T 4.36 Mistakes in encrypted data

If data is present in encrypted form and if this data is changed, it is possible that the data cannot be decrypted properly any more during decryption. Depending on the mode of operation of the encryption routines, this may mean that only a few bytes are encrypted improperly or that the entire data is encrypted improperly after the mistake. If there is no data backup, this may result in a total loss of data.

The mentioned mistakes in the encrypted data may be caused as follows:

• a transmission error that cannot be remedied occurs while the encrypted data is being transmitted
• an irreparable error occurs on the data medium (diskette, hard disk)
• a computer virus manipulates the data
• a third party deliberately manipulates the data by manipulating the encrypted data at a few locations using an editor program, for example

In unfavourable cases, e.g. in the event of a bit loss or when the amounts of data changed are too large and the error is propagated, the data cannot be recovered even if the cryptographic procedure and the keys used for encryption are known.

A mistake in the cryptographic keys used may be even more critical. When a single bit of a cryptographic key is changed, this may already result in the data encrypted using this key no longer being decryptable. In the absence of a data backup of the cryptographic key, this data is lost.