How do I encrypt a 12 character network adapter mac address so i can get in result with a 20 to 25 cipher text length?
A MAC address is actually 48 bits long, which can be displayed as 12 base-16 characters (4-bit hexidecimal).
If you encrypt the MAC address with a 64-bit block cipher such as Blowfish or DES, the resultant 64-bit value will be 16 base-16 characters.
If you encrypt the MAC address with a 128-bit block cipher such as AES or Twofish, it will be 32 base-16 characters, which exceeds your limit. However you can encode this value to base-64 (6-bit), and it will take 22 characters, less any padding which can be eliminated in implementation, since you have a known decode length. This length fits your requirement, if you are able to accept the encoding for whatever you are using this for.