Remote Attestation: isv_app yields 0x2003 (Signature invalid) while processing msg2

I would like to implement Service Provider functionality in a Node JS server. However, even though I think I follow the algorithm in the RA Sample, the SGX-enabled client (in my case, isv_app from RemoteAttestation project in sgxsdk SampleCode) does not accept MSG2 from the SP, yielding **0x2003 error** (invalid signature), returned by **sgx_ra_proc_msg2_ex** function.

The following JS code is the SP functionality to process msg1 and generate msg2:

```const crypto = require('crypto');
const aesCmac = require('node-aes-cmac').aesCmac;
const EC = require('elliptic').ec;
const ec = new EC('p256');
const pepECDH = crypto.createECDH('prime256v1');

const SPID = 'C8C56121818E4F00FE4AB868389F3059';
const X_LENGTH = 32;
const Y_LENGTH = 64;

//Utilities
const swap = function(l_index, h_index, array) {
let aux;
while (l_index < h_index) {
aux = array[l_index];
array[l_index] = array[h_index];
array[h_index] = aux;
l_index++;
h_index--;
}
};

const changeKeyEndianess = function(key) {
swap(0, X_LENGTH - 1, key);
swap(X_LENGTH, Y_LENGTH - 1, key);
};

const generateCMAC = function(key, data) {
const options = { returnAsBuffer: true };
return aesCmac(key, data, options);
};

const deriveKey = function(sharedKey, label) {
swap(0, sharedKey.length - 1, sharedKey);
const key0s = Buffer.from('00000000000000000000000000000000', 'hex');

const cmacKey0s = generateCMAC(key0s, Buffer.from(sharedKey));
//0x01 || SMK || 0x00 || 0x80 || 0x00
const auxInfo = Buffer.concat([
Buffer.from('01', 'hex'),
Buffer.from(label),
Buffer.from('008000', 'hex'),
]);
const derivedKey = generateCMAC(cmacKey0s, auxInfo);
return derivedKey;
};

//1. Generate a random EC key using the P-256 curve. This key will become Gb.
pepECDH.generateKeys();
var Gb_BE = pepECDH.getPublicKey().slice(1); //remove 0x04, used only for elliptic library internals
var Gb_LE = Gb_BE;
changeKeyEndianess(Gb_LE); //pep public key now in Little Endian
//2. Derive the key derivation key (KDK) from Ga and Gb:
//2.1. Compute the shared secret using the client's public session key, Ga,
//and the service provider's private session key (obtained from Step 1), Gb.
// The result of this operation will be the x coordinate of Gab, denoted as Gabx.
var Ga_raw = msg.slice(0, 64); //client's public key, Little Endian
var Ga_LE = Buffer.from(Ga_raw);
let Ga_BE = Ga_LE;
changeKeyEndianess(Ga_BE); //done for computeSecret function (to Big Endian)
Ga_BE = [0x04, ...Ga_BE]; //the elliptic lib requires 0x04 at the beginning of the key to work properly
var Gabx_BE = pepECDH.computeSecret(new Buffer(Ga_BE));
//2.2. Convert Gabx to little-endian byte order by reversing its bytes.
var Gabx_LE = Gabx_BE;
changeKeyEndianess(Gabx_LE); //now in little-endian
//2.3. Perform an AES-128 CMAC on the little-endian form of Gabx
//using a block of 0x00 bytes for the key.
//Derive the SMK from the KDK by performing an AES-128 CMAC on the byte sequence:
//0x01 || SMK || 0x00 || 0x80 || 0x00 --> SMK is a literal string without quotes
var derivedKeySMK = deriveKey(Gabx_LE, 'SMK');
//1. Determine the quote type that should be requested from the client
//(0x0 for unlinkable, and 0x1 for linkable). Note that this is a service provider
//policy decision, and the SPID must be associated with the correct quote type.
var quoteType = Buffer.from('0000', 'hex');
//2. Set the KDF_ID. Normally this is 0x1.
var kdfID = Buffer.from('0100', 'hex');
var spid = Buffer.from(SPID, 'hex');
//3. Calculate the ECDSA signature of:
//Gbx || Gby || Gax || Gay
//(traditionally written as r || s) with the service provider's EC private key.
var concatenatedPublicKeys = Buffer.concat([Gb_LE, Ga_LE]);
var pepPrivateKey = pepECDH.getPrivateKey();
var keysHash = crypto
.createHash('sha256')
.update(concatenatedPublicKeys)
.digest();

var SigSP_BE = ec.sign(keysHash, pepPrivateKey, { canonical: true });
var r = SigSP_BE.r.toBuffer('le', 32);
var s = SigSP_BE.s.toBuffer('le', 32);
var SigSP_LE = Buffer.concat([r, s]);
//4. Calculate the AES-128 CMAC of:
//Gb || SPID || Quote_Type || KDF_ID || SigSP
//using the SMK (derived in Step 3) as the key.
var toCmac = Buffer.concat([Gb_LE, spid, quoteType, kdfID, SigSP_LE]);
var Cmaced = generateCMAC(derivedKeySMK, toCmac);```

