1The DES library.
2
3Please note that this library was originally written to operate with
4eBones, a version of Kerberos that had had encryption removed when it left
5the USA and then put back in.  As such there are some routines that I will
6advise not using but they are still in the library for historical reasons.
7For all calls that have an 'input' and 'output' variables, they can be the
8same.
9
10This library requires the inclusion of 'des.h'.
11
12All of the encryption functions take what is called a des_key_schedule as an
13argument.  A des_key_schedule is an expanded form of the des key.
14A des_key is 8 bytes of odd parity, the type used to hold the key is a
15des_cblock.  A des_cblock is an array of 8 bytes, often in this library
16description I will refer to input bytes when the function specifies
17des_cblock's as input or output, this just means that the variable should
18be a multiple of 8 bytes.
19
20The define DES_ENCRYPT is passed to specify encryption, DES_DECRYPT to
21specify decryption.  The functions and global variable are as follows:
22
23int des_check_key;
24	DES keys are supposed to be odd parity.  If this variable is set to
25	a non-zero value, des_set_key() will check that the key has odd
26	parity and is not one of the known weak DES keys.  By default this
27	variable is turned off;
28
29void des_set_odd_parity(
30des_cblock *key );
31	This function takes a DES key (8 bytes) and sets the parity to odd.
32
33int des_is_weak_key(
34des_cblock *key );
35	This function returns a non-zero value if the DES key passed is a
36	weak, DES key.  If it is a weak key, don't use it, try a different
37	one.  If you are using 'random' keys, the chances of hitting a weak
38	key are 1/2^52 so it is probably not worth checking for them.
39
40int des_set_key(
41des_cblock *key,
42des_key_schedule schedule);
43	Des_set_key converts an 8 byte DES key into a des_key_schedule.
44	A des_key_schedule is an expanded form of the key which is used to
45	perform actual encryption.  It can be regenerated from the DES key
46	so it only needs to be kept when encryption or decryption is about
47	to occur.  Don't save or pass around des_key_schedule's since they
48	are CPU architecture dependent, DES keys are not.  If des_check_key
49	is non zero, zero is returned if the key has the wrong parity or
50	the key is a weak key, else 1 is returned.
51
52int des_key_sched(
53des_cblock *key,
54des_key_schedule schedule);
55	An alternative name for des_set_key().
56
57int des_rw_mode;		/* defaults to DES_PCBC_MODE */
58	This flag holds either DES_CBC_MODE or DES_PCBC_MODE (default).
59	This specifies the function to use in the enc_read() and enc_write()
60	functions.
61
62void des_encrypt(
63unsigned long *data,
64des_key_schedule ks,
65int enc);
66	This is the DES encryption function that gets called by just about
67	every other DES routine in the library.  You should not use this
68	function except to implement 'modes' of DES.  I say this because the
69	functions that call this routine do the conversion from 'char *' to
70	long, and this needs to be done to make sure 'non-aligned' memory
71	access do not occur.  The characters are loaded 'little endian',
72	have a look at my source code for more details on how I use this
73	function.
74	Data is a pointer to 2 unsigned long's and ks is the
75	des_key_schedule to use.  enc, is non zero specifies encryption,
76	zero if decryption.
77
78void des_encrypt2(
79unsigned long *data,
80des_key_schedule ks,
81int enc);
82	This functions is the same as des_encrypt() except that the DES
83	initial permutation (IP) and final permutation (FP) have been left
84	out.  As for des_encrypt(), you should not use this function.
85	It is used by the routines in my library that implement triple DES.
86	IP() des_encrypt2() des_encrypt2() des_encrypt2() FP() is the same
87	as des_encrypt() des_encrypt() des_encrypt() except faster :-).
88
89void des_ecb_encrypt(
90des_cblock *input,
91des_cblock *output,
92des_key_schedule ks,
93int enc);
94	This is the basic Electronic Code Book form of DES, the most basic
95	form.  Input is encrypted into output using the key represented by
96	ks.  If enc is non zero (DES_ENCRYPT), encryption occurs, otherwise
97	decryption occurs.  Input is 8 bytes long and output is 8 bytes.
