How to encrypt your data with Ecto

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If your data is encrypted, even if it’s leaked, no one know what is the data. That’s great.

In this post, I’m going to show you how to encrypt data with Ecto. Ecto allows developer to define their own types. And we will define a type called EncryptedText which encrypts data before persiting to database and decrypts data after loading.

1. Define encrypt/decrypt methods

This is a simple version of crypto module:

 1defmodule Crypto do
 2  @block_size 16
 3
 4  def generate_secret do
 5    :crypto.strong_rand_bytes(@block_size)
 6    |> Base.encode64()
 7  end
 8
 9  def encrypt(plaintext, secret_key) do
10    with {:ok, secret_key} <- decode_key(secret_key) do
11      iv = :crypto.strong_rand_bytes(@block_size)
12      plaintext = pad(plaintext, @block_size)
13      ciphertext = :crypto.crypto_one_time(:aes_128_cbc, secret_key, iv, plaintext, true)
14
15      {:ok, Base.encode64(iv <> ciphertext)}
16    end
17  end
18
19  def decrypt(ciphertext, secret_key) do
20    with {:ok, secret_key} <- decode_key(secret_key),
21         {:ok, <<iv::binary-@block_size, ciphertext::binary>>} <- Base.decode64(ciphertext) do
22      plaintext =
23        :crypto.crypto_one_time(:aes_128_cbc, secret_key, iv, ciphertext, false)
24        |> unpad
25
26      {:ok, plaintext}
27    else
28      {:error, _} = err -> err
29      _ -> {:error, "Bad encrypted data"}
30    end
31  end
32
33  defp pad(data, block_size) do
34    to_add = block_size - rem(byte_size(data), block_size)
35    data <> :binary.copy(<<to_add>>, to_add)
36  end
37
38  defp unpad(data) do
39    to_remove = :binary.last(data)
40    :binary.part(data, 0, byte_size(data) - to_remove)
41  end
42end

Let go through the code

1  def generate_secret do
2    :crypto.strong_rand_bytes(@block_size)
3    |> Base.encode64()
4  end

This function generate a 16 bytes secret key and encode base 64 string so you can add it to config.

  • encrypt/2 function
 1 def encrypt(plaintext, secret_key) do
 2    # check the key size
 3    with {:ok, secret_key} <- decode_key(secret_key) do
 4      
 5      # random initial vector
 6      iv = :crypto.strong_rand_bytes(@block_size)
 7      # if length of text is not multiple of @block_size
 8      # we add more data until it meets condition
 9      plaintext = pad(plaintext, @block_size)
10      # encrypt here
11      ciphertext = :crypto.crypto_one_time(:aes_128_cbc, secret_key, iv, plaintext, true)
12
13      {:ok, Base.encode64(iv <> ciphertext)}
14    end
15  end

This is the most important line

1ciphertext = :crypto.crypto_one_time(:aes_128_cbc, secret_key, iv, plaintext, true)
  • iv is initial vector. AES-128 algorithms encrypts data by block of 16 bytes, so we need initial vector to make sure that the output of blocks with same data are different from each other.
  • The last parameter is set to true to encrypt, set to false to decrypt data

And then we encode output to base 64 string. Here we concatenate iv and ciphertext so that we can extract iv to use for decrypting

1{:ok, Base.encode64(iv <> ciphertext)}
  • decrypt/2 function
 1def decrypt(ciphertext, secret_key) do
 2    # check the key
 3    with {:ok, secret_key} <- decode_key(secret_key),
 4         {:ok, <<iv::binary-@block_size, ciphertext::binary>>} <- Base.decode64(ciphertext) do
 5      plaintext =
 6        :crypto.crypto_one_time(:aes_128_cbc, secret_key, iv, ciphertext, false)
 7        |> unpad
 8
 9      {:ok, plaintext}
10    else
11      {:error, _} = err -> err
12      _ -> {:error, "Bad encrypted data"}
13    end
14  end

We extract iv and encrypted data from input

1{:ok, <<iv::binary-@block_size, ciphertext::binary>>} <- Base.decode64(ciphertext)

We use pattern matching to extract first 16 byte and assign to iv and assign remaining data to ciphertext. Then decrypting data

1plaintext =
2    :crypto.crypto_one_time(:aes_128_cbc, secret_key, iv, ciphertext, false)
3    |> unpad

This line is similar to the line which encrypts data, the difference is here we replace plaintext by ciphertext and last parameter is set to false. After data is decrypted, we need to remove padding to get the original data.

2. Build EncryptedText type

I define a type to store binary data, you can define a EncryptedMap to store map data. The most important function are dump and load where we encrypt before persisting and decrypt after loading.

 1defmodule EncryptedText do
 2  use Ecto.Type
 3
 4  # we store data as string
 5  def type, do: :string
 6
 7  def cast(value) when is_binary(value) do
 8    {:ok, value}
 9  end
10  def cast(_), do: :error
11
12  def dump(nil), do: nil
13  # encrypt data before persist to database
14  def dump(data) when is_binary(data) do
15    with {:ok, secret_key} <- Application.fetch_env(:myapp, :ecto_secret_key),
16         {:ok, data} <- Crypto.encrypt(data, secret_key) do
17      {:ok, data}
18    else
19      _ -> :error
20    end
21  end
22
23  def dump(_), do: :error
24
25  def load(nil), do: nil
26  # decrypt data after loaded from database
27  def load(data) when is_binary(data) do
28    secret_key = Application.fetch_env!(:myapp, :ecto_secret_key)
29    case Crypto.decrypt(data, secret_key) do
30      {:error, _} -> :error
31      ok -> ok
32    end
33  end
34
35  def load(_), do: :error
36
37  def embed_as(_), do: :dump
38end
39

3. Use it in your schema

  • You must add secret key to your app config first.
1config :myapp, :ecto_secret_key, "your key using Crypto.generate_secret"
  • Add to schema
1schema "users" do
2    field :name, :string
3    ...
4    field :secret, EncryptedText
5    ...
6end

Your data are safe now.

4. Conclusion

With Crypto you can implement encrypted field for any type of data you want.

There is an issue when you want to change your secret key, you have to load your data row by row, decrypt and then encrypt with new key and update to database.

I found this article which explains very well about crypto if you are interested https://www.thegreatcodeadventure.com/elixir-encryption-with-erlang-crypto/ Although she uses old crypto API so it will throw some warnings.

I implemented encrypted type for text and map for my company project here if you want to use it:

Github

Thanks for reading.