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Clarification of encryption keys

Would you please kindly confirm whether I have understood the following things right?

Triple DES supports 168-bit encryption with SHA-1 message authentication. Triple DES is the strongest cipher supported by SSL. Triple DES uses a key three times as long as the key for standard DES. Because the key size is so large, there are more possible keys than for any other cipher -- approximately 3.7 * 1050.

DES uses 56-bit keys -- only 56-bit keys. One can use smaller keys (by making sure only keys to 40 bit, for example, are generated. You cannot use keys larger than 56 bits. But, see below...

RC2 with 128-bit encryption and MD5 message authentication: Because the RC2 ciphers has 128-bit encryption, it is the second strongest next to triple DES, with 168-bit encryption. RC2 128-bit encryption permits approximately 3.4 * 1038 possible keys, making it very difficult to crack.

The larger the key space -- the set of all possible numbers from which to pick a key -- the longer a brute-force attack would take, so the better.

Provided you use, e.g. The Microsoft(r) Strong Cryptographic Provider (MSCP) and Microsoft(r) Enhanced Cryptographic Provider (MECP), what is meant by "two key" vs. "three key" in the following context:


DES

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-- MSCP: 56 bits; MECP: 56 bits
Triple DES (two key) -- MSCP: 112 bits; MECP: 112 bits
Triple DES (three key) -- MSCP: 168 bits; MECP: 168 bits

And, how is two key vs. three key achieved?


DES uses 56-bit keys. A method -- called "Triple DES" was developed to extend the life of DES. One, two or three keys are used with Triple DES.

DES is applied three times: Plaintext gets encrypted with key A, then decrypted with key B, then encrypted with key C. If you can only use 56-bit encryption (because of some law, for example) your software would generate one key and use it for key A, B and C. The most common form of 3DES uses two keys -- key A and key C are equivalent. 3DES with three keys uses three 56-bit keys, all different.

So, in order for someone to brute-force 3DES with two keys, they have a 112-bit key space to go through. See INTERNET CRYPTOGRAPHY by Rick Smith and/or APPLIED CRYPTOGRAPHY by Bruce Schneier.

I don't know Microsoft specifics. I expect the user might be able to pick the encryption algorithm used and key size. One can do that with other crypto products (PGP, for example). 128-bit AES, RC4 and IDEA, as well as 112-bit 3DES, are all considered good practice.

Remember Snyder's Razor: In the absence of other factors, always use the most secure options available.


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This was first published in June 2002

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