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
86 command encrypts and decrypts messages. It also works as a simple PEM
87 encoder and decoder. It provides a number of subcommands, by which the
88 various operations may be carried out.
90 Before the command name,
92 may be given. The following global options are supported:
94 .BR "\-h, \-\-help " [ \fIcommand ...]
95 Writes a brief summary of
97 various options to standard output, and returns a successful exit
98 status. With command names, gives help on those commands.
100 .B "\-v, \-\-version"
101 Writes the program's version number to standard output, and returns a
102 successful exit status.
105 Writes a very terse command line summary to standard output, and returns
106 a successful exit status.
108 .BI "\-k, \-\-keyring " file
109 Names the keyring file which
111 is to process. The default keyring, used if this option doesn't specify
112 one, is the file named
114 in the current directory. See
118 for more details about keyring files.
120 Algorithms to be used with a particular key are described by attributes
121 on the key, or its type. The
123 command deals with both signing and key-encapsulation keys. (Note that
125 uses signing keys in the same way as
127 .SS "Key-encapsulation keys"
128 (Key encapsulation is a means of transmitting a short, known, random
129 secret to a recipient. It differs from encryption in technical ways
130 which are largely uninteresting at this point.)
150 attribute is present on the key, then it must have this form; otherwise,
151 the key's type must have the form
154 Algorithm selections are taken from appropriately-named attributes, or,
155 failing that, from the
158 The key-encapsulation mechanism is chosen according to the setting of
162 for a list of supported KEMs.
165 This is Shoup's RSA-KEM (formerly Simple RSA); see
167 A proposal for an ISO standard for public key encryption (version 2.0)
169 .BR http://eprint.iacr.org/2000/060/ .
179 This is standard Diffie-Hellman key exchange, hashing the resulting
180 shared secret to form the key, as used in, e.g., DLIES (P1363a).
185 command, preferably with the
187 options, to generate the key.
190 This is the elliptic-curve analogue of
196 command to generate the key.
199 This is a simple symmetric encapsulation scheme. It works by hashing a
200 binary key with a randomly-generated salt. Use the
209 This is Bernstein's Curve25519, a fast Diffie-Hellman using a specific
220 The bulk crypto transform is chosen based on the
222 attribute on the key, or, failing that,
228 .B catcrypt show bulk
229 for a list of supported bulk crypto transforms.
232 A generic composition of
233 a cipher secure against chosen-plaintext attack,
234 and a message authentication code.
240 This is the default transform.
243 Use Salsa20 or ChaCha and Poly1305 to secure the bulk data.
244 This is nearly the same as the NaCl
249 uses Salsa20 or ChaCha rather than XSalsa20,
250 because it doesn't need the latter's extended nonce.
253 attribute may be set to one of
264 As well as the KEM itself, a number of supporting algorithms are used.
265 These are taken from appropriately named attributes on the key or,
266 failing that, derived from other attributes as described below.
269 This is the symmetric encryption algorithm
270 used by the bulk data transform.
277 is used; if that it absent, then the default of
280 .B catcrypt show cipher
281 for a list of supported symmetric encryption algorithms.
284 This is the hash function used to distil entropy from the shared secret
285 constructed by the raw KEM. If there is no
291 is used; if that is absent then the default of
294 .B catcrypt show hash
295 for a list of supported symmetric encryption algorithms.
298 This is the message authentication algorithm
302 to ensure integrity of the encrypted message and
303 defend against chosen-ciphertext attacks.
308 is chosen as a default. Run
310 for a list of supported message authentication algorithms.
313 This is the key derivation function used to stretch the hashed shared
314 secret to a sufficient length to select symmetric encryption and
315 authentication keys, initialization vectors and other necessary
316 pseudorandom quantities. If there is no
320 is chosen as a default. Run
322 for a list of supported key derivation functions.
324 Not all supported functions have the required security features: don't
325 override the default choice unless you know what you're doing.
335 attribute is present on the key, then it must have this form; otherwise,
336 the key's type must have the form
339 Algorithm selections are taken from appropriately-named attributes, or,
340 failing that, from the
343 The signature algorithm is chosen according to the setting of
347 for a list of supported signature algorithms.
350 This is almost the same as the RSASSA-PKCS1-v1_5 algorithm described in
351 RFC3447; the difference is that the hash is left bare rather than being
352 wrapped in a DER-encoded
354 structure. This doesn't affect security since the key can only be used
355 with the one hash function anyway, and dropping the DER wrapping permits
356 rapid adoption of new hash functions. Regardless, use of this algorithm
357 is not recommended, since the padding method has been shown vulnerable
367 This is the RSASSA-PSS algorithm described in RFC3447. It is the
368 preferred RSA-based signature scheme. Use the
377 This is the DSA algorithm described in FIPS180-1 and FIPS180-2. Use the
386 This is the ECDSA algorithm described in ANSI X9.62 and FIPS180-2. Use
396 This is the revised KCDSA (Korean Certificate-based Digital Signature
397 Algorithm) described in
398 .I The Revised Version of KCDSA
399 .RB ( http://dasan.sejong.ac.kr/~chlim/pub/kcdsa1.ps ).
