495 lines
18 KiB
Plaintext
495 lines
18 KiB
Plaintext
The DES library.
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Please note that this library was originally written to operate with
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eBones, a version of Kerberos that had had encryption removed when it left
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the USA and then put back in. As such there are some routines that I will
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advise not using but they are still in the library for historical reasons.
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For all calls that have an 'input' and 'output' variables, they can be the
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same.
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This library requires the inclusion of 'des.h'.
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All of the encryption functions take what is called a des_key_schedule as an
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argument. A des_key_schedule is an expanded form of the des key.
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A des_key is 8 bytes of odd parity, the type used to hold the key is a
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des_cblock. A des_cblock is an array of 8 bytes, often in this library
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description I will refer to input bytes when the function specifies
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des_cblock's as input or output, this just means that the variable should
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be a multiple of 8 bytes.
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The define DES_ENCRYPT is passed to specify encryption, DES_DECRYPT to
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specify decryption. The functions and global variable are as follows:
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int des_check_key;
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DES keys are supposed to be odd parity. If this variable is set to
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a non-zero value, des_set_key() will check that the key has odd
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parity and is not one of the known weak DES keys. By default this
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variable is turned off;
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void des_set_odd_parity(
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des_cblock *key );
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This function takes a DES key (8 bytes) and sets the parity to odd.
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int des_is_weak_key(
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des_cblock *key );
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This function returns a non-zero value if the DES key passed is a
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weak, DES key. If it is a weak key, don't use it, try a different
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one. If you are using 'random' keys, the chances of hitting a weak
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key are 1/2^52 so it is probably not worth checking for them.
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int des_set_key(
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des_cblock *key,
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des_key_schedule schedule);
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Des_set_key converts an 8 byte DES key into a des_key_schedule.
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A des_key_schedule is an expanded form of the key which is used to
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perform actual encryption. It can be regenerated from the DES key
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so it only needs to be kept when encryption or decryption is about
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to occur. Don't save or pass around des_key_schedule's since they
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are CPU architecture dependent, DES keys are not. If des_check_key
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is non zero, zero is returned if the key has the wrong parity or
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the key is a weak key, else 1 is returned.
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int des_key_sched(
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des_cblock *key,
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des_key_schedule schedule);
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An alternative name for des_set_key().
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int des_rw_mode; /* defaults to DES_PCBC_MODE */
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This flag holds either DES_CBC_MODE or DES_PCBC_MODE (default).
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This specifies the function to use in the enc_read() and enc_write()
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functions.
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void des_encrypt(
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unsigned long *data,
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des_key_schedule ks,
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int enc);
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This is the DES encryption function that gets called by just about
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every other DES routine in the library. You should not use this
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function except to implement 'modes' of DES. I say this because the
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functions that call this routine do the conversion from 'char *' to
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long, and this needs to be done to make sure 'non-aligned' memory
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access do not occur. The characters are loaded 'little endian',
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have a look at my source code for more details on how I use this
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function.
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Data is a pointer to 2 unsigned long's and ks is the
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des_key_schedule to use. enc, is non zero specifies encryption,
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zero if decryption.
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void des_encrypt2(
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unsigned long *data,
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des_key_schedule ks,
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int enc);
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This functions is the same as des_encrypt() except that the DES
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initial permutation (IP) and final permutation (FP) have been left
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out. As for des_encrypt(), you should not use this function.
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It is used by the routines in my library that implement triple DES.
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IP() des_encrypt2() des_encrypt2() des_encrypt2() FP() is the same
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as des_encrypt() des_encrypt() des_encrypt() except faster :-).
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void des_ecb_encrypt(
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des_cblock *input,
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des_cblock *output,
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des_key_schedule ks,
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int enc);
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This is the basic Electronic Code Book form of DES, the most basic
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form. Input is encrypted into output using the key represented by
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ks. If enc is non zero (DES_ENCRYPT), encryption occurs, otherwise
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decryption occurs. Input is 8 bytes long and output is 8 bytes.
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(the des_cblock structure is 8 chars).
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void des_ecb3_encrypt(
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des_cblock *input,
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des_cblock *output,
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des_key_schedule ks1,
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des_key_schedule ks2,
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des_key_schedule ks3,
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int enc);
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This is the 3 key EDE mode of ECB DES. What this means is that
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the 8 bytes of input is encrypted with ks1, decrypted with ks2 and
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then encrypted again with ks3, before being put into output;
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C=E(ks3,D(ks2,E(ks1,M))). There is a macro, des_ecb2_encrypt()
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that only takes 2 des_key_schedules that implements,
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C=E(ks1,D(ks2,E(ks1,M))) in that the final encrypt is done with ks1.
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void des_cbc_encrypt(
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des_cblock *input,
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des_cblock *output,
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long length,
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des_key_schedule ks,
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des_cblock *ivec,
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int enc);
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This routine implements DES in Cipher Block Chaining mode.
