Skip to main content
Information Security

Return to Question

Tweeted twitter.com/#!/StackSecurity/status/140978695383953408
added 206 characters in body
Source Link
Mister Smith
Mister Smith
  • 433
  • 4
  • 10

I'm working with a smartcard (DESFire) that supports both TripleDES and DES. In the specs it is stated that both modes of operation use 16 bytes keys. When the first 8 bytes in the key are identical to the trailing 8 bytes, the DES mode is used. Otherwise, the TDEA algorithm is selected.

This is a shock to me since it was my understanding that Triple DES uses a key bundle of 56bits*3 = 168 bits (21 bytes). What kind of Triple DES encryption could it be supporting? Is there some other way to select a key for TDEA so that it behaves as single DES, apart from selecting a key bundle k1k2k3 where k1=k2=k3?

EDIT: Seems that there's a "Keying option 2", where k1 = k3 != k2.

This is the only TDEA option that DESFire supports, I guess. The newer DESFire ev1 supports full keying option 3 TDEA along with AES.

I'm working with a smartcard (DESFire) that supports both TripleDES and DES. In the specs it is stated that both modes of operation use 16 bytes keys. When the first 8 bytes in the key are identical to the trailing 8 bytes, the DES mode is used. Otherwise, the TDEA algorithm is selected.

This is a shock to me since it was my understanding that Triple DES uses a key bundle of 56bits*3 = 168 bits (21 bytes). What kind of Triple DES encryption could it be supporting? Is there some other way to select a key for TDEA so that it behaves as single DES, apart from selecting a key bundle k1k2k3 where k1=k2=k3?

I'm working with a smartcard (DESFire) that supports both TripleDES and DES. In the specs it is stated that both modes of operation use 16 bytes keys. When the first 8 bytes in the key are identical to the trailing 8 bytes, the DES mode is used. Otherwise, the TDEA algorithm is selected.

This is a shock to me since it was my understanding that Triple DES uses a key bundle of 56bits*3 = 168 bits (21 bytes). What kind of Triple DES encryption could it be supporting? Is there some other way to select a key for TDEA so that it behaves as single DES, apart from selecting a key bundle k1k2k3 where k1=k2=k3?

EDIT: Seems that there's a "Keying option 2", where k1 = k3 != k2.

This is the only TDEA option that DESFire supports, I guess. The newer DESFire ev1 supports full keying option 3 TDEA along with AES.

edited tags
Link
Thomas Pornin
Thomas Pornin
  • 328.6k
  • 60
  • 800
  • 969
Source Link
Mister Smith
Mister Smith
  • 433
  • 4
  • 10

Select a TripleDES key so that it effectively behaves as regular DES

I'm working with a smartcard (DESFire) that supports both TripleDES and DES. In the specs it is stated that both modes of operation use 16 bytes keys. When the first 8 bytes in the key are identical to the trailing 8 bytes, the DES mode is used. Otherwise, the TDEA algorithm is selected.

This is a shock to me since it was my understanding that Triple DES uses a key bundle of 56bits*3 = 168 bits (21 bytes). What kind of Triple DES encryption could it be supporting? Is there some other way to select a key for TDEA so that it behaves as single DES, apart from selecting a key bundle k1k2k3 where k1=k2=k3?