for ((key, value) in authParams) {
      val newKey = key?.lowercase(US)
      newAuthParams[newKey] = value
    }
    this.authParams = newAuthParams.unmodifiable()
  }

  /** Returns a copy of this charset that expects a credential encoded with [charset]. */
  fun withCharset(charset: Charset): Challenge {
    val authParams = this.authParams.toMutableMap()
    authParams["charset"] = charset.name()

| Params     | Value    |
| :--        | :--      |
| *Type*     | *String* |
| *Required* | *Yes*    |

### AUTHPARAMS