Operationally, however, pruning may prove to be an
unattractive option for many blockchain solutions. Many
blockchain solutions use the blockchain to record a
base state and subsequent transactions. The only way
to ascertain the current world state from the blockchain
is to start with the base state and track through every
subsequent transaction. If a blockchain like this were to
be pruned, it would be necessary for the participants on
the network to formulate and agree, and to record in a
similarly immutable and decentralised way to the original
blockchain, a new base state that will replace the original
base state and all transactions up to the most recent block
that has been pruned. There are technical means available
to help achieve this, but the blockchain technology
employed by the solution will inevitably be somewhat
more complex.
Additionally, while pruning would assist compliance with
the obligation to delete data after it is no longer required
for the purpose for which it was collected, it is usually not
a viable means of complying with ad hoc requests from
data subjects for personal data about them to be erased
or rectified.
3 - Potential solution: Deletion by way of encryption
Alternatively, it may be possible to delete personal data
stored on the blockchain by irreversibly encrypting the
data. Under this approach, the encrypted data containing
the personal data would remain permanently on the
blockchain, but the personal data would be "deleted"
from the blockchain by deleting all keys that enable
decryption of the encrypted data. This method appears
to be a natural extension of the view held by the German
Blockchain Federation (Blockchain Bundesverband) and
the UK Anonymisation Network that data is no longer
personal data if it has been irreversibly anonymised.
However, the Article 29 Data Protection Working Party
previously classified encryption as pseudonymisation,
not anonymisation.28 One pseudonymisation technique
mentioned in the Article 29 Data Protection Working
Party opinion included using a keyed-hash function to
produce a hash and then deleting the key.29 The opinion
did note that employing this technique would make it
"computationally hard for an attacker to decrypt or replay
the function, as it would imply testing every possible key,
given that the key is not available."30
Nonetheless, it remains unclear whether the Working
Party opinion considers personal data that is irreversibly
encrypted and keyless to be anonymised for the purposes
of the GDPR and thus theoretically deleted from a
blockchain network.
It is for this reason that we are calling on the
European Data Protection Board and national data
protection authorities to settle this point and set
standards for encryption and key deletion that can
achieve an adequate level of anonymisation.
If deletion by encryption is a feasible solution, then any
blockchain network employing deletion by encryption
will need to ensure its governance framework obligates
its members to delete keys in response to a data subject's
request for erasure. If any member does not delete its key,
then the data would not be considered anonymised under
the Article 29 Working Party's definition of anonymised
data, which holds that data are only considered to be
anonymised when no person can re-identify them.31
GDPR and the Blockchain I 31