Repair EC x/y coordinates when importing JWK

CHANGELOG.md

@@ -16,6 +16,7 @@
 - Fix key base equality and spaceship operators [#569](https://github.com/jwt/ruby-jwt/pull/569) - [@magneland](https://github.com/magneland).
 - Remove explicit base64 require from x5c_key_finder [#580](https://github.com/jwt/ruby-jwt/pull/580) - [@anakinj](https://github.com/anakinj).
 - Performance improvements and cleanup of tests [#581](https://github.com/jwt/ruby-jwt/pull/581) - [@anakinj](https://github.com/anakinj).
+- Repair EC x/y coordinates when importing JWK [#585](https://github.com/jwt/ruby-jwt/pull/585) - [@julik](https://github.com/julik).
 - Your contribution here
 
 ## [v2.7.1](https://github.com/jwt/ruby-jwt/tree/v2.8.0) (2023-06-09)

lib/jwt/jwk/ec.rb

@@ -11,6 +11,7 @@ class EC < KeyBase # rubocop:disable Metrics/ClassLength
       EC_PUBLIC_KEY_ELEMENTS = %i[kty crv x y].freeze
       EC_PRIVATE_KEY_ELEMENTS = %i[d].freeze
       EC_KEY_ELEMENTS = (EC_PRIVATE_KEY_ELEMENTS + EC_PUBLIC_KEY_ELEMENTS).freeze
+      ZERO_BYTE = "\0".b.freeze
 
       def initialize(key, params = nil, options = {})
         params ||= {}
@@ -143,7 +144,6 @@ def parse_ec_key(key)
       if ::JWT.openssl_3?
         def create_ec_key(jwk_crv, jwk_x, jwk_y, jwk_d) # rubocop:disable Metrics/MethodLength
           curve = EC.to_openssl_curve(jwk_crv)
-
           x_octets = decode_octets(jwk_x)
           y_octets = decode_octets(jwk_y)
 
@@ -205,12 +205,27 @@ def create_ec_key(jwk_crv, jwk_x, jwk_y, jwk_d)
         end
       end
 
-      def decode_octets(jwk_data)
-        ::JWT::Base64.url_decode(jwk_data)
-      end
-
-      def decode_open_ssl_bn(jwk_data)
-        OpenSSL::BN.new(::JWT::Base64.url_decode(jwk_data), BINARY)
+      def decode_octets(base64_encoded_coordinate)
+        bytes = ::JWT::Base64.url_decode(base64_encoded_coordinate)
+        # Some base64 encoders on some platform omit a single 0-byte at
+        # the start of either Y or X coordinate of the elliptic curve point.
+        # This leads to an encoding error when data is passed to OpenSSL BN.
+        # It is know to have happend to exported JWKs on a Java application and
+        # on a Flutter/Dart application (both iOS and Android). All that is
+        # needed to fix the problem is adding a leading 0-byte. We know the
+        # required byte is 0 because with any other byte the point is no longer
+        # on the curve - and OpenSSL will actually communicate this via another
+        # exception. The indication of a stripped byte will be the fact that the
+        # coordinates - once decoded into bytes - should always be an even
+        # bytesize. For example, with a P-521 curve, both x and y must be 66 bytes.
+        # With a P-256 curve, both x and y must be 32 and so on. The simplest way
+        # to check for this truncation is thus to check whether the number of bytes
+        # is odd, and restore the leading 0-byte if it is.
+        if bytes.bytesize.odd?
+          ZERO_BYTE + bytes
+        else
+          bytes
+        end
       end
 
       class << self

spec/jwt/jwk/ec_spec.rb

@@ -138,6 +138,29 @@
           end
         end
       end
+
+      context 'with missing 0-byte at the start of EC coordinates' do
+        let(:example_keysets) do
+          [
+            '{"kty":"EC","crv":"P-256","x":"0Nv5IKAlkvXuAKmOmFgmrwXKR7qGePOzu_7RXg5msw","y":"FqnPSNutcjfvXNlufwb7nLJuUEnBkbMdZ3P79nY9c3k"}',
+            '{"kty":"EC","crv":"P-256","x":"xGjPg-7meZamM_yfkGeBUB2eJ5c82Y8vQdXwi5cVGw","y":"9FwKAuJacVyEy71yoVn1u1ETsQoiwF7QfkfXURGxg14"}',
+            '{"kty":"EC","crv":"P-256","x":"yTvy0bwt5s29mIg1DMq-IjZH4pDgZIN9keEEaSuWZhk","y":"a0nrmd8qz8jpZDgpY82Rgv3vZ5xiJuiAoMIuRlGnaw"}',
+            '{"kty":"EC","crv":"P-256","x":"yJen7AW4lLUTMH4luDj0wlMNSGCuOBB5R-ZoxlAU_g","y":"aMbA-M6ORHePSatiPVz_Pzu7z2XRnKMzK-HIscpfud8"}',
+            '{"kty":"EC","crv":"P-256","x":"p_D00Z1ydC7mBIpSKPUUrzVzY9Fr5NMhhGfnf4P9guw","y":"lCqM3B_s04uhm7_91oycBvoWzuQWJCbMoZc46uqHXA"}',
+            '{"kty":"EC","crv":"P-256","x":"hKS-vxV1bvfZ2xOuHv6Qt3lmHIiArTnhWac31kXw3w","y":"f_UWjrTpmq_oTdfss7YJ-9dEiYw_JC90kwAE-y0Yu-w"}',
+            '{"kty":"EC","crv":"P-256","x":"3W22hN16OJN1XPpUQuCxtwoBRlf-wGyBNIihQiTmSdI","y":"eUaveaPQ4CpyfY7sfCqEF1DCOoxHdMpPHW15BmUF0w"}',
+            '{"kty":"EC","crv":"P-256","x":"oq_00cGL3SxUZTA-JvcXALhfQya7elFuC7jcJScN7Bs","y":"1nNPIinv_gQiwStfx7vqs7Vt_MSyzoQDy9sCnZlFfg"}',
+            '{"crv":"P-521","kty":"EC","x":"AMNQr/q+YGv4GfkEjrXH2N0+hnGes4cCqahJlV39m3aJpqSK+uiAvkRE5SDm2bZBc3YHGzhDzfMTUpnvXwjugUQP","y":"fIwouWsnp44Fjh2gBmO8ZafnpXZwLOCoaT5itu/Q4Z6j3duRfqmDsqyxZueDA3Gaac2LkbWGplT7mg4j7vCuGsw="}'
+          ]
+        end
+
+        it 'prepends a 0-byte to either X or Y coordinate so that the keys decode correctly' do
+          example_keysets.each do |keyset_json|
+            jwk = described_class.import(JSON.parse(keyset_json))
+            expect(jwk).to be_kind_of(JWT::JWK::EC)
+          end
+        end
+      end
     end
   end
 end