From a358929e62e004b31ce2745d32b958dd8ac57759 Mon Sep 17 00:00:00 2001
From: Didactic Drunk <1479616+didactic-drunk@users.noreply.github.com>
Date: Tue, 28 May 2019 14:31:31 -0700
Subject: [PATCH] Add libsodium kdf.

---
 README.md             |  9 +++++++++
 spec/cox/kdf_spec.cr  | 26 ++++++++++++++++++++++++
 src/cox/kdf.cr        | 47 +++++++++++++++++++++++++++++++++++++++++++
 src/cox/lib_sodium.cr | 13 ++++++++++++
 4 files changed, 95 insertions(+)
 create mode 100644 spec/cox/kdf_spec.cr
 create mode 100644 src/cox/kdf.cr

diff --git a/README.md b/README.md
index 4bd0022..4c449e1 100644
--- a/README.md
+++ b/README.md
@@ -55,6 +55,15 @@ signature = Cox.sign_detached(message, signing_pair.secret)
 Cox.verify_detached(signature, message, signing_pair.public) # => true
 ```
 
+# Key derivation
+kdf = Cox::Kdf.new
+
+# kdf.derive(8_byte_context, subkey_size, subkey_id)
+subkey1 = kdf.derive "context1", 16, 0
+subkey2 = kdf.derive "context1", 16, 1
+subkey3 = kdf.derive "context2", 32, 0
+subkey4 = kdf.derive "context2", 64, 1
+
 ## Contributing
 
 1. Fork it ( https://github.com/andrewhamon/cox/fork )
diff --git a/spec/cox/kdf_spec.cr b/spec/cox/kdf_spec.cr
new file mode 100644
index 0000000..35bbdfe
--- /dev/null
+++ b/spec/cox/kdf_spec.cr
@@ -0,0 +1,26 @@
+require "../spec_helper"
+
+CONTEXT = "8_bytess"
+
+describe Cox::Kdf do
+  it "generates master key" do
+    kdf1 = Cox::Kdf.new
+
+    # verify loading saved key
+    kdf2 = Cox::Kdf.from_base64 kdf1.to_base64
+
+    # verify generated subkey's are the same after loading
+    key1_s1 = kdf1.derive CONTEXT, 16, 0
+    key2_s1 = kdf2.derive CONTEXT, 16, 0
+    key1_s1.should eq key2_s1
+  end
+
+  it "generates different keys" do
+    kdf1 = Cox::Kdf.new
+    subkey1 = kdf1.derive CONTEXT, 16, 0
+    subkey2 = kdf1.derive CONTEXT, 16, 1
+    subkey1.should_not eq subkey2
+  end
+
+# TODO: test exceptions
+end
diff --git a/src/cox/kdf.cr b/src/cox/kdf.cr
new file mode 100644
index 0000000..60fd277
--- /dev/null
+++ b/src/cox/kdf.cr
@@ -0,0 +1,47 @@
+module Cox
+  class Kdf
+    property bytes : Bytes
+
+    def initialize(bytes : Bytes)
+      if bytes.bytesize != LibSodium::KDF_KEY_BYTES
+        raise ArgumentError.new("bytes must be #{LibSodium::KDF_KEY_BYTES}, got #{bytes.bytesize}")
+      end
+
+      @bytes = bytes
+    end
+
+    def initialize
+      @bytes = Random::Secure.random_bytes(LibSodium::KDF_KEY_BYTES)
+    end
+
+    # context must be 8 bytes
+    # subkey_size must be 16..64 bytes as of libsodium 1.0.17
+    def derive(context, subkey_size, subkey_id = 0)
+      if context.bytesize != LibSodium::KDF_CONTEXT_BYTES
+        raise ArgumentError.new("context must be #{LibSodium::KDF_CONTEXT_BYTES}, got #{context.bytesize}")
+      end
+
+      subkey = Bytes.new subkey_size
+      if (ret = LibSodium.crypto_kdf_derive_from_key(subkey, subkey.bytesize, subkey_id, context, @bytes)) != 0
+        raise Cox::Error.new("crypto_kdf_derive_from_key returned #{ret} (subkey size is probably out of range)")
+      end
+      subkey
+    end
+
+    def pointer
+      bytes.to_unsafe
+    end
+
+    def pointer(size)
+      bytes.pointer(size)
+    end
+
+    def to_base64
+      Base64.encode(bytes)
+    end
+
+    def self.from_base64(encoded_key)
+      new(Base64.decode(encoded_key))
+    end
+  end
+end
diff --git a/src/cox/lib_sodium.cr b/src/cox/lib_sodium.cr
index 06ab66f..4d25991 100644
--- a/src/cox/lib_sodium.cr
+++ b/src/cox/lib_sodium.cr
@@ -10,6 +10,8 @@ module Cox
     fun crypto_sign_publickeybytes() : LibC::SizeT
     fun crypto_sign_secretkeybytes() : LibC::SizeT
     fun crypto_sign_bytes()          : LibC::SizeT
+    fun crypto_kdf_keybytes()        : LibC::SizeT
+    fun crypto_kdf_contextbytes()    : LibC::SizeT
 
     PUBLIC_KEY_BYTES  = crypto_box_publickeybytes()
     SECRET_KEY_BYTES  = crypto_box_secretkeybytes()
@@ -18,6 +20,8 @@ module Cox
     PUBLIC_SIGN_BYTES = crypto_sign_publickeybytes()
     SECRET_SIGN_BYTES = crypto_sign_secretkeybytes()
     SIGNATURE_BYTES   = crypto_sign_bytes()
+    KDF_KEY_BYTES     = crypto_kdf_keybytes()
+    KDF_CONTEXT_BYTES = crypto_kdf_contextbytes()
 
     fun crypto_box_keypair(
       public_key_output : Pointer(LibC::UChar),
@@ -61,5 +65,14 @@ module Cox
       message_size : LibC::ULongLong,
       public_key   : Pointer(LibC::UChar)
     ) : LibC::Int
+
+
+    fun crypto_kdf_derive_from_key(
+      subkey    : Pointer(LibC::UChar),
+      subkey_len    : LibC::SizeT,
+      subkey_id    : UInt64,
+      ctx    : Pointer(LibC::UChar),
+      key    : Pointer(LibC::UChar)
+    ) : LibC::Int
   end
 end