Transmitting Data Securely
One important aspect of computer security is the secure communication of data over a network. Although you can devise your own security protocols and use low-level APIs such as BSD sockets and Common Crypto to implement them, it is usually much more convenient to use standard protocols and higher-level APIs when they are available.
The Secure Sockets Layer (SSL) protocol and its successor, the Transport Layer Security (TLS) protocol, provide support for secure communication over a network. They are commonly used over TCP/IP connections such as the Internet. They use certificate-based authentication to ensure that you are communicating with a valid server, they validate data to prevent tampering, and they can use public-key cryptography to guard against eavesdropping or message forgery.
SSL is built into all major browsers and web servers (the most recent versions also include TLS). Whenever you use a secure website—for example, to send your credit card number to a vendor over the Internet—and see a protocol identifier of
https rather than
http at the beginning of the URL, you are using SSL or TLS for communication.
There are several ways to take advantage of SSL and TLS:
At a high level, you can use an
httpsURL with the URL Loading System (the NSURL class, CFURL functions, and related classes).
At a lower level, you can use the CFNetwork API to negotiate an SSL or TLS connection.
For maximum control, you can use the Secure Transport API in OS X or in iOS 5.0 and later.
In addition to these APIs, a number of open source tools use OpenSSL for secure networking. If you use OpenSSL in your publicly shipping apps, you must provide your own copy of the OpenSSL libraries, preferably as part of your app bundle; the OpenSSL libraries that OS X provides are deprecated.
Using the URL Loading System
The URL Loading System is a very high-level API that you can use to access the contents of
FTP:// URLs. Because URL Loading System works with secure
https:// URLs, it can be used for secure transport of data.
You should use the URL loading system when you need to download a resource from a remote server. Unlike CFNetwork, this API does not maintain a continuous data stream. This makes it a better choice for mobile use when it meets your needs (particularly with cellular networks, where your IP number may change as you switch from tower to tower).
CFNetwork is an iOS and OS X API for creating, sending, and receiving serialized messages over a network. It provides a higher-level interface than Secure Transport that can be used by apps to set up and maintain a secure SSL or TLS networking session and to add authentication information to a message.
CFNetwork includes the following security-related components:
CFHTTPMessage, which you can use to create, serialize, deserialize, and manage HTTP protocol messages. You should use the CFHTTPMessage API instead of other CFHTTP APIs if you need to add authentication information to a message.
CFStream Socket Additions, which allocates read and write streams and provides constants used with the CFReadStream and CFWriteStream APIs to set security protocols. You should use CFStream objects if you need to set up a streaming data connection (as opposed to an HTTP request).
In addition to the CFNetwork API, you use the CFReadStream and CFWriteStream APIs in the Core Foundation framework to create and manage the read and write streams that CFNetwork depends on. You can specify an SSL or TLS protocol version to encrypt and decrypt the data stream. Note that CFReadStream and CFWriteStream are “toll-free bridged” with their Cocoa Foundation counterparts, the classes
NSOutputStream. This means that each Core Foundation type is interchangeable in function or method calls with the corresponding bridged Foundation object, so you can use either C or Objective C interfaces, whichever is most convenient for you.
Secure Transport is a low-level API for working with SSL and TLS. With Secure Transport, your code must set up the network connection and provide callback functions that Secure Transport calls to perform I/O operations over the network.
Secure Transport is used in both OS X and iOS. The CFNetwork and URL Loading System APIs are built on top of Secure Transport.
You can use the Secure Transport API to set parameters for a secure session, open and maintain a session, and close a session. However, because Secure Transport is a fairly complex API, you should generally use Secure Transport directly only if you need more control than you can get with CFNetwork.
The Secure Transport API lets you:
Choose which protocols (SSL/TLS versions) and cipher suites should be allowed, and (after connecting) determine which protocol and cipher suite were actually negotiated.
Specify Diffie-Hellman parameters for key exchange
Specify whether client-side authentication should be required, and obtain that identification.
Manage certificates and trust policies—specify certificates to use for client or server identification, specify the domain name to use when determining whether the other host’s certificate is valid, provide trust policies for expired certificates and unknown or expired root certificates, add additional trusted root certificates, and so on.
Secure Transport uses certificate management and cryptography services provided by CDSA and Common Crypto. Secure Transport has no transport-layer dependencies; it can be used with BSD sockets, Open Transport, or any other transport-layer protocol available.
OS X includes a low-level command-line interface to the OpenSSL open-source cryptography toolkit; this interface is not available on iOS.
Further, although OpenSSL is commonly used in the open source community, it does not provide a stable API from version to version. For this reason, the programmatic interface to OpenSSL is deprecated in OS X and is not provided in iOS. Use of the Apple-provided OpenSSL libraries by apps is strongly discouraged.
To ensure compatibility, if your app depends on OpenSSL, you should compile it yourself and statically link a known version of OpenSSL into your app. Such use works on both iOS and OS X.
In general, however, you should use the CFNetwork API for secure networking and the Certificate, Key, and Trust Services API for cryptographic services. Alternatively, in OS X, you can use the Secure Transport API.
To Learn More
To learn about other security issues related to network communication, read Using Networking Securely in Networking Overview.
For more information on the TLS standard, see IETF's TLS Working Group site.
See URL Session Programming Guide for information about this API.
To learn more about CFNetwork, read CFNetwork Programming Guide.
To get started with Secure Transport, see Secure Transport Reference. For sample code, see SSLSample.