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Thread: Encryption modes

  1. #1
    stealthFIGHTER
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    Question Encryption modes

    Hello all,

    can somebody explain me when (or which of them) encryption modes are used? (ECB, CBC, OFB, CFB, OFBC).

    I saw this encryption modes in some kind of cryptos (IDEA, Blowfish...). Please if can somebody provide short! description of these enc. modes, it would be cool .

    Thanks,
    sF
    I promise that I have read the FAQ and tried to use the Search to answer my question.

  2. #2
    DakienDX
    Guest
    Hello stealthFIGHTER !

    I'll try to explain what I know about them.
    • ECB: Basic encryption mode for block-ciphers. The encrypted block is just stored as it is. You can start decryption anywhere in the encrypted data (of course on a block-size boundary). Same input data results in same output data. You can insert/delete blocks without affecting any other blocks.
    • CBC: All blocks are chained together. You Xor the original data with a random value (must be present for decryption), encrypt it and take the encrypted block as random value for the next encryption. This way blocks with same data look different. If you delete/insert a block, you'll be unable to decrypt the next block.
    • CFB: Block-ciphers can only work with blocks. So if you want to encrypt some data smaller than the block (for example one byte), you've a problem. So you take a random value again (needed for decryption), encrypt it, Xor the data with the leftmost byte of the encrypted random data, Shl the random value by one byte and put the Xored byte at the rightmost position (this is the encrytped byte). You can see that no decryption routine is used here. The byte is decrypted by Xoring it back with the leftmost byte of the random value. If you insert or delete a byte, all data after that byte cannot be decrypted any more.
    • OFB: Similar to CFB, but I can't tell you exactly how it's done, since I didn't understand it correctly. It uses the original random value somehow for encryption every time, not the encrypted. It is said to be unsecure if you don't use it on the whole blocksize instead of just one byte, which makes CBC possible again.
    • OFBC: Sorry, but I've never heard of that.
    I promise that I have read the FAQ and tried to use the Search to answer my question.

  3. #3
    It's amazing what a search on google can do, some people here could learn of that.

    Here is a more detailed discription on some of the modes:

    http://home.ecn.ab.ca/~jsavard/crypto/co0409.htm

    Don't forget to also lookup CTR mode which you didn't mention yourself. This is a very popular mode for AES and personally I think AES-CTR would be a good replacement for a stream cipher like RC4.

    // CyberHeg

  4. #4
    DakienDX
    Guest
    Originally posted by cyberheg
    It's amazing what a search on google can do, some people here could learn of that.
    Hello cyberheg !

    I'm really impressed how much this document informs about the OFB mode. If I start counting the letters it will probably have about 20% more letters and about 20% less information than I wrote.

    I also find it very interesting how the OFBC mode works. It is described there in exactly zero letters. And not a single mention of CTR on the whole domain.

    And it is extremly easy to understand the Base26 number system they use all over the place there. I find it much better than any stupid hex/binaray examples or even a bit of explanation.
    I promise that I have read the FAQ and tried to use the Search to answer my question.

  5. #5
    stealthFIGHTER
    Guest
    Cool DakienDX ,

    this is exactly what I want to see. Again thank you.


    Originally posted by cyberheg
    It's amazing what a search on google can do, some people here could learn of that.
    // CyberHeg
    Hello CyberHeg,

    of course, I did a search on google and I'm sure there is more info. about enc. modes than DakienDX wrote, but I don't need it (as I wrote 'short! description' will do) so detailed.

    Anyway thank you too for the link.

    Best regards,
    stealthFIGHTER
    I promise that I have read the FAQ and tried to use the Search to answer my question.

  6. #6
    I'm sorry DakienDX if you don't find the document useful. I could say the same thing from my point of view about your post but I leave it to the reader and their opinions.
    However I like personally reading facts from books and documents rather then just believe in everything written on a messageboard (no this is not a offense).

    As for CTR mode again it's easy to find the needed information with a websearch.
    Here is a doc on AES-CTR:

    http://csrc.nist.gov/encryption/modes/proposedmodes/ctr/ctr-spec.pdf

    // CyberHeg

  7. #7
    DakienDX
    Guest
    Hello cyberheg !

    This wasn't meant offensive from my point of view, just ironic.

    It was just the document you linked to. You wrote "some people here could learn of that" and posted a document not telling any more than I wrote already.

    And the Base26 number system in the examples made the whole thing completely confusing.

    Of course I didn't invent those cipher modes, I read about them also, not when posting, but some time ago. I know how ECB, CBC and CFB work and I know that I haven't understood OFB when reading it.

    Entering "ECB CBC CFB OFB" into Google gave me 14000 results, adding "OFBC" gave me 0 results. So I was quite sure I was not the only person who never heard about it.

    So I decided to be a bit ironic.
    I promise that I have read the FAQ and tried to use the Search to answer my question.

  8. #8
    הבּרוּ נשׂאי כּלי יהוה mike's Avatar
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    You can also check out my favorite reference, Applied Crypto:
    http://www.unixqi.com/crypto

  9. #9
    DakienDX
    Guest
    Hi mike !

    Thank you for the link. I know that page.
    Well, I actually didn't know that page until you posted it here, but I have the same page as .CHM file.

    Strange, the webpage doesn't contain a "Brought to you by tE!//TMG" string after the Preface.
    I promise that I have read the FAQ and tried to use the Search to answer my question.

  10. #10
    Programmer Run Amock... Bengaly's Avatar
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    this book as said by Bruce Schneier him self in his newest book "trues & lies" (something like that) that it is complete crap book (delicated words -> useless nowdays) ;D
    dont complain to me, complain to bruce
    "knowledge is now free at last, everything should be free from now on, enjoy knowledge and life and never work for everybody else"

  11. #11
    הבּרוּ נשׂאי כּלי יהוה mike's Avatar
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    No, 'Secrets & Lies' says that he was wrong about the idea that crypto can, by itself, provide security. Applied Crypto is still a great crypto reference.

    DakienDX: does the .CHM have the full-size pics in it? Or is it like this one, where all the image links are bad...?

  12. #12
    Programmer Run Amock... Bengaly's Avatar
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    ahh yes "Secrets & Lies" i remember now i took it from uni, scanned it a bit and saw it was a boring book with nothing new
    than i went on scanning William Stalling's book which is really cool [ i got +- 30-60 pages scanned in jpg no html/txt ]
    "knowledge is now free at last, everything should be free from now on, enjoy knowledge and life and never work for everybody else"

  13. #13
    DakienDX
    Guest
    Hi mike !

    The .CHM file contains the images and the links work too, but they aren't any better.

    I've found an online copy of the book where the links work fine, but the linked pictures are only resized versions of the old ones. You still don't see more, maybe even less.

    You can't expect much if you're looking at an 8kb tumbnail and get redirected to a 12kb "full" picture.

    I don't know if the big pictures are around anywhere.
    I promise that I have read the FAQ and tried to use the Search to answer my question.

  14. #14
    stealthFIGHTER
    Guest
    Hello DakienDX,

    you say a .CHM file? Is it the same book (but converted) that can be found on CrackZ's site? (html file format, include pictures)

    Thanks,
    sF
    I promise that I have read the FAQ and tried to use the Search to answer my question.

  15. #15
    Programmer Run Amock... Bengaly's Avatar
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    yeah same book in .chm format made by tE!/TMG
    i also got it if someone need it...(somewher on cd)
    "knowledge is now free at last, everything should be free from now on, enjoy knowledge and life and never work for everybody else"

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