DOC: New file for EVP_PKEY_size(), EVP_PKEY_bits() and EVP_PKEY_security_bits()

We change the description to be about the key rather than the
signature.  How the key size is related to the signature is explained
in the description of EVP_SignFinal() anyway.

Reviewed-by: Nicola Tuveri <nic.tuv@gmail.com>

(cherry picked from commit 6942a0d6feb8d3dcbbc6a1ec6be9de7ab2df1530)

Reviewed-by: Matthias St. Pierre <Matthias.St.Pierre@ncp-e.com>
(Merged from https://github.com/openssl/openssl/pull/11232)

2 files changed, 85 insertions, 19 deletions

diff --git a/doc/man3/EVP_PKEY_size.pod b/doc/man3/EVP_PKEY_size.pod
new file mode 100644
index 0000000000..786c503914
--- /dev/null
+++ b/doc/man3/EVP_PKEY_size.pod
@@ -0,0 +1,80 @@
+=pod
+
+=head1 NAME
+
+EVP_PKEY_size, EVP_PKEY_bits, EVP_PKEY_security_bits
+- EVP_PKEY information functions
+
+=head1 SYNOPSIS
+
+ #include <openssl/evp.h>
+
+ int EVP_PKEY_size(const EVP_PKEY *pkey);
+ int EVP_PKEY_bits(const EVP_PKEY *pkey);
+ int EVP_PKEY_security_bits(const EVP_PKEY *pkey);
+
+=head1 DESCRIPTION
+
+EVP_PKEY_size() returns the maximum suitable size for the output
+buffers for almost all operations that can be done with I<pkey>.
+The primary documented use is with L<EVP_SignFinal(3)> and
+L<EVP_SealInit(3)>, but it isn't limited there.  The returned size is
+also large enough for the output buffer of L<EVP_PKEY_sign(3)>,
+L<EVP_PKEY_encrypt(3)>, L<EVP_PKEY_decrypt(3)>, L<EVP_PKEY_derive(3)>.
+
+It must be stressed that, unless the documentation for the operation
+that's being performed says otherwise, the size returned by
+EVP_PKEY_size() is only preliminary and not exact, so the final
+contents of the target buffer may be smaller.  It is therefore crucial
+to take note of the size given back by the function that performs the
+operation, such as L<EVP_PKEY_sign(3)> (the I<siglen> argument will
+receive that length), to avoid bugs.
+
+EVP_PKEY_bits() returns the cryptographic length of the cryptosystem
+to which the key in I<pkey> belongs, in bits.  Note that the definition
+of cryptographic length is specific to the key cryptosystem.
+
+EVP_PKEY_security_bits() returns the number of security bits of the given
+I<pkey>, bits of security is defined in NIST SP800-57.
+
+=head1 RETURN VALUES
+
+EVP_PKEY_size(), EVP_PKEY_bits() and EVP_PKEY_security_bits() return a
+positive number, or 0 if this size isn't available.
+
+=head1 NOTES
+
+Most functions that have an output buffer and are mentioned with
+EVP_PKEY_size() have a functionality where you can pass NULL for the
+buffer and still pass a pointer to an integer and get the exact size
+that this function call delivers in the context that it's called in.
+This allows those functions to be called twice, once to find out the
+exact buffer size, then allocate the buffer in between, and call that
+function again actually output the data.  For those functions, it
+isn't strictly necessary to call EVP_PKEY_size() to find out the
+buffer size, but may be useful in cases where it's desirable to know
+the upper limit in advance.
+
+It should also be especially noted that EVP_PKEY_size() shouldn't be
+used to get the output size for EVP_DigestSignFinal(), according to
+L<EVP_DigestSignFinal(3)/NOTES>.
+
+=head1 SEE ALSO
+
+L<EVP_SignFinal(3)>,
+L<EVP_SealInit(3)>,
+L<EVP_PKEY_sign(3)>,
+L<EVP_PKEY_encrypt(3)>,
+L<EVP_PKEY_decrypt(3)>,
+L<EVP_PKEY_derive(3)>
+
+=head1 COPYRIGHT
+
+Copyright 2020 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
diff --git a/doc/man3/EVP_SignInit.pod b/doc/man3/EVP_SignInit.pod
index c26b7f7d5d..65df4acfba 100644
--- a/doc/man3/EVP_SignInit.pod
+++ b/doc/man3/EVP_SignInit.pod
@@ -2,10 +2,8 @@
 
 =head1 NAME
 
-EVP_PKEY_size,
-EVP_SignInit, EVP_SignInit_ex, EVP_SignUpdate, EVP_SignFinal,
-EVP_PKEY_security_bits - EVP signing
-functions
+EVP_SignInit, EVP_SignInit_ex, EVP_SignUpdate, EVP_SignFinal
+- EVP signing functions
 
 =head1 SYNOPSIS
 
@@ -17,9 +15,6 @@ functions
 
  void EVP_SignInit(EVP_MD_CTX *ctx, const EVP_MD *type);
 
- int EVP_PKEY_size(const EVP_PKEY *pkey);
- int EVP_PKEY_security_bits(const EVP_PKEY *pkey);
-
 =head1 DESCRIPTION
 
 The EVP signature routines are a high level interface to digital
@@ -34,32 +29,22 @@ signature context B<ctx>. This function can be called several times on the
 same B<ctx> to include additional data.
 
 EVP_SignFinal() signs the data in B<ctx> using the private key B<pkey> and
-places the signature in B<sig>. B<sig> must be at least EVP_PKEY_size(pkey)
+places the signature in B<sig>. B<sig> must be at least C<EVP_PKEY_size(pkey)>
 bytes in size. B<s> is an OUT parameter, and not used as an IN parameter.
 The number of bytes of data written (i.e. the length of the signature)
-will be written to the integer at B<s>, at most EVP_PKEY_size(pkey) bytes
+will be written to the integer at B<s>, at most C<EVP_PKEY_size(pkey)> bytes
 will be written.
 
 EVP_SignInit() initializes a signing context B<ctx> to use the default
 implementation of digest B<type>.
 
-EVP_PKEY_size() returns the maximum size of a signature in bytes. The actual
-signature returned by EVP_SignFinal() may be smaller.
-
-EVP_PKEY_security_bits() returns the number of security bits of the given B<pkey>,
-bits of security is defined in NIST SP800-57.
-
 =head1 RETURN VALUES
 
 EVP_SignInit_ex(), EVP_SignUpdate() and EVP_SignFinal() return 1
 for success and 0 for failure.
 
-EVP_PKEY_size() returns the maximum size of a signature in bytes.
-
 The error codes can be obtained by L<ERR_get_error(3)>.
 
-EVP_PKEY_security_bits() returns the number of security bits.
-
 =head1 NOTES
 
 The B<EVP> interface to digital signatures should almost always be used in
@@ -95,6 +80,7 @@ The previous two bugs are fixed in the newer EVP_SignDigest*() function.
 
 =head1 SEE ALSO
 
+L<EVP_PKEY_size(3)>, L<EVP_PKEY_bits(3)>, L<EVP_PKEY_security_bits(3)>,
 L<EVP_VerifyInit(3)>,
 L<EVP_DigestInit(3)>,
 L<evp(7)>, L<HMAC(3)>, L<MD2(3)>,