openssl: add openssl_digest_size()

Add function to get digest hash size for provided digest name.

src/shared/openssl-util.c

@@ -87,6 +87,37 @@ int openssl_hash(const EVP_MD *alg,
         return 0;
 }
 
+/* Returns the number of bytes generated by the specified digest algorithm. This can be used only for
+ * fixed-size algorithms, e.g. md5, sha1, sha256, etc. Do not use this for variable-sized digest algorithms,
+ * e.g. shake128. Returns 0 on success, -EOPNOTSUPP if the algorithm is not supported, or < 0 for any other
+ * error. */
+int openssl_digest_size(const char *digest_alg, size_t *ret_digest_size) {
+        assert(digest_alg);
+        assert(ret_digest_size);
+
+#if OPENSSL_VERSION_MAJOR >= 3
+        _cleanup_(EVP_MD_freep) EVP_MD *md = EVP_MD_fetch(NULL, digest_alg, NULL);
+#else
+        const EVP_MD *md = EVP_get_digestbyname(digest_alg);
+#endif
+        if (!md)
+                return log_debug_errno(SYNTHETIC_ERRNO(EOPNOTSUPP),
+                                       "Digest algorithm '%s' not supported.", digest_alg);
+
+        size_t digest_size;
+#if OPENSSL_VERSION_MAJOR >= 3
+        digest_size = EVP_MD_get_size(md);
+#else
+        digest_size = EVP_MD_size(md);
+#endif
+        if (digest_size == 0)
+                return log_openssl_errors("Failed to get Digest size");
+
+        *ret_digest_size = digest_size;
+
+        return 0;
+}
+
 int rsa_encrypt_bytes(
                 EVP_PKEY *pkey,
                 const void *decrypted_key,

src/shared/openssl-util.h

@@ -39,6 +39,7 @@ DEFINE_TRIVIAL_CLEANUP_FUNC_FULL(SSL*, SSL_free, NULL);
 DEFINE_TRIVIAL_CLEANUP_FUNC_FULL(BIO*, BIO_free, NULL);
 DEFINE_TRIVIAL_CLEANUP_FUNC_FULL(EVP_MD_CTX*, EVP_MD_CTX_free, NULL);
 #if OPENSSL_VERSION_MAJOR >= 3
+DEFINE_TRIVIAL_CLEANUP_FUNC_FULL(EVP_MD*, EVP_MD_free, NULL);
 DEFINE_TRIVIAL_CLEANUP_FUNC_FULL(OSSL_PARAM*, OSSL_PARAM_free, NULL);
 DEFINE_TRIVIAL_CLEANUP_FUNC_FULL(OSSL_PARAM_BLD*, OSSL_PARAM_BLD_free, NULL);
 #else
@@ -57,6 +58,8 @@ int openssl_pkey_from_pem(const void *pem, size_t pem_size, EVP_PKEY **ret);
 
 int openssl_hash(const EVP_MD *alg, const void *msg, size_t msg_len, uint8_t *ret_hash, size_t *ret_hash_len);
 
+int openssl_digest_size(const char *digest_alg, size_t *ret_digest_size);
+
 int rsa_encrypt_bytes(EVP_PKEY *pkey, const void *decrypted_key, size_t decrypted_key_size, void **ret_encrypt_key, size_t *ret_encrypt_key_size);
 
 int rsa_pkey_to_suitable_key_size(EVP_PKEY *pkey, size_t *ret_suitable_key_size);