Protecting cryptographic key data

A cryptographic module that is switchable between a key-input mode of operation and a data-input mode of operation, wherein:

• the cryptographic module comprises a cryptographic operation unit for performing a cryptographic operation and a data-input unit;
• when in the key-input mode of operation, the cryptographic module is configured to:
• receive key data, key length information and first input data, wherein the first input data is received at the data-input unit;
• combine an amount of the key data corresponding to the key length information with the first input data in a combining operation to produce combined data, wherein a key-influenced length of the combined data is a shortest length of the combined data that contains every data bit of the combined data whose value depends on the key data;
• perform said cryptographic operation on the combined data to generate first output data;
• and not output any of the first output data from the cryptographic module until after the cryptographic operation has been applied to all of the key-influenced length of the combined data;
• the cryptographic module is configured to output the first output data after the cryptographic operation has been applied to all of the key-influenced length of the combined data;
• and
• when in the data-input mode of operation, the cryptographic module is configured to:
• receive a quantity of second input data having a length that can be less than said key-influenced length, wherein the second input data is received at the data-input unit;
• perform the cryptographic operation on the second input data to generate second output data;
• and output the second output data.