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Index: sbin/dumpon/dumpon.8
===================================================================
--- sbin/dumpon/dumpon.8
+++ sbin/dumpon/dumpon.8
@@ -28,7 +28,7 @@
.\" From: @(#)swapon.8 8.1 (Berkeley) 6/5/93
.\" $FreeBSD$
.\"
-.Dd June 13, 2018
+.Dd October 23, 2018
.Dt DUMPON 8
.Os
.Sh NAME
@@ -348,3 +348,15 @@
It is currently not possible to configure both compression and encryption.
The encrypted dump format assumes that the kernel dump size is a multiple
of the cipher block size, which may not be true when the dump is compressed.
+.Sh SECURITY CONSIDERATIONS
+RSA keys smaller than 1024 bits are practical to factor and therefore weak.
+Even 1024 bit keys may not be large enough to ensure privacy for many more
+years.
+As a seatbelt,
+.Nm
+prevents users from configuring encrypted kernel dumps with weak RSA keys.
+If you do not care for cryptographic privacy guarantees, just use
+.Nm
+without specifying a
+.Fl k Ar pubkey
+option.
Index: sbin/dumpon/dumpon.c
===================================================================
--- sbin/dumpon/dumpon.c
+++ sbin/dumpon/dumpon.c
@@ -217,6 +217,7 @@
{
FILE *fp;
RSA *pubkey;
+ int keysize;
assert(pubkeyfile != NULL);
assert(kdap != NULL);
@@ -243,7 +244,23 @@
if (pubkey == NULL)
errx(1, "Unable to read data from %s.", pubkeyfile);
- kdap->kda_encryptedkeysize = RSA_size(pubkey);
+ keysize = RSA_size(pubkey);
+ /*
+ * RSA keys under 1024 bits are trivially factorable (2018). Avoid
+ * suggesting to users that their privacy is protected by encryption
+ * when the key size is insufficient to prevent compromise via
+ * factoring.
+ *
+ * Future work: Sanity check for weak 'e', and sanity check for absence
+ * of 'd' (i.e., the supplied
+ */
+ if (keysize * 8 < 1024)
+ errx(1, "RSA keys smaller than 1024b (you provided: %db) are "
+ "small enough to factor cheaply. "
+ "Please generate a larger key.",
+ keysize * 8);
+
+ kdap->kda_encryptedkeysize = keysize;
if (kdap->kda_encryptedkeysize > KERNELDUMP_ENCKEY_MAX_SIZE) {
errx(1, "Public key has to be at most %db long.",
8 * KERNELDUMP_ENCKEY_MAX_SIZE);

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