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wolfSSL Embedded SSL/TLS Library

The wolfSSL embedded SSL library (formerly CyaSSL) is a lightweight SSL/TLS library written in ANSI C and targeted for embedded, RTOS, and resource-constrained environments - primarily because of its small size, speed, and feature set. It is commonly used in standard operating environments as well because of its royalty-free pricing and excellent cross platform support. wolfSSL supports industry standards up to the current TLS 1.3 and DTLS 1.2, is up to 20 times smaller than OpenSSL, and offers progressive ciphers such as ChaCha20, Curve25519, NTRU, and Blake2b. User benchmarking and feedback reports dramatically better performance when using wolfSSL over OpenSSL.

wolfSSL is powered by the wolfCrypt cryptography library. Two versions of wolfCrypt have been FIPS 140-2 validated (Certificate #2425 and certificate #3389). FIPS 140-3 validation is in progress. For additional information, visit the wolfCrypt FIPS FAQ or contact [email protected].

Why Choose wolfSSL?

There are many reasons to choose wolfSSL as your embedded, desktop, mobile, or enterprise SSL/TLS solution. Some of the top reasons include size (typical footprint sizes range from 20-100 kB), support for the newest standards (SSL 3.0, TLS 1.0, TLS 1.1, TLS 1.2, TLS 1.3, DTLS 1.0, and DTLS 1.2), current and progressive cipher support (including stream ciphers), multi-platform, royalty free, and an OpenSSL compatibility API to ease porting into existing applications which have previously used the OpenSSL package. For a complete feature list, see Chapter 4 of the wolfSSL manual.

Notes, Please Read

Note 1) wolfSSL as of 3.6.6 no longer enables SSLv3 by default. wolfSSL also no longer supports static key cipher suites with PSK, RSA, or ECDH. This means if you plan to use TLS cipher suites you must enable DH (DH is on by default), or enable ECC (ECC is on by default), or you must enable static key cipher suites with one or more of the following defines:


Though static key cipher suites are deprecated and will be removed from future versions of TLS. They also lower your security by removing PFS. Since current NTRU suites available do not use ephemeral keys, WOLFSSL_STATIC_RSA needs to be used in order to build with NTRU suites.

When compiling ssl.c, wolfSSL will now issue a compiler error if no cipher suites are available. You can remove this error by defining WOLFSSL_ALLOW_NO_SUITES in the event that you desire that, i.e., you're not using TLS cipher suites.

Note 2) wolfSSL takes a different approach to certificate verification than OpenSSL does. The default policy for the client is to verify the server, this means that if you don't load CAs to verify the server you'll get a connect error, no signer error to confirm failure (-188).

If you want to mimic OpenSSL behavior of having SSL_connect succeed even if verifying the server fails and reducing security you can do this by calling:

wolfSSL_CTX_set_verify(ctx, SSL_VERIFY_NONE, 0);

before calling wolfSSL_new();. Though it's not recommended.

Note 3) The enum values SHA, SHA256, SHA384, SHA512 are no longer available when wolfSSL is built with --enable-opensslextra (OPENSSL_EXTRA) or with the macro NO_OLD_SHA_NAMES. These names get mapped to the OpenSSL API for a single call hash function. Instead the name WC_SHA, WC_SHA256, WC_SHA384 and WC_SHA512 should be used for the enum name.

wolfSSL Release 4.7.0 (February 16, 2021)

Release 4.7.0 of wolfSSL embedded TLS has bug fixes and new features including:

New Feature Additions

  • Compatibility Layer expansion SSL_get_verify_mode, X509_VERIFY_PARAM API, X509_STORE_CTX API added
  • WOLFSSL_PSK_IDENTITY_ALERT macro added for enabling a subset of TLS alerts
  • Function wolfSSL_CTX_NoTicketTLSv12 added to enable turning off session tickets with TLS 1.2 while keeping TLS 1.3 session tickets available
  • Implement RFC 5705: Keying Material Exporters for TLS
  • Added --enable-reproducible-build flag for making more deterministic library outputs to assist debugging
  • Added support for S/MIME (Secure/Multipurpose Internet Mail Extensions) bundles


