doc: life-cycle descritpion for RANDs

Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/14522)

diff --git a/doc/man7/life_cycle-rand.pod b/doc/man7/life_cycle-rand.pod
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+=pod
+
+=head1 NAME
+
+life_cycle-rand - The RAND algorithm life-cycle
+
+=head1 DESCRIPTION
+
+All random number generator (RANDs)
+go through a number of stages in their life-cycle:
+
+=over 4
+
+=item start
+
+This state represents the RAND before it has been allocated.  It is the
+starting state for any life-cycle transitions.
+
+=item newed
+
+This state represents the RAND after it has been allocated but unable to
+generate any output.
+
+=item instantiated
+
+This state represents the RAND when it is set up and capable of generating
+output.
+
+=item uninstantiated
+
+This state represents the RAND when it has been shutdown and it is no longer
+capable of generating output.
+
+=item freed
+
+This state is entered when the RAND is freed.  It is the terminal state
+for all life-cycle transitions.
+
+=back
+
+=head2 State Transition Diagram
+
+The usual life-cycle of a RAND is illustrated:
+
+=begin man
+
+                        +-------------------------+
+                        |          start          |
+                        +-------------------------+
+                          |
+                          | EVP_RAND_CTX_new
+                          v
+                        +-------------------------+
+                        |          newed          |
+                        +-------------------------+
+                          |
+                          | EVP_RAND_instantiate
+                          v
+    EVP_RAND_generate   +-------------------------+
+  +-------------------- |                         |
+  |                     |      instantiated       |
+  +-------------------> |                         | <+
+                        +-------------------------+  '
+                          |                          '
+                          | EVP_RAND_uninstantiate   ' EVP_RAND_instantiate
+                          v                          '
+                        +-------------------------+  '
+                        |     uninstantiated      | -+
+                        +-------------------------+
+                          |
+                          | EVP_RAND_CTX_free
+                          v
+                        +-------------------------+
+                        |          freed          |
+                        +-------------------------+
+
+=end man
+
+=for html <img src="rand.png">
+
+=head2 Formal State Transitions
+
+This section defines all of the legal state transitions.
+This is the canonical list.
+
+=begin man
+
+ Function Call              ------------------ Current State ------------------
+                            start   newed   instantiated  uninstantiated  freed
+ EVP_RAND_CTX_new           newed       
+ EVP_RAND_instantiate            instantiated
+ EVP_RAND_generate                          instantiated
+ EVP_RAND_uninstantiate                    uninstantiated
+ EVP_RAND_CTX_free          freed   freed      freed          freed
+ EVP_RAND_get_ctx_params            newed   instantiated  uninstantiated  freed
+ EVP_RAND_set_ctx_params            newed   instantiated  uninstantiated  freed
+ EVP_RAND_CTX_gettable_params       newed   instantiated  uninstantiated  freed
+ EVP_RAND_CTX_settable_params       newed   instantiated  uninstantiated  freed
+
+=end man
+
+=begin html
+
+<table style="border:1px solid; border-collapse:collapse">
+<tr><th style="border:1px solid" align="left">Function Call</th>
+    <th style="border:1px solid" colspan="5">Current State</th></tr>
+<tr><th style="border:1px solid"></th>
+    <th style="border:1px solid" align="center">start</th>
+    <th style="border:1px solid" align="center">newed</th>
+    <th style="border:1px solid" align="center">instantiated</th>
+    <th style="border:1px solid" align="center">uninstantiated</th>
+    <th style="border:1px solid" align="center">freed</th></tr>
+<tr><th style="border:1px solid" align="left">EVP_RAND_CTX_new</th>
+    <td style="border:1px solid" align="center">newed</td>
+    <td style="border:1px solid" align="center"></td>
+    <td style="border:1px solid" align="center"></td>
+    <td style="border:1px solid" align="center"></td>
+    <td style="border:1px solid" align="center"></td></tr>
+<tr><th style="border:1px solid" align="left">EVP_RAND_instantiate</th>
+    <td style="border:1px solid" align="center"></td>
+    <td style="border:1px solid" align="center">instantiated</td>
+    <td style="border:1px solid" align="center"></td>
+    <td style="border:1px solid" align="center"></td>
+    <td style="border:1px solid" align="center"></td></tr>
+<tr><th style="border:1px solid" align="left">EVP_RAND_generate</th>
+    <td style="border:1px solid" align="center"></td>
+    <td style="border:1px solid" align="center"></td>
+    <td style="border:1px solid" align="center">instantiated</td>
+    <td style="border:1px solid" align="center"></td>
+    <td style="border:1px solid" align="center"></td></tr>
+<tr><th style="border:1px solid" align="left">EVP_RAND_uninstantiate</th>
+    <td style="border:1px solid" align="center"></td>
+    <td style="border:1px solid" align="center"></td>
+    <td style="border:1px solid" align="center">uninstantiated</td>
+    <td style="border:1px solid" align="center"></td>
+    <td style="border:1px solid" align="center"></td></tr>
+<tr><th style="border:1px solid" align="left">EVP_RAND_CTX_free</th>
+    <td style="border:1px solid" align="center">freed</td>
+    <td style="border:1px solid" align="center">freed</td>
+    <td style="border:1px solid" align="center">freed</td>
+    <td style="border:1px solid" align="center">freed</td>
+    <td style="border:1px solid" align="center"></td></tr>
+<tr><th style="border:1px solid" align="left">EVP_RAND_get_ctx_params</th>
+    <td style="border:1px solid" align="center"></td>
+    <td style="border:1px solid" align="center">newed</td>
+    <td style="border:1px solid" align="center">instantiated</td>
+    <td style="border:1px solid" align="center">uninstantiated</td>
+    <td style="border:1px solid" align="center"></td></tr>
+<tr><th style="border:1px solid" align="left">EVP_RAND_set_ctx_params</th>
+    <td style="border:1px solid" align="center"></td>
+    <td style="border:1px solid" align="center">newed</td>
+    <td style="border:1px solid" align="center">instantiated</td>
+    <td style="border:1px solid" align="center">uninstantiated</td>
+    <td style="border:1px solid" align="center"></td></tr>
+<tr><th style="border:1px solid" align="left">EVP_RAND_CTX_gettable_params</th>
+    <td style="border:1px solid" align="center"></td>
+    <td style="border:1px solid" align="center">newed</td>
+    <td style="border:1px solid" align="center">instantiated</td>
+    <td style="border:1px solid" align="center">uninstantiated</td>
+    <td style="border:1px solid" align="center"></td></tr>
+<tr><th style="border:1px solid" align="left">EVP_RAND_CTX_settable_params</th>
+    <td style="border:1px solid" align="center"></td>
+    <td style="border:1px solid" align="center">newed</td>
+    <td style="border:1px solid" align="center">instantiated</td>
+    <td style="border:1px solid" align="center">uninstantiated</td>
+    <td style="border:1px solid" align="center"></td></tr>
+</table>
+
+=end html
+
+=head1 NOTES
+
+At some point the EVP layer will begin enforcing the transitions described
+herein.
+
+=head1 SEE ALSO
+
+L<provider-rand(7)>, L<EVP_RAND(3)>.
+
+=head1 HISTORY
+
+The provider RAND interface was introduced in OpenSSL 3.0.
+
+=head1 COPYRIGHT
+
+Copyright 2021 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

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