291 lines
10 KiB
Groff
291 lines
10 KiB
Groff
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.\" Thu Apr 12 19:27:10 2001
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.\" ======================================================================
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.\"
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.IX Title "des_modes 7"
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.TH des_modes 7 "0.9.6a" "2001-04-12" "OpenSSL"
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.UC
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.SH "NAME"
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Modes of \s-1DES\s0 \- the variants of \s-1DES\s0 and other crypto algorithms of OpenSSL
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.SH "DESCRIPTION"
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.IX Header "DESCRIPTION"
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Several crypto algorithms for OpenSSL can be used in a number of modes. Those
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are used for using block ciphers in a way similar to stream ciphers, among
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other things.
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.SH "OVERVIEW"
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.IX Header "OVERVIEW"
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.Sh "Electronic Codebook Mode (\s-1ECB\s0)"
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.IX Subsection "Electronic Codebook Mode (ECB)"
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Normally, this is found as the function \fIalgorithm\fR\fI_ecb_encrypt()\fR.
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.Ip "\(bu" 2
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64 bits are enciphered at a time.
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.Ip "\(bu" 2
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The order of the blocks can be rearranged without detection.
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.Ip "\(bu" 2
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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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.Ip "\(bu" 2
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An error will only affect one ciphertext block.
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.Sh "Cipher Block Chaining Mode (\s-1CBC\s0)"
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.IX Subsection "Cipher Block Chaining Mode (CBC)"
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Normally, this is found as the function \fIalgorithm\fR\fI_cbc_encrypt()\fR.
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Be aware that \fIdes_cbc_encrypt()\fR is not really \s-1DES\s0 \s-1CBC\s0 (it does
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not update the \s-1IV\s0); use \fIdes_ncbc_encrypt()\fR instead.
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.Ip "\(bu" 2
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a multiple of 64 bits are enciphered at a time.
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.Ip "\(bu" 2
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The \s-1CBC\s0 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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.Ip "\(bu" 2
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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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.Ip "\(bu" 2
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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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.Ip "\(bu" 2
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An error will affect the current and the following ciphertext blocks.
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.Sh "Cipher Feedback Mode (\s-1CFB\s0)"
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.IX Subsection "Cipher Feedback Mode (CFB)"
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Normally, this is found as the function \fIalgorithm\fR\fI_cfb_encrypt()\fR.
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.Ip "\(bu" 2
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a number of bits (j) <= 64 are enciphered at a time.
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.Ip "\(bu" 2
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The \s-1CFB\s0 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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.Ip "\(bu" 2
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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 can not be rearranged.
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.Ip "\(bu" 2
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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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.Ip "\(bu" 2
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The strength of the \s-1CFB\s0 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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.Ip "\(bu" 2
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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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.Ip "\(bu" 2
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Only multiples of j bits can be enciphered.
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.Ip "\(bu" 2
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An error will affect the current and the following ciphertext variables.
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.Sh "Output Feedback Mode (\s-1OFB\s0)"
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.IX Subsection "Output Feedback Mode (OFB)"
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Normally, this is found as the function \fIalgorithm\fR\fI_ofb_encrypt()\fR.
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.Ip "\(bu" 2
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a number of bits (j) <= 64 are enciphered at a time.
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.Ip "\(bu" 2
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The \s-1OFB\s0 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 \s-1OFB\s0 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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.Ip "\(bu" 2
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The absence of chaining makes the \s-1OFB\s0 more vulnerable to specific attacks.
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.Ip "\(bu" 2
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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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.Ip "\(bu" 2
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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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.Ip "\(bu" 2
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Only multiples of j bits can be enciphered.
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.Ip "\(bu" 2
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\&\s-1OFB\s0 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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.Ip "\(bu" 2
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\&\s-1OFB\s0 mode is not self-synchronizing. 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-initialized.
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.Ip "\(bu" 2
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Each re-initialization 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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.Sh "Triple \s-1ECB\s0 Mode"
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.IX Subsection "Triple ECB Mode"
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Normally, this is found as the function \fIalgorithm\fR\fI_ecb3_encrypt()\fR.
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.Ip "\(bu" 2
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Encrypt with key1, decrypt with key2 and encrypt with key3 again.
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.Ip "\(bu" 2
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As for \s-1ECB\s0 encryption but increases the key length to 168 bits.
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There are theoretic attacks that can be used that make the effective
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key length 112 bits, but this attack also requires 2^56 blocks of
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memory, not very likely, even for the \s-1NSA\s0.
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.Ip "\(bu" 2
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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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.Ip "\(bu" 2
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If the first and last key are the same, the key length is 112 bits.
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There are attacks that could reduce the key space to 55 bit's but it
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requires 2^56 blocks of memory.
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.Ip "\(bu" 2
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If all 3 keys are the same, this is effectively the same as normal
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ecb mode.
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.Sh "Triple \s-1CBC\s0 Mode"
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.IX Subsection "Triple CBC Mode"
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Normally, this is found as the function \fIalgorithm\fR\fI_ede3_cbc_encrypt()\fR.
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.Ip "\(bu" 2
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Encrypt with key1, decrypt with key2 and then encrypt with key3.
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.Ip "\(bu" 2
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As for \s-1CBC\s0 encryption but increases the key length to 168 bits with
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the same restrictions as for triple ecb mode.
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.SH "NOTES"
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.IX Header "NOTES"
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This text was been written in large parts by Eric Young in his original
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documentation for SSLeay, the predecessor of OpenSSL. In turn, he attributed
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it to:
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.PP
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.Vb 5
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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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.Ve
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.SH "SEE ALSO"
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.IX Header "SEE ALSO"
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blowfish(3), des(3), idea(3),
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rc2(3)
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