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c65df279 | 1 | .\" -*-nroff-*- |
2 | .de VS | |
3 | .sp 1 | |
4 | .RS | |
5 | .nf | |
6 | .ft B | |
7 | .. | |
8 | .de VE | |
9 | .ft R | |
10 | .fi | |
11 | .RE | |
12 | .sp 1 | |
13 | .. | |
14 | .ie t \{\ | |
15 | . if \n(.g \{\ | |
16 | . fam P | |
17 | . \} | |
18 | .\} | |
19 | .de hP | |
20 | .IP | |
21 | .ft B | |
22 | \h'-\w'\\$1\ 'u'\\$1\ \c | |
23 | .ft P | |
24 | .. | |
25 | .ie t .ds o \(bu | |
26 | .el .ds o o | |
27 | .TH catcrypt 1 "30 September 2004" "Straylight/Edgeware" "Catacomb cryptographic library" | |
28 | .SH NAME | |
29 | catcrypt \- encrypt and decrypt messages | |
30 | .SH SYNOPSIS | |
31 | .B catcrypt | |
32 | .RB [ \-k | |
33 | .IR keyring ] | |
34 | .I command | |
35 | .PP | |
36 | where | |
37 | .I command | |
38 | is one of: | |
39 | .PP | |
40 | .B help | |
41 | .RI [ command ...] | |
42 | .br | |
43 | .B show | |
44 | .RI [ item ...] | |
45 | .br | |
46 | .B encrypt | |
946c3f72 | 47 | .RB [ \-aC ] |
c65df279 | 48 | .RB [ \-k |
49 | .IR tag ] | |
50 | .RB [ \-f | |
51 | .IR format ] | |
52 | .RB [ \-o | |
53 | .IR output ] | |
54 | .RI [ file ] | |
55 | .br | |
56 | .B decrypt | |
946c3f72 | 57 | .RB [ \-aqvC ] |
c65df279 | 58 | .RB [ \-f |
59 | .IR format ] | |
60 | .RB [ \-o | |
61 | .IR output ] | |
62 | .RI [ file ] | |
63 | .br | |
64 | .B encode | |
65 | .RB [ \-f | |
66 | .IR format ] | |
67 | .RB [ \-b | |
68 | .IR boundary ] | |
69 | .RB [ \-o | |
70 | .IR output ] | |
71 | .RI [ file ] | |
72 | .br | |
fa54fe1e | 73 | .B decode |
c65df279 | 74 | .RB [ \-f |
75 | .IR format ] | |
76 | .RB [ \-b | |
77 | .IR boundary ] | |
78 | .RB [ \-o | |
79 | .IR output ] | |
80 | .RI [ file ] | |
81 | .SH "DESCRIPTION" | |
82 | The | |
83 | .B catcrypt | |
84 | command encrypts and decrypts messages. It also works as a simple PEM | |
85 | encoder and decoder. It provides a number of subcommands, by which the | |
86 | various operations may be carried out. | |
87 | .SS "Global options" | |
88 | Before the command name, | |
89 | .I "global options" | |
90 | may be given. The following global options are supported: | |
91 | .TP | |
92 | .BR "\-h, \-\-help " [ \fIcommand ...] | |
93 | Writes a brief summary of | |
94 | .BR catcrypt 's | |
95 | various options to standard output, and returns a successful exit | |
96 | status. With command names, gives help on those commands. | |
97 | .TP | |
98 | .B "\-v, \-\-version" | |
99 | Writes the program's version number to standard output, and returns a | |
100 | successful exit status. | |
101 | .TP | |
102 | .B "\-u, \-\-usage" | |
103 | Writes a very terse command line summary to standard output, and returns | |
104 | a successful exit status. | |
105 | .TP | |
106 | .BI "\-k, \-\-keyring " file | |
107 | Names the keyring file which | |
108 | .B key | |
109 | is to process. The default keyring, used if this option doesn't specify | |
110 | one, is the file named | |
111 | .B keyring | |
112 | in the current directory. See | |
113 | .BR key (1) | |
114 | and | |
115 | .BR keyring (5) | |
116 | for more details about keyring files. | |
117 | .SH "KEY SETUP" | |
118 | Algorithms to be used with a particular key are described by attributes | |
119 | on the key, or its type. The | |
120 | .B catcrypt | |
fa54fe1e | 121 | command deals with both signing and key-encapsulation keys. (Note that |
122 | .B catcrypt | |
45c0fd36 | 123 | uses signing keys in the same way as |
fa54fe1e | 124 | .BR catsign (1).) |
c65df279 | 125 | .SS "Key-encapsulation keys" |
126 | (Key encapsulation is a means of transmitting a short, known, random | |
127 | secret to a recipient. It differs from encryption in technical ways | |
128 | which are largely uninteresting at this point.) | |
129 | .PP | |
130 | A | |
131 | .I kemalgspec | |
132 | has the syntax | |
