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The TLS 1.1 (RFC 4346), the section for Server Key Exchange Message (7.4.3) says

  It is not legal to send the server key exchange message for the
  following key exchange methods:


But, below in the same section for structure of the message, it gives the structure as follows:

  struct {
      select (KeyExchangeAlgorithm) {
          case diffie_hellman:
              ServerDHParams params;
              Signature signed_params;
          case rsa:
              ServerRSAParams params;
              Signature signed_params;
  } ServerKeyExchange;

In which condition will the ServerRSAParams be sent? Why is it needed? In the TLS 1.2 (RFC 5246) specification, I find that the ServerKeyExchange structure doesn't indicate any parameters to be sent for RSA. Snippet from RFC 5246 below:

  struct {
      select (KeyExchangeAlgorithm) {
          case dh_anon:
              ServerDHParams params;
          case dhe_dss:
          case dhe_rsa:
              ServerDHParams params;
              digitally-signed struct {
                  opaque client_random[32];
                  opaque server_random[32];
                  ServerDHParams params;
              } signed_params;
          case rsa:
          case dh_dss:
          case dh_rsa:
              struct {} ;
             /* message is omitted for rsa, dh_dss, and dh_rsa */
          /* may be extended, e.g., for ECDH -- see [TLSECC] */
  } ServerKeyExchange;
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1 Answer 1

up vote 3 down vote accepted

As TLS 1.2 puts it:

The ServerKeyExchange message is sent by the server only when the server Certificate message (if sent) does not contain enough data to allow the client to exchange a premaster secret.

Therefore a ServerKeyExchange message containing a RSA public key would be used when the server's public key is not fit for key exchange (e.g. it is in a certificate which is marked as signature only) but client and server wish to use RSA for key exchange nonetheless.

Sending a RSA key like this is a feature which has fallen out of use, but it was used in TLS 1.0 with the RSA_EXPORT cipher suites. At that time, there were rather stringent regulations in the USA, limiting the size of cryptographic keys that were used for encryption in systems which were exported outside of the USA (these regulations were lifted in 2000); in particular, RSA encryption was limited to 512 bits (and 40 bits for symmetric encryption). The RSA_EXPORT cipher suites complied with these export regulations. When the server has a RSA public key, the client and the server agree to use one of the RSA_EXPORT suites, and the server's public key is larger than 512 bits, then the server must send a ServerKeyExchange message with a RSA key of at most 512 bits, and use its permanent RSA key (the one from its certificate) to sign the ServerKeyExchange message.

Dynamic RSA key generation is a bit cumbersome and expensive; a given SSL server can reuse such a key with several clients, but it is still heavier than DHE. The main use case was to support feeble clients who can do RSA only; however, even if RSA public key operations are very fast, ECDHE (Diffie-Hellman with elliptic curves) is competitive and much easier for the server. TLS 1.1 clients are allowed to advertise support for the RSA_EXPORT suites, but only for backward compatibility with SSL 3.0 and TLS 1.0 (section A.5 of TLS 1.1):

TLS 1.1 implementations MUST NOT negotiate these cipher suites in TLS 1.1 mode. However, for backward compatibility they may be offered in the ClientHello for use with TLS 1.0 or SSLv3-only servers. TLS 1.1 clients MUST check that the server did not choose one of these cipher suites during the handshake. These ciphersuites are listed below for informational purposes and to reserve the numbers.

The ServerRSAParams structure in TLS 1.1 is thus a survivance from TLS 1.0 which somehow evaded the wrath of the RFC 4346 editors, but was finally removed when RFC 5246 was produced.

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Thanks for the detailed answer. : ) –  Jay Dec 13 '12 at 9:12

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