259 lines
6.0 KiB
Plaintext
259 lines
6.0 KiB
Plaintext
=pod
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=head1 NAME
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des_modes - the variants of DES and other crypto algorithms of OpenSSL
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=head1 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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=head1 OVERVIEW
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=head2 Electronic Codebook Mode (ECB)
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Normally, this is found as the function I<algorithm>_ecb_encrypt().
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=over 2
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=item *
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64 bits are enciphered at a time.
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=item *
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The order of the blocks can be rearranged without detection.
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=item *
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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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=item *
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An error will only affect one ciphertext block.
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=back
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=head2 Cipher Block Chaining Mode (CBC)
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Normally, this is found as the function I<algorithm>_cbc_encrypt().
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Be aware that des_cbc_encrypt() is not really DES CBC (it does
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not update the IV); use des_ncbc_encrypt() instead.
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=over 2
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=item *
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a multiple of 64 bits are enciphered at a time.
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=item *
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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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=item *
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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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=item *
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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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=item *
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An error will affect the current and the following ciphertext blocks.
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=back
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=head2 Cipher Feedback Mode (CFB)
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Normally, this is found as the function I<algorithm>_cfb_encrypt().
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=over 2
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=item *
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a number of bits (j) <= 64 are enciphered at a time.
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=item *
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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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=item *
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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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=item *
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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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=item *
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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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=item *
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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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=item *
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Only multiples of j bits can be enciphered.
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=item *
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An error will affect the current and the following ciphertext variables.
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=back
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=head2 Output Feedback Mode (OFB)
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Normally, this is found as the function I<algorithm>_ofb_encrypt().
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=over 2
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=item *
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a number of bits (j) <= 64 are enciphered at a time.
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=item *
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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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=item *
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The absence of chaining makes the OFB more vulnerable to specific attacks.
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=item *
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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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=item *
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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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=item *
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Only multiples of j bits can be enciphered.
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=item *
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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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=item *
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OFB 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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=item *
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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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=back
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=head2 Triple ECB Mode
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Normally, this is found as the function I<algorithm>_ecb3_encrypt().
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=over 2
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=item *
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Encrypt with key1, decrypt with key2 and encrypt with key3 again.
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=item *
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As for ECB 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 NSA.
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=item *
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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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=item *
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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 effective key strength
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to only slightly more than 56 bits, but these require a lot of memory.
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=item *
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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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=back
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=head2 Triple CBC Mode
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Normally, this is found as the function I<algorithm>_ede3_cbc_encrypt().
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=over 2
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=item *
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Encrypt with key1, decrypt with key2 and then encrypt with key3.
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=item *
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As for CBC 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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=back
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=head1 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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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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=head1 SEE ALSO
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L<BF_encrypt(3)>, L<DES_crypt(3)>
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=head1 COPYRIGHT
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Copyright 2000-2017 The OpenSSL Project Authors. All Rights Reserved.
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Licensed under the OpenSSL license (the "License"). You may not use
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this file except in compliance with the License. You can obtain a copy
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in the file LICENSE in the source distribution or at
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L<https://www.openssl.org/source/license.html>.
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=cut
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