133 | .IR kem \c | |
134 | .RB [ / \c | |
135 | .IR cipher \c | |
136 | .RB [ / \c | |
137 | .IR hash ]]. | |
138 | If a | |
139 | .B kem | |
140 | attribute is present on the key, then it must have this form; otherwise, | |
141 | the key's type must have the form | |
142 | .BR cckem- \c | |
143 | .IR kemalgspec . | |
144 | Algorithm selections are taken from appropriately-named attributes, or, | |
145 | failing that, from the | |
146 | .IR kemalgspec . | |
147 | .PP | |
148 | The key-encapsulation mechanism is chosen according to the setting of | |
149 | .I kem | |
150 | as follows. Run | |
151 | .B catcrypt show kem | |
152 | for a list of supported KEMs. | |
153 | .TP | |
154 | .B rsa | |
155 | This is Shoup's RSA-KEM (formerly Simple RSA); see | |
156 | .I | |
157 | A proposal for an ISO standard for public key encryption (version 2.0) | |
158 | available at | |
159 | .BR http://eprint.iacr.org/2000/060/ . | |
160 | Use the | |
161 | .B rsa | |
162 | algorithm of the | |
163 | .B key add | |
164 | command (see | |
165 | .BR key (1)) | |
166 | to generate the key. | |
167 | .TP | |
168 | .B dh | |
169 | This is standard Diffie-Hellman key exchange, hashing the resulting | |
170 | shared secret to form the key, as used in, e.g., DLIES (P1363a). | |
171 | Use the | |
172 | .B dh | |
173 | algorithm of the | |
174 | .B key add | |
175 | command, preferably with the | |
176 | .B \-LS | |
177 | options, to generate the key. | |
178 | .TP | |
179 | .B ec | |
180 | This is the elliptic-curve analogue of | |
b98200ad | 181 | .BR dh . |
182 | Use the | |
c65df279 | 183 | .B ec |
184 | algorithm of the | |
185 | .BR key (1)) | |
186 | command to generate the key. | |
02dfbd5b MW |
187 | .TP |
188 | .B symm | |
189 | This is a simple symmetric encapsulation scheme. It works by hashing a | |
190 | binary key with a randomly-generated salt. Use the | |
191 | .B binary | |
192 | algorithm of the | |
193 | .B key add | |
194 | command (see | |
195 | .BR key (1)) | |
196 | to generate the key. | |
c65df279 | 197 | .PP |
198 | As well as the KEM itself, a number of supporting algorithms are used. | |
199 | These are taken from appropriately named attributes on the key or, | |
200 | failing that, derived from other attributes as described below. | |
201 | .TP | |
202 | .B cipher | |
203 | This is the symmetric encryption algorithm used for bulk data | |
204 | encryption. If there is no | |
205 | .B cipher | |
206 | attribute then the | |
207 | .I cipher | |
208 | in the | |
209 | .I kemalgspec | |
210 | is used; if that it absent, then the default of | |
211 | .B blowfish-cbc | |
212 | is used. Run | |
213 | .B catcrypt show cipher | |
214 | for a list of supported symmetric encryption algorithms. | |
215 | .TP | |
216 | .B hash | |
217 | This is the hash function used to distil entropy from the shared secret | |
218 | constructed by the raw KEM. If there is no | |
219 | .B hash | |
220 | attribute then the | |
221 | .I hash | |
222 | in the | |
b98200ad | 223 | .I kemalgspec |
224 | is used; if that is absent then the default of | |
c65df279 | 225 | .B rmd160 |
226 | is used. Run | |
227 | .B catcrypt show hash | |
228 | for a list of supported symmetric encryption algorithms. | |
229 | .TP | |
230 | .B mac | |
231 | This is the message authentication algorithm used during bulk data | |
232 | encryption to ensure integrity of the encrypted message and defend | |
233 | against chosen-ciphertext attacks. If there is no | |
234 | .B mac | |
235 | attribute then | |
236 | .IB hash -hmac | |
237 | is chosen as a default. Run | |
238 | .B catcrypt show mac | |
239 | for a list of supported message authentication algorithms. | |
240 | .TP | |
241 | .B kdf | |
242 | This is the key derivation function used to stretch the hashed shared | |
243 | secret to a sufficient length to select symmetric encryption and | |