411 This is an unofficial elliptic-curve analogue of the KCDSA algorithm.
421 This uses a symmetric message-authentication algorithm rather than a
422 digital signature. The precise message-authentication scheme used is
425 attribute on the key, which defaults to
427 if unspecified. Use the
435 As well as the signature algorithm itself, a hash function is used.
436 This is taken from the
438 attribute on the key, or, failing that, from the
442 or, if that is absent, determined by the signature algorithm as follows.
450 the default hash function is
457 the default hash function is
461 .B catcrypt show hash
462 for a list of supported hash functions.
464 Two encodings for the ciphertext are supported.
467 The raw format, which has the benefit of being smaller, but needs to be
468 attached to mail messages and generally handled with care.
471 PEM-encapsulated Base-64 encoded text. This format can be included
472 directly in email and picked out again automatically; but there is a
473 4-to-3 data expansion as a result.
474 .SH "COMMAND REFERENCE"
478 command behaves exactly as the
480 option. With no arguments, it shows an overview of
482 options; with arguments, it describes the named subcommands.
486 command prints various lists of tokens understood by
488 With no arguments, it prints all of the lists; with arguments, it prints
489 just the named lists, in order. The recognized lists can be enumerated
494 command. The lists are as follows.
497 The lists which can be enumerated by the
502 The key-encapsulation algorithms which can be used in a
503 key-encapsulation key's
508 The symmetric encryption algorithms which can be used in a
509 key-encapsulation key's
514 The message authentication algorithms which can be used in a
515 key-encapsulation key's
520 The signature algorithms which can be used in a signing key's
525 The hash functions which can be used in a key's
530 The encodings which can be applied to encrypted messages; see
536 command encrypts a file and writes out the appropriately-encoded
537 ciphertext. By default, it reads from standard input and writes to
538 standard output. If a filename argument is given, this file is read
539 instead (as binary data).
541 The following options are recognized.
544 Produce ASCII-armoured output. This is equivalent to specifying
550 .BI "\-f, \-\-format " format
551 Produce output encoded according to
554 .BI "\-k, \-\-key " tag
555 Use the key-encapsulation key named
557 in the current keyring; the default key is
560 .BI "\-p, \-\-progress"
561 Write a progress meter to standard error while processing large files.
563 .BI "\-s, \-\-sign-key " tag
564 Use the signature key named
566 in the current keyring; the default is not to sign the ciphertext.
568 .BI "\-o, \-\-ouptut " file
571 rather than to standard output.
573 .B "\-C, \-\-nocheck"
574 Don't check the public key for validity. This makes encryption go much
575 faster, but at the risk of using a duff key.
579 command decrypts a ciphertext and writes out the plaintext. By default,
580 it reads from standard input and writes to standard output. If a
581 filename argument is given, this file is read instead.
583 The following options are recognized.
586 Read ASCII-armoured input. This is equivalent to specifying
593 Buffer plaintext data until we're sure we've got it all. This is forced
594 on if output is to stdout, but is always available as an option.
596 .BI "\-f, \-\-format " format
597 Read input encoded according to
600 .BI "\-p, \-\-progress"
601 Write a progress meter to standard error while processing large files.
603 .B "\-v, \-\-verbose"
604 Produce more verbose messages. See below for the messages produced
605 during decryption. The default verbosity level is 1. (Currently this
606 is the most verbose setting. This might not be the case always.)
609 Produce fewer messages.
611 .BI "\-o, \-\-output " file
614 instead of to standard output. The file is written in binary mode.
615 Fixing line-end conventions is your problem; there are lots of good
616 tools for dealing with it.
618 .B "\-C, \-\-nocheck"
619 Don't check the private key for validity. This makes decryption go much
620 faster, but at the risk of using a duff key, and possibly leaking
621 information about the private key.
623 Output is written to standard output in a machine-readable format.
624 Major problems cause the program to write a diagnostic to standard error
625 and exit nonzero as usual. The quantity of output varies depending on
626 the verbosity level and whether the plaintext is also being written to
627 standard output. Output lines begin with a keyword:
630 An error prevented decryption. The program will exit nonzero.
634 encountered a situation which may or may not invalidate the decryption.
637 Decryption was successful. This is only produced if main output is
638 being sent somewhere other than standard output.