  • Fix to free mutex when cert manager is free’d
  • Compatibility layer EVP function to return the correct block size and type
  • DTLS secure renegotiation fixes including resetting timeout and retransmit on duplicate HelloRequest
  • Fix for edge case with shrink buffer and secure renegotiation
  • Compile fix for type used with curve448 and PPC64
  • Fixes for SP math all with PPC64 and other embedded compilers
  • SP math all fix when performing montgomery reduction on one word modulus
  • Fixes to SP math all to better support digit size of 8-bit
  • Fix for results of edge case with SP integer square operation
  • Stop non-ct mod inv from using register x29 with SP ARM64 build
  • Fix edge case when generating z value of ECC with SP code
  • Fixes for PKCS7 with crypto callback (devId) with RSA and RNG
  • Fix for compiling builds with RSA verify and public only
  • Fix for PKCS11 not properly exporting the public key due to a missing key type field
  • Call certificate callback with certificate depth issues
  • Fix for out-of-bounds read in TLSX_CSR_Parse()
  • Fix incorrect AES-GCM tag generation in the EVP layer
  • Fix for out of bounds write with SP math all enabled and an edge case of calling sp_tohex on the result of sp_mont_norm
  • Fix for parameter check in sp_rand_prime to handle 0 length values
  • Fix for edge case of failing malloc resulting in an out of bounds write with SHA256/SHA512 when small stack is enabled


  • Added --enable-wolftpm option for easily building wolfSSL to be used with wolfTPM
  • DTLS macro WOLFSSL_DTLS_RESEND_ONLY_TIMEOUT added for resending flight only after a timeout
  • Update linux kernel module to use kvmalloc and kvfree
  • Add user settings option to cmake build
  • Added support for AES GCM session ticket encryption
  • Thread protection for global RNG used by wolfSSL_RAND_bytes function calls
  • Sanity check on FIPs configure flag used against the version of FIPs bundle
  • --enable-aesgcm=table now is compatible with --enable-linuxkm
  • Increase output buffer size that wolfSSL_RAND_bytes can handle
  • Out of directory builds resolved, wolfSSL can now be built in a separate directory than the root wolfssl directory


  • [HIGH] CVE-2021-3336: In earlier versions of wolfSSL there exists a potential man in the middle attack on TLS 1.3 clients. Malicious attackers with a privileged network position can impersonate TLS 1.3 servers and bypass authentication. Users that have applications with client side code and have TLS 1.3 turned on, should update to the latest version of wolfSSL. Users that do not have TLS 1.3 turned on, or that are server side only, are NOT affected by this report. For the code change see Thanks to Aina Toky Rasoamanana and Olivier Levillain from Télécom SudParis for the report.
  • [LOW] In the case of using custom ECC curves there is the potential for a crafted compressed ECC key that has a custom prime value to cause a hang when imported. This only affects applications that are loading in ECC keys with wolfSSL builds that have compressed ECC keys and custom ECC curves enabled.
  • [LOW] With TLS 1.3 authenticated-only ciphers a section of the server hello could contain 16 bytes of uninitialized data when sent to the connected peer. This affects only a specific build of wolfSSL with TLS 1.3 early data enabled and using authenticated-only ciphers with TLS 1.3.

For additional vulnerability information visit the vulnerability page at

See INSTALL file for build instructions. More info can be found on-line at


wolfSSL Website

wolfSSL Wiki

FIPS 140-2/140-3 FAQ

wolfSSL Documentation

wolfSSL Manual

wolfSSL API Reference

wolfCrypt API Reference

TLS 1.3

wolfSSL Vulnerabilities

Additional wolfSSL Examples

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