des_modes(7) libdes des_modes(7)NAMEModes of DES - the variants of DES and other crypto algorithms of OpenSSLDESCRIPTIONSeveral crypto algorithms for OpenSSL can be used in a number of modes. Those are used for using block ciphers in a way similar to stream ciphers, among other things.OVERVIEWElectronic Codebook Mode (ECB)Normally, this is found as the functionalgorithm_ecb_encrypt().o64 bits are enciphered at a time.oThe order of the blocks can be rearranged without detection.oThe same plaintext block always produces the same ciphertext block (for the same key) making it vulnerable to a 'dictionary attack'.oAn error will only affect one ciphertext block.Cipher Block Chaining Mode (CBC)Normally, this is found as the functionalgorithm_cbc_encrypt(). Be aware thatdes_cbc_encrypt()is not really DES CBC (it does not update the IV); usedes_ncbc_encrypt()instead.oa multiple of 64 bits are enciphered at a time.oThe CBC mode produces the same ciphertext whenever the same plaintext is encrypted using the same key and starting variable.oThe chaining operation makes the ciphertext blocks dependent on the current and all preceding plaintext blocks and therefore blocks can not be rearranged.oThe use of different starting variables prevents the same plaintext enciphering to the same ciphertext.oAn error will affect the current and the following ciphertext blocks.Cipher Feedback Mode (CFB)Normally, this is found as the functionalgorithm_cfb_encrypt().oa number of bits (j) <= 64 are enciphered at a time.oThe CFB mode produces the same ciphertext whenever the same plaintext is encrypted using the same key and starting variable.oThe chaining operation makes the ciphertext variables dependent on the current and all preceding variables and therefore j-bit variables are chained together and can not be rearranged.oThe use of different starting variables prevents the same plaintext enciphering to the same ciphertext.oThe strength of the CFB mode depends on the size of k (maximal if j == k). In my implementation this is always the case.oSelection of a small value for j will require more cycles through the encipherment algorithm per unit of plaintext and thus cause greater processing overheads.oOnly multiples of j bits can be enciphered.oAn error will affect the current and the following ciphertext vari- ables.Output Feedback Mode (OFB)Normally, this is found as the functionalgorithm_ofb_encrypt().oa number of bits (j) <= 64 are enciphered at a time.oThe OFB mode produces the same ciphertext whenever the same plaintext enciphered using the same key and starting variable. More over, in the OFB mode the same key stream is produced when the same key and start variable are used. Consequently, for security reasons a spe- cific start variable should be used only once for a given key.oThe absence of chaining makes the OFB more vulnerable to specific attacks.oThe use of different start variables values prevents the same plain- text enciphering to the same ciphertext, by producing different key streams.oSelection of a small value for j will require more cycles through the encipherment algorithm per unit of plaintext and thus cause greater processing overheads.oOnly multiples of j bits can be enciphered.oOFB mode of operation does not extend ciphertext errors in the resul- tant plaintext output. Every bit error in the ciphertext causes only one bit to be in error in the deciphered plaintext.oOFB mode is not self-synchronizing. If the two operation of enci- pherment and decipherment get out of synchronism, the system needs to be re-initialized.oEach re-initialization should use a value of the start variable dif- ferent from the start variable values used before with the same key. The reason for this is that an identical bit stream would be produced each time from the same parameters. This would be susceptible to a 'known plaintext' attack.Triple ECB ModeNormally, this is found as the functionalgorithm_ecb3_encrypt().oEncrypt with key1, decrypt with key2 and encrypt with key3 again.oAs for ECB encryption but increases the key length to 168 bits. There are theoretic attacks that can be used that make the effective key length 112 bits, but this attack also requires 2^56 blocks of memory, not very likely, even for the NSA.oIf both keys are the same it is equivalent to encrypting once with just one key.oIf the first and last key are the same, the key length is 112 bits. There are attacks that could reduce the effective key strength to only slightly more than 56 bits, but these require a lot of memory.oIf all 3 keys are the same, this is effectively the same as normal ecb mode.Triple CBC ModeNormally, this is found as the functionalgorithm_ede3_cbc_encrypt().oEncrypt with key1, decrypt with key2 and then encrypt with key3.oAs for CBC encryption but increases the key length to 168 bits with the same restrictions as for triple ecb mode.NOTESThis text was been written in large parts by Eric Young in his original documentation for SSLeay, the predecessor of OpenSSL. In turn, he at- tributed it to: AS 2805.5.2 Australian Standard Electronic funds transfer - Requirements for interfaces, Part 5.2: Modes of operation for an n-bit block cipher algorithm Appendix ASEE ALSOblowfish(3), des(3), idea(3), rc2(3) 3rd Berkeley Distribution 0.9.6j des_modes(7)

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