Key rollover: rewrite the PGP keys manual appendix.

[PuTTY.git] / sshdh.c

diff --git a/sshdh.c b/sshdh.c
index c733b61f7e77fe8f3f0f5ea4f5f7a3364274382d..8f8ab2d26aa6ef9ad7e5fe74710edca473272245 100644 (file)
--- a/sshdh.c
+++ b/sshdh.c
@@ -218,6 +218,29 @@ Bignum dh_create_e(void *handle, int nbits)
     return ctx->e;
 }
 
+/*
+ * DH stage 2-epsilon: given a number f, validate it to ensure it's in
+ * range. (RFC 4253 section 8: "Values of 'e' or 'f' that are not in
+ * the range [1, p-1] MUST NOT be sent or accepted by either side."
+ * Also, we rule out 1 and p-1 too, since that's easy to do and since
+ * they lead to obviously weak keys that even a passive eavesdropper
+ * can figure out.)
+ */
+const char *dh_validate_f(void *handle, Bignum f)
+{
+    struct dh_ctx *ctx = (struct dh_ctx *)handle;
+    if (bignum_cmp(f, One) <= 0) {
+        return "f value received is too small";
+    } else {
+        Bignum pm1 = bigsub(ctx->p, One);
+        int cmp = bignum_cmp(f, pm1);
+        freebn(pm1);
+        if (cmp >= 0)
+            return "f value received is too large";
+    }
+    return NULL;
+}
+
 /*
  * DH stage 2: given a number f, compute K = f^x mod p.
  */