Fix: SDK not parsing headers with claims other than String

EXAMPLES.md

@@ -10,6 +10,7 @@
     - [Issued At ("iat")](#issued-at-iat)
     - [JWT ID ("jti")](#jwt-id-jti)
   - [Time Validation](#time-validation)
+  - [Header Claims](#header-claims)
   - [Private Claims](#private-claims)
   - [Claim Class](#claim-class)
     - [Primitives](#primitives)
@@ -91,6 +92,24 @@ boolean expiringSoon = jwt.expiresIn(60); // true if the token expires within th
 ```
 
 
+## Header Claims
+
+Header parameters can be read as `Claim`s, which support any JSON value type allowed by the JOSE spec (RFC 7515) — strings such as `alg` and `kid`, arrays such as the `x5c` certificate chain, and nested objects such as `jwk`. If the parameter can't be found, a `BaseClaim` is returned.
+
+```java
+String alg = jwt.getHeaderClaim("alg").asString();
+List<String> x5c = jwt.getHeaderClaim("x5c").asList(String.class);
+```
+
+You can also obtain all the header claims at once by calling `getHeaderClaims`.
+
+```java
+Map<String, Claim> headerClaims = jwt.getHeaderClaims();
+```
+
+> **Note**
+> The older `getHeader()` method (which returns a `Map<String, String>`) is deprecated. It still works and returns structured values as their JSON text, but `getHeaderClaim` should be preferred for reading non-string header parameters.
+
 ## Private Claims
 
 Additional Claims defined in the token can be obtained by calling `getClaim` and passing the Claim name. If the claim can't be found, a BaseClaim will be returned. BaseClaim will return null on every method call except for the `asList` and `asArray`.

README.md

@@ -40,6 +40,9 @@ String issuer = jwt.getIssuer(); //get registered claims
 String claim = jwt.getClaim("isAdmin").asString(); //get custom claims
 boolean isExpired = jwt.isExpired(10); // Do time validation with 10 seconds leeway
 boolean expiringSoon = jwt.expiresIn(60); // true if the token expires within the next 60 seconds
+
+String alg = jwt.getHeaderClaim("alg").asString(); //get header parameters
+List<String> x5c = jwt.getHeaderClaim("x5c").asList(String.class); //supports structured header values
 ```
 
 A `DecodeException` will raise with a detailed message if the token has:

lib/src/main/java/com/auth0/android/jwt/JWT.java

@@ -9,11 +9,14 @@
 
 import com.google.gson.Gson;
 import com.google.gson.GsonBuilder;
-import com.google.gson.reflect.TypeToken;
+import com.google.gson.JsonElement;
+import com.google.gson.JsonObject;
 
 import java.lang.reflect.Type;
 import java.nio.charset.Charset;
+import java.util.Collections;
 import java.util.Date;
+import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
 
@@ -27,6 +30,8 @@ public class JWT implements Parcelable {
     private final String token;
 
     private Map<String, String> header;
+
+    private Map<String, Claim> headerTree;
     private JWTPayload payload;
     private String signature;
 
@@ -43,14 +48,43 @@ public JWT(@NonNull String token) {
 
     /**
      * Get the Header values from this JWT as a Map of Strings.
+     * <p>
+     * Structured header parameters (e.g. the "x5c" certificate chain array or a nested
+     * "jwk" object) are returned as their JSON text representation. To read header
+     * parameters as typed values, prefer {@link #getHeaderClaim(String)}.
      *
      * @return the Header values of the JWT.
+     * @deprecated Use {@link #getHeaderClaim(String)} or {@link #getHeaderClaims()} instead,
+     * which support non-string header parameters such as "x5c" and "jwk".
      */
+    @Deprecated
     @NonNull
     public Map<String, String> getHeader() {
         return header;
     }
 
