RSA Security 5.2.2 manual RSA Keys, 4Summary of Recommended Key Sizes

Security Considerations

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.

Table 3-4Summary of Recommended Key Sizes

		Organizational or
Algorithm	User Key	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

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

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