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wolfSSL Porting Guide


This guide provides a reference for developers and engineers porting the wolfSSL lightweight SSL/TLS library to new embedded platforms, operating systems, or transport mediums (TCP/IP, bluetooth, etc.). It calls out areas in the wolfSSL codebase which typically require modification when porting wolfSSL. It should be considered a “guide” and as such, it is an evolving work. If there is something you find missing, please let us know and we’ll be happy to add instructions or clarification to the document.


This guide caters to developers or engineers porting the wolfSSL and wolfCrypt to new platforms or environments that are not supported by default.


Several steps need to be iterated through when getting wolfSSL to run on an embedded platform. Some of these steps are outlined in Section 2.4.

Apart from steps in Chapter 2 of the wolfSSL Manual, there are areas in the code which may need porting or modifications in order to accommodate a specific platform. wolfSSL abstracts many of these areas - attempting to make it as easy as possible to port wolfSSL to a new platform.

In the ./wolfssl/wolfcrypt/settings.h file, there are several defines specific to different operating systems, TCP/IP stacks, and chipsets (ex: MBED, FREESCALE_MQX, MICROCHIP_PIC32, MICRIUM, EBSNET, etc.). There are two main locations to put #defines when compiling and porting wolfSSL to a new platform:

  1. New defines for a Operating System or TCP/IP stack port are typically added to the settings.h file when a new port of wolfSSL is completed. This provides an easy way to turn on/off features as well as customize build settings that should be “default” for that build. New custom defines can be added in this file when doing a port of wolfSSL to a new platform. We encourage users to contribute ports of wolfSSL back to the master open source code branch on GitHub. This helps keep wolfSSL up to date and allows different ports to remain updated as the wolfSSL project improves and moves forward.

  2. For users not wanting to contribute back their changes to wolfSSL proper, or for users who want to customize the wolfSSL build with additional preprocessor defines, wolfSSL recommends the use of a custom user_settings.h header file. If WOLFSSL_USER_SETTINGS is defined when compiling the wolfSSL source files, wolfSSL will automatically include a custom header file called user_settings.h. This header should be created by the user and placed on the include path. This allows users to maintain one single file for their wolfSSL build, and makes it much easier to update to newer versions of wolfSSL.

wolfSSL encourages the submission of patches and code changes through either direct email (, or through GitHub pull request.

Porting wolfSSL

Data Types

Q: When do I need to read this section?

A: Setting the correct data type size for your platform is always important.

wolfSSL benefits speed-wise from having a 64-bit type available. Define SIZEOF_LONG and SIZEOF_LONG_LONG to match the result of sizeof(long) and sizeof(long long) on your platform. This can be added to a custom define in the settings.h file or to user_settings.h. For example, in settings.h under a sample define of MY_NEW_PLATFORM:

    #define SIZEOF_LONG 4
    #define SIZEOF_LONG_LONG 8

There are two additional data types used by wolfSSL and wolfCrypt, called word32 and word16. The default type mappings for these are:

#ifndef byte
    typedef unsigned char  byte;
    typedef unsigned short word16;
    typedef unsigned int   word32;
    typedef byte           word24[3];

word32 should be mapped to the compiler’s 32-bit type, and word16 to the compiler’s 16-bit type. If these default mappings are incorrect for your platform, you should define WOLFSSL_TYPES in settings.h or user_settings.h and assign your own custom typedefs for word32 and word16.

The fastmath library in wolfSSL uses the fp_digit and fp_word types. By default these are mapped in <wolfssl/wolfcrypt/tfm.h> depending on build configuration.

fp_word should be twice the size of fp_digit. If the default cases do not hold true for your platform, you should define WOLFSSL_BIGINT_TYPES in settings.h or user_settings.h and assign your own custom typedefs for fp_word and fp_digit.

wolfSSL does use a 64-bit type when available for some operations. The wolfSSL build tries to detect and set up the correct underlying data type for word64 based on what SIZEOF_LONG and SIZEOF_LONG_LONG have been set to. On some platforms that don’t have a true 64-bit type, where two 32-bit types are used in conjunction, performance can be slow. To compile out the use of 64-bit types, define NO_64BIT.


Q: When do I need to read this section?

A: Your platform is a big endian system.

Is your platform big endian or little endian? wolfSSL defaults to a little endian system. If your system is big endian, define BIG_ENDIAN_ORDER when building wolfSSL. Example of setting this in settings.h:

    #define BIG_ENDIAN_ORDER


Q: When do I need to read this section?

A: <sys/uio.h> is not available.

By default, the wolfSSL API makes available wolfSSL_writev() to applications, which simulates writev() semantics. On systems that don’t have the <sys/uio.h> header available, define NO_WRITEV to exclude this feature.

Input / Output

Q: When do I need to read this section?

A: A BSD-style socket API is not available, you are using a custom transport layer or TCP/IP stack, or only want to use static buffers.

wolfSSL defaults to using a BSD-style socket interface. If your transport layer provides a BSD socket interface, wolfSSL should integrate into it as-is, unless custom headers are needed.

wolfSSL provides a custom I/O abstraction layer which allows users to tailor wolfSSL's I/O functionality to their system. Full details can be found in Section 5.1.2.

