In general TDES with three different keys (3-key {k, k, k} TDES) has a key length of 168 bits: three 56-bit DES keys (with parity bits 3-key TDES has the total storage length of 192 bits), but due to the meet-in-the-middle attack the effective security it provides is only 112 bits. Another version, called two-key TDES (2-key TDES), uses k = k, thus reducing the key size to 112 bits and the storage length to 128 bits. However, this mode can be taken advantage of through certain chosen-plaintext or known-plaintext attacks and so TDES is treated by NIST to have only 80 bits of security.