98	(the des_cblock structure is 8 chars).
99
100void des_ecb3_encrypt(
101des_cblock *input,
102des_cblock *output,
103des_key_schedule ks1,
104des_key_schedule ks2,
105des_key_schedule ks3,
106int enc);
107	This is the 3 key EDE mode of ECB DES.  What this means is that
108	the 8 bytes of input is encrypted with ks1, decrypted with ks2 and
109	then encrypted again with ks3, before being put into output;
110	C=E(ks3,D(ks2,E(ks1,M))).  There is a macro, des_ecb2_encrypt()
111	that only takes 2 des_key_schedules that implements,
112	C=E(ks1,D(ks2,E(ks1,M))) in that the final encrypt is done with ks1.
113
114void des_cbc_encrypt(
115des_cblock *input,
116des_cblock *output,
117long length,
118des_key_schedule ks,
119des_cblock *ivec,
120int enc);
121	This routine implements DES in Cipher Block Chaining mode.
122	Input, which should be a multiple of 8 bytes is encrypted
123	(or decrypted) to output which will also be a multiple of 8 bytes.
124	The number of bytes is in length (and from what I've said above,
125	should be a multiple of 8).  If length is not a multiple of 8, I'm
126	not being held responsible :-).  ivec is the initialisation vector.
127	This function does not modify this variable.  To correctly implement
128	cbc mode, you need to do one of 2 things; copy the last 8 bytes of
129	cipher text for use as the next ivec in your application,
130	or use des_ncbc_encrypt().
131	Only this routine has this problem with updating the ivec, all
132	other routines that are implementing cbc mode update ivec.
133
134void des_ncbc_encrypt(
135des_cblock *input,
136des_cblock *output,
137long length,
138des_key_schedule sk,
139des_cblock *ivec,
140int enc);
141	For historical reasons, des_cbc_encrypt() did not update the
142	ivec with the value requires so that subsequent calls to
143	des_cbc_encrypt() would 'chain'.  This was needed so that the same
144	'length' values would not need to be used when decrypting.
145	des_ncbc_encrypt() does the right thing.  It is the same as
146	des_cbc_encrypt accept that ivec is updates with the correct value
147	to pass in subsequent calls to des_ncbc_encrypt().  I advise using
148	des_ncbc_encrypt() instead of des_cbc_encrypt();
149
150void des_xcbc_encrypt(
151des_cblock *input,
152des_cblock *output,
153long length,
154des_key_schedule sk,
155des_cblock *ivec,
156des_cblock *inw,
157des_cblock *outw,
158int enc);
159	This is RSA's DESX mode of DES.  It uses inw and outw to
160	'whiten' the encryption.  inw and outw are secret (unlike the iv)
161	and are as such, part of the key.  So the key is sort of 24 bytes.
162	This is much better than cbc des.
163
164void des_3cbc_encrypt(
165des_cblock *input,
166des_cblock *output,
167long length,
168des_key_schedule sk1,
169des_key_schedule sk2,
170des_cblock *ivec1,
171des_cblock *ivec2,
172int enc);
173	This function is flawed, do not use it.  I have left it in the
174	library because it is used in my des(1) program and will function
175	correctly when used by des(1).  If I removed the function, people
176	could end up unable to decrypt files.
177	This routine implements outer triple cbc encryption using 2 ks and
178	2 ivec's.  Use des_ede2_cbc_encrypt() instead.
179
180void des_ede3_cbc_encrypt(
181des_cblock *input,
182des_cblock *output,
183long length,
184des_key_schedule ks1,
185des_key_schedule ks2,
186des_key_schedule ks3,
187des_cblock *ivec,
188int enc);
189	This function implements inner triple CBC DES encryption with 3
190	keys.  What this means is that each 'DES' operation
191	inside the cbc mode is really an C=E(ks3,D(ks2,E(ks1,M))).
192	Again, this is cbc mode so an ivec is requires.
193	This mode is used by SSL.
194	There is also a des_ede2_cbc_encrypt() that only uses 2
195	des_key_schedule's, the first being reused for the final
196	encryption.  C=E(ks1,D(ks2,E(ks1,M))).  This form of triple DES
197	is used by the RSAref library.
