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+.\" Automatically generated by Pod::Man version 1.15
+.\" Mon Jan 13 19:29:14 2003
+.\"
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+.\" ======================================================================
+.\"
+.IX Title "des_modes 3"
+.TH des_modes 3 "0.9.7" "2003-01-13" "OpenSSL"
+.UC
+.SH "NAME"
+Modes of \s-1DES\s0 \- the variants of \s-1DES\s0 and other crypto algorithms of OpenSSL
+.SH "DESCRIPTION"
+.IX Header "DESCRIPTION"
+Several 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.
+.SH "OVERVIEW"
+.IX Header "OVERVIEW"
+.Sh "Electronic Codebook Mode (\s-1ECB\s0)"
+.IX Subsection "Electronic Codebook Mode (ECB)"
+Normally, this is found as the function \fIalgorithm\fR\fI_ecb_encrypt()\fR.
+.Ip "\(bu" 2
+64 bits are enciphered at a time.
+.Ip "\(bu" 2
+The order of the blocks can be rearranged without detection.
+.Ip "\(bu" 2
+The same plaintext block always produces the same ciphertext block
+(for the same key) making it vulnerable to a 'dictionary attack'.
+.Ip "\(bu" 2
+An error will only affect one ciphertext block.
+.Sh "Cipher Block Chaining Mode (\s-1CBC\s0)"
+.IX Subsection "Cipher Block Chaining Mode (CBC)"
+Normally, this is found as the function \fIalgorithm\fR\fI_cbc_encrypt()\fR.
+Be aware that \fIdes_cbc_encrypt()\fR is not really \s-1DES\s0 \s-1CBC\s0 (it does
+not update the \s-1IV\s0); use \fIdes_ncbc_encrypt()\fR instead.
+.Ip "\(bu" 2
+a multiple of 64 bits are enciphered at a time.
+.Ip "\(bu" 2
+The \s-1CBC\s0 mode produces the same ciphertext whenever the same
+plaintext is encrypted using the same key and starting variable.
+.Ip "\(bu" 2
+The chaining operation makes the ciphertext blocks dependent on the
+current and all preceding plaintext blocks and therefore blocks can not
+be rearranged.
+.Ip "\(bu" 2
+The use of different starting variables prevents the same plaintext
+enciphering to the same ciphertext.
+.Ip "\(bu" 2
+An error will affect the current and the following ciphertext blocks.
+.Sh "Cipher Feedback Mode (\s-1CFB\s0)"
+.IX Subsection "Cipher Feedback Mode (CFB)"
+Normally, this is found as the function \fIalgorithm\fR\fI_cfb_encrypt()\fR.
+.Ip "\(bu" 2
+a number of bits (j) <= 64 are enciphered at a time.
+.Ip "\(bu" 2
+The \s-1CFB\s0 mode produces the same ciphertext whenever the same
+plaintext is encrypted using the same key and starting variable.
+.Ip "\(bu" 2
+The 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.
+.Ip "\(bu" 2
+The use of different starting variables prevents the same plaintext
+enciphering to the same ciphertext.
+.Ip "\(bu" 2
+The strength of the \s-1CFB\s0 mode depends on the size of k (maximal if
+j == k). In my implementation this is always the case.
+.Ip "\(bu" 2
+Selection of a small value for j will require more cycles through
+the encipherment algorithm per unit of plaintext and thus cause
+greater processing overheads.
+.Ip "\(bu" 2
+Only multiples of j bits can be enciphered.
+.Ip "\(bu" 2
+An error will affect the current and the following ciphertext variables.
+.Sh "Output Feedback Mode (\s-1OFB\s0)"
+.IX Subsection "Output Feedback Mode (OFB)"
+Normally, this is found as the function \fIalgorithm\fR\fI_ofb_encrypt()\fR.
+.Ip "\(bu" 2
+a number of bits (j) <= 64 are enciphered at a time.
+.Ip "\(bu" 2
+The \s-1OFB\s0 mode produces the same ciphertext whenever the same
+plaintext enciphered using the same key and starting variable. More
+over, in the \s-1OFB\s0 mode the same key stream is produced when the same
+key and start variable are used. Consequently, for security reasons
+a specific start variable should be used only once for a given key.
+.Ip "\(bu" 2
+The absence of chaining makes the \s-1OFB\s0 more vulnerable to specific attacks.
+.Ip "\(bu" 2
+The use of different start variables values prevents the same
+plaintext enciphering to the same ciphertext, by producing different
+key streams.
+.Ip "\(bu" 2
+Selection of a small value for j will require more cycles through
+the encipherment algorithm per unit of plaintext and thus cause
+greater processing overheads.
+.Ip "\(bu" 2
+Only multiples of j bits can be enciphered.
+.Ip "\(bu" 2
+\&\s-1OFB\s0 mode of operation does not extend ciphertext errors in the
+resultant plaintext output. Every bit error in the ciphertext causes
+only one bit to be in error in the deciphered plaintext.
+.Ip "\(bu" 2
+\&\s-1OFB\s0 mode is not self-synchronizing. If the two operation of
+encipherment and decipherment get out of synchronism, the system needs
+to be re-initialized.
+.Ip "\(bu" 2
+Each re-initialization should use a value of the start variable
+different 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.
+.Sh "Triple \s-1ECB\s0 Mode"
+.IX Subsection "Triple ECB Mode"
+Normally, this is found as the function \fIalgorithm\fR\fI_ecb3_encrypt()\fR.
+.Ip "\(bu" 2
+Encrypt with key1, decrypt with key2 and encrypt with key3 again.
+.Ip "\(bu" 2
+As for \s-1ECB\s0 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 \s-1NSA\s0.
+.Ip "\(bu" 2
+If both keys are the same it is equivalent to encrypting once with
+just one key.
+.Ip "\(bu" 2
+If 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.
+.Ip "\(bu" 2
+If all 3 keys are the same, this is effectively the same as normal
+ecb mode.
+.Sh "Triple \s-1CBC\s0 Mode"
+.IX Subsection "Triple CBC Mode"
+Normally, this is found as the function \fIalgorithm\fR\fI_ede3_cbc_encrypt()\fR.
+.Ip "\(bu" 2
+Encrypt with key1, decrypt with key2 and then encrypt with key3.
+.Ip "\(bu" 2
+As for \s-1CBC\s0 encryption but increases the key length to 168 bits with
+the same restrictions as for triple ecb mode.
+.SH "NOTES"
+.IX Header "NOTES"
+This text was been written in large parts by Eric Young in his original
+documentation for SSLeay, the predecessor of OpenSSL. In turn, he attributed
+it to:
+.PP
+.Vb 5
+\& 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 A
+.Ve
+.SH "SEE ALSO"
+.IX Header "SEE ALSO"
+blowfish(3), des(3), idea(3),
+rc2(3)
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