Ansible SSH private key in source control?

It’s a bad idea to store any kind of plaintext secret in revision control, SSH private keys included. Instead, use ansible-vault to store the private key.

ansible-vault can operate on any file type. Just encrypt the file with

ansible-vault encrypt /path/to/local/private_key

then install the key:

- name: Install a private SSH key
  vars:
    source_key: /path/to/local/private_key
    dest_key: /path/to/remote/private_key
  tasks:
  - name: Ensure .ssh directory exists.
    file: 
      dest: "{{ dest_key | dirname }}"
      mode: 0700 
      owner: user 
      state: directory
  - name: Install ssh key
    copy: 
      src: "{{ source_key }}" 
      dest: "{{ dest_key }}"
      mode: 0600
      owner: user

Earlier versions of ansible-vault would only operate on variables defined in var files, so you had to do something like this:

ssh_key: |
  -----BEGIN RSA PRIVATE KEY-----
  ...
  -----END RSA PRIVATE KEY-----
key_file: /home/user/.ssh/id_rsa

Encrypt with ansible-vault:

ansible-vault encrypt /path/to/var_file

And install the key:

- name: Ensure .ssh directory exists.
  file: 
    dest: "{{ key_file | dirname }}"
    mode: 0700 
    owner: user 
    state: directory

- name: Install ssh key
  copy: 
    content: "{{ ssh_key }}" 
    dest: "{{ key_file }}"
    mode: 0600
    owner: user

Thanks to all those below who improved the answer with their comments.