MAILComp - New Compression Method Designed for Electric Mail Transfer Protocols over TLS

TLS (Transport Layer Security) is a well-known protocol that provides authentication and private communication for application protocols like HTTP (Hypertext Transfer Protocol). In TLS, data transferred from the upper layer can be compressed before it is encrypted. When an E-mail attached file is transferred over TLS, its compression ratio may be degraded since there are certain restrictions on the transfer of the E-mail, such as the need for text encoding of binary data and the recompression of already compressed data. The authors have already proposed an improved compression method for TLS, however, if compression is combined with encryption, the information concerned may become readable, since the length of the encrypted (and compressed) data might allow eavesdroppers to guess what kind of data are being transferred. In order to solve this problem, this paper proposes a compression method, MAILComp, designed for electric mail transfer protocols such as SMTP (Simple Mail Transfer Protocol), POP (Post Office Protocol), and IMAP (Internet Message Access Protocol) over TLS. From network experiments, compression ratios and file transfer times for four kinds of E-mail were observed, with simulated bandwidth restrictions to represent four typical communication media. The results show that MAILComp will provide an effective secure compression method whose overhead is virtually identical to that of the original compression method for TLS.