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NAME | DESCRIPTION | SEE ALSO | STANDARDS | COLOPHON |
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MODULI(5) File Formats Manual MODULI(5)
moduli — Diffie-Hellman moduli
The /etc/moduli file contains prime numbers and generators for
use by sshd(8) in the Diffie-Hellman Group Exchange key exchange
method.
New moduli may be generated with ssh-keygen(1) using a two-step
process. An initial candidate generation pass, using ssh-keygen
-M generate, calculates numbers that are likely to be useful. A
second primality testing pass, using ssh-keygen -M screen,
provides a high degree of assurance that the numbers are prime
and are safe for use in Diffie-Hellman operations by sshd(8).
This moduli format is used as the output from each pass.
The file consists of newline-separated records, one per modulus,
containing seven space-separated fields. These fields are as
follows:
timestamp The time that the modulus was last processed
as YYYYMMDDHHMMSS.
type Decimal number specifying the internal
structure of the prime modulus. Supported
types are:
0 Unknown, not tested.
2 "Safe" prime; (p-1)/2 is also prime.
4 Sophie Germain; 2p+1 is also prime.
Moduli candidates initially produced by
ssh-keygen(1) are Sophie Germain primes (type
4). Further primality testing with
ssh-keygen(1) produces safe prime moduli (type
2) that are ready for use in sshd(8). Other
types are not used by OpenSSH.
tests Decimal number indicating the type of
primality tests that the number has been
subjected to represented as a bitmask of the
following values:
0x00 Not tested.
0x01 Composite number – not prime.
0x02 Sieve of Eratosthenes.
0x04 Probabilistic Miller-Rabin primality
tests.
The ssh-keygen(1) moduli candidate generation
uses the Sieve of Eratosthenes (flag 0x02).
Subsequent ssh-keygen(1) primality tests are
Miller-Rabin tests (flag 0x04).
trials Decimal number indicating the number of
primality trials that have been performed on
the modulus.
size Decimal number indicating the size of the
prime in bits.
generator The recommended generator for use with this
modulus (hexadecimal).
modulus The modulus itself in hexadecimal.
When performing Diffie-Hellman Group Exchange, sshd(8) first
estimates the size of the modulus required to produce enough
Diffie-Hellman output to sufficiently key the selected symmetric
cipher. sshd(8) then randomly selects a modulus from /etc/moduli
that best meets the size requirement.
ssh-keygen(1), sshd(8)
M. Friedl, N. Provos, and W. Simpson, Diffie-Hellman Group
Exchange for the Secure Shell (SSH) Transport Layer Protocol, RFC
4419, March 2006.
This page is part of the openssh (Portable OpenSSH) project.
Information about the project can be found at
http://www.openssh.com/portable.html. If you have a bug report
for this manual page, see ⟨http://www.openssh.com/report.html⟩.
This page was obtained from the tarball openssh-9.7p1.tar.gz
fetched from
⟨http://ftp.eu.openbsd.org/pub/OpenBSD/OpenSSH/portable/⟩ on
2024-06-14. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
man-pages@man7.org
GNU April 16, 2022 MODULI(5)
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