Digital Signatures past, present and future

Security through obscurity is now a thing of the past. One can’t just hide the algorithm for too long. Kerckhoffs’s principle is commonly used in any crypto system where security strength lies in the key (its strength & secure storage) and not the algorithm. Both RSA and ECSA rely on generating unique K and P keys and hence their strength and protection defines how strong the security is. Security in digital signature hence relies on the security of these keys, specifically the K part. If K is known to the unwanted, your security is gone. K is hence kept safely with (at minimum) password protection on computers, USB tokens, HSMs while P is given out publicly to recipients so that they can verify the digital signatures. In summary security of digital signature relies on these factors:

1. How good the digital signature algorithm is
2. How good the key (K) strength is
3. How good the hashing algorithm is
4. How good the protection of K is

How good the digital signature algorithm is in general

In terms of algorithms RSA and ECDSA both are good enough. So far there is no known weakness identified in their two algorithms. They don’t have any flaws which could result in a similar outcome as in the case of the Enigma crypto machine.

K (Private Key) is just another random number. Any one guessing that means any one can send a message on your behalf or change your message during transit and sign it again to hide tampering.

On key sizes, especially talking about RSA, comes in variety from 100 to 8192 and beyond. On December 12, 2009 RSA-768 was cracked in 2 years by a group of mathematicians.  A decade ago an RSA key of 512 (1×10154 permutations), 1024 (1×10308 permutations) was termed as sufficiently strong. As of 2020, 1024 bit key pairs are considered to be weak and not recommended. As computing power is increasing every year and super computers are being built, threats of cracking big numbers are increasing. NIST now recommended to use RSA 2048 bit keys size which will be strong enough till 2030 and beyond 2030 recommends use RSA 3072 bit keys. Similar recommendations are there for Symmetric encryption key sizes i.e. 3DES till 2022 and then disallowed from 2023 and recommend AES 128 till 2030 and AES 192/256 for 2030 and beyond.

Same goes with ECDSA based keys where a key size ranging 160 to 223 is deprecated and recommends minimum 224 or more till 2030 and then 384 or more for 2030 and beyond.

As goes the saying, your security is as strong as the weakest link, how good the security be if you don’t safeguard your keys. It is more easier for hackers to get inside your computer, phone to steal your unprotected keys then to crack it using brute force attack. Some of the common ways of storing keys are:

• No security – Store in plain
• Low level security – Storing on your computer or device with password protection
• Medium level security – Storing inside USB token or smart card (again password protected)
• Highest level security – Storing inside Hardware Secure Modules (HSM)

You may keep your keys as plain text for just testing, exploring their structure although again it is a bad practice!

Most oftenly these are stored as PKCS12/PFX files protected with a password. Ensure the password must be strong enough to avoid brute force attack. You may use this protection method for keys used to login to test or production systems or signing documents. All the popular operating systems i.e. Microsoft, Linux and MAC support them.

These are small, hardware devices also called security tokens specifically designed to protect user’s private keys. These allow keys to be generated inside the embedded cryptographic chip inside and remain there throughout their lifetime. Some may disable the USB token or Smart Card if an incorrect password is used a number of times. These can be reused if locked after admin intervention. As these are for individual’s usage hence have low disk space allowing not more than few private keys. You may buy a token for less than $30. USB tokens are used to create digital signatures for desktop environments.

HSMs are similar like USB tokens but have a bigger form factor. Again these are specialized purpose built, security focused machines keeping your business sensitive keys. Depending upon their storage space, they may keep millions of keys. They have more advanced security controls as compared to a USB token. They have hardened OS and are certified against FIPs 140-2 standards. An HSM can detect an intrusion and hence may wipe the entire data inside (make sure you keep a backup before this happens!). As per webtrust a publicly known Certification Authorities must store their private keys in an HSM. Some secure deployments require a common criteria certified HSM as well. 

HSMs are expensive and could be around a few thousand dollars. They come in both network or PCI based. A somewhat non-exhaustive list of HSM can be found here. For a medium sized usage, a cheaper alternative (under $999) is from Yubico named YubiHSM. There have been reports of even HSM hacked. Having said there is no such thing as 100% secure or fool proof security. Having security in place is far better than having none!

In a nutshell digital signature is encrypting a hash with a key (private key) but based on the type of data it signs, different formats have to be devised. This has led into different standard bodies like ISO, IETF and OASIS to come up with rules of creating digital signatures and how they can be embedded inside original documents for transportation. This standardization also helped in compatibility among different operating systems and tools. Some of the digital signature standards are:

• CMS based digital signatures
• PDF based digital signatures 
• XML based digital signatures

As the need to keep digital signatures for long term storage arised, so did new standards were formulated hence the CAdES, PAdES (Part 1, Part 2, Part 3, Part 4, Part 5, Part 6) and XAdES standards.

Doing business electronically is here to stay as it’s fast, efficient and cheap. Financial institutions, Governments, Defense are now adopting paperless ways of doing business. Some example use cases are:

• Government wants public to apply for Identity cards, passports where ever they are
• Financial institution want clients to apply for account and sign documents with ease
• Businesses want interoffice discussion to be secure and also traceable with their identities

As more business is done electronically the demand of integrity and identity is growing and similarly the threats. One can sit in one part of the world and disrupt businesses across the globe. This has led to businesses relying on the time and tested science of cryptography and especially public key cryptography and PKI. 

Governments across the world are now adopting e-signatures (a term used commonly for digital signature although differs technically). Terms such as eSignature, eTimestamp and eSeal are formulated to secure businesses across international boundaries. To enforce these standard some legislations is also being done:

• USA: E-Sign Act and more state laws
• European (EIDAS): 
  • Basic Info
  • Trust Services and eID

PKI is here to stay and is evolving every year. For a glimpse into the future, quantum cryptography is yet another upcoming domain where researchers are continuously working to ensure new uncrackable ways of digital signature are found. NIST has already started working on identifying post quantum algorithms so sit tight.