SSL_CTX_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, SSL_set_tmp_dh_call‐
7       back, SSL_set_tmp_dh - handle DH keys for ephemeral key exchange
8

SYNOPSIS

10        #include <openssl/ssl.h>
11
12        void SSL_CTX_set_tmp_dh_callback(SSL_CTX *ctx,
13                   DH *(*tmp_dh_callback)(SSL *ssl, int is_export, int keylength));
14        long SSL_CTX_set_tmp_dh(SSL_CTX *ctx, DH *dh);
15
16        void SSL_set_tmp_dh_callback(SSL_CTX *ctx,
17                   DH *(*tmp_dh_callback)(SSL *ssl, int is_export, int keylength));
18        long SSL_set_tmp_dh(SSL *ssl, DH *dh)
19
20        DH *(*tmp_dh_callback)(SSL *ssl, int is_export, int keylength));
21

DESCRIPTION

23       SSL_CTX_set_tmp_dh_callback() sets the callback function for ctx to be
24       used when a DH parameters are required to tmp_dh_callback.  The call‐
25       back is inherited by all ssl objects created from ctx.
26
27       SSL_CTX_set_tmp_dh() sets DH parameters to be used to be dh.  The key
28       is inherited by all ssl objects created from ctx.
29
30       SSL_set_tmp_dh_callback() sets the callback only for ssl.
31
32       SSL_set_tmp_dh() sets the parameters only for ssl.
33
34       These functions apply to SSL/TLS servers only.
35

NOTES

37       When using a cipher with RSA authentication, an ephemeral DH key
38       exchange can take place. Ciphers with DSA keys always use ephemeral DH
39       keys as well.  In these cases, the session data are negotiated using
40       the ephemeral/temporary DH key and the key supplied and certified by
41       the certificate chain is only used for signing.  Anonymous ciphers
42       (without a permanent server key) also use ephemeral DH keys.
43
44       Using ephemeral DH key exchange yields forward secrecy, as the connec‐
45       tion can only be decrypted, when the DH key is known. By generating a
46       temporary DH key inside the server application that is lost when the
47       application is left, it becomes impossible for an attacker to decrypt
48       past sessions, even if he gets hold of the normal (certified) key, as
49       this key was only used for signing.
50
51       In order to perform a DH key exchange the server must use a DH group
52       (DH parameters) and generate a DH key. The server will always generate
53       a new DH key during the negotiation, when the DH parameters are sup‐
54       plied via callback and/or when the SSL_OP_SINGLE_DH_USE option of
55       SSL_CTX_set_options(3) is set. It will immediately create a DH key,
56       when DH parameters are supplied via SSL_CTX_set_tmp_dh() and
57       SSL_OP_SINGLE_DH_USE is not set. In this case, it may happen that a key
58       is generated on initialization without later being needed, while on the
59       other hand the computer time during the negotiation is being saved.
60
61       If "strong" primes were used to generate the DH parameters, it is not
62       strictly necessary to generate a new key for each handshake but it does
63       improve forward secrecy. If it is not assured, that "strong" primes
64       were used (see especially the section about DSA parameters below),
65       SSL_OP_SINGLE_DH_USE must be used in order to prevent small subgroup
66       attacks. Always using SSL_OP_SINGLE_DH_USE has an impact on the com‐
67       puter time needed during negotiation, but it is not very large, so
68       application authors/users should consider to always enable this option.
69
70       As generating DH parameters is extremely time consuming, an application
71       should not generate the parameters on the fly but supply the parame‐
72       ters.  DH parameters can be reused, as the actual key is newly gener‐
73       ated during the negotiation. The risk in reusing DH parameters is that
74       an attacker may specialize on a very often used DH group. Applications
75       should therefore generate their own DH parameters during the installa‐
76       tion process using the openssl dhparam(1) application. In order to
77       reduce the computer time needed for this generation, it is possible to
78       use DSA parameters instead (see dhparam(1)), but in this case
79       SSL_OP_SINGLE_DH_USE is mandatory.
80
81       Application authors may compile in DH parameters. Files dh512.pem,
82       dh1024.pem, dh2048.pem, and dh4096 in the 'apps' directory of current
83       version of the OpenSSL distribution contain the 'SKIP' DH parameters,
84       which use safe primes and were generated verifiably pseudo-randomly.
85       These files can be converted into C code using the -C option of the
86       dhparam(1) application.  Authors may also generate their own set of
87       parameters using dhparam(1), but a user may not be sure how the parame‐
88       ters were generated. The generation of DH parameters during installa‐
89       tion is therefore recommended.
90
91       An application may either directly specify the DH parameters or can
92       supply the DH parameters via a callback function. The callback approach
93       has the advantage, that the callback may supply DH parameters for dif‐
94       ferent key lengths.
95
96       The tmp_dh_callback is called with the keylength needed and the
97       is_export information. The is_export flag is set, when the ephemeral DH
98       key exchange is performed with an export cipher.
99

EXAMPLES

101       Handle DH parameters for key lengths of 512 and 1024 bits. (Error han‐
102       dling partly left out.)
103
104        ...
105        /* Set up ephemeral DH stuff */
106        DH *dh_512 = NULL;
107        DH *dh_1024 = NULL;
108        FILE *paramfile;
109
110        ...
111        /* "openssl dhparam -out dh_param_512.pem -2 512" */
112        paramfile = fopen("dh_param_512.pem", "r");
113        if (paramfile) {
114          dh_512 = PEM_read_DHparams(paramfile, NULL, NULL, NULL);
115          fclose(paramfile);
116        }
117        /* "openssl dhparam -out dh_param_1024.pem -2 1024" */
118        paramfile = fopen("dh_param_1024.pem", "r");
119        if (paramfile) {
120          dh_1024 = PEM_read_DHparams(paramfile, NULL, NULL, NULL);
121          fclose(paramfile);
122        }
123        ...
124
125        /* "openssl dhparam -C -2 512" etc... */
126        DH *get_dh512() { ... }
127        DH *get_dh1024() { ... }
128
129        DH *tmp_dh_callback(SSL *s, int is_export, int keylength)
130        {
131           DH *dh_tmp=NULL;
132
133           switch (keylength) {
134           case 512:
135             if (!dh_512)
136               dh_512 = get_dh512();
137             dh_tmp = dh_512;
138             break;
139           case 1024:
140             if (!dh_1024)
141               dh_1024 = get_dh1024();
142             dh_tmp = dh_1024;
143             break;
144           default:
145             /* Generating a key on the fly is very costly, so use what is there */
146             setup_dh_parameters_like_above();
147           }
148           return(dh_tmp);
149        }
150

RETURN VALUES

152       SSL_CTX_set_tmp_dh_callback() and SSL_set_tmp_dh_callback() do not
153       return diagnostic output.
154
155       SSL_CTX_set_tmp_dh() and SSL_set_tmp_dh() do return 1 on success and 0
156       on failure. Check the error queue to find out the reason of failure.
157

NAME

SYNOPSIS

DESCRIPTION

NOTES

EXAMPLES

RETURN VALUES

SEE ALSO