How to add private github repository as Composer dependency

Work with private repositories at GitHub and BitBucket:

JSON

{
    "require": {
        "vendor/my-private-repo": "dev-master"
    },
    "repositories": [
        {
            "type": "vcs",
            "url":  "git@bitbucket.org:vendor/my-private-repo.git"
        }
    ]
}

The only requirement is the installation of SSH keys for a git client.

Docs