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Generally speaking most modern compression algorithms give roughly the same compression, and with regard to the number of cores that you can use at once, it is up to you to decide how many you want to use. However, 7-zip is free and open source. The 7z format supports encryption with the AES algorithm with a 256-bit key. The key is generated from a user-supplied passphrase using an algorithm based on the SHA-256 hash function. The SHA-256 is executed 218 (262144) times,[4] which causes a significant delay on slow PCs before compression or extraction starts. This technique is called key stretching and is used to make a brute-force search for the passphrase more difficult. Current GPU-based, and custom hardware attacks limit the effectiveness of this particular method of key stretching,[5] so it is still important to choose a strong password. The 7z format provides the option to encrypt the filenames of a 7z archive.