diff --git a/Buildscr b/Buildscr
index c9420658..84dda5c7 100644
--- a/Buildscr
+++ b/Buildscr
@@ -150,7 +150,7 @@ in putty do perl -i~ -pe 'y/\015//d;s/$$/\015/' LICENCE
 
 delegate windows
   # FIXME: Cygwin alternative?
-  in putty/windows do/win vcvars32 && nmake -f Makefile.vc $(Makeargs)
+  in putty/windows do/win vcvars32 && nmake -f Makefile.vc $(Makeargs) all cleantestprogs
   # Code-sign the binaries, if the local bob config provides a script
   # to do so. We assume here that the script accepts an -i option to
   # provide a 'more info' URL, and an optional -n option to provide a
diff --git a/Recipe b/Recipe
index fc8e444a..0d119d13 100644
--- a/Recipe
+++ b/Recipe
@@ -323,3 +323,14 @@ PuTTY    : [MX] osxmain OSXTERM OSXMISC CHARSET U_BE_ALL NONSSH UXSSH
 
 fuzzterm : [UT] UXTERM CHARSET misc uxmisc uxucs fuzzterm time settings
 	 + uxstore be_none
+testbn   : [UT] testbn sshbn misc conf tree234 uxmisc
+testbn   : [C] testbn sshbn misc conf tree234 winmisc LIBS
+
+# ----------------------------------------------------------------------
+# On Windows, provide a means of removing local test binaries that we
+# aren't going to actually ship. (I prefer this to not building them
+# in the first place, so that we find out about build breakage early.)
+!begin vc
+cleantestprogs:
+	-del testbn.exe
+!end
diff --git a/sshbn.c b/sshbn.c
index 4f27dc9b..724cf301 100644
--- a/sshbn.c
+++ b/sshbn.c
@@ -2180,269 +2180,3 @@ char *bignum_decimal(Bignum x)
     sfree(workspace);
     return ret;
 }
-
-#ifdef TESTBN
-
-#include <stdio.h>
-#include <stdlib.h>
-#include <ctype.h>
-
-/*
- * gcc -Wall -g -O0 -DTESTBN -o testbn sshbn.c misc.c conf.c tree234.c unix/uxmisc.c -I. -I unix -I charset
- *
- * Then feed to this program's standard input the output of
- * testdata/bignum.py .
- */
-
-void modalfatalbox(const char *p, ...)
-{
-    va_list ap;
-    fprintf(stderr, "FATAL ERROR: ");
-    va_start(ap, p);
-    vfprintf(stderr, p, ap);
-    va_end(ap);
-    fputc('\n', stderr);
-    exit(1);
-}
-
-int random_byte(void)
-{
-    modalfatalbox("random_byte called in testbn");
-    return 0;
-}
-
-#define fromxdigit(c) ( (c)>'9' ? ((c)&0xDF) - 'A' + 10 : (c) - '0' )
-
-int main(int argc, char **argv)
-{
-    char *buf;
-    int line = 0;
-    int passes = 0, fails = 0;
-
-    while ((buf = fgetline(stdin)) != NULL) {
-        int maxlen = strlen(buf);
-        unsigned char *data = snewn(maxlen, unsigned char);
-        unsigned char *ptrs[5], *q;
-        int ptrnum;
-        char *bufp = buf;
-
-        line++;
-
-        q = data;
-        ptrnum = 0;
-
-        while (*bufp && !isspace((unsigned char)*bufp))
-            bufp++;
-        if (bufp)
-            *bufp++ = '\0';
-
-        while (*bufp) {
-            char *start, *end;
-            int i;
-
-            while (*bufp && !isxdigit((unsigned char)*bufp))
-                bufp++;
-            start = bufp;
-
-            if (!*bufp)
-                break;
-
-            while (*bufp && isxdigit((unsigned char)*bufp))
-                bufp++;
-            end = bufp;
-
-            if (ptrnum >= lenof(ptrs))
-                break;
-            ptrs[ptrnum++] = q;
-            
-            for (i = -((end - start) & 1); i < end-start; i += 2) {
-                unsigned char val = (i < 0 ? 0 : fromxdigit(start[i]));
