NAME
seccure - SECCURE Elliptic Curve Crypto Utility for Reliable Encryption
SYNOPSIS
seccure-key [-c curve] [-F pwfile] [-d] [-v] [-q]
seccure-encrypt [-m maclen] [-c curve] [-i infile] [-o outfile] [-v]
[-q] key
seccure-decrypt [-m maclen] [-c curve] [-i infile] [-o outfile] [-F
pwfile] [-d] [-v] [-q]
seccure-sign [-f] [-b] [-a] [-c curve] [-s sigfile] [-i infile] [-o
outfile] [-F pwfile] [-d] [-v] [-q]
seccure-verify [-f] [-b] [-a] [-c curve] [-s sigfile] [-i infile] [-o
outfile] [-v] [-q] key [sig]
seccure-signcrypt [-c sig_curve [-c enc_curve]] [-i infile] [-o
outfile] [-F pwfile] [-d] [-v] [-q] key
seccure-veridec [-c enc_curve [-c sig_curve]] [-i infile] [-o outfile]
[-F pwfile] [-d] [-v] [-q] key
seccure-dh [-c curve] [-v] [-q]
DESCRIPTION
The seccure toolset implements a selection of asymmetric algorithms
based on elliptic curve cryptography (ECC). In particular it offers
public key encryption / decryption, signature generation / verification
and key establishment.
ECC schemes offer a much better key size to security ratio than
classical systems (RSA, DSA). Keys are short enough to make direct
specification of keys on the command line possible (sometimes this is
more convenient than the management of PGP-like key rings). seccure
builds on this feature and therefore is the tool of choice whenever
lightweight asymmetric cryptography -- independent of key servers,
revocation certificates, the Web of Trust or even configuration files
-- is required.
COMMANDS
seccure-key: Prompt for a passphrase and calculate the corresponding
public key.
seccure-encrypt: Encrypt a message with public key key.
seccure-decrypt: Prompt for a passphrase and decrypt a seccure-
encrypted message.
seccure-sign: Prompt for a passphrase and digitally sign a message.
seccure-verify: Verify signature sig with public key key.
seccure-signcrypt: Sign a message first, encrypt it subsequently (in -b
-a and -m 0 mode, respectively). This is basically a shortcut for two
separate seccure invocations.
seccure-veridec: Counterpart to signcryption.
seccure-dh: Perform a Diffie-Hellman key exchange.
OPTIONS
-c curve
Use elliptic curve curve. Available are: secp112r1, secp128r1,
secp160r1, secp192r1/nistp192, secp224r1/nistp224,
secp256r1/nistp256, secp384r1/nistp384 and secp521r1/nistp521.
The curve name may be abbreviated by any non-ambiguous substring
(for instance it is suggested to specify p224 for the
secp224r1/nistp224 curve). The default curve is p160, which
provides reasonable security for everyday use. (See also HOW TO
CHOOSE THE CURVE.)
Note: If a public key is given on the command line seccure can
determine the corresponding curve on its own. It’s then
unnecessary to specify the curve explicitly.
-F pwfile
Don’t prompt for a passphrase; instead, take the first text line
of pwfile.
-m maclen
Set the MAC length to maclen bits. Only multiples of 8 in the
range from 0 to 256 are allowed. The default MAC length is 80
bits, which provides a reasonable level of integrity protection
for everyday use.
-i infile
Read from infile instead of STDIN.
-o outfile
Write to outfile instead of STDOUT.
-s sigfile
For seccure-sign: Write signature to sigfile instead of STDERR.
For seccure-verify: Read signature from sigfile instead of using
sig.
-f Filter mode: Copy all data read from STDIN verbatim to STDOUT
(eventually attaching or detaching a signature in -a mode).
-b Binary mode: Read/write signatures as binary strings. This leads
to very compact signatures.
-a Append mode:
For seccure-sign: Append signature to the end of the document.
Enforces -f mode.
For seccure-verify: Detach signature from the end of the
document.
-d Double prompt mode: When reading a passphrase from the console:
prompt twice and assure the phrases are the same.
-v Verbose mode: Print some extra information.
-q Quiet mode: Disable all unnecessary output.
EXAMPLE
Given the passphrase ’seccure is secure’, run
seccure-key
to determine the corresponding public key (which is ’2@DupCaCKykHBe-
QHpAP%d%B[’ on curve p160).
To encrypt the file ’document.msg’ with that key run
seccure-encrypt -i document.msg -o document.enc ’2@DupCaCKykHBe-
QHpAP%d%B[’
The message can be recovered with
seccure-decrypt -i document.enc
To sign the file run
seccure-sign -i document.msg -s document.sig
and enter the passphrase. The signature is stored in ’document.sig’ and
can be verified with
seccure-verify -i document.msg -s document.sig ’2@DupCaCKykHBe-
QHpAP%d%B[’
KEY ESTABLISHMENT
seccure-dh performs an interactive Diffie-Hellman key exchange. Two
instances have to be run in parallel; the token generated by the first
one is the input for the second one and vice versa. The output consists
of two shared keys: it is guaranteed that no attacker can ever find out
the established key as soon as the two parties can confirm that both
have the same verification key. The authentic comparision of the
verification keys can, for example, be realized via signed messages or
via telephone (using ’voice authentication’).
HOW TO CHOOSE THE CURVE
The number in the names of the curves measures their security level.
Rule of thumb: the workload to ’break’ a k-bit curve is 2^(k/2)
approximately (example: it takes about 2^112 steps to break secp224r1).
If the 80 bit security of the default curve doesn’t seem sufficient,
choosing a stronger curve (p192 and upwards) may, of course, be
considered. But the suggestion remains: p160 offers reasonable security
for everyday use. Warning: the curves p112 and p128 do not satisfy
demands for long-time security.
ALGORITHMS
seccure uses derivated versions of ECIES (Elliptic Curve Integrated
Encryption Scheme), ECDSA (Elliptic Curve Digital Signature Algorithm)
and ECDH (Elliptic Curve Diffie-Hellman) as encryption, signature and
key establishment scheme, respectively. For the symmetric parts (bulk
encryption, hashing, key derivation, HMAC calculation) seccure builds
on AES256 (in CTR mode), SHA256 and SHA512. To my best knowledge no
part of seccure is covered by patents. See the file PATENTS for an
explicit patent statement.
AUTHOR
This software (v0.3) was written by B. Poettering (seccure AT point-at-
infinity.org) in 2006. It is released under the terms of the GNU
General Public License (GPL). Find the latest version of seccure on the
project’s homepage: http://point-at-infinity.org/seccure/.