diff options
author | Richard Levitte <levitte@openssl.org> | 2019-11-23 10:36:16 +0100 |
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committer | Richard Levitte <levitte@openssl.org> | 2019-11-27 16:21:15 +0100 |
commit | 40d422fd3a685c3e1e62ab37eda1189567d000b5 (patch) | |
tree | 5621fc25e6be2b1aecfe186143a8cb72bc728b6b /doc/man7 | |
parent | 984cff6ba52655cf8242b3ef6b4e97a5e8b33266 (diff) |
doc/man7/proxy-certificates.pod: New guide for proxy certificates
This replaces doc/HOWTO/proxy_certificates.txt
Reviewed-by: Paul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/10507)
(cherry picked from commit 2a03823606b2d7e73e5dc890c3202e186511151f)
Diffstat (limited to 'doc/man7')
-rw-r--r-- | doc/man7/proxy-certificates.pod | 363 |
1 files changed, 363 insertions, 0 deletions
diff --git a/doc/man7/proxy-certificates.pod b/doc/man7/proxy-certificates.pod new file mode 100644 index 0000000000..df5ee1b4b5 --- /dev/null +++ b/doc/man7/proxy-certificates.pod @@ -0,0 +1,363 @@ +=pod + +=encoding UTF-8 + +=head1 NAME + +proxy-certificates - Proxy certificates in OpenSSL + +=head1 DESCRIPTION + +Proxy certificates are defined in RFC 3820. They are used to +extend rights to some other entity (a computer process, typically, or +sometimes to the user itself). This allows the entity to perform +operations on behalf of the owner of the EE (End Entity) certificate. + +The requirements for a valid proxy certificate are: + +=over 4 + +=item * + +They are issued by an End Entity, either a normal EE certificate, or +another proxy certificate. + +=item * + +They must not have the B<subjectAltName> or B<issuerAltName> +extensions. + +=item * + +They must have the B<proxyCertInfo> extension. + +=item * + +They must have the subject of their issuer, with one B<commonName> +added. + +=back + +=head2 Enabling proxy certificate verification + +OpenSSL expects applications that want to use proxy certificates to be +specially aware of them, and make that explicit. This is done by +setting an X509 verification flag: + + X509_STORE_CTX_set_flags(ctx, X509_V_FLAG_ALLOW_PROXY_CERTS); + +or + + X509_VERIFY_PARAM_set_flags(param, X509_V_FLAG_ALLOW_PROXY_CERTS); + +See L</NOTES> for a discussion on this requirement. + +=head2 Creating proxy certificates + +Creating proxy certificates can be done using the L<openssl-x509(1)> +command, with some extra extensions: + + [ v3_proxy ] + # A proxy certificate MUST NEVER be a CA certificate. + basicConstraints=CA:FALSE + + # Usual authority key ID + authorityKeyIdentifier=keyid,issuer:always + + # The extension which marks this certificate as a proxy + proxyCertInfo=critical,language:id-ppl-anyLanguage,pathlen:1,policy:text:AB + +It's also possible to specify the proxy extension in a separate section: + + proxyCertInfo=critical,@proxy_ext + + [ proxy_ext ] + language=id-ppl-anyLanguage + pathlen=0 + policy=text:BC + +The policy value has a specific syntax, I<syntag>:I<string>, where the +I<syntag> determines what will be done with the string. The following +I<syntag>s are recognised: + +=over 4 + +=item B<text> + +indicates that the string is a byte sequence, without any encoding: + + policy=text:räksmörgås + +=item B<hex> + +indicates the string is encoded hexadecimal encoded binary data, with +colons between each byte (every second hex digit): + + policy=hex:72:E4:6B:73:6D:F6:72:67:E5:73 + +=item B<file> + +indicates that the text of the policy should be taken from a file. +The string is then a filename. This is useful for policies that are +large (more than a few lines, e.g. XML documents). + +=back + +I<NOTE: The proxy policy value is what determines the rights granted +to the process during the proxy certificate. It's up to the +application to interpret and combine these policies.