Import RSA keys to iPhone keychain?

Question

I have a couple of NSString objects that represent an RSA public-private keypair (not generated by SecKeyCreatePair, but by an external crypto library). How can I create SecKeyRef objects (which are required for the SecKeyDecrypt/Encrypt methods) from these NSString objects?

Do I need to import them into the Keychain first? If so, how?

Thanks!

Answer 1

I dug this code up (BSD license) from the MYcrypto library. It seems to do what you want.

    SecKeyRef importKey(NSData *data, 
                    SecExternalItemType type,
                    SecKeychainRef keychain,
                    SecKeyImportExportParameters *params) {
    SecExternalFormat inputFormat = (type==kSecItemTypeSessionKey) ?kSecFormatRawKey :kSecFormatUnknown;
    CFArrayRef items = NULL;

    params->version = SEC_KEY_IMPORT_EXPORT_PARAMS_VERSION;
    params->flags |= kSecKeyImportOnlyOne;
    params->keyAttributes |= CSSM_KEYATTR_EXTRACTABLE;
    if (keychain) {
        params->keyAttributes |= CSSM_KEYATTR_PERMANENT;
        if (type==kSecItemTypeSessionKey)
            params->keyUsage = CSSM_KEYUSE_ENCRYPT | CSSM_KEYUSE_DECRYPT;
        else if (type==kSecItemTypePublicKey)
            params->keyUsage = CSSM_KEYUSE_ENCRYPT | CSSM_KEYUSE_VERIFY | CSSM_KEYUSE_WRAP;
        else if (type==kSecItemTypePrivateKey)
            params->keyUsage = CSSM_KEYUSE_DECRYPT | CSSM_KEYUSE_SIGN;
    }
    if (!check(SecKeychainItemImport((CFDataRef)data, NULL, &inputFormat, &type,
                                     0, params, keychain, &items),
               @"SecKeychainItemImport"))
        return nil;
    if (!items || CFArrayGetCount(items) != 1)
        return nil;
    SecKeyRef key = (SecKeyRef)CFRetain(CFArrayGetValueAtIndex(items,0));
    CFRelease(items);
    return key; // caller must CFRelease
}

Answer 2

I'm not sure if the code in this Apple developer forum thread works, but it seems like a direct answer to your question.