Pluggable, more flexible, security policies. (#429)

Extract @FredericJacobs' CertificateVerifier concept with @nlutsenko's
SRSecurityOptions into a pluggable SRSecurityPolicy model

This retains existing SSL configuration code paths, while allowing users
more flexibility to specify their own security policy.

If you are alread using AFNetworking and an `AFSecurityPolicy`, it's
intended that you can share domain trust logic by delegating
`SRSecurityPolicy evaluateTrust:ForDomain` to your AFSecurityPolicy
instance.

Inspired by original "Require TLS 1.2 & enable pinning" pull request by
Frederic Jacobs (@FredericJacobs) at:

https://github.com/facebook/SocketRocket/pull/274/files

SocketRocket/Internal/Security/SRPinningSecurityPolicy.h

@@ -0,0 +1,21 @@
+//
+// Copyright (c) 2016-present, Facebook, Inc.
+// All rights reserved.
+//
+// This source code is licensed under the BSD-style license found in the
+// LICENSE file in the root directory of this source tree. An additional grant
+// of patent rights can be found in the PATENTS file in the same directory.
+//
+
+#import <Foundation/Foundation.h>
+#import "SRSecurityPolicy.h"
+
+NS_ASSUME_NONNULL_BEGIN
+
+@interface SRPinningSecurityPolicy : SRSecurityPolicy
+
+- (instancetype)initWithCertificates:(NSArray *)pinnedCertificates;
+
+@end
+
+NS_ASSUME_NONNULL_END

SocketRocket/Internal/Security/SRPinningSecurityPolicy.m

@@ -0,0 +1,67 @@
+//
+// Copyright (c) 2016-present, Facebook, Inc.
+// All rights reserved.
+//
+// This source code is licensed under the BSD-style license found in the
+// LICENSE file in the root directory of this source tree. An additional grant
+// of patent rights can be found in the PATENTS file in the same directory.
+//
+
+#import "SRPinningSecurityPolicy.h"
+
+#import <Foundation/Foundation.h>
+
+#import "SRLog.h"
+
+NS_ASSUME_NONNULL_BEGIN
+
+@interface SRPinningSecurityPolicy ()
+
+@property (nonatomic, copy, readonly) NSArray *pinnedCertificates;
+
+@end
+
+@implementation SRPinningSecurityPolicy
+
+- (instancetype)initWithCertificates:(NSArray *)pinnedCertificates
+{
+    // Do not validate certificate chain since we're pinning to specific certificates.
+    self = [super initWithCertificateChainValidationEnabled:NO];
+    if (!self) { return self; }
+
+    if (pinnedCertificates.count == 0) {
+        @throw [NSException exceptionWithName:@"Creating security policy failed."
+                                       reason:@"Must specify at least one certificate when creating a pinning policy."
+                                     userInfo:nil];
+    }
+    _pinnedCertificates = [pinnedCertificates copy];
+
+    return self;
+}
+
+- (BOOL)evaluateServerTrust:(SecTrustRef)serverTrust forDomain:(NSString *)domain
+{
+    SRDebugLog(@"Pinned cert count: %d", self.pinnedCertificates.count);
+    NSUInteger requiredCertCount = self.pinnedCertificates.count;
+
+    NSUInteger validatedCertCount = 0;
+    CFIndex serverCertCount = SecTrustGetCertificateCount(serverTrust);
+    for (CFIndex i = 0; i < serverCertCount; i++) {
+        SecCertificateRef cert = SecTrustGetCertificateAtIndex(serverTrust, i);
+        NSData *data = CFBridgingRelease(SecCertificateCopyData(cert));
+        for (id ref in self.pinnedCertificates) {
+            SecCertificateRef trustedCert = (__bridge SecCertificateRef)ref;
+            // TODO: (nlutsenko) Add caching, so we don't copy the data for every pinned cert all the time.
+            NSData *trustedCertData = CFBridgingRelease(SecCertificateCopyData(trustedCert));
+            if ([trustedCertData isEqualToData:data]) {
+                validatedCertCount++;
+                break;
+            }
+        }
+    }
+    return (requiredCertCount == validatedCertCount);
+}
+
+@end
+
+NS_ASSUME_NONNULL_END

