7z Format

7z is a new archive format, providing high compression ratio.

The main features of the 7z format:

• Open architecture
• High compression ratio
• Strong AES-256 encryption
• Ability of using any compression, conversion or encryption method
• Supporting files with sizes up to 16000000000 GB
• Unicode file names
• Solid compressing
• Archive headers compressing

7z has open architecture, so it can support any new compression methods.

Method	Description
LZMA	Improved and optimized version of LZ77 algorithm
PPMD	Dmitry Shkarin's PPMdH with small changes
BCJ	Converter for 32-bit x86 executables
BCJ2	Converter for 32-bit x86 executables
BZip2	Standard BWT algorithm
Deflate	Standard LZ77-based algorithm

LZMA is the default and general compression method of 7z format. The main features of the LZMA method:

• High compression ratio
• Variable dictionary size (up to 4 GB)
• Compressing speed: about 1 MB/s on 2 GHz CPU
• Decompressing speed: about 10-20 MB/s on 2 GHz CPU
• Small memory requirements for decompressing (depend from dictionary size)
• Small code size for decompressing: about 5 KB
• Supporting multi-threading and P4's hyper-threading

The LZMA compression algorithm is very suitable for embedded applications. If you want to use LZMA code, you can ask for consultations, custom code programming, and required developer licenses at

www.7-zip.org/support.html

7-Zip also supports encryption with AES-256 algorithm. This algorithm uses cipher key with length of 256 bits. To create that key 7-Zip uses derivation function based on SHA-256 hash algorithm. A key derivation function produces a derived key from text password defined by user. For increasing the cost of exhaustive search for passwords 7-Zip uses big number of iterations to produce cipher key from text password.

Tips for selecting password length

We will estimate the time required for an exhaustive password search attack, when the password is a random sequence of lowercase Latin letters.

We suppose that one user can check 10 passwords per second and organization (with budget about $1 billion) can check 10 billions passwords per second. We also suppose that each 2 years processor doubles it's performance, so each additional Latin letter of long password adds about 9 years against exhaustive key search attack.

So we have such estimated time for attack: