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The SSL Conservatory: iOS Certificate Pinning
=============================================
When an iOS application only needs to communicate to a well-defined set of
servers over SSL or HTTPS, the security of the app's network communications can
be improved through SSL pinning. By requiring a specific certificate to be part
of the server's certificate chain, the threat of a rogue CA or a CA compromise
is significantly reduced.
### The ISPCertificatePinning class
#### Description
This class allows developers to whitelist a list of certificates for a given
domain in order to require at least one these "pinned" certificates to be part
of the server's certificate chain received when connecting to the domain over
SSL or HTTPS.
This gives developers the flexibility to pin the CA/anchor certificate, the
server/leaf certificate, or any intermediate certificate for a given domain.
Each option has different advantages and limitations; for example, pinning the
server/leaf certificate provides the best security but this certificate is going
to change more often than the CA/anchor certificate.
A change in the certificate presented by the server (for example because the
previous certificate expired) will result in the application being unable to
connect to the server until its pinned certificate has been updated as well.
To address this scenario, multiple certificates can be pinned to a single
domain. This gives developers the ability to transition from an expiring
certificate to a new one by releasing a new version of their application that
pins both certificates to the server's domain.
#### API
The ISPCertificatePinning class exposes two methods:
##### +(BOOL)setupSSLPinsUsingDictionnary:(NSDictionary*)domainsAndCertificates
This method takes a dictionary with domain names as keys and arrays of
DER-encoded certificates as values, and stores them in a pre-defined location on
the filesystem. The ability to specify multiple certificates for a single
domain is useful when transitioning from an expiring certificate to a new one
##### +(BOOL)verifyPinnedCertificateForTrust:(SecTrustRef)trust andDomain:(NSString*)domain
This method accesses the certificates previously loaded using the
setupSSLPinsUsingDictionnary: method and inspects the trust object's
certificate chain in order to find at least one certificate pinned to the
given domain. SecTrustEvaluate() should always be called before this method to
ensure that the certificate chain is valid.
### Convenience delegate classes for NSURLConnection and NSURLSession
This library also provides convenience classes for connections relying on
NSURLConnection and NSURLSession. The ISPPinnedNSURLConnectionDelegate and
ISPPinnedNSURLSessionDelegate implement the connection authentication methods
within respectively the NSURLConnectionDelegate and NSURLSessionDelegate
protocols, in order to automatically validate the server's certificate based on
SSL pins loaded using the setupSSLPinsUsingDictionnary: method.
To implement certificate pinning in their Apps, developers should simply extend
these classes when creating their own connection delegates.
### Sample code
The Xcode unit tests within SSLCertificatePinningTests contain sample code
demonstrating how to implement certificate pinning when using NSURLConnection
and NSURLSession.
### Changelog
* v3: Turned the Xcode project into a static library.
Added certificate pinning delegate class for NSURLSession connections.
* v2: Added the ability to pin multiple certificates to a single domain.
* v1: Initial release.
### License
See ../LICENSE.
### Author
Alban Diquet - https://github.com/nabla-c0d3
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