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Crypto API Toolkit for Intel(R) SGX

Introduction

Crypto API Toolkit for Intel(R) SGX (CTK) aims at enhancing the security of data and key protection applications by exposing interfaces that run the key generation and cryptographic operations securely inside an Intel(R) Software Guard Extensions (SGX) enclave. The operations are exposed and supported via PKCS11 interface for Linux.

The current release of CTK implements PKCS11 v2.40 Plus Errata 01

License

See LICENSE.md for details.

Prerequisites

System requirements

Intel(R) SGX capable system running Ubuntu 22.04-LTS 64-bit. For using Data Center Attestation Primitives (DCAP), the system must support Flexible Launch Control (FLC).

Please check this page for the server platforms that support Intel(R) SGX.

Software requirements

The common software requirements for building the CTK are listed below. Please refer to your distro's documentation for how to fetch and install these tools if they do not come preinstalled.

  • autotools (autoconf, automake, libtool)

  • g++ compiler with C++11 support

  • libcppunit-dev (for building and running the tests)

  • unzip

  • openssl

    For example in Ubuntu, the build tools and libraries can be obtained by running the command

    $ sudo apt-get update
    $ sudo apt-get install dkms libprotobuf-dev autoconf libcppunit-dev autotools-dev libc6-dev libtool build-essential

  • Intel(R) SGX software components

NOTE This version of CTK is configured to build with, and validated against Intel SGX SDK v2.24 and SGX in-kernel driver part of Linux kernel v5.18, and SGXSSL binaries (Release 3.0_Rev2) built with OpenSSL version 3.0.12) without any mitigation.

Building the source

Build configuration

  • The enclave is configured to have DisableDebug set to 0 in the enclave configuration XML (src/p11/enclave_config/p11Enclave.config.xml) for the purpose of debug during development and integration. This means that the enclave will be debuggable. For a production enclave, this value must be set to to 1 before building the enclave. Please refer to the section Enclave Project Configurations in the Intel(R) SGX Developer Reference for Linux* OS for more information. Please also note that the provider that loads the enclave needs to be built with NDEBUG preprocessor macro that disables the SGX_DEBUG_FLAG (will be defined as 0).
  • Based on the default StackSize (0x40000) defined in the enclave configuration XML (src/p11/enclave_config/p11Enclave.config.xml), the maximum data that can be transfered from untrusted to trusted during an OCALL is limited to 180KB. This could limit the number of persistent token objects. Please tune this parameter (StackSize) based on the requirements and system capability.
  • The HeapMaxSize in the configuration XML is by default set to 0xA00000. Please tune this parameter based on your application's requirements.

NOTE Please go through https://github.com/intel/linux-sgx/blob/master/psw/ae/ref_le/ref_le.md to learn about whitelisting the release enclave.

Preparing the source for the build

After downloading the souce, run sh autogen.sh

$ sh autogen.sh

Configuration options

The source can be configured by running ./configure

$ ./configure

The options that configure supports can be obtained by running ./configure --help. Below are the options that are specific to CTK:

Option Detail Default value/Notes
--with-sgxsdk SGX SDK installation path /opt/intel/sgxsdk
--with-sgxssl SGX SSL installation path /opt/intel/sgxssl
--with-token-path Path where PKCS11 tokens and objects will be stored /opt/intel/cryptoapitoolkit
--enable-dcap Build with DCAP support Build without DCAP support
--with-p11-kit-path p11-kit include directory path Build without p11-kit, using PKCS11 headers from CTK
--disable-multiprocess-support If the token is not expected to be simultaneously accessed for modification by multiple processes (write/update/delete), this flag can give a performance boost. The token and the objects are allowed to be modified (write/update/delete) by multiple processes simultaneously.
--enable-session-key-wrap Enable wrapping a session key when public key is not passed during build (please see the below row) Wrapping session keys is not allowed.
--with-public-key Path to RSA 3072-bit public key in PEM DER ASN.1 PKCS#1 format
--with-private-key Path to RSA 3072-bit private key in PEM DER ASN.1 PKCS#1 format This build option MUST be used only for test purposes.
--enable-mitigation Enable mitigations for CVE-2020-0551 (LVI) and other vulnerabilities Mitigations disabled for CVE-2020-0551 (LVI) and other vulnerabilities

Compiling

$ make

NOTE Please note that the enclave is signed with a test signing key. A production enclave should go through the process of signing an enclave as explained in the section Enclave Signing Tool in the Intel(R) SGX Developer Reference for Linux* OS.

