# In RSA, rationale for prime $p$ with $p-1$ having prime factor $u$ with $u-1$ having large prime factor?

In the 1978 RSA paper, it is recommended, among other things, to choose primes $p$ such that $(p-1)$ has a large prime factor $u$. This was motivated by Pollard's p-1 algorithm. Further, the authors state:

Additional security is provided by ensuring that $(u−1)$ also has a large prime factor.

What was the motivation for that?

• It should be noted that elliptic curve factorization has made this security requirement redundant. On the other hand, ensuring (p - 1) has a large prime factor requires very little extra effort.
– user2448
Jul 6 '12 at 8:44
• @Brett Hale: I think you mean that ECC has made redundant the requirement to choose primes $p$ such that $(p−1)$ has a large prime factor $u$. The question is about a different second requirement. Also, ECC may not obsolete the first requirement in some cases: more than 2 primes, and enormously many public moduli, with the adversary content factoring a single one.
– fgrieu
Mar 4 '14 at 17:18

In 1977 Simmons and Norris  discussed the following "cycling" or "superencryption" attack on the RSA cryptosystem: given a ciphertext C, consider decrypting it by repeatedly encrypting it with the same public key used to produce it in the first place, until the message appears. Thus, one looks for a fixed point of the transformation of the plaintext under modular exponentiation. Since the encryption operation effects a permutation of $\mathbb{Z}_n = \{0,1,\ldots,n-1\}$, the message can eventually be obtained in this manner. Rivest  responds to their concern by (a) showing that the odds of success are minuscule if the n is the product of two $p^{--}$-strong primes, and (b) arguing that this attack is really a factoring algorithm in disguise, and should be compared with other factoring attacks.