From d5af33da533c54342006b6f4d96c5455bb7ac2e5 Mon Sep 17 00:00:00 2001
From: Simon Tatham <anakin@pobox.com>
Date: Fri, 15 Apr 2022 17:18:23 +0100
Subject: [PATCH] Utility function mp_resize.

This reallocs an existing mp_int to have a different physical size,
e.g. to make sure there's enough space at the top of it.

Trivial, but I'm a little surprised I haven't needed it until now!
---
 crypto/mpint.c | 8 ++++++++
 mpint.h        | 7 +++++++
 2 files changed, 15 insertions(+)

diff --git a/crypto/mpint.c b/crypto/mpint.c
index 544a18b5..437c7e8c 100644
--- a/crypto/mpint.c
+++ b/crypto/mpint.c
@@ -82,6 +82,14 @@ mp_int *mp_new(size_t maxbits)
     return mp_make_sized(words);
 }
 
+mp_int *mp_resize(mp_int *mp, size_t newmaxbits)
+{
+    mp_int *copy = mp_new(newmaxbits);
+    mp_copy_into(copy, mp);
+    mp_free(mp);
+    return copy;
+}
+
 mp_int *mp_from_integer(uintmax_t n)
 {
     mp_int *x = mp_make_sized(
diff --git a/mpint.h b/mpint.h
index ae09a24f..4ddc0e64 100644
--- a/mpint.h
+++ b/mpint.h
@@ -42,6 +42,13 @@ mp_int *mp_new(size_t maxbits);
 void mp_free(mp_int *);
 void mp_clear(mp_int *x);
 
+/*
+ * Resize the physical size of existing mp_int, e.g. so that you have
+ * room to transform it in place to a larger value. Destroys the old
+ * mp_int in the process.
+ */
+mp_int *mp_resize(mp_int *, size_t newmaxbits);
+
 /*
  * Create mp_ints from various sources: little- and big-endian binary
  * data, an ordinary C unsigned integer type, a decimal or hex string