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MODULI(5)			     BSD File Formats Manual				MODULI(5)

NAME
     moduli -- Diffie Hellman moduli

DESCRIPTION
     The /etc/ssh/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 -G, calculates numbers that are likely to be
     useful.  A second primality testing pass, using ssh-keygen -T 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 sep-
     arated 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; (p+1)*2 is also prime.

			Moduli candidates initially produced by ssh-keygen(1) are Sophie Germain
			primes (type 4).  Futher 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  Probabalistic Miller-Rabin primality tests.

			The ssh-keygen(1) moduli candidate generation uses the Sieve of Eratos-
			thenes (flag 0x02).  Subsequent ssh-keygen(1) primality tests are Miller-
			Rabin tests (flag 0x04).

	   trials	Decimal number indicating of primaility 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 modu-
     lus required to produce enough Diffie Hellman output to sufficiently key the selected sym-
     metric cipher.  sshd(8) then randomly selects a modulus from /etc/ssh/moduli that best meets
     the size requirement.

SEE ALSO
     ssh-keygen(1), sshd(8),

     Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol, RFC 4419,
     2006.

BSD					February 17, 2018				      BSD
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