"Digital signature" and "electronic signature" are used interchangeably in everyday language, but technically and legally they are not the same. If you are evaluating digital signature software for your company, this distinction matters: it directly affects the legal validity of your documents and the technical requirements of the solution you choose.
In this article we explain everything: what a digital signature is, how it differs from an electronic signature, how the underlying cryptography works and what good software must have.
Digital signature vs. electronic signature: the distinction that matters
Under the eIDAS Regulation, the official terminology is:
| Concept | Technical definition | eIDAS legal validity | |---|---|---| | Electronic signature | Umbrella term: any electronic data linked to a signer | SES / AES / QES | | Digital signature | Cryptographic implementation based on PKI (public key infrastructure) | AES or QES | | Digitised signature | Scanned image of a handwritten signature | SES (no additional guarantees) |
A digital signature is essentially a specific implementation of advanced or qualified electronic signatures. Every digital signature is an electronic signature, but not every electronic signature is a digital signature.
When you search for "digital signature software", what you actually need in most cases is an advanced electronic signature platform that uses PKI cryptography to guarantee document integrity and authenticity.
How digital signatures work: the cryptography behind them
The technical foundation of digital signatures is asymmetric cryptography (also called public-key cryptography). Here is how it works:
1. The key pair
Each signer has a pair of mathematically related keys:
- Private key — secret, known only to the signer. Never leaves their device or the Hardware Security Module (HSM) where it is stored.
- Public key — known to everyone. Can be distributed freely and is included in the signer's digital certificate.
2. The signing process
When you sign a document with digital signature software:
- A SHA-256 hash of the document is calculated — a unique 64-character hexadecimal fingerprint.
- The hash is encrypted with your private key. The result is the digital signature.
- The digital signature is attached to the document along with your public certificate.
3. Verification
When someone wants to verify that the signature is authentic and the document has not been modified:
- Decrypt the digital signature using your public key (which is in the attached certificate).
- Calculate the hash of the document they received.
- Compare both hashes. If identical → the signature is valid and the document is intact.
If anyone modifies the document after signing — even changing a single character — the hash will not match and verification will fail.
The digital certificate: who vouches for your identity
Cryptography guarantees integrity, but does not by itself guarantee identity. How do I know that a public key actually belongs to who they claim to be?
The answer is the digital certificate: an electronic document issued by a Certificate Authority (CA) that links an identity (name, ID number, company) to a public key, signed with the CA's private key.
Recognised CAs in Europe include:
- FNMT-RCM (Spain) — the most widely used by individuals.
- Camerfirma — specialised in businesses and the corporate sector.
- GlobalSign — widely recognised across Europe.
- DigiCert — global CA with EU presence.
For qualified electronic signatures, the CA must be listed on the EU Trusted List.
What good digital signature software must have
Full document lifecycle management
The software should not be limited to "stamping the signature". It must manage the entire process:
- Document creation and preparation with customisable signature fields.
- Multi-signer workflow with configurable signing order (parallel or sequential).
- Automatic reminders to signers who have not completed the process.
- Real-time notifications to the sender about document status.
Remote key custody (Remote Signing)
For most companies, asking signers to have a digital certificate installed on their computer or a cryptographic smart card is operationally unviable. The best digital signature software offers remote key custody: the private key is held in an HSM managed by the provider, and the signer activates it via OTP or biometrics.
This enables advanced and qualified signatures without technical friction for the end user.
RFC 3161 timestamp
The RFC 3161 standard defines how a Time Stamping Authority (TSA) can certify that a document existed in a specific state at a precise moment. It is the key evidential piece in disputes: it proves the document was not created or altered after the signing date.
Always verify that the timestamp is issued by a recognised TSA, not simply by the provider's own server.
API and integrations
Digital signature software must integrate with:
- CRM (Salesforce, HubSpot, Pipedrive) to send contracts automatically.
- HRMS (Workday, Personio, Factorial) for employment contracts and HR documents.
- ERP (SAP, Odoo) for purchase orders and supplier contracts.
- Google Drive / SharePoint / Dropbox to access documents directly.
- Webhooks for real-time status notifications in your own systems.
Compatibility with signature standards
EU-recognised electronic signature formats are:
- PAdES (PDF Advanced Electronic Signatures) — signature is embedded in the PDF. The most common and practical.
- XAdES (XML Advanced Electronic Signatures) — for XML documents.
- CAdES (CMS Advanced Electronic Signatures) — for any file type.
All are specified in Commission Implementing Regulation (EU) 2015/1506.
Digital signature software use cases by department
Legal Department
- NDAs and confidentiality agreements
- Service agreements and professional engagements
- Powers of attorney and mandates
- Dispute resolution agreements
Human Resources
- Employment contracts and renewals
- Contract amendments and annexes
- Internal policies and codes of conduct
- Termination and settlement documents
See our electronic signature for employment contracts guide for HR-specific regulatory details.
Operations and Finance
- Supplier contracts and purchase orders
- SEPA mandates
- Service Level Agreements (SLAs)
- Tender documentation and public contracts
Sales
- Customer contracts and service agreements
- Subscription agreements
- Renewal documents
- Partnership agreements
Digital signature software deployment models: SaaS, on-premise or API?
SaaS (Software as a Service)
The most common and recommended model for most companies. The platform is hosted in the provider's cloud, accessed via browser, requires no installation. Advantages: immediate activation, automatic updates, no infrastructure maintenance.
On-premise
The software is installed on the company's own servers. Only makes sense for organisations with very strict regulatory requirements (central banks, defence, critical infrastructure) that cannot outsource document storage.
API-first
Ideal for companies with their own digital product that want to embed signing within their user flow without redirecting to an external platform. The provider exposes a REST API; you build the signing UX inside your own application.
Conclusion
Digital signature software is the legal infrastructure of any modern company. The PKI cryptography underpinning it guarantees integrity and identity in a mathematically irrefutable way; the eIDAS framework gives it legal validity across the entire EU.
When choosing, prioritise: AES support with PKI, RFC 3161 timestamp, remote key custody, well-documented REST API and EU-based servers.
SignDeal combines all of these in a platform designed for simplicity, without unnecessary complexity and with support from day one.
Also read: What is an electronic signature and how does it work? · Electronic signature platform guide