**5. Discussion**

In this paper, we have proposed a novel strategy for securing personal identity on a touch-screen sensor embedded in a mobile device, largely used nowadays. This strategy operates upstream from the verification system, at the sensor level, by enriching the information content of handwritten inputs. Specific additional inputs then reinforce the usual signature with alphanumeric and handwritten personal information, frequently used in public and legal usages.

We quantified information enrichment with PE measure that characterizes both signature complexity and stability. Several simple and hybrid signature types were proposed for our experimental study.

We assessed the effectiveness of our proposal across three well-established user categories in terms of signature complexity, signature stability and verification performance. This allowed highlighting inside categories, subtle differences in terms of relative performance enhancement, depending on the signature type. This methodology allows understanding which characteristics are relevant in the signing process to reinforce the digital identity for all persons.

Experiments were performed on 74 writers that signed on a tablet with a stylus in uncontrolled mobile conditions. Our analysis per writer category revealed a common trend to all: adding alphanumeric (date) and handwriting (place) information to the usual signature is the most powerful hybrid type in terms of verification performance. This can be explained by the fact that this hybrid type combines complementary information, and keeps the ballistic aspect of the signature, so important for identity verification. The relative improvement for all user categories is of at least 93% compared to the usual signature.
