util: add base64 decoding function

The standard glib provided g_base64_decode doesn't provide any
kind of sensible error checking on its input. Add a QEMU custom
wrapper qbase64_decode which can be used with untrustworthy
input that can contain invalid base64 characters, embedded
NUL characters, or not be NUL terminated at all.

Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>

include/qemu/base64.h

@@ -0,0 +1,58 @@
+/*
+ * QEMU base64 helpers
+ *
+ * Copyright (c) 2015 Red Hat, Inc.
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, see <http://www.gnu.org/licenses/>.
+ *
+ */
+
+#ifndef QEMU_BASE64_H__
+#define QEMU_BASE64_H__
+
+#include "qemu-common.h"
+
+
+/**
+ * qbase64_decode:
+ * @input: the (possibly) base64 encoded text
+ * @in_len: length of @input or -1 if NUL terminated
+ * @out_len: filled with length of decoded data
+ * @errp: pointer to a NULL-initialized error object
+ *
+ * Attempt to decode the (possibly) base64 encoded
+ * text provided in @input. If the @input text may
+ * contain embedded NUL characters, or may not be
+ * NUL terminated, then @in_len must be set to the
+ * known size of the @input buffer.
+ *
+ * Note that embedded NULs, or lack of a NUL terminator
+ * are considered invalid base64 data and errors
+ * will be reported to this effect.
+ *
+ * If decoding is successful, the decoded data will
+ * be returned and @out_len set to indicate the
+ * number of bytes in the decoded data. The caller
+ * must use g_free() to free the returned data when
+ * it is no longer required.
+ *
+ * Returns: the decoded data or NULL
+ */
+uint8_t *qbase64_decode(const char *input,
+                        size_t in_len,
+                        size_t *out_len,
+                        Error **errp);
+
+
+#endif /* QEMU_BUFFER_H__ */