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Diffstat (limited to 'openssl/trunk/doc/crypto/des_modes.pod')
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diff --git a/openssl/trunk/doc/crypto/des_modes.pod b/openssl/trunk/doc/crypto/des_modes.pod deleted file mode 100644 index 02664036..00000000 --- a/openssl/trunk/doc/crypto/des_modes.pod +++ /dev/null @@ -1,255 +0,0 @@ -=pod - -=for comment openssl_manual_section:7 - -=head1 NAME - -Modes of DES - 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<blowfish(3)|blowfish(3)>, L<des(3)|des(3)>, L<idea(3)|idea(3)>, -L<rc2(3)|rc2(3)> - -=cut - |