198
199void des_pcbc_encrypt(
200des_cblock *input,
201des_cblock *output,
202long length,
203des_key_schedule ks,
204des_cblock *ivec,
205int enc);
206	This is Propagating Cipher Block Chaining mode of DES.  It is used
207	by Kerberos v4.  It's parameters are the same as des_ncbc_encrypt().
208
209void des_cfb_encrypt(
210unsigned char *in,
211unsigned char *out,
212int numbits,
213long length,
214des_key_schedule ks,
215des_cblock *ivec,
216int enc);
217	Cipher Feedback Back mode of DES.  This implementation 'feeds back'
218	in numbit blocks.  The input (and output) is in multiples of numbits
219	bits.  numbits should to be a multiple of 8 bits.  Length is the
220	number of bytes input.  If numbits is not a multiple of 8 bits,
221	the extra bits in the bytes will be considered padding.  So if
222	numbits is 12, for each 2 input bytes, the 4 high bits of the
223	second byte will be ignored.  So to encode 72 bits when using
224	a numbits of 12 take 12 bytes.  To encode 72 bits when using
225	numbits of 9 will take 16 bytes.  To encode 80 bits when using
226	numbits of 16 will take 10 bytes. etc, etc.  This padding will
227	apply to both input and output.
228
229
230void des_cfb64_encrypt(
231unsigned char *in,
232unsigned char *out,
233long length,
234des_key_schedule ks,
235des_cblock *ivec,
236int *num,
237int enc);
238	This is one of the more useful functions in this DES library, it
239	implements CFB mode of DES with 64bit feedback.  Why is this
240	useful you ask?  Because this routine will allow you to encrypt an
241	arbitrary number of bytes, no 8 byte padding.  Each call to this
242	routine will encrypt the input bytes to output and then update ivec
243	and num.  num contains 'how far' we are though ivec.  If this does
244	not make much sense, read more about cfb mode of DES :-).
245
246void des_ede3_cfb64_encrypt(
247unsigned char *in,
248unsigned char *out,
249long length,
250des_key_schedule ks1,
251des_key_schedule ks2,
252des_key_schedule ks3,
253des_cblock *ivec,
254int *num,
255int enc);
256	Same as des_cfb64_encrypt() accept that the DES operation is
257	triple DES.  As usual, there is a macro for
258	des_ede2_cfb64_encrypt() which reuses ks1.
259
260void des_ofb_encrypt(
261unsigned char *in,
262unsigned char *out,
263int numbits,
264long length,
265des_key_schedule ks,
266des_cblock *ivec);
267	This is a implementation of Output Feed Back mode of DES.  It is
268	the same as des_cfb_encrypt() in that numbits is the size of the
269	units dealt with during input and output (in bits).
270
271void des_ofb64_encrypt(
272unsigned char *in,
273unsigned char *out,
274long length,
275des_key_schedule ks,
276des_cblock *ivec,
277int *num);
278	The same as des_cfb64_encrypt() except that it is Output Feed Back
279	mode.
280
281void des_ede3_ofb64_encrypt(
282unsigned char *in,
283unsigned char *out,
284long length,
285des_key_schedule ks1,
286des_key_schedule ks2,
287des_key_schedule ks3,
288des_cblock *ivec,
289int *num);
290	Same as des_ofb64_encrypt() accept that the DES operation is
291	triple DES.  As usual, there is a macro for
292	des_ede2_ofb64_encrypt() which reuses ks1.
293
294int des_read_pw_string(
295char *buf,
296int length,
297char *prompt,
298int verify);
299	This routine is used to get a password from the terminal with echo
300	turned off.  Buf is where the string will end up and length is the
301	size of buf.  Prompt is a string presented to the 'user' and if
302	verify is set, the key is asked for twice and unless the 2 copies
303	match, an error is returned.  A return code of -1 indicates a
304	system error, 1 failure due to use interaction, and 0 is success.
