60643d379b
(Including all changes for FreeBSD - importing the original eBones distribution would be too complex at this stage, since I don't have access to Piero's CVS.) (If you want to include eBones in your system, don't forget to include MAKE_EBONES in /etc/make.conf.) (This stuff is now also suppable from braae.ru.ac.za.) Bones originally from MIT SIPB. Original port to FreeBSD 1.x by Piero Serini. Moved to FreeBSD 2.0 by Doug Rabson and Geoff Rehmet. Nice bug fixes from Doug Rabson.
85 lines
4.2 KiB
Plaintext
85 lines
4.2 KiB
Plaintext
Modes of DES
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Quite a bit of the following information has been taken from
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AS 2805.5.2
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Australian Standard
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Electronic funds transfer - Requirements for interfaces,
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Part 5.2: Modes of operation for an n-bit block cipher algorithm
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Appendix A
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There are several different modes in which DES can be used, they are
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as follows.
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Electronic Codebook Mode (ECB) (des_ecb_encrypt())
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- 64 bits are enciphered at a time.
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- The order of the blocks can be rearranged without detection.
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- The same plaintext block always produces the same ciphertext block
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(for the same key) making it vulnerable to a 'dictionary attack'.
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- An error will only affect one ciphertext block.
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Cipher Block Chaining Mode (CBC) (des_cbc_encrypt())
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- a multiple of 64 bits are enciphered at a time.
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- The CBC mode produces the same ciphertext whenever the same
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plaintext is encrypted using the same key and starting variable.
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- The chaining operation makes the ciphertext blocks dependent on the
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current and all preceding plaintext blocks and therefore blocks can not
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be rearranged.
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- The use of different starting variables prevents the same plaintext
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enciphering to the same ciphertext.
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- An error will affect the current and the following ciphertext blocks.
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Cipher Feedback Mode (CFB) (des_cfb_encrypt())
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- a number of bits (j) <= 64 are enciphered at a time.
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- The CFB mode produces the same ciphertext whenever the same
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plaintext is encrypted using the same key and starting variable.
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- The chaining operation makes the ciphertext variables dependent on the
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current and all preceding variables and therefore j-bit variables are
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chained together and con not be rearranged.
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- The use of different starting variables prevents the same plaintext
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enciphering to the same ciphertext.
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- The strength of the CFB mode depends on the size of k (maximal if
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j == k). In my implementation this is always the case.
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- Selection of a small value for j will require more cycles through
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the encipherment algorithm per unit of plaintext and thus cause
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greater processing overheads.
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- Only multiples of j bits can be enciphered.
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- An error will affect the current and the following ciphertext variables.
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Output Feedback Mode (OFB) (des_ofb_encrypt())
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- a number of bits (j) <= 64 are enciphered at a time.
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- The OFB mode produces the same ciphertext whenever the same
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plaintext enciphered using the same key and starting variable. More
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over, in the OFB mode the same key stream is produced when the same
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key and start variable are used. Consequently, for security reasons
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a specific start variable should be used only once for a given key.
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- The absence of chaining makes the OFB more vulnerable to specific attacks.
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- The use of different start variables values prevents the same
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plaintext enciphering to the same ciphertext, by producing different
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key streams.
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- Selection of a small value for j will require more cycles through
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the encipherment algorithm per unit of plaintext and thus cause
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greater processing overheads.
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- Only multiples of j bits can be enciphered.
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- OFB mode of operation does not extend ciphertext errors in the
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resultant plaintext output. Every bit error in the ciphertext causes
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only one bit to be in error in the deciphered plaintext.
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- OFB mode is not self-synchronising. If the two operation of
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encipherment and decipherment get out of synchronism, the system needs
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to be re-initialised.
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- Each re-initialisation should use a value of the start variable
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different from the start variable values used before with the same
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key. The reason for this is that an identical bit stream would be
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produced each time from the same parameters. This would be
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susceptible to a ' known plaintext' attack.
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Triple ECB Mode (des_3ecb_encrypt())
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- Encrypt with key1, decrypt with key2 and encrypt with key1 again.
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- As for ECB encryption but increases the effective key length to 112 bits.
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- If both keys are the same it is equivalent to encrypting once with
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just one key.
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Triple CBC Mode (des_3cbc_encrypt())
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- Encrypt with key1, decrypt with key2 and encrypt with key1 again.
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- As for CBC encryption but increases the effective key length to 112 bits.
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- If both keys are the same it is equivalent to encrypting once with
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just one key.
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