Decrypt

This command allows secure data decryption or symmetric key derivation using an internal EC key from the device’s BIP32 seed tree. Inspired by OpenPGP smartcards, it enables asymmetric encryption and decryption using a fixed derivation path, ensuring that only the private key securely stored on the card can decrypt the content encrypted using the corresponding public key.

This command supports two operational modes:

P1 = 0: Outputs the symmetric key derived from ECDH.
P1 ≠ 0: Uses the symmetric key internally to decrypt encrypted data using AES-CBC.

def decrypt(pub_key: Any, PIN: str):
	card.decrypt(pub_key, PIN)

For example, decrypt(pub_key, ”123456”)

📘 Note

A secure channel must be opened.
Either PIN authentication or user signature authentication is required.
The card must have a seed or extended key loaded before using this command.

Application Protocol Data Unit (APDU) Components

The following table outlines the components of the Application Protocol Data Unit (APDU).

Field	Description	Value
CLA	This field specifies the class of the instruction.	0x80
INS	This field specifies the particular command or operation that the smart card or secure element should execute.	0xC4
P1	First parameter of the instruction that specifies the details about the operation being requested.	provide symmetric key OR decrypt in card
Data	Key data	PIN (in case not user auth) \| PubKey \| dataEncrypted (when P1=1)

Decrypt Command Modes

Condition	Data Value
P1 = 0x00 & Secure Channel Opened & No User Auth (PIN required) & Data Length = 74 bytes	Outputs symmetric key Data: PIN (9 bytes padded to 9) + PubKey (65 bytes) Performs SHA256(PrivateKey . PubKey) → returns 32-byte symmetric key
P1 = 0x00 & Secure Channel Opened & User Authenticated via Signature (no PIN) & Data Length = 65 bytes	Outputs symmetric key Data: PubKey (65 bytes) Performs SHA256(PrivateKey . PubKey) → returns 32-byte symmetric key
P1 = 0x01 & Secure Channel Opened & No User Auth (PIN required) & Data Length ≥ 74 bytes & EncData % 16 == 0	Decrypts in-card using internal AES-128-CBC Data: PIN (9 bytes) + PubKey (65 bytes) + EncryptedData (N bytes) ECDH Key: SHA256(PrivateKey . PubKey) → Split into `IV
P1 = 0x01 & Secure Channel Opened & User Authenticated via Signature (no PIN) & Data Length ≥ 65 bytes & EncData % 16 == 0	Decrypts in-card using internal AES-128-CBC Data: PubKey (65 bytes) + EncryptedData (N bytes) ECDH Key: SHA256(PrivateKey . PubKey) → Split into `IV

📘 Note

PubKey Format: Must be 65 bytes in X9.62 uncompressed format: 0x04 | X (32B) | Y (32B)
Encrypted Data (EncData): Must be padded to 16-byte boundaries (AES block size)
PIN: Right-padded with 0x00 to 9 bytes if present
PIN Effects:
If correct: state changes to “authed”
If incorrect: state resets (user auth invalidated)

Response

The following table outlines the possible responses that you will receive

Response Code	Description
0x9000	Success
0x6A80	The data length is not correct
0x6985	No key/seed loaded
0x63Cx	The PIN is not correct
0x6982	(outside secure channel) The data input length is far too long