-                val = val * 16 + fromxdigit(start[i+1]);
-                *q++ = val;
-            }
-
-            ptrs[ptrnum] = q;
-        }
-
-        if (!strcmp(buf, "mul")) {
-            Bignum a, b, c, p;
-
-            if (ptrnum != 3) {
-                printf("%d: mul with %d parameters, expected 3\n", line, ptrnum);
-                exit(1);
-            }
-            a = bignum_from_bytes(ptrs[0], ptrs[1]-ptrs[0]);
-            b = bignum_from_bytes(ptrs[1], ptrs[2]-ptrs[1]);
-            c = bignum_from_bytes(ptrs[2], ptrs[3]-ptrs[2]);
-            p = bigmul(a, b);
-
-            if (bignum_cmp(c, p) == 0) {
-                passes++;
-            } else {
-                char *as = bignum_decimal(a);
-                char *bs = bignum_decimal(b);
-                char *cs = bignum_decimal(c);
-                char *ps = bignum_decimal(p);
-                
-                printf("%d: fail: %s * %s gave %s expected %s\n",
-                       line, as, bs, ps, cs);
-                fails++;
-
-                sfree(as);
-                sfree(bs);
-                sfree(cs);
-                sfree(ps);
-            }
-            freebn(a);
-            freebn(b);
-            freebn(c);
-            freebn(p);
-        } else if (!strcmp(buf, "modmul")) {
-            Bignum a, b, m, c, p;
-
-            if (ptrnum != 4) {
-                printf("%d: modmul with %d parameters, expected 4\n",
-                       line, ptrnum);
-                exit(1);
-            }
-            a = bignum_from_bytes(ptrs[0], ptrs[1]-ptrs[0]);
-            b = bignum_from_bytes(ptrs[1], ptrs[2]-ptrs[1]);
-            m = bignum_from_bytes(ptrs[2], ptrs[3]-ptrs[2]);
-            c = bignum_from_bytes(ptrs[3], ptrs[4]-ptrs[3]);
-            p = modmul(a, b, m);
-
-            if (bignum_cmp(c, p) == 0) {
-                passes++;
-            } else {
-                char *as = bignum_decimal(a);
-                char *bs = bignum_decimal(b);
-                char *ms = bignum_decimal(m);
-                char *cs = bignum_decimal(c);
-                char *ps = bignum_decimal(p);
-                
-                printf("%d: fail: %s * %s mod %s gave %s expected %s\n",
-                       line, as, bs, ms, ps, cs);
-                fails++;
-
-                sfree(as);
-                sfree(bs);
-                sfree(ms);
-                sfree(cs);
-                sfree(ps);
-            }
-            freebn(a);
-            freebn(b);
-            freebn(m);
-            freebn(c);
-            freebn(p);
-        } else if (!strcmp(buf, "pow")) {
-            Bignum base, expt, modulus, expected, answer;
-
-            if (ptrnum != 4) {
-                printf("%d: pow with %d parameters, expected 4\n", line, ptrnum);
-                exit(1);
-            }
-
-            base = bignum_from_bytes(ptrs[0], ptrs[1]-ptrs[0]);
-            expt = bignum_from_bytes(ptrs[1], ptrs[2]-ptrs[1]);
-            modulus = bignum_from_bytes(ptrs[2], ptrs[3]-ptrs[2]);
-            expected = bignum_from_bytes(ptrs[3], ptrs[4]-ptrs[3]);
-            answer = modpow(base, expt, modulus);
-
-            if (bignum_cmp(expected, answer) == 0) {
-                passes++;
-            } else {
-                char *as = bignum_decimal(base);
-                char *bs = bignum_decimal(expt);
