This paper focuses on the length-preserving symmetric cryptography, with which people can encrypt messages of variable length (especially arbitrary length) to get ciphertext of length identical to that of plaintext being encrypted. Two confidentiality modes being able to achieve length-preserving encryption, i.e., OFB mode and CTR mode, are analyzed in terms of obstacles to guaranteeing message privacy. Furthermore, a new mode of operation, CBC-LP, is proposed for both achieving length-preserving encryption and exploiting advantages of implementing CBC mode over other confidentiality modes, such as OFB, CTR modes and so on.