DOC: Add more description of EVP_PKEY_fromdata(), and examples

Fixes #11131

Reviewed-by: Paul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/11220)

1 files changed, 165 insertions, 7 deletions

diff --git a/doc/man3/EVP_PKEY_fromdata.pod b/doc/man3/EVP_PKEY_fromdata.pod
index e3ddf68058..0d85e80b95 100644
--- a/doc/man3/EVP_PKEY_fromdata.pod
+++ b/doc/man3/EVP_PKEY_fromdata.pod
@@ -18,18 +18,29 @@ EVP_PKEY_param_fromdata_settable, EVP_PKEY_key_fromdata_settable
 
 =head1 DESCRIPTION
 
+The functions described here are used to create new keys from user
+provided key data, such as I<n>, I<e> and I<d> for a minimal RSA
+keypair.
+
+These functions use an B<EVP_PKEY_CTX> context, which should primarly
+be created with L<EVP_PKEY_CTX_new_from_name(3)> or
+L<EVP_PKEY_CTX_new_id(3)>.
+
+The exact key data that the user can pass depends on the key type.
+These are passed as an L<OSSL_PARAM(3)> array.
+
 EVP_PKEY_param_fromdata_init() initializes a public key algorithm context
 for creating key parameters from user data.
 
 EVP_PKEY_key_fromdata_init() initializes a public key algorithm context for
 creating a key from user data.
 
-EVP_PKEY_fromdata() creates key parameters or a key, given data from
-I<params> and a context that's been initialized with
-EVP_PKEY_param_fromdata_init() or EVP_PKEY_key_fromdata_init().  The result is
-written to I<*ppkey>. The parameters that can be used for various types of key
-are as described in the "Built-in RSA Import/Export Types" section on the
-L<provider-keymgmt(7)> page.
+EVP_PKEY_fromdata() creates the structure to store key parameters or a
+key, given data from I<params> and a context that's been initialized with
+EVP_PKEY_param_fromdata_init() or EVP_PKEY_key_fromdata_init().  The result
+is written to I<*ppkey>.  The parameters that can be used are specific to
+the L<provider-keymgmt(7)> implementations, please see
+L<EVP_PKEY_CTX_new_from_name(3)> for further information.
 
 EVP_PKEY_param_fromdata_settable() and EVP_PKEY_key_fromdata_settable()
 get a constant B<OSSL_PARAM> array that describes the settable parameters
@@ -50,9 +61,156 @@ EVP_PKEY_fromdata() return 1 for success and 0 or a negative value for
 failure.  In particular a return value of -2 indicates the operation is
 not supported by the public key algorithm.
 
