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

CRL: CRL means "Certificate Revocation List" and is used to check to see if any certificates in the server's chain have been revoked for some reason.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 user name 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.

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 don't 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's 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.

The mentioned libraries: OpenSSL, GnuTLS, NSS, wolfSSL, mbed TLS, Secure Channel, Secure Transport.

More comparisons in the extensive feature-by-feature comparison on wikipedia.

curl's documentation of SSL problems.

Please mail us corrections if this table is incorrect, or tell us other features we should compare!