Triple Pass DES
Hewlett-Packard Company Virtual Private Networking Concepts Guide
2-5
Triple Pass DES
Triple Pass DES
Triple Pass DES
Triple Pass DES
Triple Pass DES is a cryptographic system that uses multiple
passes of the DES algorithm to increase the effective key space
available to the system. In triple pass DES, the clear text data is
first encrypted with a 56-bit key. The resulting cipher text is then
decrypted with a different key. Decrypting cipher text with the
wrong key will result in unreadable data. Finally the unreadable
data is encrypted again with the first key. This implementation of
triple pass DES is known as EDE (for Encrypt, Decrypt, Encrypt)
and the technique increases the effective key length from 56 bits
to 112 bits. Note that 90-bit keys should protect encrypted data
for about 20 years.
Go back to the simple Symmetric Cryptographic Systems
(page 2-3) to illustrate the EDE technique. Assuming that the
clear text is AT
, the following steps are involved:
1.
Encrypt with the key set to 3.
DW = shift-right( AT , K1 = 3 )
2.
Decrypt the result DW with a different key (for example, 5 ).
YR = shift-left( DW , K2 = 5 )
Note that the result in this case is not the original clear text.
Now encrypt the result YR with the key used in the first step.
BU = shift-right( YR , K1 = 3 )
The final cipher text is BU. When this cipher text is received, the
decoding process must be performed in reverse (DED). The
decoder must know the 2 keys (K1 = 3 and K2 = 5) and then make
3 passes:
1.
Decrypt with the key set to 3.
YR = shift-left( BU , K1 = 3 )
2.
Encrypt with the key set to 5.
DW = shift-right( YR , K2 = 5 )
3.
Decrypt with the key set to 3.
AT = shift-left( DW , K1 = 3 )
The steps for both the triple pass DES technique and the 3DES
technique are illustrated with the simple symmetric
cryptographic system in the following table.