SocketRocket/SRSecurityPolicy.h

@@ -0,0 +1,67 @@
+//
+// Copyright (c) 2016-present, Facebook, Inc.
+// All rights reserved.
+//
+// This source code is licensed under the BSD-style license found in the
+// LICENSE file in the root directory of this source tree. An additional grant
+// of patent rights can be found in the PATENTS file in the same directory.
+//
+
+#import <Foundation/Foundation.h>
+#import <Security/Security.h>
+
+NS_ASSUME_NONNULL_BEGIN
+
+@interface SRSecurityPolicy : NSObject
+
+/**
+ A default `SRSecurityPolicy` implementation specifies socket security and
+ validates the certificate chain.
+
+ Use a subclass of `SRSecurityPolicy` for more fine grained customization.
+ */
++ (instancetype)defaultPolicy;
+
+/**
+ Specifies socket security and provider certificate pinning, disregarding certificate
+ chain validation.
+
+ @param pinnedCertificates Array of `SecCertificateRef` SSL certificates to use for validation.
+ */
++ (instancetype)pinnningPolicyWithCertificates:(NSArray *)pinnedCertificates;
+
+/**
+ Specifies socket security and optional certificate chain validation.
+
+ @param enabled Whether or not to validate the SSL certificate chain. If you
+ are considering using this method because your certificate was not issued by a
+ recognized certificate authority, consider using `pinningPolicyWithCertificates` instead.
+ */
+- (instancetype)initWithCertificateChainValidationEnabled:(BOOL)enabled NS_DESIGNATED_INITIALIZER;
+
+/**
+ Updates all the security options for input and output streams, for example you
+ can set your socket security level here.
+
+ @param stream Stream to update the options in.
+ */
+- (void)updateSecurityOptionsInStream:(NSStream *)stream;
+
+/**
+ Whether or not the specified server trust should be accepted, based on the security policy.
+
+ This method should be used when responding to an authentication challenge from
+ a server. In the default implemenation, no further validation is done here, but
+ you're free to override it in a subclass. See `SRPinningSecurityPolicy.h` for
+ an example.
+
+ @param serverTrust The X.509 certificate trust of the server.
+ @param domain The domain of serverTrust.
+
+ @return Whether or not to trust the server.
+ */
+- (BOOL)evaluateServerTrust:(SecTrustRef)serverTrust forDomain:(NSString *)domain;
+
+@end
+
+NS_ASSUME_NONNULL_END

SocketRocket/SRSecurityPolicy.m

@@ -0,0 +1,66 @@
+//
+// Copyright (c) 2016-present, Facebook, Inc.
+// All rights reserved.
+//
+// This source code is licensed under the BSD-style license found in the
+// LICENSE file in the root directory of this source tree. An additional grant
+// of patent rights can be found in the PATENTS file in the same directory.
+//
+
+#import "SRSecurityPolicy.h"
+#import "SRPinningSecurityPolicy.h"
+
+NS_ASSUME_NONNULL_BEGIN
+
+@interface SRSecurityPolicy ()
+
+@property (nonatomic, assign, readonly) BOOL certificateChainValidationEnabled;
+
+@end
+
+@implementation SRSecurityPolicy
+
++ (instancetype)defaultPolicy
+{
+    return [self new];
+}
+
++ (instancetype)pinnningPolicyWithCertificates:(NSArray *)pinnedCertificates
+{
+    return [[SRPinningSecurityPolicy alloc] initWithCertificates:pinnedCertificates];
+}
+
+- (instancetype)initWithCertificateChainValidationEnabled:(BOOL)enabled
+{
+    self = [super init];
+    if (!self) { return self; }
+
+    _certificateChainValidationEnabled = enabled;
+
+    return self;
+}
+
+- (instancetype)init
+{
+    return [self initWithCertificateChainValidationEnabled:YES];
+}
+
+- (void)updateSecurityOptionsInStream:(NSStream *)stream
+{
+    // Enforce TLS 1.2
+    [stream setProperty:(__bridge id)CFSTR("kCFStreamSocketSecurityLevelTLSv1_2") forKey:(__bridge id)kCFStreamPropertySocketSecurityLevel];
+
+    // Validate certificate chain for this stream if enabled.
+    NSDictionary<NSString *, id> *sslOptions = @{ (__bridge NSString *)kCFStreamSSLValidatesCertificateChain : @(self.certificateChainValidationEnabled) };
+    [stream setProperty:sslOptions forKey:(__bridge NSString *)kCFStreamPropertySSLSettings];
+}
+
+- (BOOL)evaluateServerTrust:(SecTrustRef)serverTrust forDomain:(NSString *)domain
+{
+    // No further evaluation happens in the default policy.
+    return YES;
+}
+
+@end
+
+NS_ASSUME_NONNULL_END