Add support for setting raw private Poly1305 keys

Reviewed-by: NRichard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/5520)

crypto/poly1305/poly1305_ameth.c

@@ -13,6 +13,7 @@
 #include "internal/asn1_int.h"
 #include "internal/poly1305.h"
 #include "poly1305_local.h"
+#include "internal/evp_int.h"
 
 /*
  * POLY1305 "ASN1" method. This is just here to indicate the maximum
@@ -45,6 +46,27 @@ static int poly1305_pkey_public_cmp(const EVP_PKEY *a, const EVP_PKEY *b)
     return ASN1_OCTET_STRING_cmp(EVP_PKEY_get0(a), EVP_PKEY_get0(b));
 }
 
+static int poly1305_set_priv_key(EVP_PKEY *pkey, const unsigned char *priv,
+                                 size_t len)
+{
+    ASN1_OCTET_STRING *os;
+
+    if (pkey->pkey.ptr != NULL || len != POLY1305_KEY_SIZE)
+        return 0;
+
+    os = ASN1_OCTET_STRING_new();
+    if (os == NULL)
+        return 0;
+
+    if (!ASN1_OCTET_STRING_set(os, priv, len)) {
+        ASN1_OCTET_STRING_free(os);
+        return 0;
+    }
+
+    pkey->pkey.ptr = os;
+    return 1;
+}
+
 const EVP_PKEY_ASN1_METHOD poly1305_asn1_meth = {
     EVP_PKEY_POLY1305,
     EVP_PKEY_POLY1305,
@@ -63,5 +85,17 @@ const EVP_PKEY_ASN1_METHOD poly1305_asn1_meth = {
 
     poly1305_key_free,
     poly1305_pkey_ctrl,
-    0, 0
+    NULL,
+    NULL,
+
+    NULL,
+    NULL,
+    NULL,
+
+    NULL,
+    NULL,
+    NULL,
+
+    poly1305_set_priv_key,
+    NULL,
 };