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Derive Native Script Hash

Description

Given a native script, defined in the Cardano CDDL, compute it's hash.

Derive Native Script Hash Limitations

The native script structure is recursive (with unlimited depth for nested scripts). Ledger, however, limits the depth to 10, due to the limited memory available on Ledger.

Derivation

A native script can be complex and contain nested native scripts. Therefore, the exchange can consist of multiple APDUs. Ledger keeps track of the internal state and controls the correctness of the APDUs received. If an unexpected or out of place APDU is received Ledger will abort the transaction.

Ledger internally distinguishes between two native script groups:

  1. Complex native scripts - those which contain nested scripts (types: ALL, ANY, N_OF_K)
  2. Simple native scripts - those which don't contain scripts (types: PUBKEY, INVALID_BEFORE, INVALID_HEREAFTER)

For those two script groups Ledger recognizes two different calls that can be combined to represent native scripts of arbitrary complexity:

  1. Start complex script
  2. Add simple script

After the whole native script is received, Ledger expects a finish call, which returns the computed native script hash:

General command

Field Value
CLA 0xD7
INS 0x12
P1 script phase (0x01 to 0x03, see below)
P2 unused

Start complex script

Marks the beginning of a complex native script.

Command

Field Value
P1 0x01

Data

Field Length Comments
script type 1 Script type according to the CDDL
number of nested scripts 4
number of required nested scripts 4 Only when script type is N_OF_K

The number of required nested scripts must be omitted for script types ALL and ANY.

Add simple script

Adds a simple script.

Command

Field Value
P1 0x02

data

PUBKEY

Field Length Comments
script type 1 Script type according to the CDDL
pubkey type 1 See below for more info
pubkey path variable Only when pubkey type is KEY_REFERENCE_PATH, BIP44 path. See Get Public Keys call for format example
pubkey hash 28 Only when pubkey type is KEY_REFERENCE_HASH

This native script can be either specified by a key path if the public key is owned by the device (KEY_REFERENCE_PATH = 1) and can be derived, or by a specifying the pubkey hash (KEY_REFERENCE_HASH = 2). This is encoded in the pubkey type field.

INVALID_BEFORE/INVALID_HEREAFTER

Field Length Comments
script type 1 Script type according to the cddl
timelock 8

Finish native script

Explicitly state the end of the whole native script and specify in what format should the native script hash be shown on Ledger.

Command

Field Value
P1 0x03

data

Field Length Comments
display format 1 See below for possible values

Display format can be:

  • 0x01 for bech32 encoded hash
  • 0x02 for hash shown as a policy id

response

Field Length Comments
Hash 28 The native script hash