iOS Seckeyref to Nsdata

iOS SecKeyRef to NSData

• use of undeclared identifier 'publicTag'

The publicTag is just some unique identifier added to the Keychain items. In the CryptoExercise sample project it is defined as

#define kPublicKeyTag "com.apple.sample.publickey"
static const uint8_t publicKeyIdentifier[] = kPublicKeyTag;
NSData *publicTag = [[NSData alloc] initWithBytes:publicKeyIdentifier length:sizeof(publicKeyIdentifier)];

• Cast of an indirect pointer to an Objective-C pointer to 'CFTypeRef ' (aka 'const void *') is disallowed with ARC

This can be solved by using a temporary CFTypeRef variable:

CFTypeRef result;
sanityCheck = SecItemCopyMatching((__bridge CFDictionaryRef)queryPublicKey, &result);
if (sanityCheck == errSecSuccess) {
    publicKeyBits = CFBridgingRelease(result);
}

• I do not want to build a query or whatnot to extract the key from the keychain. I have it in a variable and I wish to extract it from there ...

As far as I know, you have to store the SecKeyRef to the Keychain temporarily. SecItemAdd
has the option to return the added item as data. From the documentation:

To obtain the data of the added item as an object of type CFDataRef,
specify the return type key kSecReturnData with a value of
kCFBooleanTrue.

Putting all that together, the following code should do what you want:

- (NSData *)getPublicKeyBitsFromKey:(SecKeyRef)givenKey {

    static const uint8_t publicKeyIdentifier[] = "com.your.company.publickey";
    NSData *publicTag = [[NSData alloc] initWithBytes:publicKeyIdentifier length:sizeof(publicKeyIdentifier)];

    OSStatus sanityCheck = noErr;
    NSData * publicKeyBits = nil;

    NSMutableDictionary * queryPublicKey = [[NSMutableDictionary alloc] init];
    [queryPublicKey setObject:(__bridge id)kSecClassKey forKey:(__bridge id)kSecClass];
    [queryPublicKey setObject:publicTag forKey:(__bridge id)kSecAttrApplicationTag];
    [queryPublicKey setObject:(__bridge id)kSecAttrKeyTypeRSA forKey:(__bridge id)kSecAttrKeyType];

    // Temporarily add key to the Keychain, return as data:
    NSMutableDictionary * attributes = [queryPublicKey mutableCopy];
    [attributes setObject:(__bridge id)givenKey forKey:(__bridge id)kSecValueRef];
    [attributes setObject:@YES forKey:(__bridge id)kSecReturnData];
    CFTypeRef result;
    sanityCheck = SecItemAdd((__bridge CFDictionaryRef) attributes, &result);
    if (sanityCheck == errSecSuccess) {
        publicKeyBits = CFBridgingRelease(result);

        // Remove from Keychain again:
        (void)SecItemDelete((__bridge CFDictionaryRef) queryPublicKey);
    }

    return publicKeyBits;
}

I hope that this works, I cannot test it at the moment.

• Follow-up: How can I export a private key to NSData, since I've read several time that the function I'm trying to work with only works for public keys.

I don't know.

Converting NSData to SecKeyRef

Use this function to save your public key. Pass your RAS public key and any name for peername.

- (void)addPeerPublicKey:(NSString *)peerName keyBits:(NSData *)publicKeyData {

        OSStatus sanityCheck = noErr;
        CFTypeRef persistPeer = NULL;
        [self removePeerPublicKey:peerName];

        NSData * peerTag = [[NSData alloc] initWithBytes:(const void *)[peerName UTF8String] length:[peerName length]];
        NSMutableDictionary * peerPublicKeyAttr = [[NSMutableDictionary alloc] init];
        [peerPublicKeyAttr setObject:(id)kSecClassKey forKey:(id)kSecClass];
        [peerPublicKeyAttr setObject:(id)kSecAttrKeyTypeRSA forKey:(id)kSecAttrKeyType];
        [peerPublicKeyAttr setObject:peerTag forKey:(id)kSecAttrApplicationTag];
        [peerPublicKeyAttr setObject:publicKeyData forKey:(id)kSecValueData];
        [peerPublicKeyAttr setObject:[NSNumber numberWithBool:YES] forKey:(id)kSecReturnData];
        sanityCheck = SecItemAdd((CFDictionaryRef) peerPublicKeyAttr, (CFTypeRef *)&persistPeer);

        if(sanityCheck == errSecDuplicateItem){
            TRC_DBG(@"Problem adding the peer public key to the keychain, OSStatus == %ld.", sanityCheck );
        }

