> For the complete documentation index, see [llms.txt](https://learn.cypherock.com/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://learn.cypherock.com/design-decision/cypherock-x0-hardware-architecture/why-the-x0-card-signs-your-transactions.md).

# Why the X0 card signs your transactions

Most mobile wallets rely on the smartphone to store, reconstruct, or use private keys. Cypherock X0 was designed so that the smartphone never has to be trusted with them. Every security-sensitive operation on key material happens inside the EAL6+ certified secure element of the X0 Card.

When a transaction is made with Cypherock X0:

{% stepper %}
{% step %}

## Prepare the transaction

The X0 Wallet App prepares the transaction. The user unlocks the app's shard with their biometrics or app passcode and enters the wallet PIN.
{% endstep %}

{% step %}

## Tap an X0 Card

The user taps any one of their X0 Cards on the phone. The app sends its encrypted shard, the PIN and the transaction data to be signed to the card over an encrypted NFC session.
{% endstep %}

{% step %}

## Verify the PIN and reconstruct the wallet

The X0 Card verifies the PIN and reconstructs the wallet from its own shard and the phone's shard, entirely within its secure element.
{% endstep %}

{% step %}

## Sign and erase key material

The X0 Card derives the private key, signs the transaction, and immediately erases the reconstructed wallet, the derived private key, and the phone's shard from its memory.
{% endstep %}

{% step %}

## Broadcast the transaction

Only the signature leaves the card. The X0 Wallet App completes the transaction and broadcasts it to the blockchain.
{% endstep %}
{% endstepper %}

There are three main reasons behind this design:

1. **Consistent security on every phone:** Smartphones vary enormously in their hardware security across manufacturers and models, and their operating systems are frequent targets of attacks. If signing happened on the phone, the security of a wallet would depend on which phone its owner happens to use. By performing every operation on key material inside the X0 Card, every Cypherock X0 user gets the same EAL6+ certified hardware protection, whether they use a flagship or an entry-level phone.
2. **The phone never handles your private key:** The phone stores one encrypted shard, which on its own reveals nothing about the wallet. It never reconstructs the wallet, never derives a private key, and never signs a transaction.
3. **The same principle as Cypherock X1:** In Cypherock X1, wallet reconstruction and transaction signing happen on the X1 Vault, a dedicated device, rather than on the computer running cySync. Cypherock X0 keeps that separation. The trusted computation moves from the X1 Vault into the X0 Card, and the smartphone only takes on the role of the interface.