22 \h'-\w'\\$1\ 'u'\\$1\ \c
27 .TH catcrypt 1 "30 September 2004" "Straylight/Edgeware" "Catacomb cryptographic library"
29 catcrypt \- encrypt and decrypt messages
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.
88 Before the command name,
90 may be given. The following global options are supported:
92 .BR "\-h, \-\-help " [ \fIcommand ...]
93 Writes a brief summary of
95 various options to standard output, and returns a successful exit
96 status. With command names, gives help on those commands.
99 Writes the program's version number to standard output, and returns a
100 successful exit status.
103 Writes a very terse command line summary to standard output, and returns
104 a successful exit status.
106 .BI "\-k, \-\-keyring " file
107 Names the keyring file which
109 is to process. The default keyring, used if this option doesn't specify
110 one, is the file named
112 in the current directory. See
116 for more details about keyring files.
118 Algorithms to be used with a particular key are described by attributes
119 on the key, or its type. The
121 command deals with both signing and key-encapsulation keys. (Note that
123 uses signing keys in the same way as
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.)
140 attribute is present on the key, then it must have this form; otherwise,
141 the key's type must have the form
144 Algorithm selections are taken from appropriately-named attributes, or,
145 failing that, from the
148 The key-encapsulation mechanism is chosen according to the setting of
152 for a list of supported KEMs.
155 This is Shoup's RSA-KEM (formerly Simple RSA); see
157 A proposal for an ISO standard for public key encryption (version 2.0)
159 .BR http://eprint.iacr.org/2000/060/ .
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).
175 command, preferably with the
177 options, to generate the key.
180 This is the elliptic-curve analogue of
186 command to generate the key.
188 As well as the KEM itself, a number of supporting algorithms are used.
189 These are taken from appropriately named attributes on the key or,
190 failing that, derived from other attributes as described below.
193 This is the symmetric encryption algorithm used for bulk data
194 encryption. If there is no
200 is used; if that it absent, then the default of
203 .B catcrypt show cipher
204 for a list of supported symmetric encryption algorithms.
207 This is the hash function used to distil entropy from the shared secret
208 constructed by the raw KEM. If there is no
214 is used; if that is absent then the default of
217 .B catcrypt show hash
218 for a list of supported symmetric encryption algorithms.
221 This is the message authentication algorithm used during bulk data
222 encryption to ensure integrity of the encrypted message and defend
223 against chosen-ciphertext attacks. If there is no
227 is chosen as a default. Run
229 for a list of supported message authentication algorithms.
232 This is the key derivation function used to stretch the hashed shared
233 secret to a sufficient length to select symmetric encryption and
234 authentication keys, initialization vectors and other necessary
235 pseudorandom quantities. If there is no
239 is chosen as a default. Run
241 for a list of supported key derivation functions.
243 Not all supported functions have the required security features: don't
244 override the default choice unless you know what you're doing.
254 attribute is present on the key, then it must have this form; otherwise,
255 the key's type must have the form
258 Algorithm selections are taken from appropriately-named attributes, or,
259 failing that, from the
262 The signature algorithm is chosen according to the setting of
266 for a list of supported signature algorithms.
269 This is almost the same as the RSASSA-PKCS1-v1_5 algorithm described in
270 RFC3447; the difference is that the hash is left bare rather than being
271 wrapped in a DER-encoded
273 structure. This doesn't affect security since the key can only be used
274 with the one hash function anyway, and dropping the DER wrapping permits
275 rapid adoption of new hash functions. Regardless, use of this algorithm
276 is not recommended, since the padding method has been shown vulnerable
286 This is the RSASSA-PSS algorithm described in RFC3447. It is the
287 preferred RSA-based signature scheme. Use the
296 This is the DSA algorithm described in FIPS180-1 and FIPS180-2. Use the
305 This is the ECDSA algorithm described in ANSI X9.62 and FIPS180-2. Use
315 This is the revised KCDSA (Korean Certificate-based Digital Signature
316 Algorithm) described in
317 .I The Revised Version of KCDSA
318 .RB ( http://dasan.sejong.ac.kr/~chlim/pub/kcdsa1.ps ).
