Inform the user about the minimum bit-lengths of the variables about srp6-variables HOT 2 CLOSED

This restriction applies to private random ephemeral values.

Simply create a new SRP-6 Variable that produces such values.

from srp6-variables.

This document - The Secure Remote Password - has more details on constrains regarding a and b:

a, b > log_g n The computation of g^a and g^b in GF(n) must "wrap around" to prevent an attack from taking the algebraic logarithm of g^a to recover a. The probability of this happening is infinitesimal (less than 2^-1014 for 1024-bit n), but the check is trivial.

UPDATE:

I think this is a redundant check (for all practical pruposes).

The math:

// SRP6RandomEphemeral must confirm to: log_g (N) < r
// log_g (N) <= exp, from which follows: g^exp >= N
// log_g (N) <= exp < r
// exp < r
// Therefore, "exp" should not be greater or equal to "r"!

BigInteger's pow method only accepts int exponents (32 bit). But class SRP6RandomEphemeral guarantees to produce, at a minimum, a 256-bit long integer.

from srp6-variables.