doc/internal/man7/EVP_PKEY.pod

   1 =pod
   2
   3 =head1 NAME
   4
   5 EVP_PKEY - an internal description
   6
   7 =head1 SYNOPSIS
   8
   9  #include "crypto/evp.h"
  10
  11  typedef struct evp_pkey_st EVP_PKEY;
  12
  13 =head1 DESCRIPTION
  14
  15 I<This is not a complete description yet>
  16
  17 B<EVP_PKEY> is a complex type that's essentially a container for
  18 private/public key pairs, but has had other uses as well.
  19
  20 =for comment "uses" could as well be "abuses"...
  21
  22 The private/public key pair that an B<EVP_PKEY> contains is refered to
  23 as its "internal key" or "origin" (the reason for "origin" is
  24 explained further down, in L</Export cache for provider operations>),
  25 and it can take one of the following forms:
  26
  27 =over 4
  28
  29 =item legacy origin
  30
  31 This is the form that an B<EVP_PKEY> in OpenSSL prior to 3.0 had.  The
  32 internal key in the B<EVP_PKEY> is a pointer to the low-level key
  33 types, such as B<RSA>, B<DSA> and B<EC>, or an engine driven
  34 structure, and is governed by an associated L<EVP_PKEY_METHOD(3)> and
  35 an L<EVP_PKEY_ASN1_METHOD(3)>.
  36
  37 The functions available through those two method structures get full
  38 access to the B<EVP_PKEY> and therefore have a lot of freedom to
  39 modify whatever they want.  This also means that an B<EVP_PKEY> is a
  40 shared structure between libcrypto and any ENGINE that serves such
  41 methods.
  42
  43 =item provider-native origin
  44
  45 This is a new form in OpenSSL 3.0, which permits providers to hold the
  46 key data (see L<provider-keymgmt(7)>).  The internal key in the
  47 B<EVP_PKEY> is a pointer to that key data held by the provider, and
  48 is governed by an associated L<EVP_KEYMGMT(3)> method structure.
  49
  50 The functions available through the L<EVP_KEYMGMT(3)> have no access
  51 to the B<EVP_PKEY>, and can therefore not make any direct changes.
  52 Similarly, the key data that the B<EVP_PKEY> points at is only known
  53 to the functions pointed at in the L<EVP_KEYMGMT(3)>.
  54
  55 =back
  56
  57 These two forms can never co-exist in the same B<EVP_PKEY>, the main
  58 reason being that having both at the same time will create problems
  59 with synchronising between the two forms, and potentially make it
  60 confusing which one of the two is the origin.
  61
  62 =head2 Key mutability
  63
  64 The B<EVP_PKEY> internal keys are mutable.
  65
  66 This is especially visible with internal legacy keys, since they can
  67 be extracted with functions like L<EVP_PKEY_get0_RSA(3)> and then
  68 modified at will with functions like L<RSA_set0_key(3)>.
  69
  70 Internal provider native keys are also possible to be modified, if the
  71 associated L<EVP_KEYMGMT(3)> implementation allows it.  This is done
  72 with L<EVP_PKEY_set_params(3)> and its specialised derivatives.  The
  73 OpenSSL providers allow it for the following:
  74
  75 =over 4
  76
  77 =item DH, EC, X25519, X448:
  78
  79 It's possible to set the encoded public key.  This is supported in
  80 particular through L<EVP_PKEY_set1_encoded_public_key(3)>.
  81
  82 =item EC:
  83
  84 It's possible to flip the ECDH cofactor mode.
  85
  86 =back
  87
  88 Every time the B<EVP_PKEY> internal key mutates, an internal dirty
  89 count is incremented.  The need for a dirty count is explained further
  90 in L</Export cache for provider operations>.
  91
  92 For provider native origin keys, this doesn't require any help from
  93 the L<EVP_KEYMGMT(3)>, the dirty count is maintained in the B<EVP_PKEY>
  94 itself, and is incremented every time L<EVP_PKEY_set_params(3)> or its
  95 specialised derivatives are called.
  96 For legacy origin keys, this requires the associated
  97 L<EVP_PKEY_ASN1_METHOD(3)> to implement the dirty_cnt() function.  All
  98 of OpenSSL's built-in L<EVP_PKEY_ASN1_METHOD(3)> implement this
  99 function.
