X509_digest_sig(): Improve default hash for EdDSA and allow to return the chosen default

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Paul Dale <pauli@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/15762)

apps/lib/cmp_mock_srv.c

@@ -347,7 +347,7 @@ static int process_certConf(OSSL_CMP_SRV_CTX *srv_ctx,
         return 0;
     }
 
-    if ((digest = X509_digest_sig(ctx->certOut)) == NULL)
+    if ((digest = X509_digest_sig(ctx->certOut, NULL, NULL)) == NULL)
         return 0;
     if (ASN1_OCTET_STRING_cmp(certHash, digest) != 0) {
         ASN1_OCTET_STRING_free(digest);

crypto/cmp/cmp_msg.c

@@ -810,10 +810,11 @@ OSSL_CMP_MSG *ossl_cmp_certConf_new(OSSL_CMP_CTX *ctx, int fail_info,
     if (!ASN1_INTEGER_set(certStatus->certReqId, OSSL_CMP_CERTREQID))
         goto err;
     /*
-     * the hash of the certificate, using the same hash algorithm
-     * as is used to create and verify the certificate signature
+     * The hash of the certificate, using the same hash algorithm
+     * as is used to create and verify the certificate signature.
+     * If not available, a default hash algorithm is used.
      */
-    if ((certHash = X509_digest_sig(ctx->newCert)) == NULL)
+    if ((certHash = X509_digest_sig(ctx->newCert, NULL, NULL)) == NULL)
         goto err;
 
     if (!ossl_cmp_certstatus_set0_certHash(certStatus, certHash))

crypto/x509/x_all.c

@@ -434,13 +434,20 @@ int X509_digest(const X509 *cert, const EVP_MD *md, unsigned char *data,
 }
 
 /* calculate cert digest using the same hash algorithm as in its signature */
-ASN1_OCTET_STRING *X509_digest_sig(const X509 *cert)
+ASN1_OCTET_STRING *X509_digest_sig(const X509 *cert,
+                                   EVP_MD **md_used, int *md_is_fallback)
 {
     unsigned int len;
     unsigned char hash[EVP_MAX_MD_SIZE];
     int mdnid, pknid;
     EVP_MD *md = NULL;
-    ASN1_OCTET_STRING *new = NULL;
+    const char *md_name;
+    ASN1_OCTET_STRING *new;
+
+    if (md_used != NULL)
+        *md_used = NULL;
+    if (md_is_fallback != NULL)
+        *md_is_fallback = 0;
 
     if (cert == NULL) {
         ERR_raise(ERR_LIB_X509, ERR_R_PASSED_NULL_PARAMETER);
@@ -474,10 +481,23 @@ ASN1_OCTET_STRING *X509_digest_sig(const X509 *cert)
                 /* Error code from fetch is sufficient */
                 return NULL;
         } else if (pknid != NID_undef) {
-            /* Default to SHA-256 for known algorithms without a digest */
-            if ((md = EVP_MD_fetch(cert->libctx, "SHA256",
+            /* A known algorithm, but without a digest */
+            switch (pknid) {
+            case NID_ED25519: /* Follow CMS default given in RFC8419 */
+                md_name = "SHA512";
+                break;
+            case NID_ED448: /* Follow CMS default given in RFC8419 */
+                md_name = "SHAKE256";
+                break;
+            default: /* Fall back to SHA-256 */
+                md_name = "SHA256";
+                break;
+            }
+            if ((md = EVP_MD_fetch(cert->libctx, md_name,
                                    cert->propq)) == NULL)
                 return NULL;
+            if (md_is_fallback != NULL)
+                *md_is_fallback = 1;
         } else {
             /* A completely unknown algorithm */
             ERR_raise(ERR_LIB_X509, X509_R_UNSUPPORTED_ALGORITHM);
@@ -492,13 +512,17 @@ ASN1_OCTET_STRING *X509_digest_sig(const X509 *cert)
     if (!X509_digest(cert, md, hash, &len)
             || (new = ASN1_OCTET_STRING_new()) == NULL)
         goto err;
-    if (!(ASN1_OCTET_STRING_set(new, hash, len))) {
-        ASN1_OCTET_STRING_free(new);
-        new = NULL;
+    if ((ASN1_OCTET_STRING_set(new, hash, len))) {
+        if (md_used != NULL)
+            *md_used = md;
+        else
+            EVP_MD_free(md);
+        return new;
     }
+    ASN1_OCTET_STRING_free(new);
  err:
     EVP_MD_free(md);
-    return new;
+    return NULL;
 }
 
