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First try at documenting the DES (and other algorithms) modes
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=pod | ||
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=head1 NAME | ||
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Modes of DES and other crypto algorithms of OpenSSL | ||
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=head1 DESCRIPTION | ||
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Several crypto algorithms fo OpenSSL can be used in a number of modes. The | ||
following text has been written in large parts by Eric Young in his original | ||
documentation for SSLeay, the predecessor of OpenSSL. In turn, he attributed | ||
it to: | ||
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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 | ||
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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 | ||
(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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=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 | ||
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 | ||
current and all preceding plaintext blocks and therefore blocks can not | ||
be rearranged. | ||
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=item * | ||
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The use of different starting variables prevents the same plaintext | ||
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 | ||
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 | ||
current and all preceding variables and therefore j-bit variables are | ||
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 | ||
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 | ||
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 | ||
the encipherment algorithm per unit of plaintext and thus cause | ||
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 | ||
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. | ||
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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 | ||
plaintext enciphering to the same ciphertext, by producing different | ||
key streams. | ||
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=item * | ||
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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. | ||
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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 | ||
resultant plaintext output. Every bit error in the ciphertext causes | ||
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-synchronising. If the two operation of | ||
encipherment and decipherment get out of synchronism, the system needs | ||
to be re-initialised. | ||
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=item * | ||
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Each re-initialisation 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. | ||
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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. | ||
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. | ||
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=item * | ||
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If both keys are the same it is equivalent to encrypting once with | ||
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. | ||
There are attacks that could reduce the key space to 55 bit's but it | ||
requires 2^56 blocks 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 | ||
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 | ||
the same restrictions as for triple ecb mode. | ||
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=back | ||
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=head1 SEE ALSO | ||
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L<blowfish(3)|blowfish(3)>, L<des(3)|des(3)>, L<idea(3)|idea(3)>, | ||
L<rc2(3)|rc2(3)> |