I’ve been playing around with Public-key cryptography or asymmetric cryptography. With revaltions from Eric Snowden on programs like PRISM, I feel that encryption is more important than ever.

From Wikipedia

There are two main uses for public key crypto.

1. Public key encryption. A message is encrypted with a recipient’s public key. The message can only be unencrypted by the holder of the private key.

2. Digital Signature. A message is signed with a the senders private key to generate a cryptographic hash. Using the public key, one can verify the message was indeed sent with the coresponding private key and can ensure the message has not been altered.

Keys

First, let’s generate some keys. From the private key we will also generate the public key. Remember the private and public key are mathematically linked. This is important.

[1] pry(main)> require "openssl"
=> true
[2] pry(main)> private_key = OpenSSL::PKey::RSA.new(2048)
=> #<OpenSSL::PKey::RSA:0x007ff03b379c30>
[3] pry(main)> public_key = private_key.public_key
=> #<OpenSSL::PKey::RSA:0x007ff03c3330b8>

Great. Now we have a key pair. We can write these keys to disk if we like.

## write the private key to disk.
File.open("~/.ssh/private_key", "w+") do |f|
  f.write private_key.to_pem
end
# write the public key to disk.
File.open("~/.ssh/public_key.pem", "w+") do |f|
  f.write public_key.to_pem
end

We can read the key in from key like so.

key = OpenSSL::PKey::RSA.new(File.read("~/.ssh/private_key"))

Public key enctryption

We’ll be encrypting a message with the public key. So it can only be unenctrypted by the owner of the private key.

## Use the public key to encrypt a message
[4] pry(main)> encrypted_data = public_key.public_encrypt("Some private data is here.")
=> "›Ò"lûú¢Ì;ŸÂrúˆ”\ÛZY–4ãiÛµ´<Žì¸«"aáà
Œb쮆À!È©ÞBe^Ìe܁ܘXÖ"bUJ°Bí–Ÿ}÷IýŸôQ§HœÅaá¤kxô6A% ëë: Ñ¢ÿ¤$½PŸRéӟ\r>Ž5ÙèYde`Žà½å\öƒÔmÍJš*¦õS9ÒMÕ 7’Îð¹<¯eš®Ž`­W“‘;HZëØÿÞmò*\tL2â`KÈ)O†Ú¯k4Ð\rNó%}êMheßÆåª$V¾~ŽLvœÇ´o¡3âÞât!]-Ȉ¯ãÕý`TÙÍsÿ®(<½¹"

This will result in some unreadable binary data. Next we’ll decrypt this data with the private key.

## Use the private key to decrypt
[5] pry(main)> private_key.private_decrypt(enc)
=> "Some private data is here."

Nice!

The public key can encrypt but cannot decrypt the message. This enctryption only works in one direction. Only the private key can decrypt that message. You can also encrypt messages with a private key. Those messages can be unenctrypted with the public key and the private key.

Asymmetric public/private key encryption is slow and victim to attack in cases where it is used without padding or directly to encrypt larger chunks of data. Typical use cases for RSA encryption involve “wrapping” a symmetric key with the public key of the recipient who would “unwrap” that symmetric key again using their private key.

NOTE - The enctrypted messages are in binary format. If we want to encode the data for transmission accoss the net, we’ll want to use something like base64.

[6] pry(main)> require "base64"
=> true
[7] pry(main)> enc_string = Base64.encode64(encrypted)
=> "iRBt6WHFYP2sn2Qv+qs16js/EJqBGaTWyxUH7iI/aj3UEw1oUHHbrFs/705W\nP+8dJ77p5gAaBpS/spCYLu/strU3uN06DTOh3neTcyDQrpIL5Zqs0Gl6/76m\nOQFGi18khnwWPAyW4+uVcZiQmZU9M0tlWywwNlkVoKAwkFlwkYF07YZazfCY\nMrAoQ6nusfjqjfU7HQeQKSCnMrBkzInsqan0PUm+UuGCMbxpQMdPA1de2nHB\neMs7OR7Pd5q5T93z240Iacjtwo/CV3Tcr1EyrfCcx05Jp4FKi9DPJf33asPx\noJ7J2XNa3QXqEbisMGT/b+6QZDm/LbfZXKuCIDYOjA==\n"

This works for small messages but if you try and encrpt a string larget than your key… you’ll hit an errors. You could break up the string into smaller chunks and encrypt each chunk individually. But this is not secure. This is what cypher block chaining is for. Below is an example.

