crypto: Implement TLS Pre-Shared Keys (PSK).

Pre-Shared Keys (PSK) is a simpler mechanism for enabling TLS
connections than using certificates.  It requires only a simple secret
key:

  $ mkdir -m 0700 /tmp/keys
  $ psktool -u rjones -p /tmp/keys/keys.psk
  $ cat /tmp/keys/keys.psk
  rjones:d543770c15ad93d76443fb56f501a31969235f47e999720ae8d2336f6a13fcbc

The key can be secretly shared between clients and servers.  Clients
must specify the directory containing the "keys.psk" file and a
username (defaults to "qemu").  Servers must specify only the
directory.

Example NBD client:

  $ qemu-img info \
    --object tls-creds-psk,id=tls0,dir=/tmp/keys,username=rjones,endpoint=client \
    --image-opts \
    file.driver=nbd,file.host=localhost,file.port=10809,file.tls-creds=tls0,file.export=/

Example NBD server using qemu-nbd:

  $ qemu-nbd -t -x / \
    --object tls-creds-psk,id=tls0,endpoint=server,dir=/tmp/keys \
    --tls-creds tls0 \
    image.qcow2

Example NBD server using nbdkit:

  $ nbdkit -n -e / -fv \
    --tls=on --tls-psk=/tmp/keys/keys.psk \
    file file=disk.img

Signed-off-by: Richard W.M. Jones <rjones@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>

1 files changed, 37 insertions, 0 deletions

diff --git a/qemu-doc.texi b/qemu-doc.texi
index 1cb3ba4341..d3924e928e 100644
--- a/qemu-doc.texi
+++ b/qemu-doc.texi
@@ -1262,6 +1262,7 @@ The recommendation is for the server to keep its certificates in either
 * tls_generate_server::
 * tls_generate_client::
 * tls_creds_setup::
+* tls_psk::
 @end menu
 @node tls_generate_ca
 @subsection Setup the Certificate Authority
@@ -1510,6 +1511,42 @@ example with VNC:
 $QEMU -vnc 0.0.0.0:0,tls-creds=tls0
 @end example
 
+@node tls_psk
+@subsection TLS Pre-Shared Keys (PSK)
+
+Instead of using certificates, you may also use TLS Pre-Shared Keys
+(TLS-PSK).  This can be simpler to set up than certificates but is
+less scalable.
+
+Use the GnuTLS @code{psktool} program to generate a @code{keys.psk}
+file containing one or more usernames and random keys:
+
+@example
+mkdir -m 0700 /tmp/keys
+psktool -u rich -p /tmp/keys/keys.psk
+@end example
+
+TLS-enabled servers such as qemu-nbd can use this directory like so:
+
+@example
+qemu-nbd \
+  -t -x / \
+  --object tls-creds-psk,id=tls0,endpoint=server,dir=/tmp/keys \
+  --tls-creds tls0 \
+  image.qcow2
+@end example
+
+When connecting from a qemu-based client you must specify the
+directory containing @code{keys.psk} and an optional @var{username}
+(defaults to ``qemu''):
+
+@example
+qemu-img info \
+  --object tls-creds-psk,id=tls0,dir=/tmp/keys,username=rich,endpoint=client \
+  --image-opts \
+  file.driver=nbd,file.host=localhost,file.port=10809,file.tls-creds=tls0,file.export=/
+@end example
+
 @node gdb_usage
 @section GDB usage