String indicating the cipher used during the current connection.
The values allowed for `tls.cipher` MUST be one of the `Descriptions` of the registered TLS Cipher Suits.
@example TLS_RSA_WITH_3DES_EDE_CBC_SHA @example TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA256
public const TLS_CIPHER = 'tls.cipher';