Electronic signatures exist in different forms. All forms can show that somebody agreed with something. Some forms can also protect the data the person agreed with from being changed easily, or they can legally identify the person that agreed. To do this, ideas from public-key cryptography are used: digital signatures, certificates, and hash codes. An electronic signature often includes a timestamp to show when the signature was made. Much like cryptography, electronic signatures can be used for any kind of data, there is no requirement that the data signed has a specific format. 