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Input, which should be a multiple of 8 bytes is encrypted
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(or decrypted) to output which will also be a multiple of 8 bytes.
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The number of bytes is in length (and from what I've said above,
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should be a multiple of 8). If length is not a multiple of 8, I'm
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not being held responsible :-). ivec is the initialisation vector.
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This function does not modify this variable. To correctly implement
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cbc mode, you need to do one of 2 things; copy the last 8 bytes of
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cipher text for use as the next ivec in your application,
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or use des_ncbc_encrypt().
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Only this routine has this problem with updating the ivec, all
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other routines that are implementing cbc mode update ivec.
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void des_ncbc_encrypt(
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des_cblock *input,
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des_cblock *output,
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long length,
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des_key_schedule sk,
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des_cblock *ivec,
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int enc);
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For historical reasons, des_cbc_encrypt() did not update the
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ivec with the value requires so that subsequent calls to
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des_cbc_encrypt() would 'chain'. This was needed so that the same
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'length' values would not need to be used when decrypting.
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des_ncbc_encrypt() does the right thing. It is the same as
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des_cbc_encrypt accept that ivec is updates with the correct value
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to pass in subsequent calls to des_ncbc_encrypt(). I advise using
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des_ncbc_encrypt() instead of des_cbc_encrypt();
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void des_xcbc_encrypt(
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des_cblock *input,
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des_cblock *output,
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long length,
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des_key_schedule sk,
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des_cblock *ivec,
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des_cblock *inw,
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des_cblock *outw,
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int enc);
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This is RSA's DESX mode of DES. It uses inw and outw to
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'whiten' the encryption. inw and outw are secret (unlike the iv)
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and are as such, part of the key. So the key is sort of 24 bytes.
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This is much better than cbc des.
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void des_3cbc_encrypt(
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des_cblock *input,
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des_cblock *output,
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long length,
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des_key_schedule sk1,
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des_key_schedule sk2,
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des_cblock *ivec1,
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des_cblock *ivec2,
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int enc);
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This function is flawed, do not use it. I have left it in the
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library because it is used in my des(1) program and will function
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correctly when used by des(1). If I removed the function, people
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could end up unable to decrypt files.
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This routine implements outer triple cbc encryption using 2 ks and
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2 ivec's. Use des_ede2_cbc_encrypt() instead.
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void des_ede3_cbc_encrypt(
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des_cblock *input,
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des_cblock *output,
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long length,
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des_key_schedule ks1,
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des_key_schedule ks2,
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des_key_schedule ks3,
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des_cblock *ivec,
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int enc);
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This function implements inner triple CBC DES encryption with 3
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keys. What this means is that each 'DES' operation
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inside the cbc mode is really an C=E(ks3,D(ks2,E(ks1,M))).
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Again, this is cbc mode so an ivec is requires.
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This mode is used by SSL.
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There is also a des_ede2_cbc_encrypt() that only uses 2
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des_key_schedule's, the first being reused for the final
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encryption. C=E(ks1,D(ks2,E(ks1,M))). This form of triple DES
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is used by the RSAref library.
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void des_pcbc_encrypt(
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des_cblock *input,
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des_cblock *output,
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long length,
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des_key_schedule ks,
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des_cblock *ivec,
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int enc);
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This is Propagating Cipher Block Chaining mode of DES. It is used
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by Kerberos v4. It's parameters are the same as des_ncbc_encrypt().
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void des_cfb_encrypt(
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unsigned char *in,
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unsigned char *out,
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int numbits,
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long length,
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des_key_schedule ks,
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des_cblock *ivec,
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int enc);
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Cipher Feedback Back mode of DES. This implementation 'feeds back'
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in numbit blocks. The input (and output) is in multiples of numbits
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bits. numbits should to be a multiple of 8 bits. Length is the
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number of bytes input. If numbits is not a multiple of 8 bits,
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the extra bits in the bytes will be considered padding. So if
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numbits is 12, for each 2 input bytes, the 4 high bits of the
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second byte will be ignored. So to encode 72 bits when using
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a numbits of 12 take 12 bytes. To encode 72 bits when using
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numbits of 9 will take 16 bytes. To encode 80 bits when using
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numbits of 16 will take 10 bytes. etc, etc. This padding will
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apply to both input and output.
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void des_cfb64_encrypt(
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unsigned char *in,
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unsigned char *out,
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long length,
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des_key_schedule ks,
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des_cblock *ivec,
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int *num,
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int enc);
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This is one of the more useful functions in this DES library, it
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implements CFB mode of DES with 64bit feedback. Why is this
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useful you ask? Because this routine will allow you to encrypt an
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arbitrary number of bytes, no 8 byte padding. Each call to this
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routine will encrypt the input bytes to output and then update ivec
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and num. num contains 'how far' we are though ivec. If this does
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not make much sense, read more about cfb mode of DES :-).
