Handling COSE EC keys encoded without leading 0 bytes in coordinates (#…

…64)

lib/cose/key/ec2.rb

@@ -11,6 +11,8 @@ class EC2 < CurveKey
 
       KTY_EC2 = 2
 
+      ZERO_BYTE = "\0".b
+
       def self.enforce_type(map)
         if map[LABEL_KTY] != KTY_EC2
           raise "Not an EC2 key"
@@ -68,7 +70,7 @@ def map
       def to_pkey
         if curve
           group = OpenSSL::PKey::EC::Group.new(curve.pkey_name)
-          public_key_bn = OpenSSL::BN.new("\x04" + x + y, 2)
+          public_key_bn = OpenSSL::BN.new("\x04" + pad_coordinate(group, x) + pad_coordinate(group, y), 2)
           public_key_point = OpenSSL::PKey::EC::Point.new(group, public_key_bn)
 
           # RFC5480 SubjectPublicKeyInfo
@@ -113,6 +115,14 @@ def curve
       def self.keyword_arguments_for_initialize(map)
         super.merge(y: map[LABEL_Y])
       end
+
+      def pad_coordinate(group, coordinate)
+        coordinate_length = (group.degree + 7) / 8
+        padding_required = coordinate_length - coordinate.length
+        return coordinate if padding_required <= 0
+
+        (ZERO_BYTE * padding_required) + coordinate
+      end
     end
   end
 end

spec/cose/key/ec2_spec.rb

@@ -131,6 +131,29 @@
       expect(pkey.public_key).to eq(original_pkey.public_key)
       expect(pkey.private_key).to eq(original_pkey.private_key)
     end
+
+    it "works for an EC key that omits leading zero" do
+      # x was encoded omitting a leading zero. Before calling OpenSSL we must pad it.
+      x = ")\xC6`8\xBC\xEE\xF9*\xA4S\x9E\xA7\xFA'\xCE\xB9\x8D\x8C\xF7U\x06\xD8B\xB8\x8B\x9A\xF6\x9B\xC1\n\xCB".b
+      y = "Y\x9C\xD0+y<\xCB9Vk-\xC4\xEB\x87\xA7\xA1\xFA\xFEF\xAD\xD7\xA6\xB8\x84\xBEm[\xD7\xAEN\xD6w".b
+      original_key = COSE::Key::EC2.new(
+        kid: "id".b,
+        alg: -7,
+        key_ops: 1,
+        base_iv: "init-vector".b,
+        crv: 1,
+        x: x,
+        y: y,
+      )
+
+      pkey = original_key.to_pkey
+      key = COSE::Key::EC2.from_pkey(pkey)
+
+      expect(pkey).to be_a(OpenSSL::PKey::EC)
+      expect(pkey.group.curve_name).to eq("prime256v1")
+      expect(key.x).to eq("\0".b + x)
+      expect(key.y).to eq(y)
+    end
   end
 
   describe "#serialize" do