Simplify getting an EVP_MD for all OpenSSL forks

In all versions of OpenSSL, EVP_sha{1,256}() return a pointer to
static read-only data. There is no need to copy that data into a
buffer that is immediately freed after use.

In LibreSSL and OpenSSL 1.x, EVP_PKEY_CTX_set_signature_md() is a
wrapper around EVP_PKEY_ctrl() whose void * argument isn't const
(contrary to the documented signature), so we need to cast const
away in some way. This preserves rs*_get_EVP_MD() to contain the
pragma pollution necessary due to -Werror -Wcast-qual.

The negative performance impact of calling EVP_sha*() multiple
times called out in the OpenSSL 3.x application should not apply
in this code.

Closes: #794

src/rs1.c

@@ -10,54 +10,14 @@
 
 #include "fido.h"
 
-#if defined(LIBRESSL_VERSION_NUMBER) && LIBRESSL_VERSION_NUMBER < 0x3050200fL
 static EVP_MD *
 rs1_get_EVP_MD(void)
 {
-	const EVP_MD *from;
-	EVP_MD *to = NULL;
-
-	if ((from = EVP_sha1()) != NULL && (to = malloc(sizeof(*to))) != NULL)
-		memcpy(to, from, sizeof(*to));
-
-	return (to);
-}
-
-static void
-rs1_free_EVP_MD(EVP_MD *md)
-{
-	freezero(md, sizeof(*md));
-}
-#elif OPENSSL_VERSION_NUMBER >= 0x30000000
-static EVP_MD *
-rs1_get_EVP_MD(void)
-{
-	return (EVP_MD_fetch(NULL, "SHA-1", NULL));
-}
-
-static void
-rs1_free_EVP_MD(EVP_MD *md)
-{
-	EVP_MD_free(md);
-}
-#else
-static EVP_MD *
-rs1_get_EVP_MD(void)
-{
-	const EVP_MD *md;
-
-	if ((md = EVP_sha1()) == NULL)
-		return (NULL);
-
-	return (EVP_MD_meth_dup(md));
-}
-
-static void
-rs1_free_EVP_MD(EVP_MD *md)
-{
-	EVP_MD_meth_free(md);
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wcast-qual"
+	return ((EVP_MD *)EVP_sha1());
+#pragma GCC diagnostic pop
 }
-#endif /* LIBRESSL_VERSION_NUMBER */
 
 int
 rs1_verify_sig(const fido_blob_t *dgst, EVP_PKEY *pkey,
@@ -94,7 +54,6 @@ rs1_verify_sig(const fido_blob_t *dgst, EVP_PKEY *pkey,
 	ok = 0;
 fail:
 	EVP_PKEY_CTX_free(pctx);
-	rs1_free_EVP_MD(md);
 
 	return (ok);
 }

src/rs256.c

@@ -18,54 +18,14 @@
 #define get0_RSA(x)	EVP_PKEY_get0((x))
 #endif
 
-#if defined(LIBRESSL_VERSION_NUMBER) && LIBRESSL_VERSION_NUMBER < 0x3050200fL
 static EVP_MD *
 rs256_get_EVP_MD(void)
 {
-	const EVP_MD *from;
-	EVP_MD *to = NULL;
-
-	if ((from = EVP_sha256()) != NULL && (to = malloc(sizeof(*to))) != NULL)
-		memcpy(to, from, sizeof(*to));
-
-	return (to);
-}
-
-static void
-rs256_free_EVP_MD(EVP_MD *md)
-{
-	freezero(md, sizeof(*md));
-}
-#elif OPENSSL_VERSION_NUMBER >= 0x30000000
-static EVP_MD *
-rs256_get_EVP_MD(void)
-{
-	return (EVP_MD_fetch(NULL, "SHA2-256", NULL));
-}
-
-static void
-rs256_free_EVP_MD(EVP_MD *md)
-{
-	EVP_MD_free(md);
-}
-#else
-static EVP_MD *
-rs256_get_EVP_MD(void)
-{
-	const EVP_MD *md;
-
-	if ((md = EVP_sha256()) == NULL)
-		return (NULL);
-
-	return (EVP_MD_meth_dup(md));
-}
-
-static void
-rs256_free_EVP_MD(EVP_MD *md)
-{
-	EVP_MD_meth_free(md);
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wcast-qual"
+	return ((EVP_MD *)EVP_sha256());
+#pragma GCC diagnostic pop
 }
-#endif /* LIBRESSL_VERSION_NUMBER */
 
 static int
 decode_bignum(const cbor_item_t *item, void *ptr, size_t len)
@@ -290,7 +250,6 @@ rs256_verify_sig(const fido_blob_t *dgst, EVP_PKEY *pkey,
 	ok = 0;
 fail:
 	EVP_PKEY_CTX_free(pctx);
-	rs256_free_EVP_MD(md);
 
 	return (ok);
 }