305
306unsigned long des_cbc_cksum(
307des_cblock *input,
308des_cblock *output,
309long length,
310des_key_schedule ks,
311des_cblock *ivec);
312	This function produces an 8 byte checksum from input that it puts in
313	output and returns the last 4 bytes as a long.  The checksum is
314	generated via cbc mode of DES in which only the last 8 byes are
315	kept.  I would recommend not using this function but instead using
316	the EVP_Digest routines, or at least using MD5 or SHA.  This
317	function is used by Kerberos v4 so that is why it stays in the
318	library.
319
320char *des_fcrypt(
321const char *buf,
322const char *salt
323char *ret);
324	This is my fast version of the unix crypt(3) function.  This version
325	takes only a small amount of space relative to other fast
326	crypt() implementations.  This is different to the normal crypt
327	in that the third parameter is the buffer that the return value
328	is written into.  It needs to be at least 14 bytes long.  This
329	function is thread safe, unlike the normal crypt.
330
331char *crypt(
332const char *buf,
333const char *salt);
334	This function calls des_fcrypt() with a static array passed as the
335	third parameter.  This emulates the normal non-thread safe semantics
336	of crypt(3).
337
338void des_string_to_key(
339char *str,
340des_cblock *key);
341	This function takes str and converts it into a DES key.  I would
342	recommend using MD5 instead and use the first 8 bytes of output.
343	When I wrote the first version of these routines back in 1990, MD5
344	did not exist but I feel these routines are still sound.  This
345	routines is compatible with the one in MIT's libdes.
346
347void des_string_to_2keys(
348char *str,
349des_cblock *key1,
350des_cblock *key2);
351	This function takes str and converts it into 2 DES keys.
352	I would recommend using MD5 and using the 16 bytes as the 2 keys.
353	I have nothing against these 2 'string_to_key' routines, it's just
354	that if you say that your encryption key is generated by using the
355	16 bytes of an MD5 hash, every-one knows how you generated your
356	keys.
357
358int des_read_password(
359des_cblock *key,
360char *prompt,
361int verify);
362	This routine combines des_read_pw_string() with des_string_to_key().
363
364int des_read_2passwords(
365des_cblock *key1,
366des_cblock *key2,
367char *prompt,
368int verify);
369	This routine combines des_read_pw_string() with des_string_to_2key().
370
371void des_random_seed(
372des_cblock key);
373	This routine sets a starting point for des_random_key().
374
375void des_random_key(
376des_cblock ret);
377	This function return a random key.  Make sure to 'seed' the random
378	number generator (with des_random_seed()) before using this function.
379	I personally now use a MD5 based random number system.
380
381int des_enc_read(
382int fd,
383char *buf,
384int len,
385des_key_schedule ks,
386des_cblock *iv);
387	This function will write to a file descriptor the encrypted data
388	from buf.  This data will be preceded by a 4 byte 'byte count' and
389	will be padded out to 8 bytes.  The encryption is either CBC of
390	PCBC depending on the value of des_rw_mode.  If it is DES_PCBC_MODE,
391	pcbc is used, if DES_CBC_MODE, cbc is used.  The default is to use
392	DES_PCBC_MODE.
393
394int des_enc_write(
395int fd,
396char *buf,
397int len,
398des_key_schedule ks,
399des_cblock *iv);
400	This routines read stuff written by des_enc_read() and decrypts it.
401	I have used these routines quite a lot but I don't believe they are
402	suitable for non-blocking io.  If you are after a full
403	authentication/encryption over networks, have a look at SSL instead.
404
405unsigned long des_quad_cksum(
406des_cblock *input,
407des_cblock *output,
408long length,
409int out_count,
410des_cblock *seed);
411	This is a function from Kerberos v4 that is not anything to do with
412	DES but was needed.  It is a cksum that is quicker to generate than
413	des_cbc_cksum();  I personally would use MD5 routines now.
414=====
415Modes of DES
416Quite a bit of the following information has been taken from
417	AS 2805.5.2
418	Australian Standard
419	Electronic funds transfer - Requirements for interfaces,
420	Part 5.2: Modes of operation for an n-bit block cipher algorithm
421	Appendix A
422
423There are several different modes in which DES can be used, they are
424as follows.
