fscrypt: align Base64 encoding with RFC 4648 base64url

fscrypt uses a Base64 encoding to encode no-key filenames (the filenames
that are presented to userspace when a directory is listed without its
encryption key).  There are many variants of Base64, but the most common
ones are specified by RFC 4648.  fscrypt can't use the regular RFC 4648
"base64" variant because "base64" uses the '/' character, which isn't
allowed in filenames.  However, RFC 4648 also specifies a "base64url"
variant for use in URLs and filenames.  "base64url" is less common than
"base64", but it's still implemented in many programming libraries.

Unfortunately, what fscrypt actually uses is a custom Base64 variant
that differs from "base64url" in several ways:

- The binary data is divided into 6-bit chunks differently.

- Values 62 and 63 are encoded with '+' and ',' instead of '-' and '_'.

- '='-padding isn't used.  This isn't a problem per se, as the padding
  isn't technically necessary, and RFC 4648 doesn't strictly require it.
  But it needs to be properly documented.

There have been two attempts to copy the fscrypt Base64 code into lib/
(https://lkml.kernel.org/r/20200821182813.52570-6-jlayton@kernel.org and
https://lkml.kernel.org/r/20210716110428.9727-5-hare@suse.de), and both
have been caught up by the fscrypt Base64 variant being nonstandard and
not properly documented.  Also, the planned use of the fscrypt Base64
code in the CephFS storage back-end will prevent it from being changed
later (whereas currently it can still be changed), so we need to choose
an encoding that we're happy with before it's too late.

Therefore, switch the fscrypt Base64 variant to base64url, in order to
align more closely with RFC 4648 and other implementations and uses of
Base64.  However, I opted not to implement '='-padding, as '='-padding
adds complexity, is unnecessary, and isn't required by the RFC.

Link: https://lore.kernel.org/r/20210718000125.59701-1-ebiggers@kernel.org
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Eric Biggers <ebiggers@google.com>

1 files changed, 5 insertions, 5 deletions

diff --git a/Documentation/filesystems/fscrypt.rst b/Documentation/filesystems/fscrypt.rst
index 02ec57818920..0eb799d9d05a 100644
--- a/Documentation/filesystems/fscrypt.rst
+++ b/Documentation/filesystems/fscrypt.rst
@@ -1230,12 +1230,12 @@ the user-supplied name to get the ciphertext.
 
 Lookups without the key are more complicated.  The raw ciphertext may
 contain the ``\0`` and ``/`` characters, which are illegal in
-filenames.  Therefore, readdir() must base64-encode the ciphertext for
-presentation.  For most filenames, this works fine; on ->lookup(), the
-filesystem just base64-decodes the user-supplied name to get back to
-the raw ciphertext.
+filenames.  Therefore, readdir() must base64url-encode the ciphertext
+for presentation.  For most filenames, this works fine; on ->lookup(),
+the filesystem just base64url-decodes the user-supplied name to get
+back to the raw ciphertext.
 
-However, for very long filenames, base64 encoding would cause the
+However, for very long filenames, base64url encoding would cause the
 filename length to exceed NAME_MAX.  To prevent this, readdir()
 actually presents long filenames in an abbreviated form which encodes
 a strong "hash" of the ciphertext filename, along with the optional