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_ecb3_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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