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void des_ede3_cfb64_encrypt(
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unsigned char *in,
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unsigned char *out,
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long length,
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des_key_schedule ks1,
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des_key_schedule ks2,
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des_key_schedule ks3,
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des_cblock *ivec,
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int *num,
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int enc);
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Same as des_cfb64_encrypt() accept that the DES operation is
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triple DES. As usual, there is a macro for
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des_ede2_cfb64_encrypt() which reuses ks1.
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void des_ofb_encrypt(
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unsigned char *in,
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unsigned char *out,
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int numbits,
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long length,
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des_key_schedule ks,
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des_cblock *ivec);
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This is a implementation of Output Feed Back mode of DES. It is
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the same as des_cfb_encrypt() in that numbits is the size of the
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units dealt with during input and output (in bits).
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void des_ofb64_encrypt(
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unsigned char *in,
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unsigned char *out,
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long length,
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des_key_schedule ks,
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des_cblock *ivec,
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int *num);
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The same as des_cfb64_encrypt() except that it is Output Feed Back
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mode.
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void des_ede3_ofb64_encrypt(
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unsigned char *in,
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unsigned char *out,
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long length,
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des_key_schedule ks1,
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des_key_schedule ks2,
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des_key_schedule ks3,
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des_cblock *ivec,
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int *num);
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Same as des_ofb64_encrypt() accept that the DES operation is
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triple DES. As usual, there is a macro for
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des_ede2_ofb64_encrypt() which reuses ks1.
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int des_read_pw_string(
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char *buf,
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int length,
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char *prompt,
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int verify);
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This routine is used to get a password from the terminal with echo
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turned off. Buf is where the string will end up and length is the
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size of buf. Prompt is a string presented to the 'user' and if
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verify is set, the key is asked for twice and unless the 2 copies
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match, an error is returned. A return code of -1 indicates a
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system error, 1 failure due to use interaction, and 0 is success.
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unsigned long des_cbc_cksum(
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des_cblock *input,
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des_cblock *output,
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long length,
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des_key_schedule ks,
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des_cblock *ivec);
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This function produces an 8 byte checksum from input that it puts in
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output and returns the last 4 bytes as a long. The checksum is
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generated via cbc mode of DES in which only the last 8 byes are
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kept. I would recommend not using this function but instead using
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the EVP_Digest routines, or at least using MD5 or SHA. This
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function is used by Kerberos v4 so that is why it stays in the
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library.
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char *crypt(
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const char *buf,
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const char *salt);
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This is my fast version of the unix crypt(3) function. This version
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takes only a small amount of space relative to other fast
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crypt() implementations.
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void des_string_to_key(
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char *str,
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des_cblock *key);
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This function takes str and converts it into a DES key. I would
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recommend using MD5 instead and use the first 8 bytes of output.
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When I wrote the first version of these routines back in 1990, MD5
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did not exist but I feel these routines are still sound. This
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routines is compatible with the one in MIT's libdes.
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void des_string_to_2keys(
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char *str,
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des_cblock *key1,
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des_cblock *key2);
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This function takes str and converts it into 2 DES keys.
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I would recommend using MD5 and using the 16 bytes as the 2 keys.
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I have nothing against these 2 'string_to_key' routines, it's just
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that if you say that your encryption key is generated by using the
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16 bytes of an MD5 hash, every-one knows how you generated your
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keys.
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int des_read_password(
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des_cblock *key,
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char *prompt,
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int verify);
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This routine combines des_read_pw_string() with des_string_to_key().
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int des_read_2passwords(
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des_cblock *key1,
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des_cblock *key2,
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char *prompt,
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int verify);
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This routine combines des_read_pw_string() with des_string_to_2key().
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void des_random_seed(
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des_cblock key);
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This routine sets a starting point for des_random_key().
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void des_random_key(
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des_cblock ret);
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This function return a random key. Make sure to 'seed' the random
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number generator (with des_random_seed()) before using this function.
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I personally now use a MD5 based random number system.
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int des_enc_read(
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int fd,
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char *buf,
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int len,
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des_key_schedule ks,
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des_cblock *iv);
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This function will write to a file descriptor the encrypted data
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from buf. This data will be preceded by a 4 byte 'byte count' and
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will be padded out to 8 bytes. The encryption is either CBC of
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PCBC depending on the value of des_rw_mode. If it is DES_PCBC_MODE,
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pcbc is used, if DES_CBC_MODE, cbc is used. The default is to use
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DES_PCBC_MODE.
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int des_enc_write(
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int fd,
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char *buf,
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int len,
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des_key_schedule ks,
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des_cblock *iv);
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This routines read stuff written by des_enc_read() and decrypts it.