330 This is an unofficial elliptic-curve analogue of the KCDSA algorithm.
339 As well as the signature algorithm itself, a hash function is used.
340 This is taken from the
342 attribute on the key, or, failing that, from the
346 or, if that is absent, determined by the signature algorithm as follows.
354 the default hash function is
361 the default hash function is
365 .B catcrypt show hash
366 for a list of supported hash functions.
368 Two encodings for the ciphertext are supported.
371 The raw format, which has the benefit of being smaller, but needs to be
372 attached to mail messages and generally handled with care.
375 PEM-encapsulated Base-64 encoded text. This format can be included
376 directly in email and picked out again automatically; but there is a
377 4-to-3 data expansion as a result.
378 .SH "COMMAND REFERENCE"
382 command behaves exactly as the
384 option. With no arguments, it shows an overview of
386 options; with arguments, it describes the named subcommands.
390 command prints various lists of tokens understood by
392 With no arguments, it prints all of the lists; with arguments, it prints
393 just the named lists, in order. The recognized lists can be enumerated
398 command. The lists are as follows.
401 The lists which can be enumerated by the
406 The key-encapsulation algorithms which can be used in a
407 key-encapsulation key's
412 The symmetric encryption algorithms which can be used in a
413 key-encapsulation key's
418 The message authentication algorithms which can be used in a
419 key-encapsulation key's
424 The signature algorithms which can be used in a signing key's
429 The hash functions which can be used in a key's
434 The encodings which can be applied to encrypted messages; see
440 command encrypts a file and writes out the appropriately-encoded
441 ciphertext. By default, it reads from standard input and writes to
442 standard output. If a filename argument is given, this file is read
443 instead (as binary data).
445 The following options are recognized.
448 Produce ASCII-armoured output. This is equivalent to specifying
454 .BI "\-f, \-\-format " format
455 Produce output encoded according to
458 .BI "\-k, \-\-key " tag
459 Use the key-encapsulation key named
461 in the current keyring; the default key is
464 .BI "\-s, \-\-sign-key " tag
465 Use the signature key named
467 in the current keyring; the default is not to sign the ciphertext.
469 .BI "\-o, \-\-ouptut " file
472 rather than to standard output.
476 command decrypts a ciphertext and writes out the plaintext. By default,
477 it reads from standard input and writes to standard output. If a
478 filename argument is given, this file is read instead.
480 The following options are recognized.
483 Read ASCII-armoured input. This is equivalent to specifying
490 Buffer plaintext data until we're sure we've got it all. This is forced
491 on if output is to stdout, but is always available as an option.
493 .BI "\-f, \-\-format " format
494 Read input encoded according to
497 .B "\-v, \-\-verbose"
498 Produce more verbose messages. See below for the messages produced
499 during decryption. The default verbosity level is 1. (Currently this
500 is the most verbose setting. This might not be the case always.)
503 Produce fewer messages.
505 .BI "\-o, \-\-output " file
508 instead of to standard output. The file is written in binary mode.
509 Fixing line-end conventions is your problem; there are lots of good
510 tools for dealing with it.
512 Output is written to standard output in a machine-readable format.
513 Major problems cause the program to write a diagnostic to standard error
514 and exit nonzero as usual. The quantity of output varies depending on
515 the verbosity level and whether the plaintext is also being written to
516 standard output. Output lines begin with a keyword:
519 An error prevented decryption. The program will exit nonzero.
523 encountered a situation which may or may not invalidate the decryption.
526 Decryption was successful. This is only produced if main output is
527 being sent somewhere other than standard output.
530 The plaintext follows, starting just after the next newline character or
531 sequence. This is only produced if main output is also being sent to
535 Any other information.
537 The information written at the various verbosity levels is as follows.
539 No output. Watch the exit status.
544 All output written has been checked for authenticity. However, output
545 can fail madway through for many reasons, and the resulting message may
546 therefore be truncated. Don't rely on the output being complete until
554 command encodes an input file according to one of the encodings
557 The input is read from the
559 given on the command line, or from standard input if none is specified.
560 Options provided are:
562 .BI "\-f, \-\-format " format
567 for a list of encoding formats.
