mirror of
https://github.com/openssl/openssl.git
synced 2024-12-27 06:21:43 +08:00
3187791ed3
[skip ci] Reviewed-by: Matt Caswell <matt@openssl.org> (Merged from https://github.com/openssl/openssl/pull/7831)
259 lines
6.0 KiB
Plaintext
259 lines
6.0 KiB
Plaintext
=pod
|
|
|
|
=head1 NAME
|
|
|
|
des_modes - the variants of DES and other crypto algorithms of OpenSSL
|
|
|
|
=head1 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.
|
|
|
|
=head1 OVERVIEW
|
|
|
|
=head2 Electronic Codebook Mode (ECB)
|
|
|
|
Normally, this is found as the function I<algorithm>_ecb_encrypt().
|
|
|
|
=over 2
|
|
|
|
=item *
|
|
|
|
64 bits are enciphered at a time.
|
|
|
|
=item *
|
|
|
|
The order of the blocks can be rearranged without detection.
|
|
|
|
=item *
|
|
|
|
The same plaintext block always produces the same ciphertext block
|
|
(for the same key) making it vulnerable to a 'dictionary attack'.
|
|
|
|
=item *
|
|
|
|
An error will only affect one ciphertext block.
|
|
|
|
=back
|
|
|
|
=head2 Cipher Block Chaining Mode (CBC)
|
|
|
|
Normally, this is found as the function I<algorithm>_cbc_encrypt().
|
|
Be aware that des_cbc_encrypt() is not really DES CBC (it does
|
|
not update the IV); use des_ncbc_encrypt() instead.
|
|
|
|
=over 2
|
|
|
|
=item *
|
|
|
|
a multiple of 64 bits are enciphered at a time.
|
|
|
|
=item *
|
|
|
|
The CBC mode produces the same ciphertext whenever the same
|
|
plaintext is encrypted using the same key and starting variable.
|
|
|
|
=item *
|
|
|
|
The chaining operation makes the ciphertext blocks dependent on the
|
|
current and all preceding plaintext blocks and therefore blocks can not
|
|
be rearranged.
|
|
|
|
=item *
|
|
|
|
The use of different starting variables prevents the same plaintext
|
|
enciphering to the same ciphertext.
|
|
|
|
=item *
|
|
|
|
An error will affect the current and the following ciphertext blocks.
|
|
|
|
=back
|
|
|
|
=head2 Cipher Feedback Mode (CFB)
|
|
|
|
Normally, this is found as the function I<algorithm>_cfb_encrypt().
|
|
|
|
=over 2
|
|
|
|
=item *
|
|
|
|
a number of bits (j) <= 64 are enciphered at a time.
|
|
|
|
=item *
|
|
|
|
The CFB mode produces the same ciphertext whenever the same
|
|
plaintext is encrypted using the same key and starting variable.
|
|
|
|
=item *
|
|
|
|
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.
|
|
|
|
=item *
|
|
|
|
The use of different starting variables prevents the same plaintext
|
|
enciphering to the same ciphertext.
|
|
|
|
=item *
|
|
|
|
The strength of the CFB mode depends on the size of k (maximal if
|
|
j == k). In my implementation this is always the case.
|
|
|
|
=item *
|
|
|
|
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.
|
|
|
|
=item *
|
|
|
|
Only multiples of j bits can be enciphered.
|
|
|
|
=item *
|
|
|
|
An error will affect the current and the following ciphertext variables.
|
|
|
|
=back
|
|
|
|
=head2 Output Feedback Mode (OFB)
|
|
|
|
Normally, this is found as the function I<algorithm>_ofb_encrypt().
|
|
|
|
=over 2
|
|
|
|
=item *
|
|
|
|
a number of bits (j) <= 64 are enciphered at a time.
|
|
|
|
=item *
|
|
|
|
The 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 specific start variable should be used only once for a given key.
|
|
|
|
=item *
|
|
|
|
The absence of chaining makes the OFB more vulnerable to specific attacks.
|
|
|
|
=item *
|
|
|
|
The use of different start variables values prevents the same
|
|
plaintext enciphering to the same ciphertext, by producing different
|
|
key streams.
|
|
|
|
=item *
|
|
|
|
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.
|
|
|
|
=item *
|
|
|
|
Only multiples of j bits can be enciphered.
|
|
|
|
=item *
|
|
|
|
OFB 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.
|
|
|
|
=item *
|
|
|
|
OFB mode is not self-synchronizing. If the two operation of
|
|
encipherment and decipherment get out of synchronism, the system needs
|
|
to be re-initialized.
|
|
|
|
=item *
|
|
|
|
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.
|
|
|
|
=back
|
|
|
|
=head2 Triple ECB Mode
|
|
|
|
Normally, this is found as the function I<algorithm>_ecb3_encrypt().
|
|
|
|
=over 2
|
|
|
|
=item *
|
|
|
|
Encrypt with key1, decrypt with key2 and encrypt with key3 again.
|
|
|
|
=item *
|
|
|
|
As 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.
|
|
|
|
=item *
|
|
|
|
If both keys are the same it is equivalent to encrypting once with
|
|
just one key.
|
|
|
|
=item *
|
|
|
|
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.
|
|
|
|
=item *
|
|
|
|
If all 3 keys are the same, this is effectively the same as normal
|
|
ecb mode.
|
|
|
|
=back
|
|
|
|
=head2 Triple CBC Mode
|
|
|
|
Normally, this is found as the function I<algorithm>_ede3_cbc_encrypt().
|
|
|
|
=over 2
|
|
|
|
=item *
|
|
|
|
Encrypt with key1, decrypt with key2 and then encrypt with key3.
|
|
|
|
=item *
|
|
|
|
As for CBC encryption but increases the key length to 168 bits with
|
|
the same restrictions as for triple ecb mode.
|
|
|
|
=back
|
|
|
|
=head1 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:
|
|
|
|
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
|
|
|
|
=head1 SEE ALSO
|
|
|
|
L<BF_encrypt(3)>, L<DES_crypt(3)>
|
|
|
|
=head1 COPYRIGHT
|
|
|
|
Copyright 2000-2017 The OpenSSL Project Authors. All Rights Reserved.
|
|
|
|
Licensed under the Apache License 2.0 (the "License"). You may not use
|
|
this file except in compliance with the License. You can obtain a copy
|
|
in the file LICENSE in the source distribution or at
|
|
L<https://www.openssl.org/source/license.html>.
|
|
|
|
=cut
|