crypto: add secure random reader using getrandom

This commit adds in RandReader, a cryptographically secure io.Reader
that will fail when the os has insufficient randomness. This is done
using the getrandom() syscall in non-blocking mode.
  see: http://man7.org/linux/man-pages/man2/getrandom.2.html
Any kernel new enough to have filesystem encryption will also have this
syscall.

This RandReader is preferable to the one provided by the standard
library in crypto/rand. See the bugs:
	https://github.com/golang/go/issues/11833
	https://github.com/golang/go/issues/19274
This will be removed when go updates the crypto/rand implementation.

Change-Id: Icccaf07bc6011b95cd31a5c268e7486807dcffe2

1 files changed, 31 insertions, 0 deletions

diff --git a/crypto/crypto_test.go b/crypto/crypto_test.go
index 025b5b9..7447e40 100644
--- a/crypto/crypto_test.go
+++ b/crypto/crypto_test.go
@@ -21,6 +21,7 @@ package crypto
 
 import (
 	"bytes"
+	"compress/zlib"
 	"os"
 	"testing"
 )
@@ -139,3 +140,33 @@ func TestBadAddKeys(t *testing.T) {
 		t.Error("InsertPolicyKey should fail with bad service")
 	}
 }
+
+// Check that we can create random keys. All this test does to test the
+// "randomness" is generate a page of random bytes and attempts compression.
+// If the data can be compressed it is probably not very random. This isn't
+// indented to be a sufficient test for randomness (which is impossible), but a
+// way to catch simple regressions (key is all zeros or contains a repeating
+// pattern).
+func TestRandomKeyGen(t *testing.T) {
+	key, err := NewRandomKey(os.Getpagesize())
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer key.Wipe()
+
+	if didCompress(key.data) {
+		t.Errorf("Random key (%d bytes) should not be compressible", key.Len())
+	}
+}
+
+// didCompress checks if the given data can be compressed. Specifically, it
+// returns true if running zlib on the provided input produces a shorter output.
+func didCompress(input []byte) bool {
+	var output bytes.Buffer
+
+	w := zlib.NewWriter(&output)
+	_, err := w.Write(input)
+	w.Close()
+
+	return err == nil && len(input) > output.Len()
+}