Diffie-Hellman vs. RSA: Comparing key exchange algorithms

Do you know of any algorithms that merge or combine the RSA and Diffie-Hellman algorithms? Would there be any benefit in doing so? If this is not possible, is one better than the other?

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Let me answer this question by first explaining Diffie-Hellman vs. RSA algorithms. Diffie-Hellman is a key exchange algorithm and allows two parties to establish, over an insecure communications channel, a shared secret key that only the two parties know, even without having shared anything beforehand.

The shared key is an asymmetric key, but, like all asymmetric key systems, it is inherently slow and impractical for bulk encryption. The key is used instead to securely exchange a symmetric key, such as AES (Advanced Encryption Standard) used to encrypt subsequent communications. Unlike Diffie-Hellman, the RSA algorithm can be used for signing digital signatures as well as symmetric key exchange, but it does require the exchange of a public key beforehand.

RSA and Diffie-Hellman are both based on supposedly intractable problems, the difficulty of factoring large numbers and exponentiation and modular arithmetic respectively, and with key lengths of 1,024 bits, give comparable levels of security. Both have been subjected to scrutiny by mathematicians and cryptographers, but given correct implementation, neither is significantly less secure than the other.

The nature of the Diffie-Hellman key exchange does make it susceptible to man-in-the-middle attacks since it doesn't authenticate either party involved in the exchange. This is why Diffie-Hellman is used in combination with an additional authentication method, generally digital signatures. When using RSA, a 1,024-bit key is considered suitable both for generating digital signatures and for key exchange when used with bulk encryption, while a 2048-bit key is recommended when a digital signature must be kept secure for an extended period of time, such as a certificate authority’s key.

Getting back to the question at hand, you can’t really merge the two algorithms because of the unique attributes and complexity that each one has. Most encryption systems offer a choice between them rather than combining them. SSL 3.0 supports a choice of key exchange algorithms, including the RSA key exchange when certificates are used, and Diffie-Hellman key exchange for exchanging keys without certificates and without prior communication between client and server.

This was first published in March 2011

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