 100
 101 =head2 Export cache for provider operations
 102
 103 OpenSSL 3.0 can handle operations such as signing, encrypting, etc in
 104 diverse providers, potentially others than the provider of the
 105 L<EVP_KEYMGMT(3)>.  Two providers, possibly from different vendors,
 106 can't be expected to share internal key structures.  There are
 107 therefore instances where key data will need to be exported to the
 108 provider that is going to perform the operation (this also implies
 109 that every provider that implements a key pair based operation must
 110 also implement an L<EVP_KEYMGMT(3)>).
 111
 112 For performance reasons, libcrypto tries to minimize the need to
 113 perform such an export, so it maintains a cache of such exports in the
 114 B<EVP_PKEY>.  Each cache entry has two items, a pointer to the
 115 provider side key data and the associated L<EVP_KEYMGMT(3)>.
 116
 117 I<This cache is often referred to as the "operation key cache", and
 118 the key data that the cached keys came from is the "origin", and since
 119 there are two forms of the latter, we have the "legacy origin" and the
 120 "provider native origin".>
 121
 122 The export to the operation key cache can be performed independent of
 123 what form the origin has.
 124 For a legacy origin, this requires that the associated
 125 L<EVP_PKEY_ASN1_METHOD(3)> implements the functions export_to() and
 126 dirty_cnt().
 127 For a provider native origin, this requires that the associated
 128 L<EVP_KEYMGMT(3)> implements the OSSL_FUNC_keymgmt_export() function
 129 (see L<provider-keymgmt(7)>).
 130 In all cases, the receiving L<EVP_KEYMGMT(3)> (the one associated with
 131 the exported key data) must implement OSSL_FUNC_keymgmt_import().
 132
 133 If such caching isn't supported, the operations that can be performed
 134 with that key are limited to the same backend as the origin key
 135 (ENGINE for legacy origin keys, provider for provider side origin
 136 keys).
 137
 138 =head3 Exporting implementation details
 139
 140
 141 Exporting a key to the operation cache involves the following:
 142
 143 =over 4
 144
 145 =item 1.
 146
 147 Check if the dirty count for the internal origin key has changed since
 148 the previous time.  This is done by comparing it with a copy of the
 149 dirty count, which is maintained by the export function.
 150
 151 If the dirty count has changed, the export cache is cleared.
 152
 153 =item 2.
 154
 155 Check if there's an entry in the export cache with the same
 156 L<EVP_KEYMGMT(3)> that's the same provider that an export is to be
 157 made to (which is the provider that's going to perform an operation
 158 for which the current B<EVP_PKEY> is going to be used).
 159
 160 If such an entry is found, nothing more is done, the key data and
 161 L<EVP_KEYMGMT(3)> found in that export cache entry will be used for
 162 the operation to be performed.
 163
 164 =item 3.
 165
 166 Export the internal origin key to the provider, using the appropriate
 167 method.
 168
 169 For legacy origin keys, that's done with the help of the
 170 L<EVP_PKEY_ASN1_METHOD(3)> export_to() function.
 171
 172 For provider native origin keys, that's done by retrieving the key
 173 data in L<OSSL_PARAM(3)> form from the origin keys, using the
 174 OSSL_FUNC_keymgmt_export() functions of the associated
 175 L<EVP_KEYMGMT(3)>, and sending that data to the L<EVP_KEYMGMT(3)> of
 176 the provider that's to perform the operation, using its
 177 OSSL_FUNC_keymgmt_import() function.
 178
 179 =back
 180
 181 =head2 Changing a key origin
 182
 183 It is never possible to change the origin of a key. An B<EVP_PKEY> with a legacy
 184 origin will I<never> be upgraded to become an B<EVP_PKEY> with a provider
 185 native origin. Instead, we have the operation cache as described above, that
 186 takes care of the needs of the diverse operation the application may want to
 187 perform.
 188
 189 Similarly an B<EVP_PKEY> with a provider native origin, will I<never> be
 190 downgraded to be I<transformed> into an B<EVP_PKEY> with a legacy origin.
 191 Instead we may have a cached copy of the provider key in legacy form. Once the
 192 cached copy is created it is never updated. Changes made to the provider key
 193 are not reflected back in the cached legacy copy. Similarly changes made to the
 194 cached legacy copy are not reflected back in the provider key.
 195
 196 =head1 SEE ALSO
 197
 198 L<provider-keymgmt(7)>
 199
 200 =head1 COPYRIGHT
 201
 202 Copyright 2020-2021 The OpenSSL Project Authors. All Rights Reserved.
 203
 204 Licensed under the Apache License 2.0 (the "License").  You may not use
 205 this file except in compliance with the License.  You can obtain a copy
 206 in the file LICENSE in the source distribution or at
 207 L<https://www.openssl.org/source/license.html>.
 208
 209 =cut