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Fig. 2. Processing time in JCOP 41 and JCOP J3A for messages of 160 bytes
Observing the decryption time, it is clear that there is an important gap
(approximately 150-200 milliseconds, depending on the key length and message
size) regarding the decryption process in both smart cards. Given that the applet
functionality is basically the same (all the variables are located in RAM, the
arrays have the same length in all the cases, etc.) this difference can be only
explained due to a difference in the DES cryptoprocessor used in those models
or to the countermeasures put in place in each card to protect them against side
channel attacks.
In both smart cards, the decryption process is faster than the encryption pro-
cedure, basically because in the encryption process there is one step (the gener-
ation of the ephemeral key pair) which is not performed during the decryption
process.
The quite noticeable increase in the encryption processing time for the JCOP
41 cards can be also visualized in Figure 1, where it can be seen how the en-
cryption time gap existing between JCOP 41 and JCOP J3A almost dissapears
when passing from plaintexts of 64 bytes to clear messages of 160 bytes. This
result suggests that, when encrypting plaintexts of more than 160 bytes, JCOP
41 cards will be slower than the JCOP J3A model using the same configuration.
Another consequence of the increase in the relative difference of the encryption
and decryption time in the JCOP 41 cards is that, when encrypting messages
of 160 bytes, the time needed to encrypt the plaintext nearly doubles the time
needed to decrypt the cryptogram.
Acknowledgement . This work has been partially supported by the “Fundacion
MemoriaD.SamuelSolorzano Barruso” under the Project FS/7-2010.
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