cipher      = OpenSSL::Cipher.new 'AES-128-CBC'
cipher.encrypt
iv          = cipher.random_iv
pwd         = 'tRiSsmiTp'
salt        = OpenSSL::Random.random_bytes 16
iter        = 20000
key_len     = cipher.key_len
digest      = OpenSSL::Digest::SHA256.new

key = OpenSSL::PKCS5.pbkdf2_hmac(pwd, salt, iter, key_len, digest)
cipher.key = key

File.open("enc-text.txt",'w') do |enc|
  File.open("./plain.txt") do |f|
     loop do
       r = f.read(4096)
       break unless r
       enc << cipher.update(r)
     end
     enc << cipher.final
  end
end

Digital Signature

Asymmetric digital signatures is a great way to verify integrity and authenticity of data. Create a keypair, send the public key to your receivers, and use this method to create a digital signature. By combining the data and the public key, you can verify that the signature was created by the owner of the private key.

require "openssl"

data = "A small brown fox."


digest = OpenSSL::Digest::SHA256.new
# To list available digests:
#OpenSSL::Digest.constants

signature = private_key.sign(digest, data)

public_key.verify(digest, signature, data)
# => true
public_key.verify(digest, signature, data + "altered")
# => false
public_key.verify(digest, "altered" + signature, data)
# => false

More

Using openssl library directly we can encrypt a file.

openssl aes-256-cbc -a -e -in source-file.txt -out enc-file.txt -k private_key

And to decrypt the file.

openssl aes-256-cbc -d -a -in enc-file.txt -out unenc-file.txt -k private_key

http://ruby-doc.org/stdlib-2.0/libdoc/openssl/rdoc/OpenSSL.html#label-PBKDF2+Password-based+Encryption

I’m in and out of servers all day. One thing I’ve found helpful is a ssh config file. Create a file and place it ~/.ssh/config. You can add as many entries as you like.

Host s3-rsync
  HostName ec2-54-203-43-190.us-west-2.compute.amazonaws.com
  User ec2-user
  IdentityFile "~/.ssh/private-key-location"

Host: A friendly name/handle which you’ll use.

HostName: The remote server address you’ll be connecting to.

User: The user you want to connect as.

IdentityFile: The private key to authenticate with.

Once its all setup you can connect by typing: ssh s3-rsync

This is the same as: ssh -i ~/.ssh/bs2-deploy ec2-user@ec2-54-203-43-190.us-west-2.compute.amazonaws.com.

Rails has taught me some really good best practices. Let’s look at a default rails create action

def create
  @post = Post.new(post_params)
  respond_to do |format|
    if @post.save
      format.html { redirect_to @post, notice: 'Post was successfully created.' }
    else
      format.html { render :new }
    end
  end
end

I try not to deviate too much from this and nor should you. If @post.save returns true. return a 201 status code. If @post.save returns false re-render the :new action. So you can let the user fix the form.

Often I will see things like this.

def create
  if params[:something]
    @post = Post.create(post_params)
  else
    @post = Post.create(other_params)
  end
  redirect_to posts_path
end

At first glance this looks clean. But if you look closer. There is a subtle problem here. If the post object fails validation and fails to create, The action redirects to the index action where all new instaces are creating. The @post object has lost any knowlege of the params and it will not get to oportunity to display the errors from the object. You’ll have to resort to things like flash[:notice] = "@post.errors.full_messages.join()"… ugh.

Active record gives us a nice interface for displaying errors on an object.

class Post < ActiveRecord::Base
  validates :title, presence: true
end

irb(main):001:0> p = Post.new
=> #<Post id: nil, title: nil, body: nil, created_at: nil, updated_at: nil>
irb(main):002:0> p.valid?
=> false
irb(main):003:0> p.errors
=> #<ActiveModel::Errors:0x007fe0ca9e4b68 @base=#<Post id: nil, title: nil, body: nil, created_at: nil, updated_at: nil>,
@messages={:title=>["can't be blank"]}>
irb(main):004:0>

When you call @post.save OR @post.valid? errors are added to the @post object so they can be displayed within the form.

Digging even deeper… Check out ActiveModel::Model. It will give you this same object interface without needing database backed model. Powerful stuff.

Lately I’ve been writing a lot of angularjs. At my work, we had a great debeate.. which client side framework should we use. Ultimately, we choose angular. I’m happy we did. Angularjs is awesome. I would have been just as happy to settle with ember. Two way binding is a game changer.

Rails is great, BUT, I feel that, in any rails app, you end up creating interfaces that are a reflection of your database. Adding a client side framework, you’re free to create interfaces that are decoupled from your backend.

Lately I’ve been doing a lot of server administration. It can be a lot of fun automating systems. One thing I’ve found super valuable is to setup a “message of the day” file. This will display immediately after login and can be found in /etc/motd. I like to put notes/documentation here. For example, I’ll put in how to restart redis, how to tag a deploy, or the path to the app.