 int X509_CRL_digest(const X509_CRL *data, const EVP_MD *type,

doc/man3/X509_digest.pod

@@ -17,7 +17,8 @@ PKCS7_ISSUER_AND_SERIAL_digest
 
  int X509_digest(const X509 *data, const EVP_MD *type, unsigned char *md,
                  unsigned int *len);
- ASN1_OCTET_STRING *X509_digest_sig(const X509 *cert);
+ ASN1_OCTET_STRING *X509_digest_sig(const X509 *cert,
+                                    EVP_MD **md_used, int *md_is_fallback);
 
  int X509_CRL_digest(const X509_CRL *data, const EVP_MD *type, unsigned char *md,
                      unsigned int *len);
@@ -39,21 +40,29 @@ PKCS7_ISSUER_AND_SERIAL_digest
 
 =head1 DESCRIPTION
 
-X509_digest_sig() calculates a digest of the given certificate
-using the same hash algorithm as in its signature with a fallback to B<SHA-256>
-for algorithms where the digest is an integral part of the signature algorithm
-such as with B<EdDSA> signatures.
+X509_digest_sig() calculates a digest of the given certificate I<cert>
+using the same hash algorithm as in its signature, if the digest
+is an integral part of the certificate signature algorithm identifier.
+Otherwise, a fallback hash algorithm is determined as follows:
+SHA512 if the signature alorithm is ED25519,
+SHAKE256 if it is ED448, otherwise SHA256.
+The output parmeters are assigned as follows.
+Unless I<md_used> is NULL, the hash algorithm used is provided
+in I<*md_used> and must be freed by the caller (if it is not NULL).
+Unless I<md_is_fallback> is NULL,
+the I<*md_is_fallback> is set to 1 if the hash algorithm used is a fallback,
+otherwise to 0.
 
 X509_pubkey_digest() returns a digest of the DER representation of the public
-key in the specified X509 B<data> object.
+key in the specified X509 I<data> object.
 
 All other functions described here return a digest of the DER representation
-of their entire B<data> objects.
+of their entire I<data> objects.
 
-The B<type> parameter specifies the digest to
-be used, such as EVP_sha1(). The B<md> is a pointer to the buffer where the
+The I<type> parameter specifies the digest to
+be used, such as EVP_sha1(). The I<md> is a pointer to the buffer where the
 digest will be copied and is assumed to be large enough; the constant
-B<EVP_MAX_MD_SIZE> is suggested. The B<len> parameter, if not NULL, points
+B<EVP_MAX_MD_SIZE> is suggested. The I<len> parameter, if not NULL, points
 to a place where the digest size will be stored.
 
 =head1 RETURN VALUES

include/openssl/x509.h.in

@@ -346,7 +346,8 @@ int X509_pubkey_digest(const X509 *data, const EVP_MD *type,
                        unsigned char *md, unsigned int *len);
 int X509_digest(const X509 *data, const EVP_MD *type,
                 unsigned char *md, unsigned int *len);
-ASN1_OCTET_STRING *X509_digest_sig(const X509 *cert);
+ASN1_OCTET_STRING *X509_digest_sig(const X509 *cert,
+                                   EVP_MD **md_used, int *md_is_fallback);
 int X509_CRL_digest(const X509_CRL *data, const EVP_MD *type,
                     unsigned char *md, unsigned int *len);
 int X509_REQ_digest(const X509_REQ *data, const EVP_MD *type,