Simply put, you can define WOLFSSL_USER_IO, then write your own I/O callback functions using wolfSSL's default EmbedSend() and EmbedReceive() as templates. These two functions are located in ./src/io.c.

wolfSSL uses dynamic buffers for input and output, which default to 0 bytes. If an input record is received that is greater in size than the buffer, then a dynamic buffer is temporarily used to handle the request and then freed.

If you prefer using large, 16kB static buffers which will never need dynamic memory, you can enable this option by defining LARGE_STATIC_BUFFERS.

If dynamic buffers are used and the user requests an wolfSSL_write() that is bigger than the buffer size, then a dynamic block up to MAX_RECORD_SIZE is used to send the data. Users wishing to only send the data in chunks of the current buffer size at maximum, as defined by RECORD_SIZE, can do this by defining STATIC_CHUNKS_ONLY. When using this define, RECORD_SIZE defaults to 128 bytes.


Q: When do I need to read this section?

A: No file system is available, standard file system functions are not available, or you have a custom file system.

wolfSSL uses the filesystem for loading keys and certificates into the SSL session or context. wolfSSL also allows loading these from memory buffers. If strictly using memory buffers, a filesystem is not needed.

You can disable wolfSSL's usage of the filesystem by defining NO_FILESYSTEM when building the library. This means that certificates and keys will need to be loaded from memory buffers instead of files. An example of setting this in settings.h:

    #define NO_FILESYSTEM

Test key and certificate buffers can be found in the ./wolfssl/certs_test.h header file. These will match up to corresponding certificates and keys found in the ./certs directory.

The certs_test.h header file can be updated using the ./ script if needed. Inside, there are two arrays: fileList_1024 and fileList_2048. Additional certificates or keys may be added to the respective array, depending on key size, and must be in DER format. The above mentioned arrays map a certificate/key file location with the desired buffer name. After modifying, running it from the wolfSSL root directory will update the certificate and key buffers in ./wolfssl/certs_test.h:


If you would like to use a filesystem other than the default, the filesystem abstraction layer is located in ./wolfssl/wolfcrypt/wc_port.h. Here you will see filesystem ports for various platforms including EBSNET, FREESCALE_MQX, and MICRIUM. You can add a custom define for your platform if needed - allowing you to define file system functions with XFILE, XFOPEN, XFSEEK, etc. For example, the filesystem layer in wc_port.h for Micrium's µC/OS (MICRIUM) is as follows:

#elif defined(MICRIUM)
#include <fs.h>
#define XFILE      FS_FILE*
#define XFOPEN     fs_fopen
#define XFSEEK     fs_fseek
#define XFTELL     fs_ftell
#define XREWIND    fs_rewind
#define XFREAD     fs_fread
#define XFCLOSE    fs_fclose


Q: When do I need to read this section?

A: You want to use wolfSSL in a multithreaded environment, or want to just compile it in single threaded mode.

If wolfSSL will only be used in a single threaded environment, the wolfSSL mutex layer can be disabled when compiling wolfSSL by defining SINGLE_THREADED. This will negate the need to port the wolfSSL mutex layer.

If wolfSSL needs to be used in a multithreaded environment, the wolfSSL mutex layer will need to be ported to the new environment. The mutex layer can be found in ./wolfssl/wolfcrypt/wc_port.h and ./wolfcrypt/src/wc_port.c. wolfSSL_Mutex will need to be defined for the new system in wc_port.h and the mutex functions (wc_InitMutex, wc_FreeMutex, wc_LockMutex and wc_UnLockMutex) in wc_port.c. You can search in wc_port.h and wc_port.c to see an example for some existing platform port layers (EBSNET, FREESCALE_MQX, etc.).

Random Seed

Q: When do I need to read this section?

A: Either /dev/random or /dev/urandom is not available or you want to integrate into a hardware RNG.

By default, wolfSSL uses /dev/urandom or /dev/random to generate a RNG seed. The NO_DEV_RANDOM define can be used when building wolfSSL to disable the default GenerateSeed() function. If this is defined, you need to write a custom GenerateSeed() function in ./wolfcrypt/src/random.c, specific to your target platform. This allows you to seed wolfSSL’s PRNG with a hardware-based random entropy source if available.

For examples of how GenerateSeed() needs to be written, reference wolfSSL’s existing GenerateSeed() implementations in ./wolfcrypt/src/random.c.


Q: When do I need to read this section?

A: When you don’t have standard memory functions available or are interested in memory usage differences between optional math libraries.

wolfSSL proper uses both malloc() and free() by default. When using the normal big integer math library, wolfCrypt will also use realloc().

By default wolfSSL/wolfCrypt use the normal big integer math library, which uses quite a bit of dynamic memory. When building wolfSSL, the fastmath library can be enabled, which is both faster and uses no dynamic memory for crypto operations (all on the stack). By using fastmath, wolfSSL won't need a realloc() implementation at all. As the SSL layer of wolfSSL still uses some dynamic memory, malloc() and free() are still required.