> + +With a proxy extension, creating a proxy certificate is a matter of +two commands: + + openssl req -new -config proxy.cnf \ + -out proxy.req -keyout proxy.key \ + -subj "/DC=org/DC=openssl/DC=users/CN=proxy 1" + + openssl x509 -req -CAcreateserial -in proxy.req -out proxy.crt \ + -CA user.crt -CAkey user.key -days 7 \ + -extfile proxy.cnf -extensions v3_proxy1 + +You can also create a proxy certificate using another proxy +certificate as issuer (note: using a different configuration +section for the proxy extensions): + + openssl req -new -config proxy.cnf \ + -out proxy2.req -keyout proxy2.key \ + -subj "/DC=org/DC=openssl/DC=users/CN=proxy 1/CN=proxy 2" + + openssl x509 -req -CAcreateserial -in proxy2.req -out proxy2.crt \ + -CA proxy.crt -CAkey proxy.key -days 7 \ + -extfile proxy.cnf -extensions v3_proxy2 + +=head2 Using proxy certs in applications + +To interpret proxy policies, the application would normally start with +some default rights (perhaps none at all), then compute the resulting +rights by checking the rights against the chain of proxy certificates, +user certificate and CA certificates. + +The complicated part is figuring out how to pass data between your +application and the certificate validation procedure. + +The following ingredients are needed for such processing: + +=over 4 + +=item * + +a callback function that will be called for every certificate being +validated. The callback is called several times for each certificate, +so you must be careful to do the proxy policy interpretation at the +right time. You also need to fill in the defaults when the EE +certificate is checked. + +=item * + +a data structure that is shared between your application code and the +callback. + +=item * + +a wrapper function that sets it all up. + +=item * + +an ex_data index function that creates an index into the generic +ex_data store that is attached to an X509 validation context. + +=back + +The following skeleton code can be used as a starting point: + + #include <string.h> + #include <netdb.h> + #include <openssl/x509.h> + #include <openssl/x509v3.h> + + #define total_rights 25 + + /* + * In this example, I will use a view of granted rights as a bit + * array, one bit for each possible right. + */ + typedef struct your_rights { + unsigned char rights[(total_rights + 7) / 8]; + } YOUR_RIGHTS; + + /* + * The following procedure will create an index for the ex_data + * store in the X509 validation context the first time it's + * called. Subsequent calls will return the same index. + */ + static int get_proxy_auth_ex_data_idx(X509_STORE_CTX *ctx) + { + static volatile int idx = -1; + + if (idx < 0) { + X509_STORE_lock(X509_STORE_CTX_get0_store(ctx)); + if (idx < 0) { + idx = X509_STORE_CTX_get_ex_new_index(0, + "for verify callback", + NULL,NULL,NULL); + } + X509_STORE_unlock(X509_STORE_CTX_get0_store(ctx)); + } + return idx; + } + + /* Callback to be given to the X509 validation procedure. */ + static int verify_callback(int ok, X509_STORE_CTX *ctx) + { + if (ok == 1) { + /* + * It's REALLY important you keep the proxy policy check + * within this section. It's important to know that when + * ok is 1, the certificates are checked from top to + * bottom. You get the CA root first, followed by the + * possible chain of intermediate CAs, followed by the EE + * certificate, followed by the possible proxy + * certificates. + */ + X509 *xs = X509_STORE_CTX_get_current_cert(ctx); + + if (X509_get_extension_flags(xs) & EXFLAG_PROXY) { + YOUR_RIGHTS *rights = + (YOUR_RIGHTS *)X509_STORE_CTX_get_ex_data(ctx, + get_proxy_auth_ex_data_idx(ctx)); + PROXY_CERT_INFO_EXTENSION *pci = + X509_get_ext_d2i(xs, NID_proxyCertInfo, NULL, NULL); + + switch (OBJ_obj2nid(pci->proxyPolicy->policyLanguage)) { + case NID_Independent: + /* + * Do whatever you need to grant explicit rights + * to this particular proxy