425
426Electronic Codebook Mode (ECB) (des_ecb_encrypt())
427- 64 bits are enciphered at a time.
428- The order of the blocks can be rearranged without detection.
429- The same plaintext block always produces the same ciphertext block
430  (for the same key) making it vulnerable to a 'dictionary attack'.
431- An error will only affect one ciphertext block.
432
433Cipher Block Chaining Mode (CBC) (des_cbc_encrypt())
434- a multiple of 64 bits are enciphered at a time.
435- The CBC mode produces the same ciphertext whenever the same
436  plaintext is encrypted using the same key and starting variable.
437- The chaining operation makes the ciphertext blocks dependent on the
438  current and all preceding plaintext blocks and therefore blocks can not
439  be rearranged.
440- The use of different starting variables prevents the same plaintext
441  enciphering to the same ciphertext.
442- An error will affect the current and the following ciphertext blocks.
443
444Cipher Feedback Mode (CFB) (des_cfb_encrypt())
445- a number of bits (j) <= 64 are enciphered at a time.
446- The CFB mode produces the same ciphertext whenever the same
447  plaintext is encrypted using the same key and starting variable.
448- The chaining operation makes the ciphertext variables dependent on the
449  current and all preceding variables and therefore j-bit variables are
450  chained together and can not be rearranged.
451- The use of different starting variables prevents the same plaintext
452  enciphering to the same ciphertext.
453- The strength of the CFB mode depends on the size of k (maximal if
454  j == k).  In my implementation this is always the case.
455- Selection of a small value for j will require more cycles through
456  the encipherment algorithm per unit of plaintext and thus cause
457  greater processing overheads.
458- Only multiples of j bits can be enciphered.
459- An error will affect the current and the following ciphertext variables.
460
461Output Feedback Mode (OFB) (des_ofb_encrypt())
462- a number of bits (j) <= 64 are enciphered at a time.
463- The OFB mode produces the same ciphertext whenever the same
464  plaintext enciphered using the same key and starting variable.  More
465  over, in the OFB mode the same key stream is produced when the same
466  key and start variable are used.  Consequently, for security reasons
467  a specific start variable should be used only once for a given key.
468- The absence of chaining makes the OFB more vulnerable to specific attacks.
469- The use of different start variables values prevents the same
470  plaintext enciphering to the same ciphertext, by producing different
471  key streams.
472- Selection of a small value for j will require more cycles through
473  the encipherment algorithm per unit of plaintext and thus cause
474  greater processing overheads.
475- Only multiples of j bits can be enciphered.
476- OFB mode of operation does not extend ciphertext errors in the
477  resultant plaintext output.  Every bit error in the ciphertext causes
478  only one bit to be in error in the deciphered plaintext.
479- OFB mode is not self-synchronising.  If the two operation of
480  encipherment and decipherment get out of synchronism, the system needs
481  to be re-initialised.
482- Each re-initialisation should use a value of the start variable
483 different from the start variable values used before with the same
484 key.  The reason for this is that an identical bit stream would be
485 produced each time from the same parameters.  This would be
486 susceptible to a ' known plaintext' attack.
487
488Triple ECB Mode (des_ecb3_encrypt())
489- Encrypt with key1, decrypt with key2 and encrypt with key3 again.
490- As for ECB encryption but increases the key length to 168 bits.
491  There are theoretic attacks that can be used that make the effective
492  key length 112 bits, but this attack also requires 2^56 blocks of
493  memory, not very likely, even for the NSA.
494- If both keys are the same it is equivalent to encrypting once with
495  just one key.
496- If the first and last key are the same, the key length is 112 bits.
497  There are attacks that could reduce the key space to 55 bit's but it
498  requires 2^56 blocks of memory.
499- If all 3 keys are the same, this is effectively the same as normal
500  ecb mode.
501
502Triple CBC Mode (des_ede3_cbc_encrypt())
503- Encrypt with key1, decrypt with key2 and then encrypt with key3.
504- As for CBC encryption but increases the key length to 168 bits with
505  the same restrictions as for triple ecb mode.
506