MacSigner with secret key from AWS cloudHSM not usable.

My goal is to create a (detached) jws signature using a symmetric key from an aws cloud hsm.

The MacSigner puts the SecretKey into a SecretKeySpec (see HMAC.java line 94).

This leads to an error within the aws cloudHSM JCE provider, because this needs a key as loaded from the keystore provided for Mac.init().

Library versions used:

nimbus-jose-jwt:9.31
aws cloudHSM JCE: cloudhsm-jce:5.10.0
Java 17: Corretto-17.0.8.8.1

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Kotlin-Code to reproduce this error with Nimbus MacSigner:

            if (Security.getProvider(CloudHsmProvider.PROVIDER_NAME) == null) {
                Security.addProvider(CloudHsmProvider())
            }

            // Load the HSM as a Java crypto provider
            val hsmProvider: Provider = Security.getProvider(CloudHsmProvider.PROVIDER_NAME)

            // Get a handle to the private key for signing
            val hsmKeyStore: KeyStore = KeyStore.getInstance(CloudHsmProvider.CLOUDHSM_KEYSTORE_TYPE, hsmProvider)
            hsmKeyStore.load(null)
            val secretKey = hsmKeyStore.getKey("KEY-ID", "".toCharArray()) as SecretKey

            val signer = MACSigner(secretKey)
            signer.jcaContext.provider = hsmProvider

            // prepare header and payload
            val header = JWSHeader
                .Builder(JWSAlgorithm.HS256)
                .base64URLEncodePayload(false)
                .criticalParams(setOf("b64"))
                .keyID("KEY-ID)
                .build()


            val payload = Payload("This is the signed payload.")
            val jws = JWSObject(header, payload)

            jws.sign(signer)  // <-- Exception
            val isDetached = true
            val token = jws.serialize(isDetached)

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The above code snippet throws the following exception, when JWSObject.sign() is called:

Invalid HMAC key: The provided key is an instance of class javax.crypto.spec.SecretKeySpec when it should be of class com.amazonaws.cloudhsm.jce.provider.GenericSecretKey
com.nimbusds.jose.JOSEException: Invalid HMAC key: The provided key is an instance of class javax.crypto.spec.SecretKeySpec when it should be of class com.amazonaws.cloudhsm.jce.provider.GenericSecretKey
    at com.nimbusds.jose.crypto.impl.HMAC.getInitMac(HMAC.java:65)
    at com.nimbusds.jose.crypto.impl.HMAC.compute(HMAC.java:118)
    at com.nimbusds.jose.crypto.impl.HMAC.compute(HMAC.java:94)
    at com.nimbusds.jose.crypto.MACSigner.sign(MACSigner.java:193)
    at com.nimbusds.jose.JWSObject.sign(JWSObject.java:315)

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The following code snippet demonstrates the java crypto api calls issued by Nimbus, which lead to this error, and an alternative call sequence which does not wrap the secret inside a SecretKeySpec which works fine with AWS cloudHSM JCE provider:

        val message = "The message.".toByteArray(StandardCharset.UTF_8)
        if (Security.getProvider(CloudHsmProvider.PROVIDER_NAME) == null) {
            Security.addProvider(CloudHsmProvider())
        }
        val hsmProvider: Provider = Security.getProvider(CloudHsmProvider.PROVIDER_NAME)

        val hsmKeyStore: KeyStore = KeyStore.getInstance(CloudHsmProvider.CLOUDHSM_KEYSTORE_TYPE, hsmProvider)
        hsmKeyStore.load(null)

        val secretKey = hsmKeyStore.getKey("KEY-ID", "".toCharArray()) as SecretKey
        val alg = "HMACSHA256"

        // this works:
        val macOk : Mac = Mac.getInstance(alg, hsmProvider)
        macOk.init(secretKey)
        val resultOk = macOk.doFinal();
        println("result = $resultOk")

        // this not:
        val secretKeySpec = javax.crypto.spec.SecretKeySpec(secretKey.getEncoded(), alg)
        val mac : Mac = Mac.getInstance(secretKeySpec.getAlgorithm(), hsmProvider)
        mac.init(secretKeySpec) // <----java.security.InvalidKeyException: The provided key is an instance of class javax.crypto.spec.SecretKeySpec when it should be of class com.amazonaws.cloudhsm.jce.provider.GenericSecretKey
        mac.update(message)
        val result = mac.doFinal();
        println("result = $result")

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Is there a way to use a Nimbus MacSigner with a key from AWS cloudHSM?

Is the wrapping of the SecretKey into a SecretKeySpec required?

If not, I suggest that nimbus uses the SecretKey directly without wrapping it when performing a MacSinger operation.

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