Magento 2.0.18 is the final 2.0.x release. After March 2018, Magento 2.0.x will no longer receive security patches, quality fixes, or documentation updates.
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TLS requirement for repo.magento.com

The Magento software and component repository, repo.magento.com, recently started requiring Transport Layer Security (TLS) 1.1 or later.

The PCI Security Standards Council removed SSL/TLS 1.0 as an example of strong cryptography from the PCI Data Security Standard (PCI DSS) version 3.1, stating that it can no longer be used as a security control after June 30, 2016.

For details, see Date Change for Migrating from SSL and Early TLS.

Symptoms

If you have an earlier version of TLS, you’ll see the errors discussed in this section.

Downloading a Magento metapackage

The following error displays if you attempt to run composer create-project to get a Magento metapackage:

[Composer\Downloader\TransportException]                                           
The "https://repo.magento.com/packages.json" file could not be downloaded: Failed to enable crypto                                                                  
failed to open stream: operation failed  

Using the Web Setup Wizard

Actions like saving your authentication credentials or synchronizing with Magento Marketplace fail with the following error:

SSL connect error

Solution

See the Transport Layer Security (TLS) 1.1+ requirement for repo.magento.com.