.TH IPSEC_RSASIGKEY 8 "22 July 2001"
.\" RCSID $Id: rsasigkey.8,v 1.1 2004/03/15 20:35:30 as Exp $
.SH NAME
ipsec rsasigkey \- generate RSA signature key
.SH SYNOPSIS
.B ipsec
.B rsasigkey
[
.B \-\-verbose
] [
.B \-\-random
filename
]
.B \e
.br
\ \ \ [
.B \-\-rounds
nr
] [
.B \-\-hostname
host ] [
.B \-\-noopt
] nbits
.br
.B ipsec
.B rsasigkey
[
.B \-\-verbose
] [
.B \-\-hostname
host ]
.B \e
.br
\ \ \ 
[
.B \-\-noopt
]
.B \-\-oldkey
file
.SH DESCRIPTION
.I Rsasigkey
generates an RSA public/private key pair,
suitable for digital signatures,
of (exactly)
.I nbits
bits (that is, two primes each of exactly
.IR nbits /2
bits,
and related numbers)
and emits it on standard output as ASCII (mostly hex) data.
.I nbits
must be a multiple of 16.
.PP
The public exponent is forced to the value
.BR 3 ,
which has important speed advantages for signature checking.
Beware that the resulting keys have known weaknesses as encryption keys
\fIand should not be used for that purpose\fR.
.PP
The
.B \-\-verbose
option makes
.I rsasigkey
give a running commentary on standard error.
By default, it works in silence until it is ready to generate output.
.PP
The
.B \-\-random
option specifies a source for random bits.
The default is
.I /dev/random
(see
.IR random (4)).
Normally,
.I rsasigkey
reads exactly
.I nbits
random bits from the source;
in extremely-rare circumstances it may need more.
.PP
The
.B \-\-rounds
option specifies the number of rounds to be done by the
.I mpz_probab_prime_p
probabilistic primality checker.
The default, 30, is fairly rigorous and should not normally
have to be overridden.
.PP
The
.B \-\-hostname
option specifies what host name to use in
the first line of the output (see below);
the default is what
.IR gethostname (2)
returns.
.PP
The
.B \-\-noopt
option suppresses an optimization of the private key
(to be precise, setting of the decryption exponent to
.B lcm(p\-1,q\-1)
rather than
.BR (p\-1)*(q\-1) )
which speeds up operations on it slightly
but can cause it to flunk a validity check in old RSA implementations
(notably, obsolete versions of
.IR ipsec_pluto (8)).
.PP
The
.B \-\-oldkey
option specifies that rather than generate a new key,
.I rsasigkey
should read an old key from the
.I file
(the name
.B \-
means ``standard input'')
and use that to generate its output.
Input lines which do not look like
.I rsasigkey
output are silently ignored.
This permits updating old keys to the current format.
.PP
The output format looks like this (with long numbers trimmed down
for clarity):
.PP
.ne 15
.nf
	# RSA 2048 bits   xy.example.com   Sat Apr 15 13:53:22 2000
	# for signatures only, UNSAFE FOR ENCRYPTION
	#pubkey=0sAQOF8tZ2NZt...Y1P+buFuFn/
	Modulus: 0xcc2a86fcf440...cf1011abb82d1
	PublicExponent: 0x03
	# everything after this point is secret
	PrivateExponent: 0x881c59fdf8...ab05c8c77d23
	Prime1: 0xf49fd1f779...46504c7bf3
	Prime2: 0xd5a9108453...321d43cb2b
	Exponent1: 0xa31536a4fb...536d98adda7f7
	Exponent2: 0x8e70b5ad8d...9142168d7dcc7
	Coefficient: 0xafb761d001...0c13e98d98
.fi
.PP
The first (comment) line,
indicating the nature and date of the key,
and giving a host name,
is used by
.IR ipsec_showhostkey (8)
when generating some forms of key output.
.PP
The commented-out
.B pubkey=
line contains the public key\(emthe public exponent and the modulus\(emcombined
in approximately RFC 2537 format
(the one deviation is that the combined value is given with a
.B 0s
prefix, rather than in unadorned base-64),
suitable for use in the
.I ipsec.conf
file.
.PP
The
.BR Modulus ,
.BR PublicExponent ,
and
.B PrivateExponent
lines give the basic signing and verification data.
.PP
The
.B Prime1
and
.B Prime2
lines give the primes themselves (aka
.I p
and
.IR q ),
largest first.
The
.B Exponent1
and
.B Exponent2
lines give
the private exponent mod
.IR p\-1
and
.IR q\-1
respectively.
The
.B Coefficient
line gives the Chinese Remainder Theorem coefficient,
which is the inverse of
.IR q ,
mod
.IR p .
These additional numbers (which must all be kept as secret as the
private exponent) are precomputed aids to rapid signature generation.
.PP
No attempt is made to break long lines.
.PP
The US patent on the RSA algorithm expired 20 Sept 2000.
.SH EXAMPLES
.TP
.B "ipsec rsasigkey \-\-verbose 2192 >mykey"
generates a 2192-bit signature key and puts it in the file
.IR mykey ,
with running commentary on standard error.
The file contents can be inserted verbatim into a suitable entry in the
.I ipsec.secrets
file (see
.IR ipsec.secrets (5)),
and the public key can then be extracted and edited into the
.I ipsec.conf
file (see
.IR ipsec.conf (5)).
.TP
.B "ipsec rsasigkey \-\-verbose \-\-oldkey oldie >latest"
takes the old signature key from file
.I oldie
and puts a version in the current format into the file
.IR latest ,
with running commentary on standard error.
.SH FILES
/dev/random
.SH SEE ALSO
random(4), ipsec_showhostkey(8)
.br
\fIApplied Cryptography\fR, 2nd. ed., by Bruce Schneier, Wiley 1996.
.br
RFCs 2537, 2313.
.br
\fIGNU MP, the GNU multiple precision arithmetic library, edition 2.0.2\fR,
by Torbj Granlund.
.SH HISTORY
Written for the Linux FreeS/WAN project
<http://www.freeswan.org>
by Henry Spencer.
.SH BUGS
There is an internal limit on
.IR nbits ,
currently 20000.
.PP
.IR Rsasigkey 's
run time is difficult to predict,
since
.I /dev/random
output can be arbitrarily delayed if
the system's entropy pool is low on randomness,
and the time taken by the search for primes is also somewhat unpredictable.
A reasonably typical time for a 1024-bit key on a quiet 200MHz Pentium MMX
with plenty of randomness available is 20 seconds,
almost all of it in the prime searches.
Generating a 2192-bit key on the same system usually takes several minutes.
A 4096-bit key took an hour and a half of CPU time.
.PP
The
.B \-\-oldkey
option does not check its input format as rigorously as it might.
Corrupted
.I rsasigkey
output may confuse it.