+    /**
+     * Get a header Claim given its name. If the Claim wasn't specified in the JWT header, a BaseClaim will be returned.
+     *
+     * @param name the name of the header Claim to retrieve.
+     * @return a valid Claim.
+     */
+    @NonNull
+    public Claim getHeaderClaim(@NonNull String name) {
+        final Claim claim = headerTree.get(name);
+        return claim != null ? claim : new BaseClaim();
+    }
+
+    /**
+     * Get all the header Claims.
+     *
+     * @return a valid Map of header Claims.
+     */
+    @NonNull
+    public Map<String, Claim> getHeaderClaims() {
+        return headerTree;
+    }
+
     /**
      * Get the Signature from this JWT as a Base64 encoded String.
      *
@@ -229,13 +263,43 @@ public JWT[] newArray(int size) {
 
     private void decode(String token) {
         final String[] parts = splitToken(token);
-        Type mapType = new TypeToken<Map<String, String>>() {
-        }.getType();
-        header = parseJson(base64Decode(parts[0]), mapType);
+        parseHeader(base64Decode(parts[0]));
         payload = parseJson(base64Decode(parts[1]), JWTPayload.class);
         signature = parts[2];
     }
 
+    private void parseHeader(String json) {
+        final JsonObject object;
+        try {
+            JsonElement element = getGson().fromJson(json, JsonElement.class);
+            if (element == null || !element.isJsonObject()) {
+                throw new DecodeException("The token's header had an invalid JSON format.");
+            }
+            object = element.getAsJsonObject();
+        } catch (DecodeException e) {
+            throw e;
+        } catch (Exception e) {
+            throw new DecodeException("The token's header had an invalid JSON format.", e);
+        }
+
+        Map<String, String> stringHeader = new HashMap<>();
+        Map<String, Claim> tree = new HashMap<>();
+        for (Map.Entry<String, JsonElement> entry : object.entrySet()) {
+            JsonElement value = entry.getValue();
+            tree.put(entry.getKey(), new ClaimImpl(value));
+            stringHeader.put(entry.getKey(), stringifyHeaderValue(value));
+        }
+        header = Collections.unmodifiableMap(stringHeader);
+        headerTree = Collections.unmodifiableMap(tree);
+    }
+
+    private String stringifyHeaderValue(JsonElement value) {
+        if (value.isJsonPrimitive()) {
+            return value.getAsString();
+        }
+        return value.toString();
+    }
+
     private String[] splitToken(String token) {
         String[] parts = token.split("\\.");
         if (parts.length == 2 && token.endsWith(".")) {

lib/src/test/java/com/auth0/android/jwt/JWTTest.java

@@ -84,6 +84,72 @@ public void shouldGetHeader() {
         assertThat(jwt.getHeader(), is(hasEntry("alg", "HS256")));
     }
 
