How should we implement encryption?

In detail

• Choose the right algorithm
• Choose the right key size
• Choose the right software
• Keep the key secure

Choose the right algorithm

An encryption algorithm is a mathematical function that transforms plaintext into ciphertext. Choosing the right algorithm is important because vulnerabilities may be discovered over time, or advances in computing processing power may mean that a brute-force attack (ie attempting every possible key) is no longer a time-consuming task.

You should therefore regularly assess whether your encryption method remains appropriate.

Rather than develop a custom algorithm it is recommended that you use a trusted and verified algorithm. Accredited products can provide an assurance of suitability and also permit you to demonstrate a level of compliance with legal obligations. However, it is important to keep the products being used under regular review, due to the nature of technical development over time.

Choose the right key size

Algorithms use keys to encrypt and decrypt data. Encrypting the same data with a different key will produce a different result. Just as it is important to choose the right algorithm, it is also important to ensure that the key size is sufficiently large to defend against an attack over the lifetime of the data. As computing processing power increases or new mathematical attack methods are discovered, a key must remain sufficiently large to ensure that an attack remains a practical impossibility.

Choose the right software

The way that encryption software is put together is also crucially important. Software can use a state-of-the-art algorithm and a suitably long key to output encrypted data, but if its development did not follow good practice, or the product itself is poorly tested or subject to insufficient review, there may be vulnerabilities or other opportunities for attackers to intercept data or break the encryption without the users’ knowledge. It is also possible that the encryption software includes an intentional weakness or backdoor to enable those with knowledge of that weakness to bypass the protection and access the data.

In cases where it is critical to have an assurance that these vulnerabilities do not exist, it is important to gain an external assessment of encryption software. An assessment may also assist you in defining an appropriate algorithm and key size.

It is recommended that you ensure that any solution that you, or a processor acting on your behalf, implement meets current standards such as FIPS 140-2 (cryptographic modules, software and hardware) and FIPS 197 (PDF). Encryption products certified via the National Cyber Security Centre’s CAPS scheme would also meet the current standard. (You should also note that the reference to certification here is not to certification under Articles 42 and 43 of the UK GDPR.)

You can also find additional information on the current status of encryption algorithms in the following publications:

• guidance from the European Union Agency for Network and Information Security (ENISA) on Recommended cryptographic measures as well as its Algorithms, key sizes and parameters report (2014); 
• ECRYPT-CSA’s Algorithms, Key Size and Protocols Report (2018) which builds on the ENISA publications above and may have more up-to-date information; and
• the United States National Institute of Standards and Technology Special Publication 800-131A Revision 1 (Transitions: Recommendation for Transitioning the Use of Cryptographic Algorithms and Key Lengths). 

In some instances such specific assurances may not be available. For example, many open source software products do not have sufficient capital to fund an appropriate external assessment. However, government security agencies and private IT security organisations can offer advice regarding which specific protocols or algorithms which should be considered appropriate, although you should be aware of the limited assurances when no assurance or guidance is available. An example of such advice from the NCSC on the use of TLS includes:

‘The lack of formal assurance in TLS implementations means there may be implementation weaknesses. Using recent, supported and fully patched versions of TLS implementations from reputable sources will help to manage this risk.’

This statement highlights the importance of keeping software up to date as vulnerabilities in the code may be discovered over time, eg Heartbleed and Shellshock.  

Keep the key secure

It is important to ensure that symmetric keys and private keys remain secret as these provide the ability to decrypt the data.

In many cases keys are stored in a hierarchy for ease of management. The top level key is used to encrypt the keys below it and must therefore be managed securely.
All keys should have a finite lifespan and you need processes in place to generate a new key and re-encrypt the data. The old key should then be archived and securely deleted when no longer required.

In symmetric encryption, the key is sometimes derived from a shorter, more memorable password. It is therefore imperative that any password used to derive or secure the keys also remains secret. A poor choice or a compromise of the password can significantly lower, or even eliminate, the level of protection offered by an encryption product.

In the event that the key is compromised, or even if this possibility cannot be excluded, it may be necessary to revoke the existing key and generate a new key or key pair to protect data in the future.

It is also the case that loss of the decryption key will likely mean that no-one will be able to gain access to the data. Depending on the circumstances, loss of the decryption key could constitute ‘accidental loss, destruction or damage’ to personal data and would therefore be a contravention of the UK GDPR’s security principle; additionally, if you cannot restore the data, this may also constitute a personal data breach due to a lack of availability, depending on the risks this poses.