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sas: Fix the base64 encoding of the MAC.

See merge request !16

OLMKit:  New pickle version using a pickle key provided externally

See merge request !17

Improve ObjC wrappers so that they can use a pickle key provided by the olm lib user.

This new behavior is optional to not break existing usage.

It is retro compatible and use pickle versioning already in place. 
Existing key will be unpickled with pickle v1 and pickled with pickle v2 if an external pickle key is provided. 

Since it's important to keep backwards compatibility introduce a new
function to calculate the MAC using a SAS object.

Modifying the existing functions would break compatibility with older
releases of libolm.

When calculating the MAC for a message using olm_sas_calculate_mac() and
olm_sas_calculate_mac_long_kdf() the resulting MAC will be base64
encoded using _olm_encode_base64().

The _olm_encode_base64() method requires an input buffer and output
buffer to be passed alongside the input length. The method is called
with the same buffer, containing the MAC, for the input buffer as well
as for the output buffer. This results in an incorrectly base64 encoded
MAC.

For example the byte array:
    [121, 105, 187, 19, 37, 94, 119, 248, 224, 34, 94, 29, 157, 5,
     15, 230, 246, 115, 236, 217, 80, 78, 56, 200, 80, 200, 82, 158,
     168, 179, 10, 230]

will be encoded as  eWm7NyVeVmXgbVhnYlZobllsWm9ibGxzV205aWJHeHo
instead of as       eWm7EyVed/jgIl4dnQUP5vZz7NlQTjjIUMhSnqizCuY

Notice the different value at the 10th character.

The correct result can be independently checked using Python for
example:

>>> from base64 import b64encode
>>> mac = [121, 105, 187, 19, 37, 94, 119, 248, 224, 34, 94, 29, 157, \
           5, 15, 230, 246, 115, 236, 217, 80, 78, 56, 200, 80, 200, \
           82, 158, 168, 179, 10, 230]
>>> b64encode(bytearray(mac)).rstrip(b"=")
>>> b'eWm7EyVed/jgIl4dnQUP5vZz7NlQTjjIUMhSnqizCuY'

This happens because the encode_base64() method that is used does not support
in-place encoding in the general case. This is because the remainder for a 32
bit input will always be 2 (32 % 6 == 2).

The remainder will be used over here:
https://gitlab.matrix.org/matrix-org/olm/-/blob/c01164f001d57fbe2297fe11954b58077a68dc0d/src/base64.cpp#L74

The logic that gets executed if a remainder exists depends on the original input
values, since those already got in-place encoded the whole block will behave
differently if the input buffer is the same as the output buffer.

Update index.d.ts; specify PRIVATE_KEY_LENGTH const export

See merge request !15

remove other_key checks from Python binding since it's done in C now

See merge request !14

Also make olm_sas_generate_bytes fail if their key wasn't set.

so that we're under GitLab's 1000 character limit for rendering maths

add support for fallback keys

See merge request !13