pub(crate) fn decode_secret<'a>(
    input: &[u8],
    output: &'a mut [u8],
) -> Result<&'a [u8], Error>Expand description
Decode base64 input, writing the result into output.
input is treated as secret, so efforts are made to avoid
leaking its value via side channels, such as timing,
memory accesses, and execution trace.
The following is deemed non-secret information:
- Appearance of whitespace in 
input - Erroneous characters in 
input(indeed, the first illegal character is quoted in the error type) - The length of 
input - The length of 
output 
Returns the prefix of output that was written to.