• Registered: 2021-05-14
  • Posts: 4

Topic: Certify Creation of a Key

Hello,

I'm trying to certify the creation of a object (a RSA key) using the TPM2_CertifyCreation method, but I'm not having success. I had to modify the RsaKey strucutre in Native Test example to get some parameters that are necessary to call TPM2_CertifyCreation(). This code uses any RsaKey, but I intend to use the Attestation Identity Key. Here is the code:

typedef struct tpmKey {
    TPM_HANDLE          handle;
    TPM2B_AUTH          auth;
    TPMT_SYM_DEF_OBJECT symmetric; /* used for parameter encrypt/decrypt */
    TPM2B_PRIVATE       priv;
    TPM2B_PUBLIC        pub;
    TPM2B_CREATION_DATA creationData;
    TPM2B_DIGEST creationHash;
    TPMT_TK_CREATION creationTicket;
    TPM2B_NAME          name;
} TpmKey;

typedef TpmKey TpmRsaKey;

TpmRsaKey rsaKey;

/* Load new key */
    XMEMSET(&cmdIn.load, 0, sizeof(cmdIn.load));
    cmdIn.load.parentHandle = storage.handle;
    cmdIn.load.inPrivate = rsaKey.priv;
    cmdIn.load.inPublic = rsaKey.pub;
    rc = TPM2_Load(&cmdIn.load, &cmdOut.load);
    if (rc != TPM_RC_SUCCESS) {
        printf("TPM2_Load RSA key failed 0x%x: %s\n", rc,
            TPM2_GetRCString(rc));
        goto exit;
    }
    rsaKey.handle = cmdOut.load.objectHandle;
    printf("TPM2_Load RSA Key Handle 0x%x\n", (word32)rsaKey.handle);

    /* set session auth for RSA key */
    session[0].auth.size = sizeof(usageAuth)-1;
    XMEMCPY(session[0].auth.buffer, usageAuth, session[0].auth.size);

    cmdIn.certifyCreationIn.objectHandle = rsaKey.handle;
    cmdIn.certifyCreationIn.creationHash = rsaKey.creationHash;
    cmdIn.certifyCreationIn.creationTicket = rsaKey.creationTicket; 
    cmdIn.certifyCreationIn.inScheme.scheme = TPM_ALG_RSASSA;
    cmdIn.certifyCreationIn.inScheme.details.any.hashAlg = TPM_ALG_SHA256;
    rc = TPM2_CertifyCreation(&cmdIn.certifyCreationIn, &cmdOut.certifyCreationOut);
    cmdIn.certifyCreationIn.qualifyingData.size = 0; /* optional */
    if (rc != TPM_RC_SUCCESS) {
        printf("TPM2_CertifyCreation RSA key failed 0x%x: %s\n", rc,
            TPM2_GetRCString(rc));
        goto exit;
    }

After executing the code I'm receiving this message error:
TPM2_CertifyCreation RSA key failed 0x9a2: TPM_RC_BAD_AUTH: Authorization failure without DA implications

Thanks in advance.

Share

  • jeff
  • Member
  • Offline
  • Registered: 2020-09-14
  • Posts: 28

Re: Certify Creation of a Key

Hi junioarssuncaolp,

Can you tell us more about your use-case?

I think for TPM2_CertifyCreation, the signHandle needs to be set and match the session auth. I was able to get a different error TPM_RC_SENSITIVE.

I'll try to get a working example.

Share

  • Registered: 2021-05-14
  • Posts: 4

Re: Certify Creation of a Key

Hi jeff,

It was it, I hadn't set the signHandle. After this change (see below) my code worked, so I guess is everything correct, right?

/* set session auth for RSA key */
session[0].auth.size = sizeof(usageAuth)-1;
XMEMCPY(session[0].auth.buffer, usageAuth, session[0].auth.size);

cmdIn.certifyCreationIn.signHandle = rsaKey.handle; // Setting the signHandle with the rsaKey handle
cmdIn.certifyCreationIn.objectHandle = rsaKey.handle;
cmdIn.certifyCreationIn.creationHash = rsaKey.creationHash;
cmdIn.certifyCreationIn.creationTicket = rsaKey.creationTicket; 
cmdIn.certifyCreationIn.inScheme.scheme = TPM_ALG_RSASSA;
cmdIn.certifyCreationIn.inScheme.details.any.hashAlg = TPM_ALG_SHA256;
rc = TPM2_CertifyCreation(&cmdIn.certifyCreationIn, &cmdOut.certifyCreationOut);
cmdIn.certifyCreationIn.qualifyingData.size = 0; /* optional */
if (rc != TPM_RC_SUCCESS) {
    printf("TPM2_CertifyCreation RSA key failed 0x%x: %s\n", rc, TPM2_GetRCString(rc));
    goto exit;
}

Thanks for your help!

Share

  • Registered: 2020-11-04
  • Posts: 16

Re: Certify Creation of a Key

Hi junioarssuncaolp,

I think one of the command parameters has to be adjusted. Please see my explanation below.

If you have received TPM_RC_SUCCESS from TPM2_CertifyCreation, then the command did succeed. You should have a populated certifyInfo structure and the TPM signature over that structure. Have you checked the data in cmdOut.certifyCreationOut?

typedef struct {
    TPM2B_ATTEST certifyInfo;
    TPMT_SIGNATURE signature;
} Certify_Out;

You could cast the member cmdOut.certifyCreationOut.certifyInfo.attested to TPMS_CREATION_INFO.
This way, you can have easier access to the Name of the RSA key you certified and its creationHash, and you could do a comparison using XMEMCMP. Also, it is important to verify the TPM signature in cmdOut.certifyCreationOut.signature.


About the requirements of a certifyCreation command

  • signHandle should point to a TPM signing key that is loaded.

  • signHandle should point to a different key from the one you are certifying in objectHandle.

For example

cmdIn.certifyCreationIn.signHandle = signingKey.handle;
cmdIn.certifyCreationIn.objectHandle = attestationKey.handle;
...
/* Use the public part of the signingKey to verify the TPM signature over the certifyCreation output */

Could you please tell a bit more about your use case?
What is the end goal of using CertifyCreation over an Attestation Key?

Let us know if you have more question. We are happy to help.

Thanks,
Dimi

Dimi Tomov,
wolfSSL Engineer and Founder of TPM.dev

Share

  • Registered: 2021-05-14
  • Posts: 4

Re: Certify Creation of a Key

Hi dimitom,

Thank's for the explanation. I got it.

I am using CertifyCreation just to say to some Verifier entity that at least a certification was done within that context of the AK. I pretend to use this AK as a parameter for creating a challenge (for example, as a result of the TPM2_MakeCredential approach).

Share

  • Registered: 2020-11-04
  • Posts: 16

Re: Certify Creation of a Key

Hi junioarssuncaolp,

I am happy you have creationCertify with wolfTPM working for your use case.

As someone passioned about hardware security, HSM and TPM, I am curious to learn more about what makeCredential+creationCertify offers more compared to makeCredential+ActivateCredential?

We have examples of this under ./example/attestation

Thanks,
Dimi

Dimi Tomov,
wolfSSL Engineer and Founder of TPM.dev

Share

  • Registered: 2021-05-14
  • Posts: 4

Re: Certify Creation of a Key

Hi Dimi,

creationCertify is used like a double check for AK and is used before makeCredential, just to check some properties and see if this key is correct for his purpose. After this check, makeCredential + activateCredential is used.

Share