[thirdparty/openssl.git] / doc / man7 / des_modes.pod

=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 OpenSSL license (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
Commit	Line	Data
7948febe RL	1	=pod
	2
	3	=head1 NAME
	4
954b2747	5	des_modes - the variants of DES and other crypto algorithms of OpenSSL
7948febe RL	6
	7	=head1 DESCRIPTION
	8
c8973693	9	Several crypto algorithms for OpenSSL can be used in a number of modes. Those
a5d9c6e4 RL	10	are used for using block ciphers in a way similar to stream ciphers, among
a5d9c6e4 RL	11	other things.
7948febe RL	12
	13	=head1 OVERVIEW
	14
	15	=head2 Electronic Codebook Mode (ECB)
	16
	17	Normally, this is found as the function I<algorithm>_ecb_encrypt().
	18
2f61bc2e	19	=over 2
7948febe RL	20
	21	=item *
	22
	23	64 bits are enciphered at a time.
	24
	25	=item *
	26
	27	The order of the blocks can be rearranged without detection.
	28
	29	=item *
	30
	31	The same plaintext block always produces the same ciphertext block
	32	(for the same key) making it vulnerable to a 'dictionary attack'.
	33
	34	=item *
	35
	36	An error will only affect one ciphertext block.
	37
	38	=back
	39
	40	=head2 Cipher Block Chaining Mode (CBC)
	41
	42	Normally, this is found as the function I<algorithm>_cbc_encrypt().
8ce288a8 BM	43	Be aware that des_cbc_encrypt() is not really DES CBC (it does
8ce288a8 BM	44	not update the IV); use des_ncbc_encrypt() instead.
7948febe	45
2f61bc2e	46	=over 2
7948febe RL	47
	48	=item *
	49
	50	a multiple of 64 bits are enciphered at a time.
	51
	52	=item *
	53
	54	The CBC mode produces the same ciphertext whenever the same
	55	plaintext is encrypted using the same key and starting variable.
	56
	57	=item *
	58
	59	The chaining operation makes the ciphertext blocks dependent on the
	60	current and all preceding plaintext blocks and therefore blocks can not
	61	be rearranged.
	62
	63	=item *
	64
	65	The use of different starting variables prevents the same plaintext
	66	enciphering to the same ciphertext.
	67
	68	=item *
	69
	70	An error will affect the current and the following ciphertext blocks.
	71
	72	=back
	73
	74	=head2 Cipher Feedback Mode (CFB)
	75
	76	Normally, this is found as the function I<algorithm>_cfb_encrypt().
	77
2f61bc2e	78	=over 2
7948febe RL	79
	80	=item *
	81
	82	a number of bits (j) <= 64 are enciphered at a time.
	83
	84	=item *
	85
	86	The CFB mode produces the same ciphertext whenever the same
	87	plaintext is encrypted using the same key and starting variable.
	88
	89	=item *
	90
	91	The chaining operation makes the ciphertext variables dependent on the
	92	current and all preceding variables and therefore j-bit variables are
	93	chained together and can not be rearranged.
	94
	95	=item *
	96
	97	The use of different starting variables prevents the same plaintext
	98	enciphering to the same ciphertext.
	99
	100	=item *
	101
	102	The strength of the CFB mode depends on the size of k (maximal if
	103	j == k). In my implementation this is always the case.
	104
	105	=item *
	106
	107	Selection of a small value for j will require more cycles through
	108	the encipherment algorithm per unit of plaintext and thus cause
	109	greater processing overheads.
	110
	111	=item *
	112
	113	Only multiples of j bits can be enciphered.
	114
	115	=item *
	116
	117	An error will affect the current and the following ciphertext variables.
	118
	119	=back
	120
	121	=head2 Output Feedback Mode (OFB)
	122
	123	Normally, this is found as the function I<algorithm>_ofb_encrypt().
	124
2f61bc2e	125	=over 2
7948febe RL	126
	127	=item *
	128
	129	a number of bits (j) <= 64 are enciphered at a time.
	130
	131	=item *
	132
	133	The OFB mode produces the same ciphertext whenever the same
	134	plaintext enciphered using the same key and starting variable. More
	135	over, in the OFB mode the same key stream is produced when the same
	136	key and start variable are used. Consequently, for security reasons
	137	a specific start variable should be used only once for a given key.
