8bc643efc8
Modified version of the commit ffaef3f15 in the master branch by Stephen Henson. This makes the SSL_OP_SINGLE_DH_USE option a no-op and always generates a new DH key for every handshake regardless. This is a follow on from CVE-2016-0701. This branch is not impacted by that CVE because it does not support X9.42 style parameters. It is still possible to generate parameters based on primes that are not "safe", although by default OpenSSL does not do this. The documentation does sign post that using such parameters is unsafe if the private DH key is reused. However to avoid accidental problems or future attacks this commit has been backported to this branch. Issue reported by Antonio Sanso Reviewed-by: Viktor Dukhovni <viktor@openssl.org>
131 lines
4.7 KiB
Plaintext
131 lines
4.7 KiB
Plaintext
=pod
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=head1 NAME
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SSL_CTX_set_tmp_dh_callback, SSL_CTX_set_tmp_dh, SSL_set_tmp_dh_callback, SSL_set_tmp_dh - handle DH keys for ephemeral key exchange
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=head1 SYNOPSIS
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#include <openssl/ssl.h>
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void SSL_CTX_set_tmp_dh_callback(SSL_CTX *ctx,
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DH *(*tmp_dh_callback)(SSL *ssl, int is_export, int keylength));
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long SSL_CTX_set_tmp_dh(SSL_CTX *ctx, DH *dh);
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void SSL_set_tmp_dh_callback(SSL *ctx,
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DH *(*tmp_dh_callback)(SSL *ssl, int is_export, int keylength));
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long SSL_set_tmp_dh(SSL *ssl, DH *dh)
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=head1 DESCRIPTION
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SSL_CTX_set_tmp_dh_callback() sets the callback function for B<ctx> to be
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used when a DH parameters are required to B<tmp_dh_callback>.
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The callback is inherited by all B<ssl> objects created from B<ctx>.
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SSL_CTX_set_tmp_dh() sets DH parameters to be used to be B<dh>.
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The key is inherited by all B<ssl> objects created from B<ctx>.
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SSL_set_tmp_dh_callback() sets the callback only for B<ssl>.
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SSL_set_tmp_dh() sets the parameters only for B<ssl>.
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These functions apply to SSL/TLS servers only.
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=head1 NOTES
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When using a cipher with RSA authentication, an ephemeral DH key exchange
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can take place. Ciphers with DSA keys always use ephemeral DH keys as well.
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In these cases, the session data are negotiated using the
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ephemeral/temporary DH key and the key supplied and certified
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by the certificate chain is only used for signing.
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Anonymous ciphers (without a permanent server key) also use ephemeral DH keys.
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Using ephemeral DH key exchange yields forward secrecy, as the connection
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can only be decrypted, when the DH key is known. By generating a temporary
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DH key inside the server application that is lost when the application
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is left, it becomes impossible for an attacker to decrypt past sessions,
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even if he gets hold of the normal (certified) key, as this key was
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only used for signing.
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In order to perform a DH key exchange the server must use a DH group
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(DH parameters) and generate a DH key. The server will always generate
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a new DH key during the negotiation.
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As generating DH parameters is extremely time consuming, an application
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should not generate the parameters on the fly but supply the parameters.
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DH parameters can be reused, as the actual key is newly generated during
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the negotiation. The risk in reusing DH parameters is that an attacker
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may specialize on a very often used DH group. Applications should therefore
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generate their own DH parameters during the installation process using the
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openssl L<dhparam(1)|dhparam(1)> application. This application
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guarantees that "strong" primes are used.
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Files dh2048.pem, and dh4096.pem in the 'apps' directory of the current
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version of the OpenSSL distribution contain the 'SKIP' DH parameters,
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which use safe primes and were generated verifiably pseudo-randomly.
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These files can be converted into C code using the B<-C> option of the
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L<dhparam(1)|dhparam(1)> application. Generation of custom DH
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parameters during installation should still be preferred to stop an
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attacker from specializing on a commonly used group. Files dh1024.pem
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and dh512.pem contain old parameters that must not be used by
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applications.
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An application may either directly specify the DH parameters or
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can supply the DH parameters via a callback function.
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Previous versions of the callback used B<is_export> and B<keylength>
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parameters to control parameter generation for export and non-export
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cipher suites. Modern servers that do not support export ciphersuites
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are advised to either use SSL_CTX_set_tmp_dh() or alternatively, use
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the callback but ignore B<keylength> and B<is_export> and simply
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supply at least 2048-bit parameters in the callback.
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=head1 EXAMPLES
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Setup DH parameters with a key length of 2048 bits. (Error handling
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partly left out.)
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Command-line parameter generation:
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$ openssl dhparam -out dh_param_2048.pem 2048
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Code for setting up parameters during server initialization:
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...
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SSL_CTX ctx = SSL_CTX_new();
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...
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/* Set up ephemeral DH parameters. */
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DH *dh_2048 = NULL;
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FILE *paramfile;
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paramfile = fopen("dh_param_2048.pem", "r");
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if (paramfile) {
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dh_2048 = PEM_read_DHparams(paramfile, NULL, NULL, NULL);
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fclose(paramfile);
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} else {
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/* Error. */
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}
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if (dh_2048 == NULL) {
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/* Error. */
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}
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if (SSL_CTX_set_tmp_dh(ctx, dh_2048) != 1) {
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/* Error. */
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}
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...
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=head1 RETURN VALUES
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SSL_CTX_set_tmp_dh_callback() and SSL_set_tmp_dh_callback() do not return
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diagnostic output.
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SSL_CTX_set_tmp_dh() and SSL_set_tmp_dh() do return 1 on success and 0
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on failure. Check the error queue to find out the reason of failure.
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=head1 SEE ALSO
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L<ssl(3)|ssl(3)>, L<SSL_CTX_set_cipher_list(3)|SSL_CTX_set_cipher_list(3)>,
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L<SSL_CTX_set_tmp_rsa_callback(3)|SSL_CTX_set_tmp_rsa_callback(3)>,
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L<SSL_CTX_set_options(3)|SSL_CTX_set_options(3)>,
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L<ciphers(1)|ciphers(1)>, L<dhparam(1)|dhparam(1)>
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=cut
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