Transaction Inputs

Introduction

These rules apply to transaction inputs, excluding miner transactions.

Necessary Functions/Definitions

Default Minimum Decoys

This is the default number of decoys an input must at least have.

There are exceptions to this being the minimum decoy size for all transactions. See further down in Rules.

Hard-Fork	Minimum Decoys¹
1	N/A
2 to 5	2
6	4
7	6
8 to 14	10
15+	15

Minimum And Maximum Decoys Used

To check a transaction inputs ring size we must first get the minimum and maximum number of decoys used in the transactions inputs².

So if this was our transactions:

Input	1	2	3
Ring size	12	8	16

The minium and maximum amount of decoys would be 7 and 15 respectively.

Mixable And Un-Mixable Inputs

A mixable input is one that has enough outputs on the chain with the same amount to be able to build a ring with the minimum amount of decoys needed.

A ringCT input, aka an output with 0 amount, is always considered mixable³.

For other inputs you first get the amount of outputs on chain with that amount and check if thats less than or equal to the default minimum amount of decoys if it is then the input is un-mixable otherwise it is mixable⁴.

Rules

No Empty Inputs

The transaction must have at least 1 input⁵.

No Empty decoys

All inputs must have decoys⁶.

Input Type

All inputs must be of type txin_to_key⁷.

Inputs Must Not Overflow

The inputs when summed must not overflow a u64 and the outputs when summed must not either⁸.

Unique Ring Members

From hard-fork 6 all ring members in an input must be unique, this is done by checking that no key_offset after the first is 0⁹.

Unique Key Image

The key image must be unique in a transaction¹⁰ and the whole chain ¹¹.

Torsion Free Key Image

The key image must be a canonical prime order point¹².

Minimum Decoys

These rules are in effect from hard fork 2.

First you get the minimum number of decoys used in the transaction.

Then you get the amount of mixable and un-mixable inputs.

Now get the default minimum decoys allowed for the current hard-fork.

If the minimum amount of decoys used in the transaction is less than the default minimum decoys allowed then the transaction is only allowed if there is at least one input which is un-mixable¹³.

If there is an un-mixable then the transaction is not allowed to have more than 1 mixable input as well.

Special rules¹⁴:

For hard-fork 15 both 10 and 15 decoys are allowed.
From hard-fork 8 upwards the minimum amount of decoys used in a transaction must be equal to the minimum allowed.