Installation

$ sudo make install

Running the tests

The tests will be built into an executable and can be executed by running ./p11test from the directory $(srcroot)/src/tests

Please note that the built libraries must be installed before running the tests.

Uninstallation

$ sudo make uninstall

APIs, Mechanisms and Attributes

CTK implements the APIs according to PKCS11 v2.40 (Plus Errata 01) specification with some restrictions (please see the section on Restrictions below in this document). The developers are strongly encouraged to go through the PKCS11 specification for the details on the APIs, mechanisms and attributes.

APIs

CTK does not support the APIs listed below.

C_GetOperationState
C_SetOperationState
C_DigestKey
C_SignRecoverInit
C_SignRecover
C_VerifyRecoverInit
C_VerifyRecover
C_DigestEncryptUpdate
C_DecryptDigestUpdate
C_SignEncryptUpdate
C_DecryptVerifyUpdate
C_DeriveKey
C_SeedRandom
C_WaitForSlotEvent

Mechanisms

CTK supports only the mechanisms listed below.

CKM_AES_KEY_GEN
CKM_AES_CTR
CKM_AES_GCM
CKM_AES_CBC
CKM_AES_CBC_PAD
CKM_AES_KEY_WRAP
CKM_AES_KEY_WRAP_PAD
CKM_RSA_PKCS_KEY_PAIR_GEN
CKM_RSA_PKCS
CKM_RSA_PKCS_OAEP
CKM_SHA1_RSA_PKCS1
CKM_SHA224_RSA_PKCS1
CKM_SHA256_RSA_PKCS
CKM_SHA384_RSA_PKCS
CKM_SHA512_RSA_PKCS
CKM_RSA_PKCS_PSS
CKM_SHA1_RSA_PKCS_PSS1
CKM_SHA224_RSA_PKCS_PSS1
CKM_SHA256_RSA_PKCS_PSS
CKM_SHA384_RSA_PKCS_PSS
CKM_SHA512_RSA_PKCS_PSS
CKM_RSA_X_509
CKM_SHA256
CKM_SHA384
CKM_SHA512
CKM_SHA256_HMAC
CKM_SHA384_HMAC
CKM_SHA512_HMAC
CKM_EC_KEY_PAIR_GEN
CKM_EC_EDWARDS_KEY_PAIR_GEN
CKM_ECDSA
CKM_EDDSA
CKM_EXPORT_ECDSA_QUOTE_RSA_PUBLIC_KEY2

Supported Key Sizes

  • AES: 128, 192, 256 bits
  • RSA: 2048-16K bits
  • ECDSA: P-256, P-384, P-521 curves
  • EDDSA: Ed25519

1 IMPORTANT Intel Corporation strongly recommends not to use RSA sign & verify mechanisms based on SHA1 and SHA224 digest schemes as they are not considered cryptographically strong and vulnerable to collision based attacks. They are supported only for interoperability and backward compatibility towards existing applications that require these mechanisms. They are deprecated and will be removed in a future update.  

2 Custom mechanism to support ECDSA based quote generation based on RSA public key.

Attributes

CTK supports most of the attributes listed in the PKCS11 specification with some restrictions. Please see the section on Restrictions below in this document.

Support for wrapping and unwrapping

CTK by default disables support for wrapping and exporting keys outside the enclave except with CKM_EXPORT_ECDSA_QUOTE_RSA_PUBLIC_KEY for generating the quote, but can be configured to support wrapping of the keys at build time by using one of the following methods.

Enabling session keys wrap

By configuring the build with --enable-session-key-wrap CTK can be built to support wrapping and exporting session keys (in-memory) that are created.

Enabling authorized unwrapping

To authorize only a trusted application to unwrap the keys inside the enclave, Crypto API Toolkit necessitates that a 384-bit hash of an RSA 3072-bit public key be embedded as part of the enclave binary. This is done via passing the public key using the option --with-public-key while configuring the build. For unwrapping a key blob inside the enclave, the private portion of this keypair (which remains with the application that owns it in a secure location) must be used to sign the wrapped key blob and pass it during the C_UnwrapKey operation along with the signature and the public portion of the key as described below.