569 .BI "\-b, \-\-boundary " label
570 Set the PEM boundary string to
572 i.e., assuming we're encoding in PEM format, the output will have
573 .BI "\-\-\-\-\-BEGIN " label "\-\-\-\-\-"
575 .BI "\-\-\-\-\-END " label "\-\-\-\-\-"
576 at the bottom. The default
581 .BI "\-o, \-\-output " file
584 instead of to standard output.
588 command decodes an input file encoded according to one of the encodings
591 The input is read from the
593 given on the command line, or from standard input if none is specified.
594 Options provided are:
596 .BI "\-f, \-\-format " format
601 for a list of encoding formats.
603 .BI "\-b, \-\-boundary " label
604 Set the PEM boundary string to
606 i.e., assuming we're encoding in PEM format, start processing input
608 .BI "\-\-\-\-\-BEGIN " label "\-\-\-\-\-"
610 .BI "\-\-\-\-\-END " label "\-\-\-\-\-"
611 lines. Without this option,
613 will start reading at the first plausible boundary string, and continue
614 processing until it reaches the matching end boundary.
616 .BI "\-o, \-\-output " file
619 instead of to standard output.
620 .SH "SECURITY PROPERTIES"
621 Assuming the security of the underlying primitive algorithms, the
622 following security properties of the ciphertext hold.
624 An adversary given the public key-encapsulation key and capable of
625 requesting encryption of arbitrary plaintexts of his own devising is
626 unable to decide whether he is given ciphertexts corresponding to his
627 chosen plaintexts or random plaintexts of the same length. This holds
628 even if the adversary is permitted to request decryption of any
629 ciphertext other than one produced as a result of an encryption request.
630 This property is called
633 An adversary given the public key-encapsulation and verification keys,
634 and capable of requesting encryption of arbitrary plaintext of his own
635 devising is unable to produce a new ciphertext which will be accepted as
636 genuine. This property is called
639 An adversary given the public key-encapsulation and verification keys,
640 and capable of requesting encryption of arbitrary plaintext of his own
641 devising is unable to decide whether the ciphertexts he is given are
642 correctly signed. This property doesn't seem to have a name.
644 Not all is rosy. If you leak intermediate values during decryption then
645 an adversary can construct a new correctly-signed message. Don't do
646 that, then \(en leaking intermediate values often voids security
647 warranties. But it does avoid the usual problem with separate signing
648 and encryption that a careful leak by the recipient can produce evidence
649 that you signed some incriminating message.
655 provide `non-repudiation' in any useful way. This is deliberate: the
656 purpose of signing is to convince the recipient of the sender's
657 identity, rather than to allow the recipient to persuade anyone else.
658 Indeed, given an encrypted and signed message, the recipient can
659 straightforwardly construct a new message, apparently from the same
660 sender, and whose signature still verifies, but with arbitrarily chosen
662 .SH "CRYPTOGRAPHIC THEORY"
663 Encryption of a message proceeds as follows.
665 Emit a header packet containing the key-ids for the key-encapsulation
666 key, and signature key if any.
668 Use the KEM to produce a public value and a shared secret the recipient
669 will be able to extract from the public value using his private key.
670 Emit a packet containing the public value.
672 Hash the shared secret. Use the KDF to produce a pseudorandom keystream
673 of indefinite length.
675 Use the first bits of the keystream to key a symmetric encryption
676 scheme; use the next bits to key a message authentication code.
678 If we're signing the message then extract 1024 bytes from the keystream,
679 sign them, and emit a packet containing the signature. The signature
680 packet doesn't contain the signed message, just the signature.
682 Split the message into blocks. For each block, pick a random IV from
683 the keystream, encrypt the block and emit a packet containing the
684 IV, ciphertext, and a MAC tag over the ciphertext and a sequence number.
686 The last chunk is the encryption of an empty plaintext block. No
687 previous plaintext block is empty. This lets us determine the
688 difference between a complete file and one that's been maliciously
691 That's it. Nothing terribly controversial, really.
699 Mark Wooding, <mdw@nsict.org>
projects / u / mdw / catacomb / blob