Function decode_into_constant_time 

pub const fn decode_into_constant_time(
    output: &mut [u8],
    encoded_data: &[u8],
    alphabet: Alphabet,
) -> Result<(), DecodeError>

Constant-time base64 decoding. Processes all input data without secret-dependent branches or memory accesses, making it suitable for cryptographic applications.

Consumers may prefer the faster decode_into which dispatches to a SIMD-accelerated path when available (non constant-time).