641 The plaintext follows, starting just after the next newline character or
642 sequence. This is only produced if main output is also being sent to
646 Any other information.
648 The information written at the various verbosity levels is as follows.
650 No output. Watch the exit status.
655 All output written has been checked for authenticity. However, output
656 can fail midway through for many reasons, and the resulting message may
657 therefore be truncated. Don't rely on the output being complete until
665 command encodes an input file according to one of the encodings
668 The input is read from the
670 given on the command line, or from standard input if none is specified.
671 Options provided are:
673 .BI "\-p, \-\-progress"
674 Write a progress meter to standard error while processing large files.
676 .BI "\-f, \-\-format " format
681 for a list of encoding formats.
683 .BI "\-b, \-\-boundary " label
684 Set the PEM boundary string to
686 i.e., assuming we're encoding in PEM format, the output will have
687 .BI "\-\-\-\-\-BEGIN " label "\-\-\-\-\-"
689 .BI "\-\-\-\-\-END " label "\-\-\-\-\-"
690 at the bottom. The default
695 .BI "\-o, \-\-output " file
698 instead of to standard output.
702 command decodes an input file encoded according to one of the encodings
705 The input is read from the
707 given on the command line, or from standard input if none is specified.
708 Options provided are:
710 .BI "\-f, \-\-format " format
715 for a list of encoding formats.
717 .BI "\-b, \-\-boundary " label
718 Set the PEM boundary string to
720 i.e., assuming we're encoding in PEM format, start processing input
722 .BI "\-\-\-\-\-BEGIN " label "\-\-\-\-\-"
724 .BI "\-\-\-\-\-END " label "\-\-\-\-\-"
725 lines. Without this option,
727 will start reading at the first plausible boundary string, and continue
728 processing until it reaches the matching end boundary.
730 .BI "\-p, \-\-progress"
731 Write a progress meter to standard error while processing large files.
733 .BI "\-o, \-\-output " file
736 instead of to standard output.
737 .SH "SECURITY PROPERTIES"
738 Assuming the security of the underlying primitive algorithms, the
739 following security properties of the ciphertext hold.
741 An adversary given the public key-encapsulation key and capable of
742 requesting encryption of arbitrary plaintexts of his own devising is
743 unable to decide whether he is given ciphertexts corresponding to his
744 chosen plaintexts or random plaintexts of the same length. This holds
745 even if the adversary is permitted to request decryption of any
746 ciphertext other than one produced as a result of an encryption request.
747 This property is called
750 An adversary given the public key-encapsulation and verification keys,
751 and capable of requesting encryption of arbitrary plaintext of his own
752 devising is unable to produce a new ciphertext which will be accepted as
753 genuine. This property is called
756 An adversary given the public key-encapsulation and verification keys,
757 and capable of requesting encryption of arbitrary plaintext of his own
758 devising is unable to decide whether the ciphertexts he is given are
759 correctly signed. This property doesn't seem to have a name.
761 Not all is rosy. If you leak intermediate values during decryption then
762 an adversary can construct a new correctly-signed message. Don't do
763 that, then \(en leaking intermediate values often voids security
764 warranties. But it does avoid the usual problem with separate signing
765 and encryption that a careful leak by the recipient can produce evidence
766 that you signed some incriminating message.
772 provide `non-repudiation' in any useful way. This is deliberate: the
773 purpose of signing is to convince the recipient of the sender's
774 identity, rather than to allow the recipient to persuade anyone else.
775 Indeed, given an encrypted and signed message, the recipient can
776 straightforwardly construct a new message, apparently from the same
777 sender, and whose signature still verifies, but with arbitrarily chosen
779 .SH "CRYPTOGRAPHIC THEORY"
780 Encryption of a message proceeds as follows.
782 Emit a header packet containing the key-ids for the key-encapsulation
783 key, and signature key if any.
785 Use the KEM to produce a public value and a shared secret the recipient
786 will be able to extract from the public value using his private key.
787 Emit a packet containing the public value.
789 Hash the shared secret. Use the KDF to produce a pseudorandom keystream
790 of indefinite length.
792 Use the first bits of the keystream to key a symmetric encryption
793 scheme; use the next bits to key a message authentication code.
795 If we're signing the message then extract 1024 bytes from the keystream,
796 sign the header and public value, and the keystream bytes; emit a packet
797 containing the signature. The signature packet doesn't contain the
798 signed message, just the signature.
800 Split the message into blocks. For each block, pick a random IV from
801 the keystream, encrypt the block and emit a packet containing the
802 IV, ciphertext, and a MAC tag over the ciphertext and a sequence number.
804 The last chunk is the encryption of an empty plaintext block. No
805 previous plaintext block is empty. This lets us determine the
806 difference between a complete file and one that's been maliciously
809 That's it. Nothing terribly controversial, really.
817 Mark Wooding, <mdw@distorted.org.uk>
~mdw / catacomb / blob