Fix text hmac field to use the get_encrypt_sql method

[ruanpotgieter95]

Hi I started to use TextHMACField ( need the key to make it more secure ) for hashing. What I found was that the key isn't being used. When looking at the code I believe the implementation for using the key was swapped between TextDigestField and TextHMACField.

The SQL for the two above classes look like this:

DIGEST_SQL = "digest(%s, 'sha512')"
HMAC_SQL = "hmac(%s, '{}', 'sha512')"

As you can see the HMAC is using the PGCRYPTO extensions hmac function that takes in a key '{}'. Where digest can't.

You can see that the implementation to get_encrypt_sql is in the TextDigestField class and not in the TextHMACField.

class TextDigestField(HashMixin, models.TextField):
    """Text digest field for postgres."""
    encrypt_sql = DIGEST_SQL

    def get_encrypt_sql(self, connection):
        """Get encrypt sql."""
        return self.encrypt_sql.format(get_setting(connection, 'PGCRYPTO_KEY'))

class TextHMACField(HashMixin, models.TextField):
    """Text HMAC field for postgres."""
    encrypt_sql = HMAC_SQL

This makes it that the TextHMACField will never have the key used. When you want to hash a value in plain SQL to check the where clause, you will need to add '{}' as the key for it to work.

The fix is as simple as moving the get_encrypt_sql into TextHMACField. I have tested this locally and it is working as expected.

class TextHMACField(HashMixin, models.TextField):
    """Text HMAC field for postgres."""
    encrypt_sql = HMAC_SQL

    def get_encrypt_sql(self, connection):
        """Get encrypt sql."""
        return self.encrypt_sql.format(get_setting(connection, 'PGCRYPTO_KEY'))