Cryptography Reference
In-Depth Information
plaintext
1
Encrypt
using DES
Key
K
1
2
Decrypt
using DES
Key
K
2
3
Encrypt
using DES
ciphertext
Key
K
3
Figure 4.5.
Encryption using 3TDES
Decryption of a 3TDES ciphertext consists of the reverse process. In other
words, we first decrypt using
K
3
, then
encrypt
with
K
2
, and finally decrypt
with
K
1
.
At first encounter 3TDES may seem rather confusing because the second
encryption step is actually a single DES
decryption
. We could replace this
with a single DES encryption, thus making 3TDES a triple encryption process
using single DES, however, this is not preferred for implementation reasons. By
structuring Triple DES as an encrypt-decrypt-encrypt process, it is possible to use
a 3TDES (hardware) implementation for single DES by setting
K
1
,
K
2
and
K
3
to be
the same value. This provides backwards compatibility, since an organisation can
convert to 3TDESwhile continuing to be able to secure links to other organisations
that are still using legacy systems based on single DES.
2-KEY TRIPLE DES
The variant of Triple DES known as 2TDES is identical to 3TDES except that
K
3
is replaced by
K
1
. In other words we encrypt with
K
1
, then decrypt with
K
2
, and
then encrypt with
K
1
again. Hence 2TDES has a key length of 112 bits. Note that
although both the first and third steps involve encrypting with
K
1
, the results in
each case are different since the 'plaintexts' on which the encryption operation is
conducted are different.
TRIPLE DES IN PRACTICE
Triple DES had a brief spell of being the 'default' global encryption algorithm,
corresponding to the period between the practical evolution of DES key searches
and the adoption of AES.
The lack of attacks on the underlying DES mechanism carry through to Triple
DES so, as an encryption algorithm, Triple DES is highly regarded. Note, however,
