Implement constant time decoding

[05nelsonm]

Add as a config option the ability to set constant time.

Some operations (e.g. encryption/decryption) require processing the entire contents of the input before throwing exception in order to mitigate timing attacks.

As module :library:encoding-core does not have knowledge of the higher level implementation details, this will need to be something added to them individually.

• When an EncoderDecoder is configured for constant time and encounters an unrecognized input, it should substitute that unknown input so operations continue (e.g. character TABLE[0]) and set an error flag.
• When Feed.doFinalProtected is invoked, it should process the remaining contents in its buffer, and then throw an exception.
  • invoking flush() should not produce an exception, only upon after Feed.doFinal is invoked. Implementations can check isClosed within their doFinalProtected function to check whether flush or doFinal has been invoked.

[05nelsonm]

Think about adding a static EncoderDecoder configured specifically for this, too.

public class Base64(
    config: Base64.Config
) : EncoderDecoder<Base64.Config>(config) {

    // ...

    public object Default {

        @JvmField
        public val CT: Base64 = Base64 {
            lineBreakInterval = 64
            constantTime = true
        }
    }

    // ...
}

[05nelsonm]

Actually, this could be implemented in core by adding another constructor with the new parameter (and deprecating the prior constructor), then handled in the Decoder.Feed abstraction.

I think the parameter would need to be of type Char?, where by if it is null, constant time is not enabled, and if it is non-null, that character will be substituted in the event of an encoding exception ocurrs.

May need to add a new exception type, like InvalidDecoderInputException that can be caught to trigger the functionality in order to substitute with the defined alternative.

EDIT: Actually, throwing an exception would defeat the purpose because that would generate a stacktrace (which takes time). Implementations need to handle substitution somehow. Need to rethink and play with implementation/abstractions to see how best to do this.