Problem
If the system verifies that a transaction hash is valid on-chain but does not record it as used, the same transaction hash can be replayed indefinitely to access the AI service for free after paying once:
func verifyPayment(txHash string) bool {
// Only checks if tx exists and has correct amount on-chain.
// Does NOT check if this tx was already used.
return checkBlockchain(txHash)
}An attacker pays once, receives a valid transaction hash, and replays it thousands of times.
Proposed Fix
Store used transaction hashes in a Redis set (fast, with TTL matching the on-chain finality window) or a database:
import "github.com/redis/go-redis/v9"
var rdb = redis.NewClient(&redis.Options{Addr: "localhost:6379"})
func isTransactionUsed(ctx context.Context, txHash string) (bool, error) {
return rdb.SIsMember(ctx, "used_tx_hashes", txHash).Result()
}
func markTransactionUsed(ctx context.Context, txHash string) error {
// Keep for 30 days (transaction replay window)
pipe := rdb.Pipeline()
pipe.SAdd(ctx, "used_tx_hashes", txHash)
pipe.Expire(ctx, "used_tx_hashes", 30*24*time.Hour)
_, err := pipe.Exec(ctx)
return err
}
func handlePayment(w http.ResponseWriter, r *http.Request) {
// ...after verifying amount...
used, err := isTransactionUsed(ctx, req.TxHash)
if err != nil || used {
http.Error(w, "Transaction already used", http.StatusPaymentRequired)
return
}
if err := markTransactionUsed(ctx, req.TxHash); err != nil {
http.Error(w, "Internal error", http.StatusInternalServerError)
return
}
serveAIResponse(w, req.Prompt)
}Acceptance Criteria
Problem
If the system verifies that a transaction hash is valid on-chain but does not record it as used, the same transaction hash can be replayed indefinitely to access the AI service for free after paying once:
An attacker pays once, receives a valid transaction hash, and replays it thousands of times.
Proposed Fix
Store used transaction hashes in a Redis set (fast, with TTL matching the on-chain finality window) or a database:
Acceptance Criteria