matrix-sdk-crypto-ffi | Scan QR code method is sensitive to base 64 padding when decoding

[BillCarsonFr]

Root cause of element-hq/element-ios#7660

Some clients will pass QR code data as base64 with padding and currently the rust-crypto-ffi will fail to process such qr codes due to the unexpected padding.

The sdk should be able to decode base64 indifferently of the padding.
The decoder used by the ffi crate is here

matrix-rust-sdk/bindings/matrix-sdk-crypto-ffi/src/verification.rs

Line 20 in 4a01294

const STANDARD_NO_PAD: GeneralPurpose =

The one used by other parts of the SDK is this one:

matrix-rust-sdk/crates/matrix-sdk-crypto/src/utilities.rs

Lines 31 to 35 in 4a01294

	const STANDARD_NO_PAD: GeneralPurpose = GeneralPurpose::new(
	&alphabet::STANDARD,
	general_purpose::NO_PAD
	.with_decode_padding_mode(base64::engine::DecodePaddingMode::Indifferent),
	);

The later is correctly using DecodePaddingMode::Indifferent

Notice that vodozemac as also a similar utility crate that is not public https://github.com/matrix-org/vodozemac/blob/e3b658526f6f1dd0a9065c1c96346b796712c425/src/utilities/mod.rs#L28

Proposed solution => Make the base64 encode/decode utility from vodozemac public and use it everywhere in the rust SDK