244 | authentication keys, initialization vectors and other necessary | |
245 | pseudorandom quantities. If there is no | |
246 | .B kdf | |
247 | attribute then | |
248 | .IB hash -mgf | |
249 | is chosen as a default. Run | |
250 | .B catcrypt show kdf | |
251 | for a list of supported key derivation functions. | |
252 | .B Caution! | |
253 | Not all supported functions have the required security features: don't | |
254 | override the default choice unless you know what you're doing. | |
255 | .SS "Signing keys" | |
256 | A | |
257 | .I sigalgspec | |
258 | has the form | |
259 | .IR sig \c | |
260 | .RB [ / \c | |
261 | .IR hash ]. | |
262 | If a | |
263 | .B sig | |
264 | attribute is present on the key, then it must have this form; otherwise, | |
265 | the key's type must have the form | |
266 | .BI ccsig- \c | |
267 | .IR sigalgspec . | |
268 | Algorithm selections are taken from appropriately-named attributes, or, | |
269 | failing that, from the | |
270 | .IR sigalgspec . | |
271 | .PP | |
272 | The signature algorithm is chosen according to the setting of | |
273 | .I sig | |
274 | as follows. Run | |
275 | .B catcrypt show sig | |
276 | for a list of supported signature algorithms. | |
277 | .TP | |
278 | .B rsapkcs1 | |
279 | This is almost the same as the RSASSA-PKCS1-v1_5 algorithm described in | |
280 | RFC3447; the difference is that the hash is left bare rather than being | |
45c0fd36 | 281 | wrapped in a DER-encoded |
c65df279 | 282 | .B DigestInfo |
283 | structure. This doesn't affect security since the key can only be used | |
284 | with the one hash function anyway, and dropping the DER wrapping permits | |
285 | rapid adoption of new hash functions. Regardless, use of this algorithm | |
286 | is not recommended, since the padding method has been shown vulnerable | |
287 | to attack. Use the | |
288 | .B rsa | |
289 | algorithm of the | |
290 | .B key add | |
291 | command (see | |
292 | .BR key (1)) | |
293 | to generate the key. | |
294 | .TP | |
295 | .B rsapss | |
296 | This is the RSASSA-PSS algorithm described in RFC3447. It is the | |
297 | preferred RSA-based signature scheme. Use the | |
298 | .B rsa | |
299 | algorithm of the | |
300 | .B key add | |
301 | command (see | |
302 | .BR key (1)) | |
303 | to generate the key. | |
304 | .TP | |
305 | .B dsa | |
45c0fd36 | 306 | This is the DSA algorithm described in FIPS180-1 and FIPS180-2. Use the |
c65df279 | 307 | .B dsa |
308 | algorithm of the | |
309 | .B key add | |
310 | command (see | |
311 | .BR key (1)) | |
312 | to generate the key. | |
313 | .TP | |
314 | .B ecdsa | |
315 | This is the ECDSA algorithm described in ANSI X9.62 and FIPS180-2. Use | |
316 | the | |
317 | .B ec | |
318 | algorithm of the | |
319 | .B key add | |
320 | command (see | |
321 | .BR key (1)) | |
322 | to generate the key. | |
323 | .TP | |
324 | .B kcdsa | |
325 | This is the revised KCDSA (Korean Certificate-based Digital Signature | |
326 | Algorithm) described in | |
327 | .I The Revised Version of KCDSA | |
328 | .RB ( http://dasan.sejong.ac.kr/~chlim/pub/kcdsa1.ps ). | |
329 | Use the | |
330 | .B dh | |
331 | algorithm of the | |
332 | .B key add | |
333 | command with the | |
334 | .B \-LS | |
335 | options (see | |
336 | .BR key (1)) | |
337 | to generate the key. | |
338 | .TP | |
339 | .B eckcdsa | |
340 | This is an unofficial elliptic-curve analogue of the KCDSA algorithm. | |
341 | Use the | |
342 | .B ec | |
343 | algorithm of the | |
344 | .B key add | |
345 | command (see | |
346 | .BR key (1)) | |
347 | to generate the key. | |
02dfbd5b MW |
348 | .TP |
349 | .B mac | |
350 | This uses a symmetric message-authentication algorithm rather than a | |
351 | digital signature. The precise message-authentication scheme used is | |
352 | determined by the | |
