Cloud Signing: advanced PKI digital signatures made easy!

Posted by Liaquat Khan on 29-Jul-2015 14:12:00


A quick background:  Advanced digital signatures require each user to have their own unique signing key. The security of the system then relies on the fact that the user's private signing key is not accessible to anyone else other than the owner.  If implemented properly it allows an independent judge to determine that any digital signatures produced with the user's private key must have been created by the owner and no one else - thereby delivering the "non-repudiation" property where signers can't reasonably deny the signatures they have created. 

The issue with digital signature technology in the past however has been the cost and complexity of issuing each user with their own private signing key in a secure manner!  Now with the advent of cloud computing and in particular cloud HSMs (Hardware Security Modules) the situation has changed dramatically - today advanced digital signature technology can be low-cost, easy to use and secure so that it can be applied to any business use case, even on a mass scale.  

In this blog we look at why cloud signing is such a hot topic right now and how to implement it properly.  


Advanced digital signature requirements 

There is much confusion between electronic signatures and PKI digital signatures. You can learn more about it here, but as a quick note basic e-signing just adds the user's mark on a document and does nothing to protect the integrity of the signed document or to prove that the user actually made that mark.

With PKI digtial signatures, cryptographic codes are created using privately-held signing keys under the control of the signer which ensure data integrity and strong authentication of the user - cryptographically-binding the user's authenticated digital identity to their signed documents. 

There are many cloud e-sign providers who simply implement basic electronic signature squiggles on a document with no cryptographic evidence embedded into the signed document to independently prove it was indeed the user who made that mark.  Most high-trust schemes however, require PKI-based digital signatures where each user has their own private signing key.  As an example consider:

EU Qualified Signatures - these are recognised as equivalent to handwritten ink signatures in a court of law and require use of unique user keys held in secure cryptographic hardware. 

Adobe AATL Signatures - this is a trust scheme run by Adobe for its Reader/Actobat product range.  It again requires unique user keys and protection of these in secure cryptograhic hardware.  Adobe software will automatically mark signatures as "trusted" if your signing key was certified by an AATL recognised Certificate Authority (CA). 

Traditionally the protection of the user's private signing key has been achieved by storing it within tamper-resistant cryptographic hardware devices like smartcards and secure USB tokens.  These are PIN-protected and kept under the control of their users. 

Problems with smartcards / tokens & the rise of server-held keys

Although there are many examples of e-Trust schemes relying on smartcards/tokens, in particular electronic ID (eID) cards issued by many governments, the general purpose use of such devices has been limited.  This is mainly due to the following reasons:

  • complex to use - in the case of smartcards the user needs specialist reader devices, which are not generally available.  Using such devices on mobile phones is even harder.
  • Forgotten tokens - often users forget to bring their tokens when needed or lose/misplace them.  Also use of such tokens in public areas is sometimes blocked or there is no readers available. 
  • Expense to deploy - the cost of providing the secure devices (and readers) to each end-user is often too high for most business applications where a large number of users are involved.
  • Browser compatability issues - to use these smartcards/USB tokens requires web applications to deploy Java applets, and latest versions of browsers (e.g. Google Chrome) are blocking such technology because of various security issues.  Even where the browser allows Java, the frequent pop-up warning messages make non-technical users nervous.

To overcome this the industry has been moving for sometime to server-held signing keys i.e. each user's signing key is managed in a Hardware Security Module (HSM) held centrally. As an example the new EU eIDAS Regulations allow EU Qualified Signatures to be created using server-held signing keys as long as it's managed securely.  Similarly Adobe AATL Signatures can be created using server-held keys also. 

Before Cloud HSMs however the situation was quite complex if you wanted to deploy a server-side signing solution.  Basically you needed to purhcase an HSM appliance and install, configure, patch and maintain these security devices.  So although the complexity of smartcards/USB signing devices was hidden from the end-users perspective by using HSMs, IT departments still had the complexity of managing these security devices.

Today both Azure and Amazon cloud platforns offer cloud HSMs as part of their service.  This means you can now deploy an advanced digital signature solution using unique user signing keys with strong hadware-based protection, at a fraction of the cost and complexity compared to an on-premise HSM solution, and at the same time meet the needs of high-trust schemes like Adobe AATL and EU regulations. 

Cloud Signing with SigningHub 

SigningHub has extensive support for secure cloud-based digital signatures which includes:

  • Support Azure Key Vault HSMs - for generating and managing unique user keys and creating advanced, EU Qualified and Adobe AATL signatures.  SigningHub is the first global signing platform to integrate with the Azure Key Vault, see the Microsoft blog here for more details.
  • Ability to host your own private PKI infrastructure components like private Certificate Authorities, OCSP Validation Authorities and Time Stamp Authority (TSA) servers using Ascertia ADSS Server the above cloud platforms.


Cloud-based digital signatures using unique individual PKI signing keys for every user has many benefits:
  • Easy of use for end-users: no need for card/token readers, specialist desktop software to be installed or Java runtime environment 
  • Ability to sign from anywhere, anytime: avoid lost or forgotten tokens 
  • Easy Management: no need to purchase, install, configure, patch or maintain on-premise HSMs
  • Immediate deployment: issue keys and start creating advanced PKI digital signatures in minutes 
  • Enhance protection and compliance: by using strong unqiue cryptographic keys for every user protected in FIPS 140-2 Level 2 HSMs
  • Reduce latency and achieve global redundancy: cloud HSM services scale rapidly to meet your business application needs for more keys during peak demands without the complexities of on-premise dedicated HSMs.  You can also implement global redundancy by using cloud HSMs in multiple data centers.
  • Reduce costs: the use of cloud HSMs is dramatically lower than a dedicated on-premise HSM in terms of the hardware - this is not to even mention the admin management overheads - which can be even higher! 

Contact us to disucss how cloud signing can help transform your business.

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