GitHub 官方发布：https://blog.github.com/2018-02-23-weak-cryptographic-standards-removed/
从 2018 年 02 月 23 日起，GitHub 官方永久删除了对弱加密标准的支持，导致 cocoaPods 无法连接 GitHub 进行更新，报错信息：“[!] Failed to connect to GitHub to update the CocoaPods/Specs specs repo - Please check if you are offline, or that GitHub is down”。
解决该问题需要更新本地 openssl，rube，cocoaPod。参考更新步骤：
$ which openssl
/usr/bin/openssl
$ openssl version
OpenSSL 0.9.8zh 14 Jan 2016
$ /usr/bin/ruby -e "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/master/install)"
$ brew update
$ brew install openssl
$ brew upgrade openssl
`` If you need to have this software first in your PATH run: echo 'export PATH="/usr/local/opt/openssl/bin:$PATH"' >> ~/.bash_profile
$ echo 'export PATH="/usr/local/opt/openssl/bin:$PATH"' >> ~/.bash_profile
$ source ~/.bash_profile
$ which openssl
/usr/local/opt/openssl/bin/openssl
$ openssl version
OpenSSL 1.0.2n 7 Dec 2017
$ brew install rbenv ruby-build
$ echo 'export PATH="$HOME/.rbenv/bin:$PATH"' >> ~/.bash_profile
$ echo 'if which rbenv > /dev/null; then eval "$(rbenv init -)"; fi' >> ~/.bash_profile
$ source ~/.bash_profile
$ rbenv install --list
Available versions:
1.8.5-p52
1.8.5-p113
1.8.5-p114
1.8.5-p115
1.8.5-p231
1.8.6
:
2.1.0
2.1.1
2.1.2
:
$ rbenv install 2.5.0
$ rbenv versions
* system (set by /Users/username/.rbenv/version)
2.5.0
$ ruby --version
ruby 2.0.0p648 (2015-12-16 revision 53162) [universal.x86_64-darwin16]
$ rbenv global 2.5.0
$ rbenv versions
system
* 2.5.0 (set by /Users/username/.rbenv/version)
$ ruby --version
ruby 2.5.0p0 (2013-12-25 revision 44422) [x86_64-darwin16.0]
$ gem install cocoapods -n /usr/local/bin
$ which pod
/usr/local/bin/pod
$ pod --version
1.4.0
然后执行命令：
$ pod update
参考链接：https://qiita.com/satoshi-iwaki/items/ed2c267127dcc80baa21 （注意：更新 ruby 时请选择较高的版本，部分低版本将被淘汰）。