> 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/cypherock-x0-features/signing-inside-the-x0-card.md).

# Signing inside the X0 Card

This is the defining innovation behind Cypherock X0. Many mobile wallets rely on the smartphone to reconstruct private keys or to sign transactions. Cypherock X0 does neither on the phone.

Every X0 Card carries an EAL6+ certified secure element, the same class of security hardware trusted in the banking industry for debit and credit cards. Inside this secure element, the X0 Card handles every security-sensitive operation of your wallet:

* Wallet generation
* Shard storage
* Wallet reconstruction
* Private key derivation
* Transaction signing
* Secure erasure of temporary secrets

Since every X0 Card can do all of this on its own, any one of your 4 X0 Cards can sign your transactions together with your phone.

## Automatic erasure after every transaction

Security doesn't stop after signing. As soon as a transaction is signed, the X0 Card erases the reconstructed wallet, the derived private key and the shard it received from your phone. Only the card's own encrypted shard remains. This keeps sensitive data in the card's memory for the shortest possible time.

## Your phone is just the interface

During a transaction, your phone:

* Prepares the transaction
* Securely stores one encrypted shard
* Relays encrypted data between you and the X0 Card
* Sends the signed transaction to the blockchain

Your phone never reconstructs your wallet, derives keys or signs. To learn why Cypherock X0 was designed this way, read [Why the X0 Card signs your transactions](file:///design-decisions/cypherock-x0-hardware-architecture/why-the-x0-card-signs-your-transactions.md).