§ 51 BDSG
Consent
(1)Where the processing of personal data may be based on consent pursuant to a legal provision, the controller must be able to demonstrate the consent of the data subject.
(2)Where the data subject's consent is given in the context of a written declaration which also concerns other matters, the request for consent must be presented in a manner which is clearly distinguishable from the other matters, in an intelligible and easily accessible form, using clear and plain language.
(3)The data subject shall have the right to withdraw his or her consent at any time. The withdrawal of consent shall not affect the lawfulness of processing based on consent before its withdrawal. The data subject shall be informed thereof prior to giving consent.
(4)Consent shall be effective only if it is based on the data subject's free decision. In assessing whether consent was freely given, the circumstances of giving consent shall be taken into account. The data subject shall be informed of the intended purpose of the processing. Where the circumstances of the individual case so require or where the data subject so requests, the data subject shall also be informed of the consequences of refusing consent.
(5)Where special categories of personal data are processed, the consent must expressly relate to those data.