353 | .B mac | |
354 | attribute on the key, which defaults to | |
355 | .IB hash -hmac | |
356 | if unspecified. Use the | |
357 | .B binary | |
358 | algorithm of the | |
359 | .B key add | |
360 | command (see | |
361 | .BR key (1)) | |
362 | to generate the key. | |
c65df279 | 363 | .PP |
364 | As well as the signature algorithm itself, a hash function is used. | |
365 | This is taken from the | |
366 | .B hash | |
367 | attribute on the key, or, failing that, from the | |
368 | .I hash | |
369 | specified in the | |
370 | .IR sigalgspec , | |
371 | or, if that is absent, determined by the signature algorithm as follows. | |
372 | .hP \*o | |
373 | For | |
374 | .BR rsapkcs1 , | |
375 | .BR rsapss , | |
376 | .BR dsa , | |
377 | and | |
378 | .BR ecdsa , | |
379 | the default hash function is | |
380 | .BR sha . | |
381 | .hP \*o | |
382 | For | |
45c0fd36 | 383 | .BR kcdsa |
c65df279 | 384 | and |
385 | .BR eckcdsa , | |
386 | the default hash function is | |
387 | .BR has160 . | |
388 | .PP | |
389 | Run | |
390 | .B catcrypt show hash | |
391 | for a list of supported hash functions. | |
392 | .SH "ENCODINGS" | |
393 | Two encodings for the ciphertext are supported. | |
394 | .TP | |
395 | .B binary | |
396 | The raw format, which has the benefit of being smaller, but needs to be | |
397 | attached to mail messages and generally handled with care. | |
398 | .TP | |
399 | .B pem | |
400 | PEM-encapsulated Base-64 encoded text. This format can be included | |
401 | directly in email and picked out again automatically; but there is a | |
402 | 4-to-3 data expansion as a result. | |
403 | .SH "COMMAND REFERENCE" | |
404 | .SS help | |
405 | The | |
406 | .B help | |
407 | command behaves exactly as the | |
408 | .B \-\-help | |
409 | option. With no arguments, it shows an overview of | |
410 | .BR catcrypt 's | |
411 | options; with arguments, it describes the named subcommands. | |
412 | .SS show | |
413 | The | |
414 | .B show | |
415 | command prints various lists of tokens understood by | |
416 | .BR catcrypt . | |
417 | With no arguments, it prints all of the lists; with arguments, it prints | |
418 | just the named lists, in order. The recognized lists can be enumerated | |
419 | using the | |
420 | .VS | |
421 | catcrypt show list | |
422 | .VE | |
423 | command. The lists are as follows. | |
424 | .TP | |
425 | .B list | |
426 | The lists which can be enumerated by the | |
427 | .B show | |
428 | command. | |
429 | .TP | |
430 | .B kem | |
431 | The key-encapsulation algorithms which can be used in a | |
432 | key-encapsulation key's | |
433 | .B kem | |
434 | attribute. | |
435 | .TP | |
436 | .B cipher | |
437 | The symmetric encryption algorithms which can be used in a | |
438 | key-encapsulation key's | |
439 | .B cipher | |
440 | attribute. | |
441 | .TP | |
442 | .B mac | |
443 | The message authentication algorithms which can be used in a | |
444 | key-encapsulation key's | |
445 | .B mac | |
446 | attribute. | |
447 | .TP | |
448 | .B sig | |
449 | The signature algorithms which can be used in a signing key's | |
450 | .B sig | |
451 | attribute. | |
452 | .TP | |
453 | .B hash | |
454 | The hash functions which can be used in a key's | |
455 | .B hash | |
456 | attribute. | |
457 | .TP | |
458 | .B enc | |
45c0fd36 | 459 | The encodings which can be applied to encrypted messages; see |
c65df279 | 460 | .B ENCODINGS |
461 | above. | |
462 | .SS encrypt | |
463 | The | |
464 | .B encrypt | |
465 | command encrypts a file and writes out the appropriately-encoded | |
466 | ciphertext. By default, it reads from standard input and writes to | |
467 | standard output. If a filename argument is given, this file is read | |
468 | instead (as binary data). | |
469 | .PP | |
470 | The following options are recognized. | |
471 | .TP | |