For a comparison of resource usage (stack/heap) between the big integer math library and fastmath library, ask us to see our Resource Use document.

To enable fastmath, define USE_FAST_MATH and build in ./wolfcrypt/src/tfm.c instead of ./wolfcrypt/src/integer.c. Since the stack memory can be large when using fastmath, we recommend defining TFM_TIMING_RESISTANT as well.

If the normal malloc(), free(), and possibly realloc() functions are not available, define XMALLOC_USER, then provide custom memory function hooks in ./wolfssl/wolfcrypt/types.h specific to the target environment.

Please read section for details about using XMALLOC_USER.


Q: When do I need to read this section?

A: When standard time functions (time(), gmtime()) are not available, or you need to specify a custom clock tick function.

By default, wolfSSL uses time(), gmtime(), and ValidateDate(), as specified in ./wolfcrypt/src/asn.c. These are abstracted to XTIME, XGMTIME, and XVALIDATE_DATE. If the standard time functions, and time.h, are not available, the user can define USER_TIME. After defining USER_TIME, the user can define their own XTIME, XGMTIME, and XVALIDATE_DATE functions.

wolfSSL uses time(0) by default for the clock tick function. This is located in ./src/internal.c inside of the LowResTimer() function.

Defining USER_TICKS allows the user to define their own clock tick function if time(0) is not wanted. The custom function needs second accuracy, but doesn’t have to be correlated to EPOCH. See LowResTimer() function in ./src/internal.c for reference.

C Standard Library

Q: When do I need to read this section?

A: When you don’t have a C standard library available, or have a custom one.

wolfSSL can be built without the C standard library to provide a higher level of portability and flexibility to developers. When doing so, the user needs to map functions they wish to use instead of the C standard ones.

Section 7, above, covered memory functions. In addition to memory function abstraction, wolfSSL also abstracts string function and math functions, where the specific functions are typically abstracted to a define in the form of X<FUNC>, where <FUNC> is the name of the function being abstracted.

Please read Section 5.1 for details.


Q: When do I need to read this section?

A: You want to enable debug messages but don’t have stderr available.

By default, wolfSSL provides debug output through stderr. In order for debug messages to be enabled, wolfSSL must be compiled with DEBUG_WOLFSSL defined, and wolfSSL_Debugging_ON() must be called from the application code. wolfSSL_Debugging_OFF() may be used by the application layer to turn off wolfSSL debug messages.

For environments which do not have stderr available, or wish to output debug messages over a different output stream or in a different format, wolfSSL allows applications to register a logging callback.

Please read Section 8.1 for details.

Public Key Operations

Q: When do I need to read this section?

A: You want to use your own public key implementation with wolfSSL.

wolfSSL allows users to write their own public key callbacks which will be called when the SSL/TLS layer needs to do public key operations. The user can optionally define 6 functions:

  1. ECC sign callback
  2. ECC verify callback
  3. RSA sign callback
  4. RSA verify callback
  5. RSA encrypt callback
  6. RSA decrypt callback

For full details, please read Section 6.4.

Atomic Record Layer Processing

Q: When do I need to read this section?

A: You want to do your own processing of record layers, specifically MAC/encrypt and decrypt/verify operations.

By default, wolfSSL handles record layer processing for the user using its cryptography library, wolfCrypt. wolfSSL provides Atomic Record Processing callbacks for users who wish to have more control over MAC/encrypt and decrypt/verify functionality during the SSL/TLS connection.

The user will need to define 2 functions:

  1. MAC/encrypt callback function
  2. Decrypt/verify callback function

For full details, please read Section 6.3.


Q: When do I need to read this section?

A: When you want to disable features.

Features can be disabled when building wolfSSL by using the appropriate defines. For a list of defines available, please refer to Chapter 2.

Next Steps

wolfCrypt Test Application

After getting wolfSSL proper to build on the target platform, a good next step is to port the wolfCrypt test application. Running this application on the target system will verify that all the crypto algorithms are working correctly, using NIST test vectors.

If this step is skipped, and you instead proceed directly to establishing an SSL connection, it can be more difficult to debug problems caused by underlying crypto operations failing.

The wolfCrypt test application is located in ./wolfcrypt/test/test.c. If an embedded application has its own main() function, then NO_MAIN_DRIVER must be defined when compiling ./wolfcrypt/test/test.c. This will allow the application’s main() to call each cipher/algorithm test individually on its own.

If an embedded device does not have enough resources to run the entire wolfCrypt test application, individual tests can be broken out of test.c and compiled individually. Please ensure that correct header files needed for the specific test case are included in the build when extracting isolated crypto tests from test.c.


General support questions may be sent directly to wolfSSL either through email, support forums, or wolfSSL’s Zendesk ticket tracking system.


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wolfSSL offers several support packages as well as consulting services to help users and customers port wolfSSL to new environments.

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