-                char *cs = bignum_decimal(modulus);
-                char *ds = bignum_decimal(answer);
-                char *ps = bignum_decimal(expected);
-                
-                printf("%d: fail: %s ^ %s mod %s gave %s expected %s\n",
-                       line, as, bs, cs, ds, ps);
-                fails++;
-
-                sfree(as);
-                sfree(bs);
-                sfree(cs);
-                sfree(ds);
-                sfree(ps);
-            }
-            freebn(base);
-            freebn(expt);
-            freebn(modulus);
-            freebn(expected);
-            freebn(answer);
-        } else if (!strcmp(buf, "divmod")) {
-            Bignum n, d, expect_q, expect_r, answer_q, answer_r;
-            int fail;
-
-            if (ptrnum != 4) {
-                printf("%d: divmod with %d parameters, expected 4\n", line, ptrnum);
-                exit(1);
-            }
-
-            n = bignum_from_bytes(ptrs[0], ptrs[1]-ptrs[0]);
-            d = bignum_from_bytes(ptrs[1], ptrs[2]-ptrs[1]);
-            expect_q = bignum_from_bytes(ptrs[2], ptrs[3]-ptrs[2]);
-            expect_r = bignum_from_bytes(ptrs[3], ptrs[4]-ptrs[3]);
-            answer_q = bigdiv(n, d);
-            answer_r = bigmod(n, d);
-
-            fail = FALSE;
-            if (bignum_cmp(expect_q, answer_q) != 0) {
-                char *as = bignum_decimal(n);
-                char *bs = bignum_decimal(d);
-                char *cs = bignum_decimal(answer_q);
-                char *ds = bignum_decimal(expect_q);
-
-                printf("%d: fail: %s / %s gave %s expected %s\n",
-                       line, as, bs, cs, ds);
-                fail = TRUE;
-
-                sfree(as);
-                sfree(bs);
-                sfree(cs);
-                sfree(ds);
-            }
-            if (bignum_cmp(expect_r, answer_r) != 0) {
-                char *as = bignum_decimal(n);
-                char *bs = bignum_decimal(d);
-                char *cs = bignum_decimal(answer_r);
-                char *ds = bignum_decimal(expect_r);
-
-                printf("%d: fail: %s mod %s gave %s expected %s\n",
-                       line, as, bs, cs, ds);
-                fail = TRUE;
-
-                sfree(as);
-                sfree(bs);
-                sfree(cs);
-                sfree(ds);
-            }
-
-            freebn(n);
-            freebn(d);
-            freebn(expect_q);
-            freebn(expect_r);
-            freebn(answer_q);
-            freebn(answer_r);
-
-            if (fail)
-                fails++;
-            else
-                passes++;
-        } else {
-            printf("%d: unrecognised test keyword: '%s'\n", line, buf);
-            exit(1);
-        }
-
-        sfree(buf);
-        sfree(data);
-    }
-
-    printf("passed %d failed %d total %d\n", passes, fails, passes+fails);
-    return fails != 0;
-}
-
-#endif
diff --git a/testbn.c b/testbn.c
new file mode 100644
index 00000000..55471bd2
--- /dev/null
+++ b/testbn.c
@@ -0,0 +1,265 @@
+/*
+ * testbn.c: standalone test program for the bignum code.
+ */
+
+/*
+ * Accepts input on standard input, in the form generated by
+ * testdata/bignum.py.
+ */
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <ctype.h>
+
+#include "ssh.h"
+
+void modalfatalbox(const char *p, ...)
