SSL_set_tmp_dh_callback(3ssl)

1SSL_CTX_set_tmp_dh_callback(3)      OpenSSL     SSL_CTX_set_tmp_dh_callback(3)
2
3
4

NAME

6       SSL_CTX_set_tmp_dh_callback, SSL_CTX_set_tmp_dh,
7       SSL_set_tmp_dh_callback, SSL_set_tmp_dh - handle DH keys for ephemeral
8       key exchange
9

SYNOPSIS

11        #include <openssl/ssl.h>
12
13        void SSL_CTX_set_tmp_dh_callback(SSL_CTX *ctx,
14                   DH *(*tmp_dh_callback)(SSL *ssl, int is_export, int keylength));
15        long SSL_CTX_set_tmp_dh(SSL_CTX *ctx, DH *dh);
16
17        void SSL_set_tmp_dh_callback(SSL *ctx,
18                   DH *(*tmp_dh_callback)(SSL *ssl, int is_export, int keylength));
19        long SSL_set_tmp_dh(SSL *ssl, DH *dh)
20

DESCRIPTION

22       SSL_CTX_set_tmp_dh_callback() sets the callback function for ctx to be
23       used when a DH parameters are required to tmp_dh_callback.  The
24       callback is inherited by all ssl objects created from ctx.
25
26       SSL_CTX_set_tmp_dh() sets DH parameters to be used to be dh.  The key
27       is inherited by all ssl objects created from ctx.
28
29       SSL_set_tmp_dh_callback() sets the callback only for ssl.
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31       SSL_set_tmp_dh() sets the parameters only for ssl.
32
33       These functions apply to SSL/TLS servers only.
34

NOTES

36       When using a cipher with RSA authentication, an ephemeral DH key
37       exchange can take place. Ciphers with DSA keys always use ephemeral DH
38       keys as well.  In these cases, the session data are negotiated using
39       the ephemeral/temporary DH key and the key supplied and certified by
40       the certificate chain is only used for signing.  Anonymous ciphers
41       (without a permanent server key) also use ephemeral DH keys.
42
43       Using ephemeral DH key exchange yields forward secrecy, as the
44       connection can only be decrypted, when the DH key is known. By
45       generating a temporary DH key inside the server application that is
46       lost when the application is left, it becomes impossible for an
47       attacker to decrypt past sessions, even if he gets hold of the normal
48       (certified) key, as this key was only used for signing.
49
50       In order to perform a DH key exchange the server must use a DH group
51       (DH parameters) and generate a DH key. The server will always generate
52       a new DH key during the negotiation.
53
54       As generating DH parameters is extremely time consuming, an application
55       should not generate the parameters on the fly but supply the
56       parameters.  DH parameters can be reused, as the actual key is newly
57       generated during the negotiation. The risk in reusing DH parameters is
58       that an attacker may specialize on a very often used DH group.
59       Applications should therefore generate their own DH parameters during
60       the installation process using the openssl dhparam(1) application. This
61       application guarantees that "strong" primes are used.
62
63       Files dh2048.pem, and dh4096.pem in the 'apps' directory of the current
64       version of the OpenSSL distribution contain the 'SKIP' DH parameters,
65       which use safe primes and were generated verifiably pseudo-randomly.
66       These files can be converted into C code using the -C option of the
67       dhparam(1) application. Generation of custom DH parameters during
68       installation should still be preferred to stop an attacker from
69       specializing on a commonly used group. Files dh1024.pem and dh512.pem
70       contain old parameters that must not be used by applications.
71
72       An application may either directly specify the DH parameters or can
73       supply the DH parameters via a callback function.
74
75       Previous versions of the callback used is_export and keylength
76       parameters to control parameter generation for export and non-export
77       cipher suites. Modern servers that do not support export ciphersuites
78       are advised to either use SSL_CTX_set_tmp_dh() or alternatively, use
79       the callback but ignore keylength and is_export and simply supply at
80       least 2048-bit parameters in the callback.
81

EXAMPLES

83       Setup DH parameters with a key length of 2048 bits. (Error handling
84       partly left out.)
85
86        Command-line parameter generation:
87        $ openssl dhparam -out dh_param_2048.pem 2048
88
89        Code for setting up parameters during server initialization:
90
91        ...
92        SSL_CTX ctx = SSL_CTX_new();
93        ...
94
95        /* Set up ephemeral DH parameters. */
96        DH *dh_2048 = NULL;
97        FILE *paramfile;
98        paramfile = fopen("dh_param_2048.pem", "r");
99        if (paramfile) {
100          dh_2048 = PEM_read_DHparams(paramfile, NULL, NULL, NULL);
101          fclose(paramfile);
102        } else {
103          /* Error. */
104        }
105        if (dh_2048 == NULL) {
106         /* Error. */
107        }
108        if (SSL_CTX_set_tmp_dh(ctx, dh_2048) != 1) {
109          /* Error. */
110        }
111        ...
112

RETURN VALUES

114       SSL_CTX_set_tmp_dh_callback() and SSL_set_tmp_dh_callback() do not
115       return diagnostic output.
116
117       SSL_CTX_set_tmp_dh() and SSL_set_tmp_dh() do return 1 on success and 0
118       on failure. Check the error queue to find out the reason of failure.
119

NAME

SYNOPSIS

DESCRIPTION

NOTES

EXAMPLES

RETURN VALUES

SEE ALSO