curl / Docs / Protocols / SSL libraries compared

Compare SSL libraries

This comparison only involves SSL/TLS libraries that libcurl can be built to use.

Feature	OpenSSL[1]	GnuTLS	wolfSSL	mbedTLS	Schannel	Secure Transport	rustls	BearSSL
Supported	yes	yes	yes	yes	yes	yes	no	yes
Native cert check	yes	yes	yes	yes	yes	yes	yes	yes
CRL	manual	manual	manual	manual	automatic	automatic	manual	manual
TLSv1.0	yes	yes	yes	yes	yes	yes	yes	yes
TLSv1.1	yes	yes	yes	yes	yes	yes	yes	yes
TLSv1.2	yes	yes	yes	yes	yes	yes	yes	yes
TLSv1.3	yes	yes	yes	yes	yes	no	yes	no
TLS SRP	yes	yes	no	no	no	no	no	no
TLS ECC	yes	yes	yes	yes	yes	yes	yes	yes
ALPN	yes	yes	yes	yes	yes	yes	yes	yes
QUIC	yes [2]	yes	yes	no	no	no	no	no
Small	no	no	yes	yes	yes	yes	no	yes
Platforms	POSIX, Windows, VMS	POSIX, Windows	POSIX, Windows	POSIX, Windows	Windows	macOS, iOS, tvOS etc	POSIX, Windows	POSIX
Uses Certificate Files	yes	yes	yes	yes	no	no	yes	yes
Uses Certificate db	no	no	no	no	yes	yes	no	no
Crypto module/token support	PKCS#11 [8]	PKCS#11	no	no	Microsoft CryptoAPI	Keychain	no	no
Select Certificates/Keys with PKCS#11 URI	yes [8]	yes	no	no	no	no	no	no
Integrates with system token database	yes	yes	yes	no	yes	yes	no	no
FIPS-140	yes	yes	yes	no	yes	yes	no	no
OpenSSL-like API	yes	no	yes	no	no	no	no	no
Vendor	OpenSSL Project	Free Software Foundation	wolfSSL	mbed TLS	Microsoft Corporation	Apple Inc.	Open Source team	Thomas Pornin
License	Apache-2.0	LGPL	GPLv2	Apache-2.0 / GPLv2	Proprietary	APSL 2.0	Apache-2.0 / MIT / ISC	MIT
First release	1998	2004?	2006	2006	2000	2003?	2016	2016
Famous User	Apache HTTPD	GNOME	MySQL	Hiawatha HTTPD	Microsoft Internet Explorer	Apple Safari	?	?

[1] = Mostly the same feature set is also provided by LibreSSL, BoringSSL, AWS-LC and quictls

[2] = OpenSSL 3.2 has a QUIC stack that curl works with experimentally. The OpenSSL forks LibreSSL, BoringSSL, AWS-LC and quictls support the QUIC API that curl works with using ngtcp2.

[8] = Via external engine_pkcs11;

Glossary of Terms

Supported: "no" here means the TLS library is still experimental in curl and we discourage using it in production.

Native cert check: The TLS library can verify server certificates against the platform's native CA cert store.

CRL: CRL means "Certificate Revocation List" and is used to check to see if any certificates in the server's chain have been revoked for some reason. If automatic, then the engine will automatically download a CRL and use it to evaluate the trust of the server's certificate chain when performing the TLS handshake. If manual, then the engine will not automatically use a CRL, but you can provide one that has been downloaded separately by using the CURLOPT_CRL option. If no, then the CURLOPT_CRL option will be ignored.

SSLv2: This was the first public release of the SSL protocol. It is deprecated and really should no longer be used, because it has a number of serious security problems.

SSLv3: It is deprecated and should no longer be used, because it has a number of serious security problems.

TLSv1.0: TLS is a slight variation on SSLv3 that was the first version of the protocol to be approved of by the Internet Engineering Task Force (IETF). This version of TLS has been available since 1999 and is by far the most widely supported version on the public Internet. There have been a few minor security vulnerabilities found in TLSv1.0 which were fixed later, but all of them (so far) have been easily worked around, which has contributed to the longevity of this version of TLS.

TLSv1.1: TLSv1.1 is similar to v1.0, except that it has a better fix for the CBC (Cipher Block Chain) cipher-suite attack that lead to the BEAST (Browser Exploit Against SSL/TLS) vulnerability in TLSv1.0.

TLSv1.2: TLSv1.2 provides better security than earlier versions, with support for many all-new cipher suites that are even more difficult to crack.

TLSv1.3: This is the most recent version. TLSv1.3 provides even better security than TLSv1.2, with more data encrypted in the handshake etc. It also offers less roundtrip handshakes.

TLS SRP: SRP means "Secure Remote Password" and it is a method of performing client-side authentication with a TLS server by using a username and password, sometimes coupled with a certificate. It is not yet widely supported, but for the engines that do support it, you can provide the credentials to curl by using the CURLOPT_TLSAUTH_USERNAME and CURLOPT_TLSAUTH_PASSWORD options.

TLS ECC: ECC means "Elliptic Curve Cryptography" and it is an advanced set of cipher-suites that are used in TLS connections (typically with TLSv1.2). Not all engines support ECC.

QUIC: Needed for HTTP/3 support.

Small: can be built with a small foot-print. The system native ones also count here since they come "for free" for users.

Uses Certificate/Key Files: Some engines, such as OpenSSL, read certificates and keys from files rather than a central database. These engines require you to use a certificate bundle in order to verify a server's certificate chain; this is usually set at build time but can also be set by using the CURLOPT_CAINFO option.

Uses Certificate/Key Database: Some engines, such as Apple's Security framework, use a central database instead of separate files to store certificates and keys. Apple's Security framework database, for instance, is called the Keychain. For engines that use a database and do not also support files, the CURLOPT_CAINFO option is ignored.

Crypto module/token support: Support for cryptographic hardware tokens and software databases is typically provided via ; PKCS#11 on POSIX platforms, and via platform-specific APIs on Windows and Darwin. Examples of PKCS#11 software tokens include the GNOME keyring, and the NSS "soft token" database.

Integrates with system token database: Platforms often have a system-wide configuration which specifies which crypto modules/token should be visible in which applications. Many Linux distributions have chosen to use p11-kit; to provide this configuration, and some now consider it a bug for applications not to automatically use the tokens configured therein.

Select Certificates/Keys with PKCS#11 URI: RFC 7512 defines a standard URI format for specifying objects within PKCS#11 tokens/databases.

FIPS-140: FIPS-140 is a security standard used by the United States and Canada for transferring information that is sensitive but not classified. If yes, and you are using curl or a libcurl-based application in the US or Canadian government, or in a government contractor, then it is okay for you to use the engine when building curl/libcurl.

License: If you are deploying an application that uses libcurl, then the license used by the engine may affect whether or not you are able to distribute your application legally. OpenSSL's 4-clause BSD license, for instance, is not compatible with the GNU GPL.

Compare SSL libraries

Glossary of Terms

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