  1. Extract the modulus and the exponent from RSA public key used during CTK build (via --with-public-key-path option). This must be a 3072-bit RSA key.
  2. Sign the wrapped key to be unwrapped inside the enclave with the private portion of the key that was passed during the build using --with-public-key option using the mechanism CKM_SHA384_RSA_PKCS_PSS.
  3. Fill the following structure
typedef struct CK_UNWRAP_KEY_PARAMS{
    CK_ULONG         modulusLen;
    CK_ULONG         exponentLen;
    CK_ULONG         signatureLen;
    CK_ULONG         wrappedKeyLen;
    CK_MECHANISM_PTR pMechanism;
} CK_UNWRAP_KEY_PARAMS;
  1. Fill pWrappedKey parameter of the C_UnwrapKey API in the format below. The parameter ulWrappedKeyLen of the same API must be set to the size of this structure.
--------------------------------------------------------------------------------------------------------------
|          CK_UNWRAP_KEY_PARAMS       |    modulus   |    exponent   |  signature  |      wrappedKey         |
--------------------------------------------------------------------------------------------------------------

The CK_RSA_PKCS_PSS_PARAMS structure passed as pMechanism must set hashAlg to CKM_SHA384 and its mgf to CKG_MGF1_SHA384.

Please refer getUnwrapParams from src/test/UnwrapKeyHelper.cpp to see how the above steps are done. The RSA keypair can be generated using the openssl commands below, if needed:

  • The private portion of the keypair can be generated using openssl genrsa -out privateKey.pem 3072
  • Using the private key generated above, the corresponding public portion of the corresponding keypair can be generated using openssl rsa -in privateKey.pem -outform PEM -RSAPublicKey_out -out publicKey.pem

Quote Generation and Verification

Quote Generation

After performing the initialization (load library, initialize and opening the session) and after creating the key pair (whose public key is to be exported along with the enclave quote), the application

  1. Sets the mechanism to CKM_EXPORT_ECDSA_QUOTE_RSA_PUBLIC_KEY and fills the mechanism parameters in the structure CK_ECDSA_QUOTE_RSA_PUBLIC_KEY_PARAMS.
typedef struct CK_ECDSA_QUOTE_RSA_PUBLIC_KEY_PARAMS {
    CK_LONG qlPolicy;
    CK_BYTE nonce[NONCE_LENGTH];
} CK_ECDSA_QUOTE_RSA_PUBLIC_KEY_PARAMS;

NOTE: NONCE_LENGTH is defined in src/p11/trusted/SoftHSMv2/common/QuoteGenerationDefs.h, set as 32 bytes (default) and can be expanded upto 256 bytes based on the requirement.

  1. The application calls C_WrapKey with the mechanism details, the object handle of the key to be exported and passes a NULL_PTR for the hWrappingKey and destination buffer.
  2. If the key handle is valid, the enclave returns the size of the destination buffer required to hold the exported public key and quote which is the total size of the actual public key buffer and sizeof(CK_RSA_PUBLIC_KEY_PARAMS) and the size of the quote itself. The quote is of the format sgx_quote_t and contains the hash of the public key and the quote generated based on this.
  3. If the call is successful, the application allocates the buffer as per the returned buffer size.
  4. The application calls C_WrapKey with the same parameters, with the allocated buffer for pWrappedKey parameter and the allocated size in pulWrappedKeyLen.
  5. The Crypto API Toolkit for Intel(R) SGX enclave calculates the hash of the public key, generates a quote and returns it in the pWrappedKey buffer (in the below format).
--------------------------------------------------------------------------------------------------------------
| Size |   CK_RSA_PUBLIC_KEY_PARAMS   | ulExponentLen | ulModulusLen |              sgx_quote_t              |
--------------------------------------------------------------------------------------------------------------
| Data | ulExponentLen | ulModulusLen |    exponent   |    modulus   | SHA256 hash of the public key | quote |
--------------------------------------------------------------------------------------------------------------

Please refer testeQuoteGeneration() from src/test/AsymWrapUnwrapTests.cpp to see how the quote from the enclave based on the public key’s hash can be retrieved.

