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Digital signature library for Android

Digital Signatures can be considered as the electronic equivalent of a physical signature with ink on paper. However an electronic signature also provides certain advantages over an ink-and-paper signature. It can secure and protect a digital document by creating a signing fingerprint uniquely identifying a sender. The recipient of the document and signature confidence of the sender's identity validates that the document has not been altered by anyone else since it was signed.

Some of these main advantages are:

Authentication
A digital signature allows precise identification of who created/signed a document. It can be safely assumed that when a signature is valid, you know who signed it.

Integrity
A digital signature allows users to easily validate whether the contents of a document were changed after it was signed.

Non-repudiation
A digital signature ensures that the signer cannot deny that they signed the document.

PDFTron SDK benefits include:

  • Import signature fields via XFDF/FDF
  • Export signature data via XFDF/FDF
  • Built-in support for PKI signing (and PFX digital certificates)
  • Support for custom signature handlers
  • Sign with images, ink annotations, or entirely custom appearances

linkHow does a Digital Signature work?

A digital signature will generate a unique value (hash / digest) from the combination of the document data and private key. During verification, the document data and public key is used to generate the exact same unique value (hash / digest). If these unique values match then we can say the data has not been altered and the digital signature is valid.

ds-diagram

linkDigital Signature vs e-Signature

An e-signature is an annotation that appears in the document but has no additional identifiable information about the creator other than an author field which can be altered.

A digital signature on the other hand uses a cryptographic algorithm to uniquely identify the author and any alterations to the document including the annotations or e-signature would result in an invalid digital signature validation.

linkCertificate Authority (CA)

A cryptographic digital signature can use a certificate authority (CA) to act as a trusted third party between a sender and other parties.

ca

linkHow does a CA work?

The CA will issue a digital certificate which contains a public key and the identity of the owner. A matching private key is not made available publicly, but kept secret by the end user who generated the key pair. The certificate is also a confirmation or validation by the CA that the public key contained in the certificate belongs to the person, organization, server or other entity noted in the certificate. A CA's obligation in such schemes is to verify an applicant's credentials, so that users and relying parties can trust the information in the CA's certificates. CAs use a variety of standards and tests to check this information. In essence, the certificate authority is responsible for saying "yes, this person is who they say they are, and we, the CA, certify that".

ca-diagram

linkHow a digital signature and a CA work together?

For security and legal purposes, a public key owner must be verifiable and it is common to use a public key infrastructure (PKI) where the public key owner is validated by a CA. Since a public key is used to validate a cryptographic signature then a digital signature and a CA work together to authenticate the owner and the data.

dc

linkIs a CA required for digital signatures?

No. The CA is required in use cases where a third party entity needs to be involved between a sender and other parties. If a CA is not used then a digital signature can instead use a self-signed certificate as shown in our digital signature sample or demo for example. PDFTron does not provide CA services so it is the responsibility of users creating a digital signature workflow to use a CA if it is required for your use cases.

linkSignature tool

Android has a built in signature tool that lets users create signature annotations. In fact, signature annotations are just freehand annotations, or ink annotations according to the PDF specification. It's possible to preview signatures, save signatures and apply default signatures directly.

One significant difference between digital signatures and signature annotations is that digital signatures employ asymmetric cryptography and provide an extra layer of security whereas signature annotations are just created superficially and can move around the document.

linkGet started

Signature tool
To add a signature with or without signing.

Signing a PDF Document

Certifying a PDF Document

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