+=head1 EXAMPLES
+
+These examples are very terse for the sake of staying on topic, which
+is the EVP_PKEY_fromdata() set of functions.  In real applications,
+BIGNUMs would be handled and converted to byte arrays with
+BN_bn2nativepad(), but that's off topic here.
+
+=begin comment
+
+TODO Write a set of cookbook documents and link to them.
+
+=end comment
+
+=head2 Creating an RSA keypair using raw key data
+
+ #include <openssl/evp.h>
+
+ /*
+  * These are extremely small to make this example simple.  A real
+  * and secure application will not use such small numbers.  A real
+  * and secure application is expected to use BIGNUMs, and to build
+  * this array dynamically.
+  */
+ const unsigned long rsa_n = 0xbc747fc5;
+ const unsigned long rsa_e = 0x10001;
+ const unsigned long rsa_d = 0x7b133399;
+ const OSSL_PARAM[] = {
+     OSSL_PARAM_ulong("n", &rsa_n),
+     OSSL_PARAM_ulong("e", &rsa_e),
+     OSSL_PARAM_ulong("d", &rsa_d),
+     OSSL_PARAM_END
+ };
+     
+ int main()
+ {
+     EVP_PKEY_CTX *ctx = EVP_PKEY_CTX_new_from_name(NULL, "RSA", NULL);
+     EVP_PKEY *pkey = NULL;
+
+     if (ctx == NULL
+         || !EVP_PKEY_key_fromdata_init(ctx)
+         || !EVP_PKEY_fromdata(ctx, &pkey, params))
+         exit(1);
+
+     /* Do what you want with |pkey| */
+ }
+
+=head2 Creating an ECC keypair using raw key data
+
+ #include <openssl/evp.h>
+
+ /*
+  * These arrays represent large numbers, big endian organization.
+  * In a real application, these would probably be bignums that get
+  * converted to the native integer organization with BN_bn2nativepad().
+  * We're not doing that here, since this is not an example of BIGNUM
+  * functionality, but an example of EVP_PKEY_fromdata().
+  */
+ #ifndef B_ENDIAN
+ # error "We haven't prepared little endian arrays"
+ #endif
+ const unsigned char priv[] = {
+     0xb9, 0x2f, 0x3c, 0xe6, 0x2f, 0xfb, 0x45, 0x68,
+     0x39, 0x96, 0xf0, 0x2a, 0xaf, 0x6c, 0xda, 0xf2,
+     0x89, 0x8a, 0x27, 0xbf, 0x39, 0x9b, 0x7e, 0x54,
+     0x21, 0xc2, 0xa1, 0xe5, 0x36, 0x12, 0x48, 0x5d
+ };
+ const unsigned char pub[] = {
+     0x04, 0xcf, 0x20, 0xfb, 0x9a, 0x1d, 0x11, 0x6c,
+     0x5e, 0x9f, 0xec, 0x38, 0x87, 0x6c, 0x1d, 0x2f,
+     0x58, 0x47, 0xab, 0xa3, 0x9b, 0x79, 0x23, 0xe6,
+     0xeb, 0x94, 0x6f, 0x97, 0xdb, 0xa3, 0x7d, 0xbd,
+     0xe5, 0x26, 0xca, 0x07, 0x17, 0x8d, 0x26, 0x75,
+     0xff, 0xcb, 0x8e, 0xb6, 0x84, 0xd0, 0x24, 0x02,
+     0x25, 0x8f, 0xb9, 0x33, 0x6e, 0xcf, 0x12, 0x16,
+     0x2f, 0x5c, 0xcd, 0x86, 0x71, 0xa8, 0xbf, 0x1a,
+     0x47
+ };
+ const OSSL_PARAM params[] = {
+     OSSL_PARAM_utf8_string("curve-name", "prime256v1"),
+     OSSL_PARAM_BN("priv", priv, sizeof(priv)),
+     OSSL_PARAM_BN("pub", pub, sizeof(pub)),
+     OSSL_PARAM_END
+ };
+
+ int main()
+ {
+     EVP_PKEY_CTX *ctx = EVP_PKEY_CTX_new_from_name(NULL, "EC", NULL);
+     EVP_PKEY *pkey = NULL;
+
+     if (ctx == NULL
+         || !EVP_PKEY_key_fromdata_init(ctx)
+         || !EVP_PKEY_fromdata(ctx, &pkey, params))
+         exit(1);
+
+     /* Do what you want with |pkey| */
+ }
+
+=head2 Finding out params for an unknown key type
+
+ #include <openssl/evp.h>
+
+ /* Program expects a key type as first argument */
+ int main(int argc, char *argv[])
+ {
+     EVP_PKEY_CTX *ctx = EVP_PKEY_CTX_new_from_name(NULL, argv[1], NULL);
+     const *OSSL_PARAM *settable_params = NULL;
+
+     if (ctx == NULL
+         || (settable_params = EVP_PKEY_key_fromdata_settable(ctx)) == NULL)
+         exit(1);
+
+     for (; settable_params->key != NULL; settable_params++) {
+         const char *datatype = NULL;
+
+         switch (settable_params->data_type) {
+         case OSSL_PARAM_INTEGER:
+             datatype = "integer";
+             break;
+         case OSSL_PARAM_UNSIGNED_INTEGER:
+             datatype = "unsigned integer";
+             break;
+         case OSSL_PARAM_UTF8_STRING:
+             datatype = "printable string (utf-8 encoding expected)";
+             break;
+         case OSSL_PARAM_UTF8_PTR:
+             datatype = "printable string pointer (utf-8 encoding expected)";
+             break;
+         case OSSL_PARAM_OCTET_STRING:
+             datatype = "octet string";
+             break;
+         case OSSL_PARAM_OCTET_PTR:
+             datatype = "octet string pointer";
+             break;
+         }
+         printf("%s : %s ", settable_params->key, datatype);
+         if (settable_params->data_size == 0)
+             printf("(unlimited size)");
+         else
+             printf("(maximum size %zu)", settable_params->data_size);
+     }
+ }
+
+The descriptor L<OSSL_PARAM(3)> returned by
+EVP_PKEY_key_fromdata_settable() may also be used programmatically, for
+example with L<OSSL_PARAM_allocate_from_text(3)>.
+
 =head1 SEE ALSO
 
-L<EVP_PKEY_CTX_new(3)>, L<provider(7)>, L<EVP_PKEY_gettable_params(3)>
+L<EVP_PKEY_CTX_new(3)>, L<provider(7)>, L<EVP_PKEY_gettable_params(3)>,
+L<OSSL_PARAM(3)>
 
 =head1 HISTORY