How does eCash prevent double spending?

eCash is designed to be fast, low-cost, private, and secure. One of the key challenges it solves is double spending – the risk that someone could try to spend the same piece of digital money more than once.

eCash prevents this by using blind signatures, a cryptographic technique that ensures notes are valid without revealing their details to anyone except the holder, allowing eCash to truly work like digital cash.

Defining Double Spending

Double spending is when the same digital notes representing money are used more than once. Unlike physical cash, which changes hands and cannot be reused, digital files can be copied. Without safeguards, someone could attempt to copy eCash notes and spend them more than once.

How Blind Signatures Work

Blind signatures allow the federation issuing or minting the eCash to sign the notes in a way that proves they are valid without ever opening them or seeing their unique details, hence the term "blind." 

This feature is critical during the minting process because once the notes are opened by the federation when they are being redeemed for bitcoin, they are marked as spent and are no longer valid.

Preventing Double Spending

Each eCash note has a unique serial number that only becomes visible (to the issuing federation) when you spend it (ie redeem it in exchange for bitcoin). Here’s how double spending is prevented:

• When you spend eCash: You present the unopened signed note back to the issuing federation.

• The federation checks: It verifies its signature and opens the note to record the serial number as spent.

• If someone tries to reuse it: Any second attempt to spend the same note is immediately rejected because the federation already knows it has been used based on its serial number.

Why This Matters

As explained above, the use of blind signatures prevents double spending, ensuring that:

• Privacy is preserved: A federation cannot link your redemption of eCash to your later spending.

• Security is guaranteed: No one can spend the same note twice because serial numbers are tracked.

• Trust is minimized: You don’t need to trust guardians to behave honestly with your transaction details, since blind signatures ensure that transaction activity cannot be linked to individuals.