Security Considerations
98 RSA BSAFE Crypto-C Developer’s Guide
Table 3-4 gives a summary of the recommended key sizes for the algorithms
supported in Crypto-C. These recommendations were current at the time this manual
went to press. Please note, however, that such recommendations are always
provisional and can be affected by changes in the cryptographic state of the art.
RSA Keys
The security of the RSA algorithm is based on the difficulty of factoring large integers.
Therefore, the choice for the key size depends on the efficiency of integer-factoring
algorithms. Because users will probably want a key pair to last a few years, it is
advisable to choose a size that will not only remain secure against current state of the
art factoring, but will probably defeat improved factoring attempts of the future. The
RSA Laboratories publication, “Frequently Asked Questions About Today’s
Cryptography,” describes current factoring capabilities.
For normal user data, RSA Security recommends a modulus size of 768 bits. For
organization keys or for long-term applications, a 1024-bit modulus is advisable. For
root keys, RSA Security recommends a 2048-bit modulus. This safeguards against
Table 3-4 Summary of Recommended Key Sizes
Algorithm User Key
Organizational or
Long-Term Key Root Key
AES 128 (192 or 256 is also
acceptable)
Diffie-Hellman 768-bit prime 1024-bit prime 2048-bit prime
DSA 768-bit prime 1024-bit prime 2048-bit prime
ECAES 160-170-bit modulus Not recommended
at this time
EC Diffie-Hellman 160-170-bit modulus Not recommended
at this time
ECDSA 160-170-bit modulus Not recommended
at this time
RC2 –––––––––––––––––––––––80-128 effective key bits ––––––––––––––––––––––
RC4 –––––––––––––––––––––––––– 80-128 key bits –––––––––––––––––––––––––
RC5 –––––––––––––––––––––––– 80-128 key bits with
–––––––––––– 16 rounds for 32-bit word or 20 rounds for 64-bit word –––––––––––
RC6 –––––––––––––––––––– 80-128 key bits with 20 rounds –––––––––––––––––––
RSA 768-bit modulus 1024-bit modulus 2048-bit modulus