':return:
A unicode string of "rsassa_pkcs1v15", "rsassa_pss", "dsa" or
"ecdsa"'

@property
def signature_algo(self):

    algorithm = self[u'algorithm'].native
    algo_map = {u'md2_rsa': u'rsassa_pkcs1v15', u'md5_rsa': u'rsassa_pkcs1v15', u'sha1_rsa': u'rsassa_pkcs1v15', u'sha224_rsa': u'rsassa_pkcs1v15', u'sha256_rsa': u'rsassa_pkcs1v15', u'sha384_rsa': u'rsassa_pkcs1v15', u'sha512_rsa': u'rsassa_pkcs1v15', u'rsassa_pkcs1v15': u'rsassa_pkcs1v15', u'rsassa_pss': u'rsassa_pss', u'sha1_dsa': u'dsa', u'sha224_dsa': u'dsa', u'sha256_dsa': u'dsa', u'dsa': u'dsa', u'sha1_ecdsa': u'ecdsa', u'sha224_ecdsa': u'ecdsa', u'sha256_ecdsa': u'ecdsa', u'sha384_ecdsa': u'ecdsa', u'sha512_ecdsa': u'ecdsa', u'ecdsa': u'ecdsa'}
    if (algorithm in algo_map):
        return algo_map[algorithm]
    raise ValueError(unwrap(u'\n                                                Signature    algorithm    not    known    for    %s\n                                                ', algorithm))