472 | .B "\-a, \-\-armour" | |
473 | Produce ASCII-armoured output. This is equivalent to specifying | |
474 | .BR "\-f pem" . | |
475 | The variant spelling | |
476 | .B "\-\-armor" | |
477 | is also accepted. | |
478 | .TP | |
479 | .BI "\-f, \-\-format " format | |
480 | Produce output encoded according to | |
481 | .IR format . | |
482 | .TP | |
483 | .BI "\-k, \-\-key " tag | |
484 | Use the key-encapsulation key named | |
485 | .I tag | |
486 | in the current keyring; the default key is | |
487 | .BR ccrypt . | |
488 | .TP | |
489 | .BI "\-s, \-\-sign-key " tag | |
490 | Use the signature key named | |
491 | .I tag | |
492 | in the current keyring; the default is not to sign the ciphertext. | |
493 | .TP | |
494 | .BI "\-o, \-\-ouptut " file | |
495 | Write output to | |
496 | .I file | |
497 | rather than to standard output. | |
946c3f72 | 498 | .TP |
499 | .B "\-C, \-\-nocheck" | |
500 | Don't check the public key for validity. This makes encryption go much | |
501 | faster, but at the risk of using a duff key. | |
c65df279 | 502 | .SS decrypt |
503 | The | |
504 | .B decrypt | |
505 | command decrypts a ciphertext and writes out the plaintext. By default, | |
506 | it reads from standard input and writes to standard output. If a | |
507 | filename argument is given, this file is read instead. | |
508 | .PP | |
509 | The following options are recognized. | |
510 | .TP | |
511 | .B "\-a, \-\-armour" | |
fa54fe1e | 512 | Read ASCII-armoured input. This is equivalent to specifying |
c65df279 | 513 | .BR "\-f pem" . |
514 | The variant spelling | |
515 | .B "\-\-armor" | |
516 | is also accepted. | |
517 | .TP | |
fa54fe1e | 518 | .B "\-b, \-\-buffer" |
519 | Buffer plaintext data until we're sure we've got it all. This is forced | |
520 | on if output is to stdout, but is always available as an option. | |
521 | .TP | |
c65df279 | 522 | .BI "\-f, \-\-format " format |
523 | Read input encoded according to | |
524 | .IR format . | |
525 | .TP | |
526 | .B "\-v, \-\-verbose" | |
527 | Produce more verbose messages. See below for the messages produced | |
528 | during decryption. The default verbosity level is 1. (Currently this | |
529 | is the most verbose setting. This might not be the case always.) | |
530 | .TP | |
531 | .B "\-q, \-\-quiet" | |
532 | Produce fewer messages. | |
533 | .TP | |
534 | .BI "\-o, \-\-output " file | |
535 | Write output to | |
536 | .I file | |
537 | instead of to standard output. The file is written in binary mode. | |
538 | Fixing line-end conventions is your problem; there are lots of good | |
539 | tools for dealing with it. | |
946c3f72 | 540 | .TP |
541 | .B "\-C, \-\-nocheck" | |
542 | Don't check the private key for validity. This makes decryption go much | |
543 | faster, but at the risk of using a duff key, and possibly leaking | |
544 | information about the private key. | |
c65df279 | 545 | .PP |
546 | Output is written to standard output in a machine-readable format. | |
547 | Major problems cause the program to write a diagnostic to standard error | |
548 | and exit nonzero as usual. The quantity of output varies depending on | |
549 | the verbosity level and whether the plaintext is also being written to | |
fa54fe1e | 550 | standard output. Output lines begin with a keyword: |
c65df279 | 551 | .TP |
552 | .BI "FAIL " reason | |
553 | An error prevented decryption. The program will exit nonzero. | |
554 | .TP | |
555 | .BI "WARN " reason | |
556 | .B catcrypt | |
557 | encountered a situation which may or may not invalidate the decryption. | |
45c0fd36 | 558 | .TP |
c65df279 | 559 | .BI "OK " message |
560 | Decryption was successful. This is only produced if main output is | |
561 | being sent somewhere other than standard output. | |
562 | .TP | |
563 | .B "DATA" | |
564 | The plaintext follows, starting just after the next newline character or | |