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I have used these routines quite a lot but I don't believe they are
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suitable for non-blocking io. If you are after a full
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authentication/encryption over networks, have a look at SSL instead.
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unsigned long des_quad_cksum(
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des_cblock *input,
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des_cblock *output,
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long length,
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int out_count,
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des_cblock *seed);
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This is a function from Kerberos v4 that is not anything to do with
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DES but was needed. It is a cksum that is quicker to generate than
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des_cbc_cksum(); I personally would use MD5 routines now.
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=====
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Modes of DES
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Quite a bit of the following information has been taken from
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AS 2805.5.2
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Australian Standard
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Electronic funds transfer - Requirements for interfaces,
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Part 5.2: Modes of operation for an n-bit block cipher algorithm
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Appendix A
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There are several different modes in which DES can be used, they are
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as follows.
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Electronic Codebook Mode (ECB) (des_ecb_encrypt())
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- 64 bits are enciphered at a time.
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- The order of the blocks can be rearranged without detection.
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- The same plaintext block always produces the same ciphertext block
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(for the same key) making it vulnerable to a 'dictionary attack'.
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- An error will only affect one ciphertext block.
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Cipher Block Chaining Mode (CBC) (des_cbc_encrypt())
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- a multiple of 64 bits are enciphered at a time.
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- The CBC mode produces the same ciphertext whenever the same
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plaintext is encrypted using the same key and starting variable.
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- The chaining operation makes the ciphertext blocks dependent on the
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current and all preceding plaintext blocks and therefore blocks can not
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be rearranged.
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- The use of different starting variables prevents the same plaintext
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enciphering to the same ciphertext.
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- An error will affect the current and the following ciphertext blocks.
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Cipher Feedback Mode (CFB) (des_cfb_encrypt())
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- a number of bits (j) <= 64 are enciphered at a time.
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- The CFB mode produces the same ciphertext whenever the same
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plaintext is encrypted using the same key and starting variable.
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- The chaining operation makes the ciphertext variables dependent on the
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current and all preceding variables and therefore j-bit variables are
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chained together and can not be rearranged.
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- The use of different starting variables prevents the same plaintext
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enciphering to the same ciphertext.
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- The strength of the CFB mode depends on the size of k (maximal if
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j == k). In my implementation this is always the case.
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- Selection of a small value for j will require more cycles through
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the encipherment algorithm per unit of plaintext and thus cause
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greater processing overheads.
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- Only multiples of j bits can be enciphered.
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- An error will affect the current and the following ciphertext variables.
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Output Feedback Mode (OFB) (des_ofb_encrypt())
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- a number of bits (j) <= 64 are enciphered at a time.
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- The OFB mode produces the same ciphertext whenever the same
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plaintext enciphered using the same key and starting variable. More
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over, in the OFB mode the same key stream is produced when the same
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key and start variable are used. Consequently, for security reasons
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a specific start variable should be used only once for a given key.
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- The absence of chaining makes the OFB more vulnerable to specific attacks.
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- The use of different start variables values prevents the same
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plaintext enciphering to the same ciphertext, by producing different
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key streams.
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- Selection of a small value for j will require more cycles through
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the encipherment algorithm per unit of plaintext and thus cause
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greater processing overheads.
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- Only multiples of j bits can be enciphered.
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- OFB mode of operation does not extend ciphertext errors in the
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resultant plaintext output. Every bit error in the ciphertext causes
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only one bit to be in error in the deciphered plaintext.
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- OFB mode is not self-synchronising. If the two operation of
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encipherment and decipherment get out of synchronism, the system needs
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to be re-initialised.
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- Each re-initialisation should use a value of the start variable
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different from the start variable values used before with the same
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key. The reason for this is that an identical bit stream would be
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produced each time from the same parameters. This would be
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susceptible to a ' known plaintext' attack.
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Triple ECB Mode (des_ecb3_encrypt())
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- Encrypt with key1, decrypt with key2 and encrypt with key3 again.
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- As for ECB encryption but increases the key length to 168 bits.
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There are theoretic attacks that can be used that make the effective
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key length 112 bits, but this attack also requires 2^56 blocks of
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memory, not very likely, even for the NSA.
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- If both keys are the same it is equivalent to encrypting once with
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just one key.
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- If the first and last key are the same, the key length is 112 bits.
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There are attacks that could reduce the key space to 55 bit's but it
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requires 2^56 blocks of memory.
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- If all 3 keys are the same, this is effectively the same as normal
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|
ecb mode.
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Triple CBC Mode (des_ede3_cbc_encrypt())
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|
- Encrypt with key1, decrypt with key2 and then encrypt with key3.
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|
- As for CBC encryption but increases the key length to 168 bits with
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the same restrictions as for triple ecb mode.
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