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Section: Maintenance Commands (8)
Updated: 24 Jan 2000
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ipsec rsasigkey - generate RSA signature key  


ipsec rsasigkey [ --verbose ] [ --random filename ] [ --rounds nr ] nbits  


Rsasigkey generates an RSA public/private key pair, suitable for digital 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. The value of nbits must be a multiple of 8.

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 purpose.

The --verbose option makes rsasigkey give a running commentary on standard error. By default, it works in silence until it is ready to generate output.

The --random option specifies a source for random bits. The default is /dev/random (see random(4)).

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 output format looks like this (with long numbers trimmed down for clarity):


        # for signatures only, UNSAFE FOR ENCRYPTION
        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

The commented-out pubkey= line contains the public key---the public exponent and the modulus---combined in approximately RFC 2537 format. (The one deviation is that the combined value is given in hex with a 0x prefix, rather than in unadorned base-64.)

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 verification.

No attempt is made to break long lines.  


ipsec --verbose rsasigkey 1024 >mykey
generates a 1024-bit signature key and puts it in the file mykey, with running commentary on standard error. The file contents 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.conf(5)).




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.xs4all.nl/~freeswan/> by Henry Spencer.  


There is an internal limit on nbits, currently 4096.

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 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 200MHz Pentium with plenty of randomness available is 20 seconds, almost all of it in the prime searches.

This program makes no use of code furnished by RSA Data Security Inc. or Public Key Partners. However, the techniques used would appear to be covered (in the USA) by the US patent on the RSA algorithm, which expires 20 Sept 2000.





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