Get the YubiKey's Key Agreement public key

Command APDU info

CLA	INS	P1	P2	Lc	Data	Le
00	10	00	00	06	06 A2 01 02 02 02	(absent)

The Ins byte (instruction) is 10, which is the byte for CTAPHID_CBOR. That means the command information is in a CBOR encoded structure in the Data.

The data consists of the CTAP Command Byte and the CBOR encoding of the command's parameters. In this case, the CTAP Command Byte is 06, which is the command "authenticatorClientPin". The CBOR encoding is the following:

  A2         map containing two elements
     01      key (of key/value) specifying ...
        0x   ... PIN/UV protocol (x=1 for protocol one, x=2 for protocol two)
     02      key specifying ...
        02   ... subcommand, 02 = KeyAgreement

Response APDU info

Response APDU for a successful get

Total Length: variable + 2
Data Length: variable

Data	SW1	SW2
encoded info	90	00

The info returned is CBOR encoded. It has a structure similar to the following.

  A5
     01 --int--
     03 --int--
     20 --int--
     21 --byte string--
     22 --byte string--

The integers describe the algorithm and curve, and the byte strings are the x- and y-coordinates of the public key.

The lengths of the byte string are dependent on the algorithm. Currently only one algorithm is supported, ECDH using the NIST curve P-256. That means the byte strings are both 32 bytes long. The total length of the encoding will be 78 bytes. Hence, the total length of the response will be 80 bytes.

Response APDU when no protocol is given

Total Length: 2
Data Length: 0

Data	SW1	SW2
(no data)	6F	14

Response APDU when an unsupported protocol is specified

Total Length: 2
Data Length: 0

Data	SW1	SW2
(no data)	6F	33