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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def encrypt(plaintext_str, keys):
 """Encrypt data and return the encrypted form. :param bytes plaintext_str: the mail to encrypt :param key: optionally, a list of keys to encrypt with :type key: list[gpg.gpgme.gpgme_key_t] or None :returns: encrypted mail :rtype: str """

    assert keys, 'Must provide at least one key to encrypt with'
    ctx = gpg.core.Context(armor=True)
    out = ctx.encrypt(plaintext_str, recipients=keys, sign=False,
                      always_trust=True)[0]
    return out