        TRC_DBG(@"SecItemAdd OSStATUS = %ld", sanityCheck);

//        TRC_DBG(@"PersistPeer privatekey data after import into keychain %@", persistPeer);
        persistPeer = NULL;
        [peerPublicKeyAttr removeObjectForKey:(id)kSecValueData];
        sanityCheck = SecItemCopyMatching((CFDictionaryRef) peerPublicKeyAttr, (CFTypeRef*)&persistPeer);

        TRC_DBG(@"SecItemCopying OSStATUS = %ld", sanityCheck);
//        TRC_DBG(@"SecItem copy matching returned this public key data %@", persistPeer);
        // The nice thing about persistent references is that you can write their value out to disk and
        // then use them later. I don't do that here but it certainly can make sense for other situations
        // where you don't want to have to keep building up dictionaries of attributes to get a reference.
        //
        // Also take a look at SecKeyWrapper's methods (CFTypeRef)getPersistentKeyRefWithKeyRef:(SecKeyRef)key
        // & (SecKeyRef)getKeyRefWithPersistentKeyRef:(CFTypeRef)persistentRef.
        [peerTag release];
        [peerPublicKeyAttr release];
        if (persistPeer) CFRelease(persistPeer);
    }

This is the function to retrieve the public key ref. Pass the same name which one is used for save.

-(SecKeyRef)getPublicKeyReference:(NSString*)peerName{

       OSStatus sanityCheck = noErr;

       SecKeyRef pubKeyRefData = NULL;
       NSData * peerTag = [[NSData alloc] initWithBytes:(const void *)[peerName UTF8String] length:[peerName length]];
       NSMutableDictionary * peerPublicKeyAttr = [[NSMutableDictionary alloc] init];

       [peerPublicKeyAttr setObject:(id)kSecClassKey forKey:(id)kSecClass];
       [peerPublicKeyAttr setObject:(id)kSecAttrKeyTypeRSA forKey:(id)kSecAttrKeyType];
       [peerPublicKeyAttr setObject:peerTag forKey:(id)kSecAttrApplicationTag];
       [peerPublicKeyAttr setObject:[NSNumber numberWithBool:YES] forKey:       (id)kSecReturnRef];
       sanityCheck = SecItemCopyMatching((CFDictionaryRef) peerPublicKeyAttr, (CFTypeRef*)&pubKeyRefData);
       [peerTag release];
       [peerPublicKeyAttr release];

       TRC_DBG(@"SecItemCopying OSStATUS = %ld", sanityCheck);
       if(pubKeyRefData){
           TRC_DBG(@"SecItem copy matching returned this publickeyref  %@", pubKeyRefData);
           return pubKeyRefData;
       }else{
           TRC_DBG(@"pubKeyRef is NULL");
           return nil;
       }
   }

Pass your public key data to this function before addPeerPublicKey

- (NSData *)stripPublicKeyHeader:(NSData *)d_key
{
    // Skip ASN.1 public key header
    if (d_key == nil) return(nil);

    unsigned int len = [d_key length];
    if (!len) return(nil);

    unsigned char *c_key = (unsigned char *)[d_key bytes];
    unsigned int  idx    = 0;

    if (c_key[idx++] != 0x30) return(nil);

    if (c_key[idx] > 0x80) idx += c_key[idx] - 0x80 + 1;
    else idx++;

    // PKCS #1 rsaEncryption szOID_RSA_RSA
    static unsigned char seqiod[] =
    { 0x30,   0x0d, 0x06, 0x09, 0x2a, 0x86, 0x48, 0x86, 0xf7, 0x0d, 0x01, 0x01,
        0x01, 0x05, 0x00 };
    if (memcmp(&c_key[idx], seqiod, 15)) return(nil);

    idx += 15;

    if (c_key[idx++] != 0x03) return(nil);

    if (c_key[idx] > 0x80) idx += c_key[idx] - 0x80 + 1;
    else idx++;

    if (c_key[idx++] != '\0') return(nil);

    // Now make a new NSData from this buffer
    return([NSData dataWithBytes:&c_key[idx] length:len - idx]);

}

iPhone: How do you export a SecKeyRef or an NSData containing public key bits to the PEM format?

I think you will want to look at http://www.openssl.org/docs/crypto/pem.html#
maybe:

int PEM_write_PrivateKey(FILE *fp, EVP_PKEY *x, const EVP_CIPHER *enc,
                                    unsigned char *kstr, int klen,
                                    pem_password_cb *cb, void *u);

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