	138
	139	=item *
	140
	141	The absence of chaining makes the OFB more vulnerable to specific attacks.
	142
	143	=item *
	144
	145	The use of different start variables values prevents the same
	146	plaintext enciphering to the same ciphertext, by producing different
	147	key streams.
	148
	149	=item *
	150
	151	Selection of a small value for j will require more cycles through
	152	the encipherment algorithm per unit of plaintext and thus cause
	153	greater processing overheads.
	154
	155	=item *
	156
	157	Only multiples of j bits can be enciphered.
	158
	159	=item *
	160
	161	OFB mode of operation does not extend ciphertext errors in the
	162	resultant plaintext output. Every bit error in the ciphertext causes
	163	only one bit to be in error in the deciphered plaintext.
	164
	165	=item *
	166
c8973693	167	OFB mode is not self-synchronizing. If the two operation of
7948febe	168	encipherment and decipherment get out of synchronism, the system needs
c8973693	169	to be re-initialized.
7948febe RL	170
	171	=item *
	172
c8973693	173	Each re-initialization should use a value of the start variable
7948febe RL	174	different from the start variable values used before with the same
	175	key. The reason for this is that an identical bit stream would be
	176	produced each time from the same parameters. This would be
	177	susceptible to a 'known plaintext' attack.
	178
	179	=back
	180
	181	=head2 Triple ECB Mode
	182
	183	Normally, this is found as the function I<algorithm>_ecb3_encrypt().
	184
2f61bc2e	185	=over 2
7948febe RL	186
	187	=item *
	188
	189	Encrypt with key1, decrypt with key2 and encrypt with key3 again.
	190
	191	=item *
	192
	193	As for ECB encryption but increases the key length to 168 bits.
	194	There are theoretic attacks that can be used that make the effective
	195	key length 112 bits, but this attack also requires 2^56 blocks of
	196	memory, not very likely, even for the NSA.
	197
	198	=item *
	199
	200	If both keys are the same it is equivalent to encrypting once with
	201	just one key.
	202
	203	=item *
	204
	205	If the first and last key are the same, the key length is 112 bits.
2c17323e BM	206	There are attacks that could reduce the effective key strength
2c17323e BM	207	to only slightly more than 56 bits, but these require a lot of memory.
7948febe RL	208
	209	=item *
	210
	211	If all 3 keys are the same, this is effectively the same as normal
	212	ecb mode.
	213
	214	=back
	215
	216	=head2 Triple CBC Mode
	217
	218	Normally, this is found as the function I<algorithm>_ede3_cbc_encrypt().
	219
2f61bc2e	220	=over 2
7948febe RL	221
	222	=item *
	223
	224	Encrypt with key1, decrypt with key2 and then encrypt with key3.
	225
	226	=item *
	227
	228	As for CBC encryption but increases the key length to 168 bits with
	229	the same restrictions as for triple ecb mode.
	230
	231	=back
	232
a5d9c6e4 RL	233	=head1 NOTES
	234
	235	This text was been written in large parts by Eric Young in his original
	236	documentation for SSLeay, the predecessor of OpenSSL. In turn, he attributed
	237	it to:
	238
1bc74519 RS	239	AS 2805.5.2
	240	Australian Standard
	241	Electronic funds transfer - Requirements for interfaces,
	242	Part 5.2: Modes of operation for an n-bit block cipher algorithm
	243	Appendix A
a5d9c6e4	244
7948febe RL	245	=head1 SEE ALSO
7948febe RL	246
9e183d22	247	L<BF_encrypt(3)>, L<DES_crypt(3)>
369782ac	248
e2f92610 RS	249	=head1 COPYRIGHT
e2f92610 RS	250
9e183d22	251	Copyright 2000-2017 The OpenSSL Project Authors. All Rights Reserved.
e2f92610 RS	252
	253	Licensed under the OpenSSL license (the "License"). You may not use
	254	this file except in compliance with the License. You can obtain a copy
	255	in the file LICENSE in the source distribution or at
	256	L<https://www.openssl.org/source/license.html>.
	257
	258	=cut