory cache of seen stamps for replay protection.
type Verifier struct {
difficulty int
mu sync.Mutex
seen map[string]time.Time
}
func NewVerifier(difficultyBits int) *Verifier {
return &Verifier{
difficulty: difficultyBits,
seen: make(map[string]time.Time),
}
}
// Verify validates stamp "<unix-ts>:<nonce>": well-formed, not expired,
// meets the required leading-zero-bit difficulty, and not replayed.
func (v *Verifier) Verify(stamp string) error {
tsPart, _, ok := strings.Cut(stamp, ":")
if !ok {
return errors.New("malformed stamp")
}
tsSec, err := strconv.ParseInt(tsPart, 10, 64)
if err != nil {
return errors.New("malformed timestamp")
}
ts := time.Unix(tsSec, 0)
now := time.Now()
if now.Sub(ts) > MaxAge || ts.Sub(now) > maxClockSkew {
return errors.New("stamp expired or in the future")
}
sum := sha256.Sum256([]byte(stamp))
if leadingZeroBits(sum[:]) < v.difficulty {
return errors.New("insufficient proof of work")
}
v.mu.Lock()
defer v.mu.Unlock()
v.cleanupLocked(now)
if _, dup := v.seen[stamp]; dup {
return errors.New("replayed stamp")
}
v.seen[stamp] = now
return nil
}
func (v *Verifier) cleanupLocked(now time.Time) {
for stamp, seenAt := range v.seen {
if now.Sub(seenAt) > MaxAge {
delete(v.seen, stamp)
}
}
}
func leadingZeroBits(b []byte) int {
count := 0
for _, by := range b {
if by == 0 {
count += 8
continue
}
count += bits.LeadingZeros8(by)
break
}
return count
}
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