certificate, usually + * by pulling them from some database. If there + * are none to be found, clear all rights (making + * this and any subsequent proxy certificate void + * of any rights). + */ + memset(rights->rights, 0, sizeof(rights->rights)); + break; + case NID_id_ppl_inheritAll: + /* + * This is basically a NOP, we simply let the + * current rights stand as they are. + */ + break; + default: + /* + * This is usually the most complex section of + * code. You really do whatever you want as long + * as you follow RFC 3820. In the example we use + * here, the simplest thing to do is to build + * another, temporary bit array and fill it with + * the rights granted by the current proxy + * certificate, then use it as a mask on the + * accumulated rights bit array, and voilà, you + * now have a new accumulated rights bit array. + */ + { + int i; + YOUR_RIGHTS tmp_rights; + memset(tmp_rights.rights, 0, + sizeof(tmp_rights.rights)); + + /* + * process_rights() is supposed to be a + * procedure that takes a string and its + * length, interprets it and sets the bits + * in the YOUR_RIGHTS pointed at by the + * third argument. + */ + process_rights((char *) pci->proxyPolicy->policy->data, + pci->proxyPolicy->policy->length, + &tmp_rights); + + for(i = 0; i < total_rights / 8; i++) + rights->rights[i] &= tmp_rights.rights[i]; + } + break; + } + PROXY_CERT_INFO_EXTENSION_free(pci); + } else if (!(X509_get_extension_flags(xs) & EXFLAG_CA)) { + /* We have an EE certificate, let's use it to set default! */ + YOUR_RIGHTS *rights = + (YOUR_RIGHTS *)X509_STORE_CTX_get_ex_data(ctx, + get_proxy_auth_ex_data_idx(ctx)); + + /* + * The following procedure finds out what rights the + * owner of the current certificate has, and sets them + * in the YOUR_RIGHTS structure pointed at by the + * second argument. + */ + set_default_rights(xs, rights); + } + } + return ok; + } + + static int my_X509_verify_cert(X509_STORE_CTX *ctx, + YOUR_RIGHTS *needed_rights) + { + int ok; + int (*save_verify_cb)(int ok,X509_STORE_CTX *ctx) = + X509_STORE_CTX_get_verify_cb(ctx); + YOUR_RIGHTS rights; + + X509_STORE_CTX_set_verify_cb(ctx, verify_callback); + X509_STORE_CTX_set_ex_data(ctx, get_proxy_auth_ex_data_idx(ctx), + &rights); + X509_STORE_CTX_set_flags(ctx, X509_V_FLAG_ALLOW_PROXY_CERTS); + ok = X509_verify_cert(ctx); + + if (ok == 1) { + ok = check_needed_rights(rights, needed_rights); + } + + X509_STORE_CTX_set_verify_cb(ctx, save_verify_cb); + + return ok; + } + +If you use SSL or TLS, you can easily set up a callback to have the +certificates checked properly, using the code above: + + SSL_CTX_set_cert_verify_callback(s_ctx, my_X509_verify_cert, + &needed_rights); + +=head1 NOTES + +To this date, it seems that proxy certificates have only been used in +environments that are aware of them, and no one seems to have +investigated how they can be used or misused outside of such an +environment. + +For that reason, OpenSSL requires that applications aware of proxy +certificates must also make that explicit. + +B<subjectAltName> and B<issuerAltName> are forbidden in proxy +certificates, and this is enforced in OpenSSL. The subject must be +the same as the issuer, with one commonName added on. + +=head1 SEE ALSO + +L<X509_STORE_CTX_set_flags(3)>, +L<X509_STORE_CTX_set_verify_cb(3)>, +L<X509_VERIFY_PARAM_set_flags(3)>, +L<SSL_CTX_set_cert_verify_callback(3)>, +L<openssl-req(1)>, L<openssl-x509(1)>, +L<RFC 3820|https://tools.ietf.org/html/rfc3820> + +=head1 COPYRIGHT + +Copyright 2019 The OpenSSL Project Authors. All Rights Reserved. + +Licensed under the Apache License 2.0 (the "License"). You may not use +this file except in compliance with the License. You can obtain a copy +in the file LICENSE in the source distribution or at +L<https://www.openssl.org/source/license.html>. + +=cut |