Stacks App

General structure

The general structure of commands and responses is as follows:

Commands

Field	Type	Content	Note
CLA	byte (1)	Application Identifier	0x09
INS	byte (1)	Instruction ID
P1	byte (1)	Parameter 1
P2	byte (1)	Parameter 2
L	byte (1)	Bytes in payload
PAYLOAD	byte (L)	Payload

Response

Field	Type	Content	Note
ANSWER	byte (?)	Answer	depends on the command
SW1-SW2	byte (2)	Return code	see list of return codes

Return codes

Return code	Description
0x6400	Execution Error
0x6982	Empty buffer
0x6983	Output buffer too small
0x6986	Command not allowed
0x6D00	INS not supported
0x6E00	CLA not supported
0x6F00	Unknown
0x9000	Success

---

Command definition

GET_VERSION

Command

Field	Type	Content	Expected
CLA	byte (1)	Application Identifier	0x09
INS	byte (1)	Instruction ID	0x00
P1	byte (1)	Parameter 1	ignored
P2	byte (1)	Parameter 2	ignored
L	byte (1)	Bytes in payload	0

Response

Field	Type	Content	Note
TEST	byte (1)	Test Mode	0xFF means test mode is enabled
MAJOR	byte (1)	Version Major
MINOR	byte (1)	Version Minor
PATCH	byte (1)	Version Patch
LOCKED	byte (1)	Device is locked
SW1-SW2	byte (2)	Return code	see list of return codes

INS_GET_ADDR_SECP256K1

Command

Field	Type	Content	Expected
CLA	byte (1)	Application Identifier	0x09
INS	byte (1)	Instruction ID	0x01
P1	byte (1)	Request User confirmation	No = 0
P2	byte (1)	Parameter 2	ignored
L	byte (1)	Bytes in payload	(depends)
Path[0]	byte (4)	Derivation Path Data	0x8000002c
Path[1]	byte (4)	Derivation Path Data	0x8000167d
Path[2]	byte (4)	Derivation Path Data	?
Path[3]	byte (4)	Derivation Path Data	?
Path[4]	byte (4)	Derivation Path Data	?

Response

Field	Type	Content	Note
PK	byte (65)	Public Key
ADDR_RAW_LEN	byte (1)	ADDR_RAW Length
ADDR_RAW	byte (??)	Address as Raw Bytes
ADDR_HUMAN_LEN	byte (1)	ADDR_HUMAN Len
ADDR_HUMAN	byte (??)	Address as String
SW1-SW2	byte (2)	Return code	see list of return codes

---

INS_GET_AUTH_PUBKEY

Command

Field	Type	Content	Expected
CLA	byte (1)	Application Identifier	0x09
INS	byte (1)	Instruction ID	0x03
P1	byte (1)	Retrieval mode	0 (Only retrieve)
P2	byte (1)	Parameter 2	0 (ignored)
L	byte (1)	Bytes in payload	(depends)
Path	byte (?)	Derivation Path Data	(see below)

Derivation Path

• Starts with "m"
• Can be either 6 or 4 levels deep
• For 6 levels: "m/44'/5757'/5'/0/3"
• For 4 levels (Identity): "m/888'/0'/<account>"
• Each level is serialized as a 4-byte little-endian unsigned integer
• Hardened levels (with ') have 0x80000000 added to their value

Response

Field	Type	Content	Note
publicKey	byte (??)	Public Key
address	byte (??)	Address as String
SW1-SW2	byte (2)	Return code	0x9000 for success

Processing

1. The command is sent with the serialized derivation path as payload.
2. The device derives the public key and address for the given path.
3. The device returns the public key and address in the response.

Notes

• The CLA (0x09) is specific to this application.
• The INS (0x03) identifies this as a GET_AUTH_PUBKEY operation.
• P1 is set to 0, indicating "only retrieve" mode.
• P2 is ignored in this command.
• The expected successful return code is 0x9000.
• In case of an error, the response will include a return code and an error message instead of the public key and address.

---

INS_SIGN_SECP256K1

Command

Field	Type	Content	Expected
CLA	byte (1)	Application Identifier	0x09
INS	byte (1)	Instruction ID	0x02
P1	byte (1)	Payload desc	0 = init
			1 = add
			2 = last
P2	byte (1)	----	not used
L	byte (1)	Bytes in payload	(depends)

The first packet/chunk includes only the derivation path

All other packets/chunks contain data chunks that are described below

First Packet

Field	Type	Content	Expected
Path[0]	byte (4)	Derivation Path Data	0x8000002c
Path[1]	byte (4)	Derivation Path Data	0x8000167d
Path[2]	byte (4)	Derivation Path Data	?
Path[3]	byte (4)	Derivation Path Data	?
Path[4]	byte (4)	Derivation Path Data	?

Other Chunks/Packets

Field	Type	Content	Expected
Data	bytes...	Message

Data is defined as:

Field	Type	Content	Expected
Message	bytes..	Data to sign

Response

Field	Type	Content	Note
secp256k1 R	byte (32)	Signature
secp256k1 S	byte (32)	Signature
secp256k1 V	byte (1)	Signature
SIG	byte (variable)	Signature	DER format
SW1-SW2	byte (2)	Return code	see list of return codes

INS_SIGN_JWT_SECP256K1

Command

Field	Type	Content	Expected
CLA	byte (1)	Application Identifier	0x09
INS	byte (1)	Instruction ID	0x04
P1	byte (1)	Chunk index	1 to N
P2	byte (1)	Total chunks	N
L	byte (1)	Bytes in payload	(depends)
Path	byte (?)	Derivation Path Data	(see below)
Message	byte (?)	JWT message to sign	(variable)

Derivation Path

• Starts with "m"
• Can be either 6 or 4 levels deep
• For 6 levels: "m/44'/5757'/5'/0/3"
• For 4 levels (Identity): "m/888'/0'/<account>"
• Each level is serialized as a 4-byte little-endian unsigned integer
• Hardened levels (with ') have 0x80000000 added to their value

Response

Field	Type	Content	Note
returnCode	byte (2)	Return code	see list of return codes
errorMessage	byte (??)	Error message string	Optional
postSignHash	byte (32)	Hash after signing	Optional
signatureCompact	byte (65)	Compact signature	Optional
signatureDER	byte (??)	DER-encoded signature	Optional

Processing

1. The message is split into chunks of up to 250 bytes each.
2. Each chunk is sent in a separate APDU command.
3. P1 indicates the current chunk index (starting from 1).
4. P2 indicates the total number of chunks.
5. The first chunk includes the derivation path.
6. Subsequent chunks only contain message data.
7. The device processes all chunks and returns the final result.

Notes

• The CLA (0x09) is specific to this application.
• The INS (0x04) identifies this as a SIGN_JWT_SECP256K1 operation.
• The message length is determined by the total payload across all chunks.
• Error responses may not include all fields of the success response.