This public key, after verification, may be used to provision a secret key inside the enclave.

NOTE: If the enclave has only the public key available (imported) and the private key is not available as part of the key pair, the quote generation will fail.

Quote Verification

CTK does not support verifiying the quote. Please refer to the sample application in Intel(R) SGX DCAP to verify the quote (https://github.com/intel/SGXDataCenterAttestationPrimitives/tree/master/QuoteVerification).

Multithreading support

CTK is multithread safe, but the enclave is configured to not support multithreaded applications by default. Support for multithreaded applications can be enabled by updating the TCSNum tag in the enclave configuration XML. The enclave support for threads is limited by the number of TCSs and the available EPC memory. The maximum number of threads that an enclave can run simultaneously inside the enclave is the same as the number of logical processors in the system. This is typically the value set in the TCSNum tag. Please refer to the Intel(R) SGX Developer Reference for Linux* OS for configuring stack size and heap size in the XML for multithreaded applications.

Restrictions

CTK imposes certain restrictions to further harden the security. They are listed below:

  • CTK does not support application provided function pointers or callbacks and mutexes.

    • C_Initialize: The members CreateMutex, DestroyMutex, LockMutex and UnlockMutex in CK_C_INITIALIZE_ARGS are not supported and must be set to NULL_PTR.
    • C_OpenSession: The members pApplication and Notify are not supported and must be set to NULL_PTR.

  • C_CreateObject: CTK does not support creation of secret key objects using C_CreateObject API using the attribute CKA_VALUE. This is to prevent memory scraping attacks getting access to the key when it is being created in this manner. As an alternative, an application can call C_CreateObject to create a key inside the enclave by passing the mechanism and key's length in the template via CKA_VALUE_LEN.

  • C_GetAttributeValue, C_SetAttributeValue: CTK does not allow setting and getting attributes that can expose the keys in the clear or affect its stability. The attributes listed below are not supported for C_GetAttributeValue and C_SetAttributeValue APIs:

    • CKA_VALUE
    • CKA_PRIVATE_EXPONENT
    • CKA_VALUE_LEN (restricted only for C_SetAttributeValue API)
    • CKA_PRIME_1
    • CKA_PRIME_2
    • CKA_EXPONENT_1
    • CKA_EXPONENT_2
    • CKA_COEEFICIENT

  • C_CopyObject: A token object can only be copied as a token object and a session object can only be copied as a session object using C_CopyObject API.

  • C_WrapKey, C_UnwrapKey:

    • A wrapped key can get unwrapped only inside the enclave. It cannot be extracted to come out in the clear.
    • A key used for wrapping cannot be used for encryption or decryption.
    • If the key that is used for wrapping another key was used for encrypting data earlier, that encrypted data cannot be decrypted after the wrapping operation.
    • The key used to generate a quote can only be an RSA public key.
    • The key used to generate a quote cannot be created with CKA_TOKEN attribute set to true. It must be a session key object.
    • The key used to generate a quote cannot be used for sign and verify operations in addition to encryption and decryption operations.
    • The key that is used for wrap and unwrap operations can only be a session key.
    • The key used to unwrap another key is destroyed automatically on a successful call to C_UnwrapKey.

Using Crypto API Toolkit

This section demonstrates how to use pkcs11-tool to create a token and an RSA keypair. The CTK build results in two shared object files: libp11sgx.so (untrusted) and libp11SgxEnclave.signed.so (trusted). The untrusted shared object (lip11sgx.so from the installation directory) should be used in the place of PKCS11 module for use with tools like pkcs11-tool.

Creating a token

$pkcs11-tool --module /usr/local/lib/libp11sgx.so --init-token --label "ctk" --slot 0 --so-pin 1234 --init-pin --pin 1234

Creating an RSA keypair

$pkcs11-tool --module /usr/local/lib/libp11sgx.so --login --pin 1234 --id 0001 --token "ctk" --keypairgen --key-type rsa:3072 --label "cert-key" --usage-sign

Listing the objects

$pkcs11-tool --module /usr/local/lib/libp11sgx.so --list-objects -login --pin 1234 --login-type user

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