def rsa_init(spec):
    """
    Initiates a :py:class:`oidcmsg.keybundle.KeyBundle` instance
    containing newly minted RSA keys according to a spec.
    
    Example of specification::
        {'size':2048, 'use': ['enc', 'sig'] }
         
    Using the spec above 2 RSA keys would be minted, one for 
    encryption and one for signing.
        
    :param spec:
    :return: KeyBundle
    """

    try:
        size = spec['size']
    except KeyError:
        size = 2048

    kb = KeyBundle(keytype="RSA")
    if 'use' in spec:
        for use in harmonize_usage(spec["use"]):
            _key = new_rsa_key(use=use, key_size=size)
            kb.append(_key)
    else:
        _key = new_rsa_key(key_size=size)
        kb.append(_key)

    return kb