+{
+    va_list ap;
+    fprintf(stderr, "FATAL ERROR: ");
+    va_start(ap, p);
+    vfprintf(stderr, p, ap);
+    va_end(ap);
+    fputc('\n', stderr);
+    exit(1);
+}
+
+int random_byte(void)
+{
+    modalfatalbox("random_byte called in testbn");
+    return 0;
+}
+
+#define fromxdigit(c) ( (c)>'9' ? ((c)&0xDF) - 'A' + 10 : (c) - '0' )
+
+int main(int argc, char **argv)
+{
+    char *buf;
+    int line = 0;
+    int passes = 0, fails = 0;
+
+    while ((buf = fgetline(stdin)) != NULL) {
+        int maxlen = strlen(buf);
+        unsigned char *data = snewn(maxlen, unsigned char);
+        unsigned char *ptrs[5], *q;
+        int ptrnum;
+        char *bufp = buf;
+
+        line++;
+
+        q = data;
+        ptrnum = 0;
+
+        while (*bufp && !isspace((unsigned char)*bufp))
+            bufp++;
+        if (bufp)
+            *bufp++ = '\0';
+
+        while (*bufp) {
+            char *start, *end;
+            int i;
+
+            while (*bufp && !isxdigit((unsigned char)*bufp))
+                bufp++;
+            start = bufp;
+
+            if (!*bufp)
+                break;
+
+            while (*bufp && isxdigit((unsigned char)*bufp))
+                bufp++;
+            end = bufp;
+
+            if (ptrnum >= lenof(ptrs))
+                break;
+            ptrs[ptrnum++] = q;
+            
+            for (i = -((end - start) & 1); i < end-start; i += 2) {
+                unsigned char val = (i < 0 ? 0 : fromxdigit(start[i]));
+                val = val * 16 + fromxdigit(start[i+1]);
+                *q++ = val;
+            }
+
+            ptrs[ptrnum] = q;
+        }
+
+        if (!strcmp(buf, "mul")) {
+            Bignum a, b, c, p;
+
+            if (ptrnum != 3) {
+                printf("%d: mul with %d parameters, expected 3\n", line, ptrnum);
+                exit(1);
+            }
+            a = bignum_from_bytes(ptrs[0], ptrs[1]-ptrs[0]);
+            b = bignum_from_bytes(ptrs[1], ptrs[2]-ptrs[1]);
+            c = bignum_from_bytes(ptrs[2], ptrs[3]-ptrs[2]);
+            p = bigmul(a, b);
+
+            if (bignum_cmp(c, p) == 0) {
+                passes++;
+            } else {
+                char *as = bignum_decimal(a);
+                char *bs = bignum_decimal(b);
+                char *cs = bignum_decimal(c);
+                char *ps = bignum_decimal(p);
+                
+                printf("%d: fail: %s * %s gave %s expected %s\n",
+                       line, as, bs, ps, cs);
+                fails++;
+
+                sfree(as);
+                sfree(bs);
+                sfree(cs);
+                sfree(ps);
+            }
+            freebn(a);
+            freebn(b);
+            freebn(c);
+            freebn(p);
+        } else if (!strcmp(buf, "modmul")) {
+            Bignum a, b, m, c, p;
+
+            if (ptrnum != 4) {
+                printf("%d: modmul with %d parameters, expected 4\n",
+                       line, ptrnum);
+                exit(1);
+            }
+            a = bignum_from_bytes(ptrs[0], ptrs[1]-ptrs[0]);
+            b = bignum_from_bytes(ptrs[1], ptrs[2]-ptrs[1]);
+            m = bignum_from_bytes(ptrs[2], ptrs[3]-ptrs[2]);
+            c = bignum_from_bytes(ptrs[3], ptrs[4]-ptrs[3]);
+            p = modmul(a, b, m);
+
+            if (bignum_cmp(c, p) == 0) {
+                passes++;
+            } else {
+                char *as = bignum_decimal(a);
+                char *bs = bignum_decimal(b);
+                char *ms = bignum_decimal(m);
+                char *cs = bignum_decimal(c);
+                char *ps = bignum_decimal(p);
+                
+                printf("%d: fail: %s * %s mod %s gave %s expected %s\n",
+                       line, as, bs, ms, ps, cs);
+                fails++;
+
+                sfree(as);
+                sfree(bs);
+                sfree(ms);