```
Then, the SP contacts IAS and receives code 200 with empty SigRL. Here comes the isv_app output (msg2 printed out after parsing):

```
Sent MSG1 to remote attestation service provider. Received the following MSG2:
176 bytes:
{
0x2, 0x0, 0x0, 0xa8, 0x0, 0x0, 0x0, 0x0,
0x7b, 0x7f, 0xe3, 0x71, 0x6, 0x6f, 0xf2, 0xd4,
0x86, 0x14, 0xa6, 0x87, 0x1a, 0x50, 0x5f, 0x9b,
0x13, 0x11, 0x11, 0x7a, 0x73, 0xaa, 0x8a, 0x80,
0xad, 0xbc, 0xcb, 0x5a, 0xdb, 0xf4, 0x77, 0x2f,
0x5c, 0xb9, 0x12, 0xf3, 0x20, 0x47, 0x34, 0x88,
0x1, 0x75, 0x46, 0xab, 0xb1, 0x24, 0x8d, 0x49,
0xa0, 0xab, 0x73, 0xf9, 0xe6, 0x43, 0x74, 0x4b,
0xe9, 0x1b, 0x5b, 0xba, 0x67, 0xd, 0x5, 0x16,
0xc8, 0xc5, 0x61, 0x21, 0x81, 0x8e, 0x4f, 0x0,
0xfe, 0x4a, 0xb8, 0x68, 0x38, 0x9f, 0x30, 0x59,
0x0, 0x0, 0x1, 0x0, 0xea, 0x50, 0x1c, 0xcb,
0x72, 0xa5, 0x59, 0xb3, 0x35, 0x46, 0xa3, 0x4d,
0xd1, 0xe1, 0xf8, 0xb2, 0x17, 0x8e, 0xea, 0x40,
0xd0, 0x65, 0xc3, 0x54, 0xc, 0x67, 0xc9, 0x10,
0x34, 0xcc, 0x6f, 0x49, 0x11, 0xa9, 0x90, 0x91,
0xba, 0x63, 0x6a, 0x70, 0xd5, 0xed, 0xe2, 0xa4,
0x3c, 0x57, 0x87, 0x84, 0x92, 0x13, 0xc0, 0x5e,
0x28, 0x28, 0x5e, 0x46, 0x31, 0xf8, 0x80, 0x84,
0x7, 0xaa, 0x99, 0x0, 0xb5, 0x3f, 0x5d, 0x23,
0x9f, 0x55, 0x6b, 0x28, 0x9, 0x8, 0xe9, 0x5d,
0xf6, 0x4a, 0x9a, 0xfe, 0x0, 0x0, 0x0, 0x0
}

A more descriptive representation of MSG2:
RESPONSE TYPE: 0x2
RESPONSE STATUS: 0x0 0x0
RESPONSE BODY SIZE: 168
MSG2 gb - 64 bytes:
{
0x7b, 0x7f, 0xe3, 0x71, 0x6, 0x6f, 0xf2, 0xd4,
0x86, 0x14, 0xa6, 0x87, 0x1a, 0x50, 0x5f, 0x9b,
0x13, 0x11, 0x11, 0x7a, 0x73, 0xaa, 0x8a, 0x80,
0xad, 0xbc, 0xcb, 0x5a, 0xdb, 0xf4, 0x77, 0x2f,
0x5c, 0xb9, 0x12, 0xf3, 0x20, 0x47, 0x34, 0x88,
0x1, 0x75, 0x46, 0xab, 0xb1, 0x24, 0x8d, 0x49,
0xa0, 0xab, 0x73, 0xf9, 0xe6, 0x43, 0x74, 0x4b,
0xe9, 0x1b, 0x5b, 0xba, 0x67, 0xd, 0x5, 0x16
}
MSG2 spid - 16 bytes:
{
0xc8, 0xc5, 0x61, 0x21, 0x81, 0x8e, 0x4f, 0x0,
0xfe, 0x4a, 0xb8, 0x68, 0x38, 0x9f, 0x30, 0x59
}
MSG2 quote_type : 0
MSG2 kdf_id : 1
MSG2 sign_gb_ga - 64 bytes:
{
0xea, 0x50, 0x1c, 0xcb, 0x72, 0xa5, 0x59, 0xb3,
0x35, 0x46, 0xa3, 0x4d, 0xd1, 0xe1, 0xf8, 0xb2,
0x17, 0x8e, 0xea, 0x40, 0xd0, 0x65, 0xc3, 0x54,
0xc, 0x67, 0xc9, 0x10, 0x34, 0xcc, 0x6f, 0x49,
0x11, 0xa9, 0x90, 0x91, 0xba, 0x63, 0x6a, 0x70,
0xd5, 0xed, 0xe2, 0xa4, 0x3c, 0x57, 0x87, 0x84,
0x92, 0x13, 0xc0, 0x5e, 0x28, 0x28, 0x5e, 0x46,
0x31, 0xf8, 0x80, 0x84, 0x7, 0xaa, 0x99, 0x0
}
MSG2 mac - 16 bytes:
{
0xb5, 0x3f, 0x5d, 0x23, 0x9f, 0x55, 0x6b, 0x28,
0x9, 0x8, 0xe9, 0x5d, 0xf6, 0x4a, 0x9a, 0xfe
}
MSG2 sig_rl -
( null )

Error, call sgx_ra_proc_msg2_ex fail. p_msg3 = 0x(nil) [main]. Ret: 0x00002003
Call enclave_ra_close success.
Enter a character before exit ...
```

I have already posted this problem on Intel Forum, where I learned that I am preparing the MSG2 correctly and Stack Overflow (no answers so far). I am really out of ideas what to do; the fields are passed correctly, only the signature does not pass the check. Maybe there is a bug in sgx_ra_proc_msg2_ex function?

Thanks for help.
Regards,
Kris

Start with the Node.js MSG2 construction shown in the issue and the RemoteAttestation SampleCode path that calls sgx_ra_proc_msg2_ex. Compare the generated gb, sign_gb_ga, and mac fields with the sample's expected inputs, then reproduce the isv_app run until the signature-invalid error is resolved.

Written by the indexing model from the issue text.