From 3da71f7de6ad612b3fbb571c1d90c5be8558feb8 Mon Sep 17 00:00:00 2001
From: Francesco Noacco <francesco.noacco2000@gmail.com>
Date: Mon, 24 Nov 2025 17:16:40 +0100
Subject: [PATCH 1/2] chore: update lock

---
 mix.lock | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mix.lock b/mix.lock
index bb4c1a4..2dccf58 100644
--- a/mix.lock
+++ b/mix.lock
@@ -1,5 +1,5 @@
 %{
-  "b58": {:git, "https://github.com/dwyl/base58.git", "711b29b62f50394eef3602a2b52dc28d536a5894", []},
+  "b58": {:hex, :b58, "1.0.3", "d300d6ae5a3de956a54b9e8220e924e4fee1a349de983df2340fe61e0e464202", [:mix], [], "hexpm", "af62a98a8661fd89978cf3a3a4b5b2ebe82209de6ac6164f0b112e36af72fc59"},
   "cbor": {:hex, :cbor, "1.0.0", "35d33a26f6420ce3d2d01c0b1463a748b34c537d5609fc40116daf3666700d36", [:mix], [], "hexpm", "cc5e21e0fa5a0330715a3806c67bc294f8b65d07160f751b5bd6058bed1962ac"},
   "hkdf_erlang": {:hex, :hkdf_erlang, "0.1.1", "b35538edfffefc44d6855b2bfe2dc00909c2a4365f41e47d2fe493811006955e", [:rebar3], [], "hexpm", "eb784bda0df4e964fd69f622310752198fb0218543e1a7393912abc7a5ce926e"},
 }

From e182d7ffd4270a781ef31510e4623e48401d04b9 Mon Sep 17 00:00:00 2001
From: Francesco Noacco <francesco.noacco2000@gmail.com>
Date: Mon, 24 Nov 2025 17:16:40 +0100
Subject: [PATCH 2/2] feat: add encrypt0 message

---
 lib/cose/messages/encrypt0.ex | 82 +++++++++++++++++++++++++++++++++++
 1 file changed, 82 insertions(+)
 create mode 100644 lib/cose/messages/encrypt0.ex

diff --git a/lib/cose/messages/encrypt0.ex b/lib/cose/messages/encrypt0.ex
new file mode 100644
index 0000000..4cf0ff7
--- /dev/null
+++ b/lib/cose/messages/encrypt0.ex
@@ -0,0 +1,82 @@
+defmodule COSE.Messages.Encrypt0 do
+  defstruct [:phdr, :uhdr, :ciphertext, :payload, :aad]
+
+  @spec build(binary, map, map) :: map
+  def build(payload, phdr \\ %{}, uhdr \\ %{}) do
+    %__MODULE__{
+      phdr: phdr,
+      uhdr: uhdr,
+      payload: payload,
+      aad: COSE.tag_as_byte(<<>>)
+    }
+  end
+
+  def encrypt_encode(msg, cipher_suite, key, iv) do
+    msg
+    |> encrypt(cipher_suite, key, iv)
+    |> encode()
+  end
+
+  def encrypt(msg, cipher_suite, key, iv, external_aad \\ <<>>) do
+    aad = msg |> enc_structure(external_aad) |> CBOR.encode()
+
+    {encrypted, tag} =
+      :crypto.crypto_one_time_aead(cipher_suite, key.k, iv, msg.payload, aad, 8, true)
+
+    Map.put(msg, :ciphertext, COSE.tag_as_byte(encrypted <> tag))
+  end
+
+  def encode(msg) do
+    cose_values = [
+      COSE.Headers.tag_phdr(msg.phdr),
+      COSE.Headers.translate(msg.uhdr),
+      msg.ciphertext
+    ]
+
+    CBOR.encode(%CBOR.Tag{tag: 16, value: cose_values})
+  end
+
+  def decrypt_decode(msg_cbor, cipher_suite, key) do
+    msg = decode(msg_cbor)
+    decrypt(msg, cipher_suite, key, msg.uhdr.iv.value)
+  end
+
+  def decrypt(msg, cipher_suite, key, iv, external_aad \\ <<>>) do
+    aad = msg |> enc_structure(external_aad) |> CBOR.encode()
+    {encrypted, tag} = split_encrypted_tag(msg.ciphertext.value)
+
+    :crypto.crypto_one_time_aead(cipher_suite, key.k, iv, encrypted, aad, tag, false)
+    |> case do
+      payload when is_binary(payload) ->
+        {:ok, Map.put(msg, :payload, payload)}
+
+      error ->
+        error
+    end
+  end
+
+  def decode(encoded_msg) do
+    {:ok, %CBOR.Tag{tag: 16, value: [phdr, uhdr, ciphertext]}, _} =
+      CBOR.decode(encoded_msg)
+
+    %__MODULE__{
+      phdr: COSE.Headers.decode_phdr(phdr),
+      uhdr: COSE.Headers.translate(uhdr),
+      ciphertext: ciphertext
+    }
+  end
+
+  def enc_structure(msg, external_aad \\ <<>>) do
+    [
+      "Encrypt0",
+      (msg.phdr == %{} && COSE.tag_as_byte(<<>>)) || COSE.Headers.tag_phdr(msg.phdr),
+      external_aad
+    ]
+  end
+
+  def split_encrypted_tag(ciphertext, tag_len \\ 8) do
+    encrypted_len = byte_size(ciphertext) - tag_len
+    <<encrypted::binary-size(encrypted_len), tag::binary-size(tag_len)>> = ciphertext
+    {encrypted, tag}
+  end
+end