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authorJung-uk Kim <jkim@FreeBSD.org>2020-02-21 00:33:21 +0000
committerJung-uk Kim <jkim@FreeBSD.org>2020-02-21 00:33:21 +0000
commit6b4382c4054d7c169c6140feaa144ca360a90ead (patch)
tree176bf7fdbae226099058e11e9a429805b6e0af42 /secure/lib/libcrypto/man/man7/des_modes.7
parent24722aacdb06a5ac6780beef7db856a0f99095c3 (diff)
downloadsrc-6b4382c4054d7c169c6140feaa144ca360a90ead.tar.gz
src-6b4382c4054d7c169c6140feaa144ca360a90ead.zip
MFC: r356963
Install man5 and man7 for OpenSSL.
Notes
Notes: svn path=/stable/12/; revision=358188
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+.\" ========================================================================
+.\"
+.IX Title "DES_MODES 7"
+.TH DES_MODES 7 "2019-09-10" "1.1.1d" "OpenSSL"
+.\" For nroff, turn off justification. Always turn off hyphenation; it makes
+.\" way too many mistakes in technical documents.
+.if n .ad l
+.nh
+.SH "NAME"
+des_modes \- the variants of DES 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"
+.SS "Electronic Codebook Mode (\s-1ECB\s0)"
+.IX Subsection "Electronic Codebook Mode (ECB)"
+Normally, this is found as the function \fIalgorithm\fR\fB_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.
+.SS "Cipher Block Chaining Mode (\s-1CBC\s0)"
+.IX Subsection "Cipher Block Chaining Mode (CBC)"
+Normally, this is found as the function \fIalgorithm\fR\fB_cbc_encrypt()\fR.
+Be aware that \fBdes_cbc_encrypt()\fR is not really \s-1DES CBC\s0 (it does
+not update the \s-1IV\s0); use \fBdes_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.
+.SS "Cipher Feedback Mode (\s-1CFB\s0)"
+.IX Subsection "Cipher Feedback Mode (CFB)"
+Normally, this is found as the function \fIalgorithm\fR\fB_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.
+.SS "Output Feedback Mode (\s-1OFB\s0)"
+.IX Subsection "Output Feedback Mode (OFB)"
+Normally, this is found as the function \fIalgorithm\fR\fB_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.
+.SS "Triple \s-1ECB\s0 Mode"
+.IX Subsection "Triple ECB Mode"
+Normally, this is found as the function \fIalgorithm\fR\fB_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.
+.SS "Triple \s-1CBC\s0 Mode"
+.IX Subsection "Triple CBC Mode"
+Normally, this is found as the function \fIalgorithm\fR\fB_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"
+\&\fBBF_encrypt\fR\|(3), \fBDES_crypt\fR\|(3)
+.SH "COPYRIGHT"
+.IX Header "COPYRIGHT"
+Copyright 2000\-2017 The OpenSSL Project Authors. All Rights Reserved.
+.PP
+Licensed under the OpenSSL license (the \*(L"License\*(R"). You may not use
+this file except in compliance with the License. You can obtain a copy
+in the file \s-1LICENSE\s0 in the source distribution or at
+<https://www.openssl.org/source/license.html>.