How can I compute a SHA-2 (ideally SHA 256 or SHA 512) hash in iOS?

The Security services API doesn't appear to allow me to compute a hash directly. There are plenty of public domain and liberally licensed versions available, but I'd rather use a system library implementation if possible.

The data is accessible via NSData, or plain pointers.

The cryptographic strength of the hash is important to me. SHA-256 is the minimum acceptable hash size.

This is what I'm using for SHA1:

#import <CommonCrypto/CommonDigest.h>

+ (NSData *)sha1:(NSData *)data {
    unsigned char hash[CC_SHA1_DIGEST_LENGTH];
    if ( CC_SHA1([data bytes], [data length], hash) ) {
        NSData *sha1 = [NSData dataWithBytes:hash length:CC_SHA1_DIGEST_LENGTH];        
        return sha1;
    }
return nil;
}

Replace CC_SHA1 with CC_SHA256 (or whichever you need), as well as CC_SHA1_DIGEST_LENGTH with CC_SHA256_DIGEST_LENGTH.

Here's a pretty similar one based on NSString

+ (NSString *)hashed_string:(NSString *)input
{
    const char *cstr = [input cStringUsingEncoding:NSUTF8StringEncoding];
    NSData *data = [NSData dataWithBytes:cstr length:input.length];
    uint8_t digest[CC_SHA256_DIGEST_LENGTH];

    // This is an iOS5-specific method.
    // It takes in the data, how much data, and then output format, which in this case is an int array.
    CC_SHA256(data.bytes, data.length, digest);

    NSMutableString* output = [NSMutableString stringWithCapacity:CC_SHA256_DIGEST_LENGTH * 2];

    // Parse through the CC_SHA256 results (stored inside of digest[]).
    for(int i = 0; i < CC_SHA256_DIGEST_LENGTH; i++) {
        [output appendFormat:@"%02x", digest[i]];
    }

    return output;
}

(Credits go to http://www.raywenderlich.com/6475/basic-security-in-ios-5-tutorial-part-1)