why we use encrypt-decrypt-encrypt sequence in 3DES [closed]

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Closed 7 years ago.

I'm wondering why we use encrypt-decrypt-encrypt sequence in 3DES with three keys instead of three times encryption with three different keys ?

thanks

Answer 1

I'm largely restating what is said here: https://security.stackexchange.com/questions/1886/why-triple-des-used-in-ede-mode

Encrypt-decrypt-encrypt (EDE) is the preferred method because if a single key is used for all 3 operations it is equivalent to regular 56-bit DES. That is, a 56-bit DES implementation can decrypt that message. This makes this version of 3DES backwards compatible with DES.

Encrypt-encrypt-encrypt (EEE) is also a valid method though. It is no more or less valid than EDE. However, EDE is usually preferred for the reasons mentioned above.

Answer 2

The answer by Luke is correct. In symmetric-key cryptography, encryption is the reverse of decryption if you apply the same key otherwise it is encryption again. So, encrypt-decrypt-encrypt is equivalent to encrypt-encrypt-encrypt IF the keys are different.