What is triple encryption? How many keys are used in this encryption technique? Why is the middle portion of 3DES decryption rather than an encryption ?
Subject Computer and Network Security
NU Year Set: 3.(b) Marks: 1+1+3 Year: 2014

Triple Data Encryption Standard (DES) is a type of computerized cryptography where block cipher algorithms are applied three times to each data block. The key size is increased in Triple DES to ensure additional security through encryption capabilities. Each block contains 64 bits of data. Three keys are referred to as bundle keys with 56 bits per key. 

The TDEA is basically three applications of the DES cipher. Each part of the "key bundle" is used with one or more executions of the DES cipher algorithm (see also the Feistel cipher);

  • for a single key, it is used three times (equates to classic DES but is no longer recommended), K1 = K2 = K3;
  • for a double key, the first part of the key is used twice, K1 and K2 are independent and K3 = K1;
  • and for the triple key length, each key part is used once, all parts are independent.

What you are seeing as "two" (or "three") keys is most likely the double (or triple) key length being used, each part being provided separately.

The documentation for each API should provide details on how the keys are provided/expected.

A few test cases to check interoperability never hurt either.


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