## Cryptology ePrint Archive: Report 1999/019

**Security of all RSA and Discrete Log Bits **

*Johan Hastad and Mats Naslund *

**Abstract: **We study the security of individual bits in an RSA
encrypted message E_N(x). We show that given E_N(x), predicting any
single bit in x with only a non-negligible advantage over the trivial
guessing strategy, is (through a polynomial time reduction) as hard as
breaking RSA. Moreover, we prove that blocks of O(log log N) bits of x
are computationally indistinguishable from random bits. The results
carry over to the Rabin encryption scheme.

Considering the discrete exponentiation function, g^x modulo p, with
probability 1-o(1) over random choices of the prime p, the analog
results are demonstrated. Finally, we prove that the bits of ax+b
modulo p give hard core predicates for any one-way function f.

**Category / Keywords: **public key encryption, RSA, discrete log, bit security, hard core.

**Publication Info: **Appeared in the THEORY OF CRYPTOGRAPHY LIBRARY and has been included in the ePrint Archive.

**Date: **received August 27, 1999. Work performed 1998/early 1999. (Preliminary versions in FOCS '98 and in Naslund's PhD thesis from Aug. 1998).

**Contact author: **mats naslund at era-t ericsson se

**Available format(s): **Postscript (PS) | Compressed Postscript (PS.GZ) | BibTeX Citation

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