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
28 tmp_rsa_callback. The callback is inherited by all SSL objects newly
29 created from ctx with <SSL_new(3)|SSL_new(3)>. Already created SSL
30 objects are not 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
55 certified 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
63 connection can only be decrypted, when the RSA key is known. By
64 generating a temporary RSA key inside the server application that is
65 lost when the application is left, it becomes impossible for an
66 attacker to decrypt past sessions, even if he gets hold of the normal
67 (certified) RSA key, as this key was used for signing only. The
68 downside is that creating a 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
73 purposes 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 DHE (Ephemeral Diffie-Hellman) key exchange
76 instead in order to achieve forward secrecy (see
77 SSL_CTX_set_tmp_dh_callback(3)).
78 An application may either directly specify the key or can supply the
80 key via a callback function. The callback approach has the advantage,
81 that the callback may generate the key only in case it is actually
82 needed. As the generation of a RSA key is however costly, it will lead
83 to a significant delay in the handshake procedure. Another advantage
84 of the callback function is that it can supply keys of different size
85 while the explicit setting of the key is only useful for key size of
86 512 bits to satisfy the export restricted ciphers and does give away
87 key length if a longer key would be allowed.
88 The tmp_rsa_callback is called with the keylength needed and the
90 is_export information. The is_export flag is set, when the ephemeral
91 RSA key exchange is performed with an export cipher.
92
94 Generate temporary RSA keys to prepare ephemeral RSA key exchange. As
95 the generation of a RSA key costs a lot of computer time, they saved
96 for later reuse. For demonstration purposes, two keys for 512 bits and
97 1024 bits respectively are generated.
98 ...
100 /* Set up ephemeral RSA stuff */
101 RSA *rsa_512 = NULL;
102 RSA *rsa_1024 = NULL;
103 rsa_512 = RSA_generate_key(512,RSA_F4,NULL,NULL);
105 if (rsa_512 == NULL)
106 evaluate_error_queue();
107 rsa_1024 = RSA_generate_key(1024,RSA_F4,NULL,NULL);
109 if (rsa_1024 == NULL)
110 evaluate_error_queue();
111 ...
113 RSA *tmp_rsa_callback(SSL *s, int is_export, int keylength)
115 {
116 RSA *rsa_tmp=NULL;
117 switch (keylength) {
119 case 512:
120 if (rsa_512)
121 rsa_tmp = rsa_512;
122 else { /* generate on the fly, should not happen in this example */
123 rsa_tmp = RSA_generate_key(keylength,RSA_F4,NULL,NULL);
124 rsa_512 = rsa_tmp; /* Remember for later reuse */
125 }
126 break;
127 case 1024:
128 if (rsa_1024)
129 rsa_tmp=rsa_1024;
130 else
131 should_not_happen_in_this_example();
132 break;
133 default:
134 /* Generating a key on the fly is very costly, so use what is there */
135 if (rsa_1024)
136 rsa_tmp=rsa_1024;
137 else
138 rsa_tmp=rsa_512; /* Use at least a shorter key */
139 }
140 return(rsa_tmp);
141 }
142
144 SSL_CTX_set_tmp_rsa_callback() and SSL_set_tmp_rsa_callback() do not
145 return diagnostic output.
146 SSL_CTX_set_tmp_rsa() and SSL_set_tmp_rsa() do return 1 on success and
148 0 on failure. Check the error queue to find out the reason of failure.
149 SSL_CTX_need_tmp_rsa() and SSL_need_tmp_rsa() return 1 if a temporary
151 RSA key is needed and 0 otherwise.
152
154 ssl(3), SSL_CTX_set_cipher_list(3), SSL_CTX_set_options(3),
155 SSL_CTX_set_tmp_dh_callback(3), SSL_new(3), ciphers(1)
1561.0.2o 2020-01-28 SSL_CTX_set_tmp_rsa_callback(3)