First try at documenting the DES (and other algorithms) modes

doc/crypto/des_modes.pod

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+=pod
+
+=head1 NAME
+
+Modes of DES and other crypto algorithms of OpenSSL
+
+=head1 DESCRIPTION
+
+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:
+
+	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 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().
+
+=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-synchronising.  If the two operation of
+encipherment and decipherment get out of synchronism, the system needs
+to be re-initialised.
+
+=item *
+
+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.
+
+=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 key space to 55 bit's but it
+requires 2^56 blocks 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 SEE ALSO
+
+L<blowfish(3)|blowfish(3)>, L<des(3)|des(3)>, L<idea(3)|idea(3)>,
+L<rc2(3)|rc2(3)>