diff --git a/objectivec/GPBExtensionRegistry.m b/objectivec/GPBExtensionRegistry.m
index 65534b67..01eb761f 100644
--- a/objectivec/GPBExtensionRegistry.m
+++ b/objectivec/GPBExtensionRegistry.m
@@ -34,6 +34,8 @@
 #import "GPBDescriptor.h"
 
 @implementation GPBExtensionRegistry {
+  // TODO(dmaclach): Reimplement with CFDictionaries that don't use
+  // objects as keys.
   NSMutableDictionary *mutableClassMap_;
 }
 
@@ -63,16 +65,13 @@
   return result;
 }
 
-- (CFMutableDictionaryRef)extensionMapForContainingMessageClass:
+- (NSMutableDictionary *)extensionMapForContainingMessageClass:
         (Class)containingMessageClass {
-  CFMutableDictionaryRef extensionMap = (CFMutableDictionaryRef)
+  NSMutableDictionary *extensionMap =
       [mutableClassMap_ objectForKey:containingMessageClass];
   if (extensionMap == nil) {
-    // Use a custom dictionary here because the keys are numbers and conversion
-    // back and forth from NSNumber isn't worth the cost.
-    extensionMap = CFDictionaryCreateMutable(kCFAllocatorDefault, 0, NULL,
-                                             &kCFTypeDictionaryValueCallBacks);
-    [mutableClassMap_ setObject:(id)extensionMap
+    extensionMap = [NSMutableDictionary dictionary];
+    [mutableClassMap_ setObject:extensionMap
                          forKey:(id<NSCopying>)containingMessageClass];
   }
   return extensionMap;
@@ -84,28 +83,17 @@
   }
 
   Class containingMessageClass = extension.containingMessageClass;
-  CFMutableDictionaryRef extensionMap =
+  NSMutableDictionary *extensionMap =
       [self extensionMapForContainingMessageClass:containingMessageClass];
-  ssize_t key = extension.fieldNumber;
-  CFDictionarySetValue(extensionMap, (const void *)key, extension);
+  [extensionMap setObject:extension forKey:@(extension.fieldNumber)];
 }
 
 - (GPBExtensionDescriptor *)extensionForDescriptor:(GPBDescriptor *)descriptor
                                        fieldNumber:(NSInteger)fieldNumber {
   Class messageClass = descriptor.messageClass;
-  CFMutableDictionaryRef extensionMap = (CFMutableDictionaryRef)
+  NSDictionary *extensionMap =
       [mutableClassMap_ objectForKey:messageClass];
-  ssize_t key = fieldNumber;
-  GPBExtensionDescriptor *result =
-      (extensionMap
-       ? CFDictionaryGetValue(extensionMap, (const void *)key)
-       : nil);
-  return result;
-}
-
-static void CopyKeyValue(const void *key, const void *value, void *context) {
-  CFMutableDictionaryRef extensionMap = (CFMutableDictionaryRef)context;
-  CFDictionarySetValue(extensionMap, key, value);
+  return [extensionMap objectForKey:@(fieldNumber)];
 }
 
 - (void)addExtensions:(GPBExtensionRegistry *)registry {
@@ -114,16 +102,13 @@ static void CopyKeyValue(const void *key, const void *value, void *context) {
     return;
   }
   NSMutableDictionary *otherClassMap = registry->mutableClassMap_;
-  [otherClassMap enumerateKeysAndObjectsUsingBlock:^(id key, id value, BOOL * stop) {
-#pragma unused(stop)
-    Class containingMessageClass = key;
-    CFMutableDictionaryRef otherExtensionMap = (CFMutableDictionaryRef)value;
-
-    CFMutableDictionaryRef extensionMap =
+  for (Class containingMessageClass in otherClassMap) {
+    NSMutableDictionary *extensionMap =
         [self extensionMapForContainingMessageClass:containingMessageClass];
-
-    CFDictionaryApplyFunction(otherExtensionMap, CopyKeyValue, extensionMap);
-  }];
+    NSMutableDictionary *otherExtensionMap =
+        [registry extensionMapForContainingMessageClass:containingMessageClass];
+    [extensionMap addEntriesFromDictionary:otherExtensionMap];
+  }
 }
 
 #pragma clang diagnostic pop
diff --git a/objectivec/GPBUnknownFieldSet.m b/objectivec/GPBUnknownFieldSet.m
index a7335f05..9ba1d65c 100644
--- a/objectivec/GPBUnknownFieldSet.m
+++ b/objectivec/GPBUnknownFieldSet.m
@@ -36,6 +36,39 @@
 #import "GPBUtilities.h"
 #import "GPBWireFormat.h"
 
+#pragma mark CFDictionaryKeyCallBacks
+
+// We use a custom dictionary here because our keys are numbers and
+// conversion back and forth from NSNumber was costing us performance.
+// If/when we move to C++ this could be done using a std::map and some
+// careful retain/release calls.
+
+static const void *GPBUnknownFieldSetKeyRetain(CFAllocatorRef allocator,
+                                               const void *value) {
+#pragma unused(allocator)
+  return value;
+}
+
+static void GPBUnknownFieldSetKeyRelease(CFAllocatorRef allocator,
+                                         const void *value) {
+#pragma unused(allocator)
+#pragma unused(value)
+}
+
+static CFStringRef GPBUnknownFieldSetCopyKeyDescription(const void *value) {
+  return CFStringCreateWithFormat(kCFAllocatorDefault, NULL, CFSTR("%d"),
+                                  (int)value);
+}
+
+static Boolean GPBUnknownFieldSetKeyEqual(const void *value1,
+                                          const void *value2) {
+  return value1 == value2;
+}
+
+static CFHashCode GPBUnknownFieldSetKeyHash(const void *value) {
+  return (CFHashCode)value;
+}
+
 #pragma mark Helpers
 
 static void checkNumber(int32_t number) {
@@ -258,9 +291,13 @@ static void GPBUnknownFieldSetSerializedSizeAsMessageSet(const void *key,
   int32_t number = [field number];
   checkNumber(number);
   if (!fields_) {
-    // Use a custom dictionary here because the keys are numbers and conversion
-    // back and forth from NSNumber isn't worth the cost.
-    fields_ = CFDictionaryCreateMutable(kCFAllocatorDefault, 0, NULL,
+    CFDictionaryKeyCallBacks keyCallBacks = {
+        // See description above for reason for using custom dictionary.
+        0, GPBUnknownFieldSetKeyRetain, GPBUnknownFieldSetKeyRelease,
+        GPBUnknownFieldSetCopyKeyDescription, GPBUnknownFieldSetKeyEqual,
+        GPBUnknownFieldSetKeyHash,
+    };
+    fields_ = CFDictionaryCreateMutable(kCFAllocatorDefault, 0, &keyCallBacks,
                                         &kCFTypeDictionaryValueCallBacks);
   }
   ssize_t key = number;