Ignore the digest in req app if using EdDSA

This follows on from the previous commit, and makes the same change to
ignore the digest if we are using EdDSA.
Reviewed-by: NViktor Dukhovni <viktor@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6901)

apps/req.c

@@ -1601,10 +1601,19 @@ static int do_sign_init(EVP_MD_CTX *ctx, EVP_PKEY *pkey,
                         const EVP_MD *md, STACK_OF(OPENSSL_STRING) *sigopts)
 {
     EVP_PKEY_CTX *pkctx = NULL;
-    int i;
+    int i, def_nid;
 
     if (ctx == NULL)
         return 0;
+    /*
+     * EVP_PKEY_get_default_digest_nid() returns 2 if the digest is mandatory
+     * for this algorithm.
+     */
+    if (EVP_PKEY_get_default_digest_nid(pkey, &def_nid) == 2
+            && def_nid == NID_undef) {
+        /* The signing algorithm requires there to be no digest */
+        md = NULL;
+    }
     if (!EVP_DigestSignInit(ctx, &pkctx, md, NULL, pkey))
         return 0;
     for (i = 0; i < sk_OPENSSL_STRING_num(sigopts); i++) {

doc/man1/req.pod

@@ -209,7 +209,7 @@ the configuration file.
 
 Some public key algorithms may override this choice. For instance, DSA
 signatures always use SHA1, GOST R 34.10 signatures always use
-GOST R 34.11-94 (B<-md_gost94>).
+GOST R 34.11-94 (B<-md_gost94>), Ed25519 and Ed448 never use any digest.
 
 =item B<-config filename>
 
@@ -394,10 +394,10 @@ option. For compatibility B<encrypt_rsa_key> is an equivalent option.
 
 =item B<default_md>
 
-This option specifies the digest algorithm to use.
-Any digest supported by the OpenSSL B<dgst> command can be used.
-If not present then MD5 is used.
-This option can be overridden on the command line.
+This option specifies the digest algorithm to use. Any digest supported by the
+OpenSSL B<dgst> command can be used. This option can be overridden on the
+command line. Certain signing algorithms (i.e. Ed25519 and Ed448) will ignore
+any digest that has been set.
 
 =item B<string_mask>