Blum integer

In mathematics, a natural number n is a Blum integer if n = p×q is a semiprime for which p and q are distinct prime numbers congruent to 3 mod 4. That is, p and q must be of the form 4t + 3, for some integer t. Integers of this form are referred to as Blum primes. This means that the factors of a Blum integer are Gaussian primes with no imaginary part. The first few Blum integers are

Blum integers were named for computer scientist Manuel Blum.

Given n = p×q a Blum integer, Q_n the set of all quadratic residues modulo n, and a ∈ Q_n. Then:

Before modern factoring algorithms, such as MPQS and NFS, were developed, it was thought to be useful to select Blum integers as RSA moduli. This is no longer regarded as a useful precaution, since MPQS and NFS are able to factor Blum integers with the same ease as RSA moduli constructed from randomly selected primes.

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