129 lines
4.6 KiB
Text
129 lines
4.6 KiB
Text
=pod
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=head1 NAME
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DH_set_default_method, DH_get_default_method,
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DH_set_method, DH_new_method, DH_OpenSSL - select DH method
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=head1 SYNOPSIS
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#include <openssl/dh.h>
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#include <openssl/engine.h>
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void DH_set_default_method(const DH_METHOD *meth);
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const DH_METHOD *DH_get_default_method(void);
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int DH_set_method(DH *dh, const DH_METHOD *meth);
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DH *DH_new_method(ENGINE *engine);
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const DH_METHOD *DH_OpenSSL(void);
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=head1 DESCRIPTION
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A B<DH_METHOD> specifies the functions that OpenSSL uses for Diffie-Hellman
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operations. By modifying the method, alternative implementations
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such as hardware accelerators may be used. IMPORTANT: See the NOTES section for
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important information about how these DH API functions are affected by the use
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of B<ENGINE> API calls.
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Initially, the default DH_METHOD is the OpenSSL internal implementation, as
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returned by DH_OpenSSL().
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DH_set_default_method() makes B<meth> the default method for all DH
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structures created later. B<NB>: This is true only whilst no ENGINE has been set
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as a default for DH, so this function is no longer recommended.
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DH_get_default_method() returns a pointer to the current default DH_METHOD.
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However, the meaningfulness of this result is dependent on whether the ENGINE
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API is being used, so this function is no longer recommended.
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DH_set_method() selects B<meth> to perform all operations using the key B<dh>.
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This will replace the DH_METHOD used by the DH key and if the previous method
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was supplied by an ENGINE, the handle to that ENGINE will be released during the
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change. It is possible to have DH keys that only work with certain DH_METHOD
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implementations (eg. from an ENGINE module that supports embedded
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hardware-protected keys), and in such cases attempting to change the DH_METHOD
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for the key can have unexpected results.
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DH_new_method() allocates and initializes a DH structure so that B<engine> will
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be used for the DH operations. If B<engine> is NULL, the default ENGINE for DH
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operations is used, and if no default ENGINE is set, the DH_METHOD controlled by
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DH_set_default_method() is used.
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=head1 THE DH_METHOD STRUCTURE
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typedef struct dh_meth_st
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{
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/* name of the implementation */
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const char *name;
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/* generate private and public DH values for key agreement */
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int (*generate_key)(DH *dh);
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/* compute shared secret */
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int (*compute_key)(unsigned char *key, BIGNUM *pub_key, DH *dh);
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/* compute r = a ^ p mod m (May be NULL for some implementations) */
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int (*bn_mod_exp)(DH *dh, BIGNUM *r, BIGNUM *a, const BIGNUM *p,
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const BIGNUM *m, BN_CTX *ctx,
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BN_MONT_CTX *m_ctx);
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/* called at DH_new */
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int (*init)(DH *dh);
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/* called at DH_free */
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int (*finish)(DH *dh);
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int flags;
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char *app_data; /* ?? */
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} DH_METHOD;
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=head1 RETURN VALUES
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DH_OpenSSL() and DH_get_default_method() return pointers to the respective
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B<DH_METHOD>s.
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DH_set_default_method() returns no value.
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DH_set_method() returns non-zero if the provided B<meth> was successfully set as
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the method for B<dh> (including unloading the ENGINE handle if the previous
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method was supplied by an ENGINE).
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DH_new_method() returns NULL and sets an error code that can be obtained by
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L<ERR_get_error(3)|ERR_get_error(3)> if the allocation fails. Otherwise it
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returns a pointer to the newly allocated structure.
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=head1 NOTES
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As of version 0.9.7, DH_METHOD implementations are grouped together with other
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algorithmic APIs (eg. RSA_METHOD, EVP_CIPHER, etc) in B<ENGINE> modules. If a
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default ENGINE is specified for DH functionality using an ENGINE API function,
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that will override any DH defaults set using the DH API (ie.
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DH_set_default_method()). For this reason, the ENGINE API is the recommended way
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to control default implementations for use in DH and other cryptographic
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algorithms.
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=head1 SEE ALSO
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L<dh(3)|dh(3)>, L<DH_new(3)|DH_new(3)>
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=head1 HISTORY
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DH_set_default_method(), DH_get_default_method(), DH_set_method(),
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DH_new_method() and DH_OpenSSL() were added in OpenSSL 0.9.4.
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DH_set_default_openssl_method() and DH_get_default_openssl_method() replaced
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DH_set_default_method() and DH_get_default_method() respectively, and
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DH_set_method() and DH_new_method() were altered to use B<ENGINE>s rather than
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B<DH_METHOD>s during development of the engine version of OpenSSL 0.9.6. For
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0.9.7, the handling of defaults in the ENGINE API was restructured so that this
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change was reversed, and behaviour of the other functions resembled more closely
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the previous behaviour. The behaviour of defaults in the ENGINE API now
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transparently overrides the behaviour of defaults in the DH API without
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requiring changing these function prototypes.
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=cut
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