// VerifyTransaction checks that the inputs to the transaction exist,
// that the transaction does not create or destroy coins and that the
// signatures on the transaction are valid
func (bc Blockchain) VerifyTransaction(tx coin.Transaction) error {
//CHECKLIST: DONE: check for duplicate ux inputs/double spending
//CHECKLIST: DONE: check that inputs of transaction have not been spent
//CHECKLIST: DONE: check there are no duplicate outputs
// Q: why are coin hours based on last block time and not
// current time?
// A: no two computers will agree on system time. Need system clock
// indepedent timing that everyone agrees on. fee values would depend on
// local clock
// Check transaction type and length
// Check for duplicate outputs
// Check for duplicate inputs
// Check for invalid hash
// Check for no inputs
// Check for no outputs
// Check for non 1e6 multiple coin outputs
// Check for zero coin outputs
// Check valid looking signatures
if err := tx.Verify(); err != nil {
return err
}
uxIn, err := bc.unspent.GetMultiple(tx.In)
if err != nil {
return err
}
// Checks whether ux inputs exist,
// Check that signatures are allowed to spend inputs
if err := tx.VerifyInput(uxIn); err != nil {
return err
}
// Get the UxOuts we expect to have when the block is created.
uxOut := coin.CreateUnspents(bc.Head().Head, tx)
// Check that there are any duplicates within this set
if uxOut.HasDupes() {
return errors.New("Duplicate unspent outputs in transaction")
}
if DebugLevel1 {
// Check that new unspents don't collide with existing. This should
// also be checked in verifyTransactions
for i := range uxOut {
if bc.unspent.Has(uxOut[i].Hash()) {
return errors.New("New unspent collides with existing unspent")
}
}
}
// Check that no coins are lost, and sufficient coins and hours are spent
err = coin.VerifyTransactionSpending(bc.Time(), uxIn, uxOut)
if err != nil {
return err
}
return nil
}