diff --git a/src/lib/crypto/Base64.cxx b/src/lib/crypto/Base64.cxx
new file mode 100644
index 000000000..58910a020
--- /dev/null
+++ b/src/lib/crypto/Base64.cxx
@@ -0,0 +1,58 @@
+/*
+ * Copyright 2019 Max Kellermann <max.kellermann@gmail.com>
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ *
+ * - Redistributions of source code must retain the above copyright
+ * notice, this list of conditions and the following disclaimer.
+ *
+ * - Redistributions in binary form must reproduce the above copyright
+ * notice, this list of conditions and the following disclaimer in the
+ * documentation and/or other materials provided with the
+ * distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
+ * FOR A PARTICULAR PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE
+ * FOUNDATION OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
+ * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
+ * (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+ * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+ * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
+ * STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED
+ * OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#include "Base64.hxx"
+#include "lib/ffmpeg/Error.hxx"
+#include "util/StringView.hxx"
+#include "util/WritableBuffer.hxx"
+
+extern "C" {
+#include <libavutil/base64.h>
+}
+
+#include <string>
+
+size_t
+DecodeBase64(WritableBuffer<void> out, StringView in)
+{
+	/* since av_base64_decode() wants a null-terminated string, we
+	   need to make a copy here and null-terminate it */
+	const std::string copy(in.data, in.size);
+	return DecodeBase64(out, copy.c_str());
+}
+
+size_t
+DecodeBase64(WritableBuffer<void> out, const char *in)
+{
+	int nbytes = av_base64_decode((uint8_t *)out.data, in, out.size);
+	if (nbytes < 0)
+		throw MakeFfmpegError(nbytes, "Base64 decoder failed");
+
+	return nbytes;
+}
diff --git a/src/lib/crypto/Base64.hxx b/src/lib/crypto/Base64.hxx
new file mode 100644
index 000000000..54bffde1b
--- /dev/null
+++ b/src/lib/crypto/Base64.hxx
@@ -0,0 +1,56 @@
+/*
+ * Copyright 2019 Max Kellermann <max.kellermann@gmail.com>
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ *
+ * - Redistributions of source code must retain the above copyright
+ * notice, this list of conditions and the following disclaimer.
+ *
+ * - Redistributions in binary form must reproduce the above copyright
+ * notice, this list of conditions and the following disclaimer in the
+ * documentation and/or other materials provided with the
+ * distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
+ * FOR A PARTICULAR PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE
+ * FOUNDATION OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
+ * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
+ * (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+ * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+ * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
+ * STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED
+ * OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#ifndef BASE64_HXX
+#define BASE64_HXX
+
+#include <stddef.h>
+
+template<typename T> struct WritableBuffer;
+struct StringView;
+
+constexpr size_t
+CalculateBase64OutputSize(size_t in_size) noexcept
+{
+	return in_size * 3 / 4;
+}
+
+/**
+ * Throws on error.
+ */
+size_t
+DecodeBase64(WritableBuffer<void> out, StringView in);
+
+/**
+ * Throws on error.
+ */
+size_t
+DecodeBase64(WritableBuffer<void> out, const char *in);
+
+#endif
diff --git a/src/lib/crypto/meson.build b/src/lib/crypto/meson.build
index 5ea661baf..da543a2be 100644
--- a/src/lib/crypto/meson.build
+++ b/src/lib/crypto/meson.build
@@ -16,3 +16,22 @@ else
 endif
 
 conf.set('HAVE_MD5', crypto_md5_dep.found())
+
+if libavutil_dep.found()
+  crypto_base64 = static_library(
+    'crypto_base64',
+    'Base64.cxx',
+    include_directories: inc,
+    dependencies: [
+      libavutil_dep,
+    ],
+  )
+
+  crypto_base64_dep = declare_dependency(
+    link_with: crypto_base64,
+  )
+else
+  crypto_base64_dep = dependency('', required: false)
+endif
+
+conf.set('HAVE_BASE64', crypto_base64_dep.found())