b98200ad | 565 | sequence. This is only produced if main output is also being sent to |
fa54fe1e | 566 | standard output. |
c65df279 | 567 | .TP |
568 | .BI "INFO " note | |
569 | Any other information. | |
570 | .PP | |
571 | The information written at the various verbosity levels is as follows. | |
572 | .hP 0. | |
573 | No output. Watch the exit status. | |
574 | .hP 1. | |
575 | All messages. | |
576 | .PP | |
577 | .B Warning! | |
fa54fe1e | 578 | All output written has been checked for authenticity. However, output |
579 | can fail madway through for many reasons, and the resulting message may | |
45c0fd36 | 580 | therefore be truncated. Don't rely on the output being complete until |
4224d0b9 | 581 | .B OK |
582 | is printed or | |
c65df279 | 583 | .B catcrypt decrypt |
fa54fe1e | 584 | exits successfully. |
c65df279 | 585 | .SS "encode" |
586 | The | |
587 | .B encode | |
588 | command encodes an input file according to one of the encodings | |
589 | described above in | |
590 | .BR ENCODINGS . | |
45c0fd36 | 591 | The input is read from the |
c65df279 | 592 | .I file |
593 | given on the command line, or from standard input if none is specified. | |
594 | Options provided are: | |
595 | .TP | |
596 | .BI "\-f, \-\-format " format | |
597 | Produce output in | |
598 | .IR format . | |
599 | Run | |
600 | .B catcrypt show enc | |
601 | for a list of encoding formats. | |
602 | .TP | |
603 | .BI "\-b, \-\-boundary " label | |
604 | Set the PEM boundary string to | |
605 | .IR label ; | |
606 | i.e., assuming we're encoding in PEM format, the output will have | |
607 | .BI "\-\-\-\-\-BEGIN " label "\-\-\-\-\-" | |
608 | at the top and | |
609 | .BI "\-\-\-\-\-END " label "\-\-\-\-\-" | |
610 | at the bottom. The default | |
611 | .I label | |
612 | is | |
613 | .BR MESSAGE . | |
614 | .TP | |
615 | .BI "\-o, \-\-output " file | |
616 | Write output to | |
617 | .I file | |
618 | instead of to standard output. | |
619 | .SS "decode" | |
620 | The | |
621 | .B decode | |
622 | command decodes an input file encoded according to one of the encodings | |
623 | described above in | |
624 | .BR ENCODINGS . | |
45c0fd36 | 625 | The input is read from the |
c65df279 | 626 | .I file |
627 | given on the command line, or from standard input if none is specified. | |
628 | Options provided are: | |
629 | .TP | |
630 | .BI "\-f, \-\-format " format | |
631 | Decode input in | |
632 | .IR format . | |
633 | Run | |
634 | .B catcrypt show enc | |
635 | for a list of encoding formats. | |
636 | .TP | |
637 | .BI "\-b, \-\-boundary " label | |
638 | Set the PEM boundary string to | |
639 | .IR label ; | |
640 | i.e., assuming we're encoding in PEM format, start processing input | |
641 | between | |
642 | .BI "\-\-\-\-\-BEGIN " label "\-\-\-\-\-" | |
45c0fd36 | 643 | and |
c65df279 | 644 | .BI "\-\-\-\-\-END " label "\-\-\-\-\-" |
645 | lines. Without this option, | |
646 | .B catcrypt | |
647 | will start reading at the first plausible boundary string, and continue | |
648 | processing until it reaches the matching end boundary. | |
649 | .TP | |
650 | .BI "\-o, \-\-output " file | |
651 | Write output to | |
652 | .I file | |
653 | instead of to standard output. | |
654 | .SH "SECURITY PROPERTIES" | |
655 | Assuming the security of the underlying primitive algorithms, the | |
656 | following security properties of the ciphertext hold. | |
657 | .hP \*o | |
658 | An adversary given the public key-encapsulation key and capable of | |
659 | requesting encryption of arbitrary plaintexts of his own devising is | |
660 | unable to decide whether he is given ciphertexts corresponding to his | |
661 | chosen plaintexts or random plaintexts of the same length. This holds | |
662 | even if the adversary is permitted to request decryption of any | |
