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What is encryption?

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In more detail

What is encryption?

Encryption is a mathematical function using a secret value—the key—which encodes data so that only users with access to that key can read the information.
In many cases encryption can provide an appropriate safeguard against the unauthorised or unlawful processing of personal data, especially in cases where it is not possible to implement alternative measures.

Example

An organisation issues laptops to employees for remote working together with secure storage lockers for use at home and locking devices for use outside the home. However, there is still the risk of loss or theft of the devices (eg whilst being used outside of the office).
To address this risk, the organisation requires all data stored on laptops to be encrypted. This significantly reduces the chance of unauthorised or unlawful processing of the data in the event of loss or theft. 

Encryption in practice

Information is encrypted and decrypted using a secret key. (Some algorithms use a different key for encryption and decryption). Without the key the information cannot be accessed and is therefore protected from unauthorised or unlawful processing.

Whilst it is possible to attempt decryption without the key (eg, by trying every possible key in turn), in practical terms it will take such a long time to find the right key—ie many millions of years, depending on the computing power available and the type of key—that it becomes effectively impossible. However, as computing power increases, the length of time taken to try a large number of keys will reduce so it is important that you keep algorithms and key sizes under consideration, normally by establishing a review period.

You should consider encryption alongside a range of other technical and organisational security measures. You also need to ensure that your use of encryption is effective against the risks you are trying to address, as it cannot be used in every processing operation.

Therefore, you should consider the benefits that encryption will offer in the context of your processing, as well as the residual risks. You should also consider whether there are other security measures that may be appropriate to put in place, either instead of encryption or alongside it.

You can do this by means of a Data Protection Impact Assessment (DPIA), which, depending on your processing activities, you may be required to undertake under Article 35 of the UK GDPR. In any case, a DPIA will also help you to assess your processing, document any decisions and the reasons for them, and can ensure that you are only using the minimum personal data necessary for the purpose.

Further reading

Data protection impact assessments 

Accountability

Documentation 

If we encrypt personal data, does this count as processing?

Yes. Article 4(2) of the UK GDPR defines ‘processing’ as any operation or set of operations performed on personal data, including ‘adaptation or alteration’. The process of converting personal data from plaintext into ciphertext represents ‘adaptation or alteration’ of that data.

Whether you are a controller or a processor, if you have encrypted personal data yourself and are responsible for managing the key then you will still be processing data covered by the UK GDPR.

If you also subsequently store, retrieve, consult or otherwise use that encrypted data, you will also be processing data covered by the UK GDPR.

You should therefore ensure that you do not view the use of encryption as an anonymisation technique or think the encrypted data is not subject to the UK GDPR. If you were responsible for encrypting the data and are the holder of the key, you have the ability to re-identify individuals through decryption of that dataset.

In this respect, encryption can be regarded as a pseudonymisation technique. It is a security measure designed to protect personal data.

What are the other considerations?

You should not underestimate the importance of good key management - make sure that you keep the keys secret in order for encryption to be effective.

Encryption can take many different forms. Whilst it is not the intention to review each of these in turn, it is important to recognise when and where encryption can provide protection to certain types of data processing activities. Later in this guidance, we outline a number of scenarios where encryption may be beneficial to you.

Encryption is also governed by laws and regulations, which may differ by country. For example, in the UK you may be required to provide access to an encryption key in the event you receive a court order to do so.

Finally, not all processing activities can be completely protected from end to end using encryption. This is because in general information needs to exist in a plaintext form whilst being ‘actively processed’. For example, data contained within a spreadsheet can be stored in an encrypted format but in order for the spreadsheet software to open it and the user to analyse it, that data must first be decrypted. The same is true for information sent over the internet – it can be encrypted whilst it is in transit but must be decrypted in order for the recipient to read the information.

Developments in the state of the art may eventually enable computation of encrypted data more widely. This may change some of the considerations you need to have regarding encryption. Irrespective of this, the security requirements mean you need to keep your encryption solution under regular review, including taking account of the state of the art (see ‘How should we implement encryption?’).

When should we use encryption?

When processing data, there are a number of areas that can benefit from the use of encryption. You should assess the benefits and risks of using encryption at these different points in the processing lifecycle separately. When first considering your processing, you should also ensure that you adopt a data protection by design approach, and using encryption can be one example of the measures that you put in place as part of this approach.

The two main purposes for which you should consider using encryption are data storage and data transfer. These two activities can also be referred to as data at rest and data in transit.

Recommendation

You should have a policy governing the use of encryption, including guidelines that enable staff to understand when they should and should not use it.

For example, there may be a guideline stating that any email containing sensitive personal data (either in the body or within an attachment) should be sent encrypted or that all mobile devices should be encrypted and secured with a password complying with a specific format.

You should also be aware of any industry or sector-specific guidelines that may include a minimum standard or recommend a specific policy for encrypting personal data. Examples include: