1SSL_CTX_set_tmp_rsa_callback(3) OpenSSL SSL_CTX_set_tmp_rsa_callback(3)
2
6 SSL_CTX_set_tmp_rsa_callback, SSL_CTX_set_tmp_rsa,
7 SSL_CTX_need_tmp_rsa, SSL_set_tmp_rsa_callback, SSL_set_tmp_rsa,
8 SSL_need_tmp_rsa - handle RSA keys for ephemeral key exchange
9
11 #include <openssl/ssl.h>
12 void SSL_CTX_set_tmp_rsa_callback(SSL_CTX *ctx,
14 RSA *(*tmp_rsa_callback)(SSL *ssl, int is_export, int keylength));
15 long SSL_CTX_set_tmp_rsa(SSL_CTX *ctx, RSA *rsa);
16 long SSL_CTX_need_tmp_rsa(SSL_CTX *ctx);
17 void SSL_set_tmp_rsa_callback(SSL_CTX *ctx,
19 RSA *(*tmp_rsa_callback)(SSL *ssl, int is_export, int keylength));
20 long SSL_set_tmp_rsa(SSL *ssl, RSA *rsa)
21 long SSL_need_tmp_rsa(SSL *ssl)
22 RSA *(*tmp_rsa_callback)(SSL *ssl, int is_export, int keylength));
24
26 SSL_CTX_set_tmp_rsa_callback() sets the callback function for ctx to be
27 used when a temporary/ephemeral RSA key is required to tmp_rsa_call‐
28 back. The callback is inherited by all SSL objects newly created from
29 ctx with <SSL_new(3)⎪SSL_new(3)>. Already created SSL objects are not
30 affected.
31 SSL_CTX_set_tmp_rsa() sets the temporary/ephemeral RSA key to be used
33 to be rsa. The key is inherited by all SSL objects newly created from
34 ctx with <SSL_new(3)⎪SSL_new(3)>. Already created SSL objects are not
35 affected.
36 SSL_CTX_need_tmp_rsa() returns 1, if a temporary/ephemeral RSA key is
38 needed for RSA-based strength-limited 'exportable' ciphersuites because
39 a RSA key with a keysize larger than 512 bits is installed.
40 SSL_set_tmp_rsa_callback() sets the callback only for ssl.
42 SSL_set_tmp_rsa() sets the key only for ssl.
44 SSL_need_tmp_rsa() returns 1, if a temporary/ephemeral RSA key is
46 needed, for RSA-based strength-limited 'exportable' ciphersuites
47 because a RSA key with a keysize larger than 512 bits is installed.
48 These functions apply to SSL/TLS servers only.
50
52 When using a cipher with RSA authentication, an ephemeral RSA key
53 exchange can take place. In this case the session data are negotiated
54 using the ephemeral/temporary RSA key and the RSA key supplied and cer‐
55 tified by the certificate chain is only used for signing.
56 Under previous export restrictions, ciphers with RSA keys shorter (512
58 bits) than the usual key length of 1024 bits were created. To use these
59 ciphers with RSA keys of usual length, an ephemeral key exchange must
60 be performed, as the normal (certified) key cannot be directly used.
61 Using ephemeral RSA key exchange yields forward secrecy, as the connec‐
63 tion can only be decrypted, when the RSA key is known. By generating a
64 temporary RSA key inside the server application that is lost when the
65 application is left, it becomes impossible for an attacker to decrypt
66 past sessions, even if he gets hold of the normal (certified) RSA key,
67 as this key was used for signing only. The downside is that creating a
68 RSA key is computationally expensive.
69 Additionally, the use of ephemeral RSA key exchange is only allowed in
71 the TLS standard, when the RSA key can be used for signing only, that
72 is for export ciphers. Using ephemeral RSA key exchange for other pur‐
73 poses violates the standard and can break interoperability with
74 clients. It is therefore strongly recommended to not use ephemeral RSA
75 key exchange and use EDH (Ephemeral Diffie-Hellman) key exchange
76 instead in order to achieve forward secrecy (see
77 SSL_CTX_set_tmp_dh_callback(3)).
78 On OpenSSL servers ephemeral RSA key exchange is therefore disabled by
80 default and must be explicitly enabled using the SSL_OP_EPHEMERAL_RSA
81 option of SSL_CTX_set_options(3), violating the TLS/SSL standard. When
82 ephemeral RSA key exchange is required for export ciphers, it will
83 automatically be used without this option!
84 An application may either directly specify the key or can supply the
86 key via a callback function. The callback approach has the advantage,
87 that the callback may generate the key only in case it is actually
88 needed. As the generation of a RSA key is however costly, it will lead
89 to a significant delay in the handshake procedure. Another advantage
90 of the callback function is that it can supply keys of different size
91 (e.g. for SSL_OP_EPHEMERAL_RSA usage) while the explicit setting of the
92 key is only useful for key size of 512 bits to satisfy the export
93 restricted ciphers and does give away key length if a longer key would
94 be allowed.
95 The tmp_rsa_callback is called with the keylength needed and the
97 is_export information. The is_export flag is set, when the ephemeral
98 RSA key exchange is performed with an export cipher.
99
101 Generate temporary RSA keys to prepare ephemeral RSA key exchange. As
102 the generation of a RSA key costs a lot of computer time, they saved
103 for later reuse. For demonstration purposes, two keys for 512 bits and
104 1024 bits respectively are generated.
105 ...
107 /* Set up ephemeral RSA stuff */
108 RSA *rsa_512 = NULL;
109 RSA *rsa_1024 = NULL;
110 rsa_512 = RSA_generate_key(512,RSA_F4,NULL,NULL);
112 if (rsa_512 == NULL)
113 evaluate_error_queue();
114 rsa_1024 = RSA_generate_key(1024,RSA_F4,NULL,NULL);
116 if (rsa_1024 == NULL)
117 evaluate_error_queue();
118 ...
120 RSA *tmp_rsa_callback(SSL *s, int is_export, int keylength)
122 {
123 RSA *rsa_tmp=NULL;
124 switch (keylength) {
126 case 512:
127 if (rsa_512)
128 rsa_tmp = rsa_512;
129 else { /* generate on the fly, should not happen in this example */
130 rsa_tmp = RSA_generate_key(keylength,RSA_F4,NULL,NULL);
131 rsa_512 = rsa_tmp; /* Remember for later reuse */
132 }
133 break;
134 case 1024:
135 if (rsa_1024)
136 rsa_tmp=rsa_1024;
137 else
138 should_not_happen_in_this_example();
139 break;
140 default:
141 /* Generating a key on the fly is very costly, so use what is there */
142 if (rsa_1024)
143 rsa_tmp=rsa_1024;
144 else
145 rsa_tmp=rsa_512; /* Use at least a shorter key */
146 }
147 return(rsa_tmp);
148 }
149
151 SSL_CTX_set_tmp_rsa_callback() and SSL_set_tmp_rsa_callback() do not
152 return diagnostic output.
153 SSL_CTX_set_tmp_rsa() and SSL_set_tmp_rsa() do return 1 on success and
155 0 on failure. Check the error queue to find out the reason of failure.
156 SSL_CTX_need_tmp_rsa() and SSL_need_tmp_rsa() return 1 if a temporary
158 RSA key is needed and 0 otherwise.
159
161 ssl(3), SSL_CTX_set_cipher_list(3), SSL_CTX_set_options(3),
162 SSL_CTX_set_tmp_dh_callback(3), SSL_new(3), ciphers(1)
1630.9.8b 2001-09-07 SSL_CTX_set_tmp_rsa_callback(3)