'Base64-decode and decrypt a value using the crypto_meta provided.
:param value: a base64-encoded value to decrypt
:param key: crypto key to use
:param crypto_meta: a crypto-meta dict of form returned by
:py:func:`~swift.common.middleware.crypto.Crypto.get_crypto_meta`
:returns: decrypted value'

def decrypt_value(self, value, key, crypto_meta):

    if (not value):
        return ''
    crypto_ctxt = self.crypto.create_decryption_ctxt(key, crypto_meta['iv'], 0)
    return crypto_ctxt.update(base64.b64decode(value))
