Provided by: openswan_2.4.6+dfsg.2-1.1build2_i386
ipsec rsasigkey - generate RSA signature key
ipsec rsasigkey [ --verbose ] [ --random filename ] \
[ --rounds nr ] [ --hostname host ] [ --noopt ] nbits
ipsec rsasigkey [ --verbose ] [ --hostname host ] \
[ --noopt ] --oldkey file
Rsasigkey generates an RSA public/private key pair, suitable for digi‐
tal signatures, of (exactly) nbits bits (that is, two primes each of
exactly nbits/2 bits, and related numbers) and emits it on standard
output as ASCII (mostly hex) data. nbits must be a multiple of 16.
The public exponent is forced to the value 3, which has important speed
advantages for signature checking. Beware that the resulting keys have
known weaknesses as encryption keys and should not be used for that
The --verbose option makes rsasigkey give a running commentary on stan‐
dard error. By default, it works in silence until it is ready to gen‐
The --random option specifies a source for random bits. The default is
/dev/random (see random(4)). Normally, rsasigkey reads exactly nbits
random bits from the source; in extremely-rare circumstances it may
The --rounds option specifies the number of rounds to be done by the
mpz_probab_prime_p probabilistic primality checker. The default, 30,
is fairly rigorous and should not normally have to be overridden.
The --hostname option specifies what host name to use in the first line
of the output (see below); the default is what gethostname(2) returns.
The --noopt option suppresses an optimization of the private key (to be
precise, setting of the decryption exponent to lcm(p-1,q-1) rather than
(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 ipsec_pluto(8)).
The --oldkey option specifies that rather than generate a new key,
rsasigkey should read an old key from the file (the name - means
‘‘standard input’’) and use that to generate its output. Input lines
which do not look like rsasigkey output are silently ignored. This
permits updating old keys to the current format.
The output format looks like this (with long numbers trimmed down for
# RSA 2048 bits xy.example.com Sat Apr 15 13:53:22 2000
# for signatures only, UNSAFE FOR ENCRYPTION
# everything after this point is secret
The first (comment) line, indicating the nature and date of the key,
and giving a host name, is used by ipsec_showhostkey(8) when generating
some forms of key output.
The commented-out pubkey= line contains the public key—the public expo‐
nent and the modulus—combined in approximately RFC 2537 format (the one
deviation is that the combined value is given with a 0s prefix, rather
than in unadorned base-64), suitable for use in the ipsec.conf file.
The Modulus, PublicExponent, and PrivateExponent lines give the basic
signing and verification data.
The Prime1 and Prime2 lines give the primes themselves (aka p and q),
largest first. The Exponent1 and Exponent2 lines give the private
exponent mod p-1 and q-1 respectively. The Coefficient line gives the
Chinese Remainder Theorem coefficient, which is the inverse of q, mod
p. These additional numbers (which must all be kept as secret as the
private exponent) are precomputed aids to rapid signature generation.
No attempt is made to break long lines.
The US patent on the RSA algorithm expired 20 Sept 2000.
ipsec rsasigkey --verbose 2192 >mykey
generates a 2192-bit signature key and puts it in the file
mykey, with running commentary on standard error. The file con‐
tents can be inserted verbatim into a suitable entry in the
ipsec.secrets file (see ipsec.secrets(5)), and the public key
can then be extracted and edited into the ipsec.conf file (see
ipsec rsasigkey --verbose --oldkey oldie >latest
takes the old signature key from file oldie and puts a version
in the current format into the file latest, with running commen‐
tary on standard error.
Applied Cryptography, 2nd. ed., by Bruce Schneier, Wiley 1996.
RFCs 2537, 2313.
GNU MP, the GNU multiple precision arithmetic library, edition 2.0.2,
by Torbj Granlund.
Written for the Linux FreeS/WAN project <http://www.freeswan.org> by
There is an internal limit on nbits, currently 20000.
Rsasigkey’s run time is difficult to predict, since /dev/random output
can be arbitrarily delayed if the system’s entropy pool is low on ran‐
domness, 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.
The --oldkey option does not check its input format as rigorously as it
might. Corrupted rsasigkey output may confuse it.
22 July 2001 IPSEC_RSASIGKEY(8)