+    @Test
+    public void shouldGetHeaderClaimAsArray() {
+        // header: {"alg":"RS256","x5c":["MIICert1","MIICert2"]}
+        JWT jwt = jwtWithHeader("{\"alg\":\"RS256\",\"x5c\":[\"MIICert1\",\"MIICert2\"]}");
+        assertThat(jwt, is(notNullValue()));
+        assertThat(jwt.getHeaderClaim("x5c"), is(instanceOf(ClaimImpl.class)));
+        assertThat(jwt.getHeaderClaim("x5c").asList(String.class), is(hasSize(2)));
+        assertThat(jwt.getHeaderClaim("x5c").asList(String.class), is(hasItems("MIICert1", "MIICert2")));
+    }
+
+    @Test
+    public void shouldGetHeaderClaimAsObject() {
+        // header: {"alg":"RS256","jwk":{"kty":"RSA","kid":"abc"}}
+        JWT jwt = jwtWithHeader("{\"alg\":\"RS256\",\"jwk\":{\"kty\":\"RSA\",\"kid\":\"abc\"}}");
+        assertThat(jwt, is(notNullValue()));
+        @SuppressWarnings("unchecked")
+        Map<String, String> jwk = jwt.getHeaderClaim("jwk").asObject(Map.class);
+        assertThat(jwk, is(notNullValue()));
+        assertThat(jwk, is(hasEntry("kty", "RSA")));
+        assertThat(jwk, is(hasEntry("kid", "abc")));
+    }
+
+    @Test
+    public void shouldGetHeaderClaimAsString() {
+        JWT jwt = jwtWithHeader("{\"alg\":\"HS256\",\"typ\":\"JWT\"}");
+        assertThat(jwt, is(notNullValue()));
+        assertThat(jwt.getHeaderClaim("alg").asString(), is("HS256"));
+        assertThat(jwt.getHeaderClaim("typ").asString(), is("JWT"));
+    }
+
+    @Test
+    public void shouldGetBaseClaimIfHeaderClaimIsMissing() {
+        JWT jwt = jwtWithHeader("{\"alg\":\"HS256\"}");
+        assertThat(jwt, is(notNullValue()));
+        assertThat(jwt.getHeaderClaim("notExisting"), is(notNullValue()));
+        assertThat(jwt.getHeaderClaim("notExisting"), is(not(instanceOf(ClaimImpl.class))));
+        assertThat(jwt.getHeaderClaim("notExisting"), is(instanceOf(BaseClaim.class)));
+    }
+
+    @Test
+    public void shouldGetAllHeaderClaims() {
+        JWT jwt = jwtWithHeader("{\"alg\":\"RS256\",\"x5c\":[\"MIICert1\"]}");
+        assertThat(jwt, is(notNullValue()));
+        Map<String, Claim> claims = jwt.getHeaderClaims();
+        assertThat(claims, is(notNullValue()));
+        assertThat(claims.get("alg").asString(), is("RS256"));
+        assertThat(claims.get("x5c").asList(String.class), is(hasItems("MIICert1")));
+    }
+
+    @Test
+    public void shouldGetLegacyHeaderStringForStructuredValue() {
+        JWT jwt = jwtWithHeader("{\"alg\":\"RS256\",\"x5c\":[\"MIICert1\",\"MIICert2\"]}");
+        assertThat(jwt, is(notNullValue()));
+        // Legacy Map<String,String> must not throw on structured values; returns the JSON text.
+        assertThat(jwt.getHeader(), is(hasEntry("alg", "RS256")));
+        assertThat(jwt.getHeader(), is(hasEntry("x5c", "[\"MIICert1\",\"MIICert2\"]")));
+    }
+
+    @Test
+    public void shouldThrowIfHeaderHasInvalidJSONFormat() {
+        exception.expect(DecodeException.class);
+        exception.expectMessage("The token's header had an invalid JSON format.");
+        // header decodes to the non-JSON-object string "notJson"
+        new JWT(String.format("%s.e30.sig", encodeString("notJson")));
+    }
+
     @Test
     public void shouldGetSignature() {
         JWT jwt = new JWT("eyJhbGciOiJIUzI1NiJ9.e30.XmNK3GpH3Ys_7wsYBfq4C3M6goz71I7dTgUkuIa5lyQ");
@@ -466,6 +532,19 @@ private JWT customTimeJWT(@Nullable Long iatMs, @Nullable Long expMs) {
         return new JWT(String.format("%s.%s.%s", header, body, signature));
     }
 
+    /**
+     * Creates a new JWT with a custom header JSON, an empty payload and a dummy signature.
+     *
+     * @param headerJson the raw JSON to use as the token header.
+     * @return a JWT
+     */
+    private JWT jwtWithHeader(String headerJson) {
+        String header = encodeString(headerJson);
+        String body = encodeString("{}");
+        String signature = "sign";
+        return new JWT(String.format("%s.%s.%s", header, body, signature));
+    }
+
     private String encodeString(String source) {
         byte[] bytes = Base64.encode(source.getBytes(), Base64.URL_SAFE | Base64.NO_WRAP | Base64.NO_PADDING);
         return new String(bytes, Charset.defaultCharset());