defmodule Plug.Session.COOKIE do
@moduledoc """
Stores the session in a cookie.
This cookie store is based on `Plug.Crypto.MessageVerifier`
and `Plug.Crypto.MessageEncryptor` which encrypts and signs
each cookie to ensure they can't be read nor tampered with.
Since this store uses crypto features, it requires you to
set the `:secret_key_base` field in your connection. This
can be easily achieved with a plug:
plug :put_secret_key_base
def put_secret_key_base(conn, _) do
put_in conn.secret_key_base, "-- LONG STRING WITH AT LEAST 64 BYTES --"
end
## Options
* `:secret_key_base` - the secret key base to built the cookie
signing/encryption on top of. If one is given on initialization,
the cookie store can precompute all relevant values at compilation
time. Otherwise, the value is taken from `conn.secret_key_base`
and cached.
* `:encryption_salt` - a salt used with `conn.secret_key_base` to generate
a key for encrypting/decrypting a cookie, can be either a binary or
an MFA returning a binary;
* `:signing_salt` - a salt used with `conn.secret_key_base` to generate a
key for signing/verifying a cookie, can be either a binary or
an MFA returning a binary;
* `:key_iterations` - option passed to `Plug.Crypto.KeyGenerator`
when generating the encryption and signing keys. Defaults to 1000;
* `:key_length` - option passed to `Plug.Crypto.KeyGenerator`
when generating the encryption and signing keys. Defaults to 32;
* `:key_digest` - option passed to `Plug.Crypto.KeyGenerator`
when generating the encryption and signing keys. Defaults to `:sha256`;
* `:serializer` - cookie serializer module that defines `encode/1` and
`decode/1` returning an `{:ok, value}` tuple. Defaults to
`:external_term_format`.
* `:log` - Log level to use when the cookie cannot be decoded.
Defaults to `:debug`, can be set to false to disable it.
* `:rotating_options` - additional list of options to use when decrypting and
verifying the cookie. These options are used only when the cookie could not
be decoded using primary options and are fetched on init so they cannot be
changed in runtime. Defaults to `[]`.
## Examples
plug Plug.Session, store: :cookie,
key: "_my_app_session",
encryption_salt: "cookie store encryption salt",
signing_salt: "cookie store signing salt",
key_length: 64,
log: :debug
"""
require Logger
@behaviour Plug.Session.Store
alias Plug.Crypto.KeyGenerator
alias Plug.Crypto.MessageVerifier
alias Plug.Crypto.MessageEncryptor
@impl true
def init(opts) do
build_opts(opts)
|> build_rotating_opts(opts[:rotating_options])
|> Map.delete(:secret_key_base)
end
@impl true
def get(conn, raw_cookie, opts) do
opts = Map.put(opts, :secret_key_base, conn.secret_key_base)
[opts | opts.rotating_options]
|> Enum.find_value(:error, &read_raw_cookie(raw_cookie, &1))
|> decode(opts.serializer, opts.log)
end
@impl true
def put(conn, _sid, term, opts) do
%{serializer: serializer, key_opts: key_opts, signing_salt: signing_salt} = opts
binary = encode(term, serializer)
case opts do
%{encryption_salt: nil} ->
MessageVerifier.sign(binary, derive(conn.secret_key_base, signing_salt, key_opts))
%{encryption_salt: encryption_salt} ->
MessageEncryptor.encrypt(
binary,
derive(conn.secret_key_base, encryption_salt, key_opts),
derive(conn.secret_key_base, signing_salt, key_opts)
)
end
end
@impl true
def delete(_conn, _sid, _opts) do
:ok
end
defp encode(term, :external_term_format) do
:erlang.term_to_binary(term)
end
defp encode(term, serializer) do
{:ok, binary} = serializer.encode(term)
binary
end
defp decode({:ok, binary}, :external_term_format, log) do
{:term,
try do
Plug.Crypto.non_executable_binary_to_term(binary)
rescue
e ->
Logger.log(
log,
"Plug.Session could not decode incoming session cookie. Reason: " <>
Exception.message(e)
)
%{}
end}
end
defp decode({:ok, binary}, serializer, _log) do
case serializer.decode(binary) do
{:ok, term} -> {:custom, term}
_ -> {:custom, %{}}
end
end
defp decode(:error, _serializer, false) do
{nil, %{}}
end
defp decode(:error, _serializer, log) do
Logger.log(
log,
"Plug.Session could not verify incoming session cookie. " <>
"This may happen when the session settings change or a stale cookie is sent."
)
{nil, %{}}
end
defp prederive(secret_key_base, value, key_opts)
when is_binary(secret_key_base) and is_binary(value) do
{:prederived, derive(secret_key_base, value, Keyword.delete(key_opts, :cache))}
end
defp prederive(_secret_key_base, value, _key_opts) do
value
end
defp derive(_secret_key_base, {:prederived, value}, _key_opts) do
value
end
defp derive(secret_key_base, {module, function, args}, key_opts) do
derive(secret_key_base, apply(module, function, args), key_opts)
end
defp derive(secret_key_base, key, key_opts) do
secret_key_base
|> validate_secret_key_base()
|> KeyGenerator.generate(key, key_opts)
end
defp validate_secret_key_base(nil),
do: raise(ArgumentError, "cookie store expects conn.secret_key_base to be set")
defp validate_secret_key_base(secret_key_base) when byte_size(secret_key_base) < 64,
do: raise(ArgumentError, "cookie store expects conn.secret_key_base to be at least 64 bytes")
defp validate_secret_key_base(secret_key_base), do: secret_key_base
defp check_signing_salt(opts) do
case opts[:signing_salt] do
nil -> raise ArgumentError, "cookie store expects :signing_salt as option"
salt -> salt
end
end
defp check_serializer(serializer) when is_atom(serializer), do: serializer
defp check_serializer(_),
do: raise(ArgumentError, "cookie store expects :serializer option to be a module")
defp read_raw_cookie(raw_cookie, opts) do
signing_salt = derive(opts.secret_key_base, opts.signing_salt, opts.key_opts)
case opts do
%{encryption_salt: nil} ->
MessageVerifier.verify(raw_cookie, signing_salt)
%{encryption_salt: _} ->
encryption_salt = derive(opts.secret_key_base, opts.encryption_salt, opts.key_opts)
MessageEncryptor.decrypt(raw_cookie, encryption_salt, signing_salt)
end
|> case do
:error -> nil
result -> result
end
end
defp build_opts(opts) do
encryption_salt = opts[:encryption_salt]
signing_salt = check_signing_salt(opts)
iterations = Keyword.get(opts, :key_iterations, 1000)
length = Keyword.get(opts, :key_length, 32)
digest = Keyword.get(opts, :key_digest, :sha256)
log = Keyword.get(opts, :log, :debug)
secret_key_base = Keyword.get(opts, :secret_key_base)
key_opts = [iterations: iterations, length: length, digest: digest, cache: Plug.Keys]
serializer = check_serializer(opts[:serializer] || :external_term_format)
%{
secret_key_base: secret_key_base,
encryption_salt: prederive(secret_key_base, encryption_salt, key_opts),
signing_salt: prederive(secret_key_base, signing_salt, key_opts),
key_opts: key_opts,
serializer: serializer,
log: log
}
end
defp build_rotating_opts(opts, rotating_opts) when is_list(rotating_opts) do
Map.put(opts, :rotating_options, Enum.map(rotating_opts, &build_opts/1))
end
defp build_rotating_opts(opts, _), do: Map.put(opts, :rotating_options, [])
end