Abstract: A cryptographic device and method of operation for encrypting messages. The device can be incorporated into a postage metering system to provide cryptographically secured postal indicia. The device and method provide increased security against side-channel attacks such as differential power analysis (DPA). An encryption key is transformed with a first function to generate temporary key as a function of a random number. A message is encrypted with the temporary key to generate a modified message. The modified message is transformed with a second function to generate an encryption. The encryption generated is identical to a direct encryption of the message with the untransformed key. The temporary key is changed frequently to protect against side-channel attacks.

Abstract: A method for assigning keys to a plurality of devices in a public key cryptographic system includes generating a set of more than one master private keys; calculating for each master private key a corresponding master public key; installing in each of the plurality of devices a corresponding device private key derived as a linear combination of at least two of the master private keys from the set of master private keys whereby knowledge of the corresponding device private key installed in any one of the plurality of devices is not sufficient to determine the corresponding device private key of any other of the plurality of devices. A device public key can be calculated as a corresponding combination of the master public keys.