CSKH-02.2020. Abstract—We propose a new double-blocklength compression function which is called Alpha-DBL. This scheme uses two parallel secure single block length schemes based on a block cipher with 𝟐𝒏-bit key and 𝒏-bit block size to compress a 𝟑𝒏-bit string to a 𝟐𝒏-bit one. We show that the Alpha-DBL scheme attains nearly optimal collision security and preimage security bounds (up to 𝟐^𝒏 and 𝟐^𝟐𝒏 queries for finding a collision and a preimage, respectively). More precisely, for 𝒏 = 𝟏𝟐𝟖, no adversary making less than 𝟐^𝒏−𝟏.𝟐𝟕 = 𝟐^𝟏𝟐𝟔.𝟕𝟑 queries can find a collision with probability greater than 1/2. To our knowledge, this collision security bound is nearly better than other such compression functions. In addition, we provide a preimage security analysis of Alpha-DBL that shows security bound of 𝟐^𝟐𝒏−𝟓 = 𝟐 𝟐𝟓𝟏 queries for 𝒏 = 𝟏𝟐𝟖. Using this scheme in the iterated hash function construction can preserve the collision resistance security and the preimage resistance security.