+                sfree(cs);
+                sfree(ps);
+            }
+            freebn(a);
+            freebn(b);
+            freebn(m);
+            freebn(c);
+            freebn(p);
+        } else if (!strcmp(buf, "pow")) {
+            Bignum base, expt, modulus, expected, answer;
+
+            if (ptrnum != 4) {
+                printf("%d: pow with %d parameters, expected 4\n", line, ptrnum);
+                exit(1);
+            }
+
+            base = bignum_from_bytes(ptrs[0], ptrs[1]-ptrs[0]);
+            expt = bignum_from_bytes(ptrs[1], ptrs[2]-ptrs[1]);
+            modulus = bignum_from_bytes(ptrs[2], ptrs[3]-ptrs[2]);
+            expected = bignum_from_bytes(ptrs[3], ptrs[4]-ptrs[3]);
+            answer = modpow(base, expt, modulus);
+
+            if (bignum_cmp(expected, answer) == 0) {
+                passes++;
+            } else {
+                char *as = bignum_decimal(base);
+                char *bs = bignum_decimal(expt);
+                char *cs = bignum_decimal(modulus);
+                char *ds = bignum_decimal(answer);
+                char *ps = bignum_decimal(expected);
+                
+                printf("%d: fail: %s ^ %s mod %s gave %s expected %s\n",
+                       line, as, bs, cs, ds, ps);
+                fails++;
+
+                sfree(as);
+                sfree(bs);
+                sfree(cs);
+                sfree(ds);
+                sfree(ps);
+            }
+            freebn(base);
+            freebn(expt);
+            freebn(modulus);
+            freebn(expected);
+            freebn(answer);
+        } else if (!strcmp(buf, "divmod")) {
+            Bignum n, d, expect_q, expect_r, answer_q, answer_r;
+            int fail;
+
+            if (ptrnum != 4) {
+                printf("%d: divmod with %d parameters, expected 4\n", line, ptrnum);
+                exit(1);
+            }
+
+            n = bignum_from_bytes(ptrs[0], ptrs[1]-ptrs[0]);
+            d = bignum_from_bytes(ptrs[1], ptrs[2]-ptrs[1]);
+            expect_q = bignum_from_bytes(ptrs[2], ptrs[3]-ptrs[2]);
+            expect_r = bignum_from_bytes(ptrs[3], ptrs[4]-ptrs[3]);
+            answer_q = bigdiv(n, d);
+            answer_r = bigmod(n, d);
+
+            fail = FALSE;
+            if (bignum_cmp(expect_q, answer_q) != 0) {
+                char *as = bignum_decimal(n);
+                char *bs = bignum_decimal(d);
+                char *cs = bignum_decimal(answer_q);
+                char *ds = bignum_decimal(expect_q);
+
+                printf("%d: fail: %s / %s gave %s expected %s\n",
+                       line, as, bs, cs, ds);
+                fail = TRUE;
+
+                sfree(as);
+                sfree(bs);
+                sfree(cs);
+                sfree(ds);
+            }
+            if (bignum_cmp(expect_r, answer_r) != 0) {
+                char *as = bignum_decimal(n);
+                char *bs = bignum_decimal(d);
+                char *cs = bignum_decimal(answer_r);
+                char *ds = bignum_decimal(expect_r);
+
+                printf("%d: fail: %s mod %s gave %s expected %s\n",
+                       line, as, bs, cs, ds);
+                fail = TRUE;
+
+                sfree(as);
+                sfree(bs);
+                sfree(cs);
+                sfree(ds);
+            }
+
+            freebn(n);
+            freebn(d);
+            freebn(expect_q);
+            freebn(expect_r);
+            freebn(answer_q);
+            freebn(answer_r);
+
+            if (fail)
+                fails++;
+            else
+                passes++;
+        } else {
+            printf("%d: unrecognised test keyword: '%s'\n", line, buf);
+            exit(1);
+        }
+
+        sfree(buf);
+        sfree(data);
+    }
+
+    printf("passed %d failed %d total %d\n", passes, fails, passes+fails);
+    return fails != 0;
+}