安装CUDA：进行sudo apt-get update时报错

安装完nvidia cuda的仓库安装包，进行sudo apt-get update时报错：
W: http://extras.ubuntu.com/ubuntu/dists/trusty/Release.gpg: Signature by key C47415DFF48C09645B78609416126D3A3E5C1192 uses weak digest algorithm (SHA1）
已经修改过好几次源了，只是不知道这个错误需要怎样修改
所以接下来进行sudo apt-get install cuda时也报错：
Reading package lists... Done
Building dependency tree

Reading state information... Done
E: Unable to locate package cuda

https://askubuntu.com/questions/933003/apt-get-update-signature-uses-weak-digest-algorithm-sha1