security - Generate key pair on iphone and print to log as NSString

Question

Following Apple example code in: http://developer.apple.com/library/ios/#documentation/Security/Conceptual/CertKeyTrustProgGuide/iPhone_Tasks/iPhone_Tasks.html

I'm able to successfully generate key-pair with the code snippet below, but unable to print the keys...

The function SecKeyGeneratePair() - returns the keys as SecKeyRef type.

I have no idea how to handle this type, I understand that this is the keychain representation but how can I actually view the key-pair as NSString?? More specifically, how to convert SecKeyRef to NSString??

static const UInt8 publicKeyIdentifier[] = "com.apple.sample.publickey";
static const UInt8 privateKeyIdentifier[] = "com.apple.sample.privatekey";
                                                            // 1


- (void)generateKeyPairPlease
{
    OSStatus status = noErr;
    NSMutableDictionary *privateKeyAttr = [[NSMutableDictionary alloc] init];
    NSMutableDictionary *publicKeyAttr = [[NSMutableDictionary alloc] init];
    NSMutableDictionary *keyPairAttr = [[NSMutableDictionary alloc] init];
                                                                // 2

    NSData * publicTag = [NSData dataWithBytes:publicKeyIdentifier
                                length:strlen((const char *)publicKeyIdentifier)];
    NSData * privateTag = [NSData dataWithBytes:privateKeyIdentifier
                               length:strlen((const char *)privateKeyIdentifier)];
                                                                // 3

    SecKeyRef publicKey = NULL;
    SecKeyRef privateKey = NULL;                                // 4

    [keyPairAttr setObject:(id)kSecAttrKeyTypeRSA
                                   forKey:(id)kSecAttrKeyType]; // 5
    [keyPairAttr setObject:[NSNumber numberWithInt:1024]
                             forKey:(id)kSecAttrKeySizeInBits]; // 6

    [privateKeyAttr setObject:[NSNumber numberWithBool:YES]
                               forKey:(id)kSecAttrIsPermanent]; // 7
    [privateKeyAttr setObject:privateTag
                            forKey:(id)kSecAttrApplicationTag]; // 8

    [publicKeyAttr setObject:[NSNumber numberWithBool:YES]
                               forKey:(id)kSecAttrIsPermanent]; // 9
    [publicKeyAttr setObject:publicTag
                            forKey:(id)kSecAttrApplicationTag]; // 10

    [keyPairAttr setObject:privateKeyAttr
                               forKey:(id)kSecPrivateKeyAttrs]; // 11
    [keyPairAttr setObject:publicKeyAttr
                                forKey:(id)kSecPublicKeyAttrs]; // 12

    status = SecKeyGeneratePair((CFDictionaryRef)keyPairAttr,
                                      &publicKey, &privateKey); // 13
//    error handling...


    if(privateKeyAttr) [privateKeyAttr release];
    if(publicKeyAttr) [publicKeyAttr release];
    if(keyPairAttr) [keyPairAttr release];
    if(publicKey) CFRelease(publicKey);
    if(privateKey) CFRelease(privateKey);                       // 14
}

Answer 1

You can use SecItemCopyMatching to get key's NSData. Check getPublicKeyBits method in Apple's CryptoExercise, it implements exactly what you need.

Then you can convert NSData to a string. Perhaps, Base64 encoding will suite your needs. Here you can find Base64 encoding/decoding sample for iPhone. Alternatively, this answer may also be useful for Base64 encoding.