663 | ciphertext other than one produced as a result of an encryption request. | |
664 | This property is called | |
665 | .BR IND-CCA2 . | |
666 | .hP \*o | |
667 | An adversary given the public key-encapsulation and verification keys, | |
668 | and capable of requesting encryption of arbitrary plaintext of his own | |
669 | devising is unable to produce a new ciphertext which will be accepted as | |
670 | genuine. This property is called | |
671 | .BR INT-CTXT . | |
672 | .hP \*o | |
673 | An adversary given the public key-encapsulation and verification keys, | |
674 | and capable of requesting encryption of arbitrary plaintext of his own | |
675 | devising is unable to decide whether the ciphertexts he is given are | |
676 | correctly signed. This property doesn't seem to have a name. | |
677 | .PP | |
678 | Not all is rosy. If you leak intermediate values during decryption then | |
679 | an adversary can construct a new correctly-signed message. Don't do | |
680 | that, then \(en leaking intermediate values often voids security | |
681 | warranties. But it does avoid the usual problem with separate signing | |
682 | and encryption that a careful leak by the recipient can produce evidence | |
683 | that you signed some incriminating message. | |
4224d0b9 | 684 | .PP |
685 | Note that | |
686 | .BR catcrypt 's | |
687 | signatures do | |
688 | .I not | |
689 | provide `non-repudiation' in any useful way. This is deliberate: the | |
690 | purpose of signing is to convince the recipient of the sender's | |
691 | identity, rather than to allow the recipient to persuade anyone else. | |
692 | Indeed, given an encrypted and signed message, the recipient can | |
693 | straightforwardly construct a new message, apparently from the same | |
694 | sender, and whose signature still verifies, but with arbitrarily chosen | |
695 | content. | |
c65df279 | 696 | .SH "CRYPTOGRAPHIC THEORY" |
697 | Encryption of a message proceeds as follows. | |
698 | .hP 0. | |
699 | Emit a header packet containing the key-ids for the key-encapsulation | |
700 | key, and signature key if any. | |
701 | .hP 1. | |
702 | Use the KEM to produce a public value and a shared secret the recipient | |
703 | will be able to extract from the public value using his private key. | |
704 | Emit a packet containing the public value. | |
705 | .hP 2. | |
706 | Hash the shared secret. Use the KDF to produce a pseudorandom keystream | |
707 | of indefinite length. | |
708 | .hP 3. | |
709 | Use the first bits of the keystream to key a symmetric encryption | |
710 | scheme; use the next bits to key a message authentication code. | |
711 | .hP 4. | |
712 | If we're signing the message then extract 1024 bytes from the keystream, | |
f9e51332 | 713 | sign the header and public value, and the keystream bytes; emit a packet |
714 | containing the signature. The signature packet doesn't contain the | |
715 | signed message, just the signature. | |
c65df279 | 716 | .hP 5. |
717 | Split the message into blocks. For each block, pick a random IV from | |
718 | the keystream, encrypt the block and emit a packet containing the | |
aaa2361e | 719 | IV, ciphertext, and a MAC tag over the ciphertext and a sequence number. |
720 | .hP 6. | |
721 | The last chunk is the encryption of an empty plaintext block. No | |
722 | previous plaintext block is empty. This lets us determine the | |
723 | difference between a complete file and one that's been maliciously | |
724 | truncated. | |
c65df279 | 725 | .PP |
726 | That's it. Nothing terribly controversial, really. | |
727 | .SH "SEE ALSO" | |
728 | .BR key (1), | |
fa54fe1e | 729 | .BR catsign (1), |
c65df279 | 730 | .BR dsig (1), |
731 | .BR hashsum (1), | |
732 | .BR keyring (5). | |
733 | .SH AUTHOR | |
f387fcb1 | 734 | Mark Wooding, <mdw@distorted.org.uk> |