What is the Key Length of the des-ede-cbc cipher?

问题:

I know that DES has a key length of 56, but what does the ede mean and does it effect the key length?

Openssl
des-ede-cbc

回答1:

Triple DES, DES-EDE or TDEA (formally speaking) can be used with no less than 3 key sizes.

The most logical form uses 3 separate keys for each of the phases (Encrypt, Decrypt and then Encrypt again, which is the meaning of EDE). It has a key size of 3 times 56 bits or 168 bits, but those are usually encoded with parity bits (the least significant bit of each byte), making 192 bits in total. Due to a meet-in-the-middle attack (already known at the design phase) the security is only around 112 bits, so don't be fooled by the key size alone.

The two key DES-EDE uses the same keys for the Encrypt phases. The key size is therefore 112 bits, encoded as 128 bits and a security of just around 80 bits, due to various attacks. 80 bits is probably just a bit on the short side nowadays.

Finally single key DES-EDE is mainly used for backwards compatibility. The first encrypt and decrypt cancel each other out so you're left with just one encrypt. You can guess the key sizes; single key TDES is never used in software and may not be supported (it just makes sense in hardware, where you don't want to supply a separate implementation of DES in addition to DES-EDE).

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DES-EDE is much slower than a good implementation of AES, and AES has a security of around 126,8 for a key size of 128 bits (using a very complicated attack). So if you have any chance, choose AES instead.