#file-format #object-storage #data-transformation #compression #encryption #component #aruna


Library and component for encrypting / compressing .aruna files, including specification

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#1118 in Cryptography

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Short guidance for usage of the ArunaReadWriter custom component. For the formal file specification click here.

An overhauled generic version of customisable data transformer component for the Aruna Object Storage (AOS). The idea is simple, you implement these simple base trait with your custom data transformation logic:

pub trait Transformer {
    async fn process_bytes(&mut self, buf: &mut bytes::BytesMut, finished: bool) -> Result<bool>;

And afterwards the structs implementing Transformer can be registered in the ArunaReadWriter to be plugged between the Read and Write parts of a ReadWriter.


        let file = b"This is a very very important test".to_vec();
        let mut file2 = Vec::new();

        // Create a new ArunaReadWriter
        ArunaReadWriter::new_with_writer(file.as_ref(), &mut file2)
            .add_transformer(ZstdEnc::new(1, false))
            .add_transformer(ZstdEnc::new(2, false)) // Double compression because we can
                ChaCha20Enc::new(false, b"wvwj3485nxgyq5ub9zd3e7jsrq7a92ea".to_vec()).unwrap(),
                ChaCha20Enc::new(false, b"99wj3485nxgyq5ub9zd3e7jsrq7a92ea".to_vec()).unwrap(),
            .add_transformer(Filter::new(Range { from: 0, to: 3 }))
        assert_eq!(file2, b"Thi".to_vec());

This example creates a Vec<u8> from a bytes array (implements AsyncRead) and sinks it in another Vec<u8> (impl AsynWrite). In between, custom data transformations can take place.

The example compresses the vector two times with a custom padded Zstandard compression component and afterwards encrypts the result also two times with ChaCha20-Poly1305. Afterwards all steps are reversed resulting in the original data.

Notes for own implementations

The main logic is build around, the process_bytes function.

async fn process_bytes(&mut self, buf: &mut bytes::Bytes, finished: bool, should_flush: bool) -> Result<bool>;

The idea is that your Transformer receives a mutable buffer with bytes that you MUST transform. If you have transformed (either all or via an internal buffer) the data is put back into the buffer for the next transformers process_bytes method. If should_flush is true all internal buffers should be flushed and cleared immediately.

The ARUNA file format

This document contains the formal description for the aruna (.aruna equivalent to .zst.c4gh) file format. A file format that enables compression and encryption while still maintaining a resonable performant indexing solution for large multi-gigabyte files. Optimized for usage with object storage solutions, like S3.


The core of the aruna file format is the combination of GA4GH's crypt4gh encryption format with the zstandard compression algorithm (RFC8878). This is extended by an optional custom footer block containing positional information for decrypting and decompressing blocks within larger files.


Aruna files consist of three distinct parts. A Header section followed by blocks of compressed and encrypted data and an optional footer section containing indirect index information and block sizes.

Data structure

For Compression the data SHOULD first be split into raw data chunks with exactly 5 Mib size (except the last one). These chunks MUST be compressed using the zstandard algorithm with a compression level of choice and MAY optionally end with a MAC. Each compressed frame MUST be followed by a skippable frame as defined in RFC8878 if the resulting compressed size is not a multiple of 65536 Bytes (64 Kib) and the raw file size was more than 5 Mib. The skippable frame SHOULD use 0x184D2A50 as Magic_Number and SHOULD avoid 0x184D2A51 and 0x184D2A52 to avoid confusion with the custom footer section. A skippable frame MUST be used to align the total compressed size to a multiple of the encryption block size of 65536 Bytes (except for the last block) if more than one chunk exists. Because skippable frames have a minimum size of 8 Bytes they extend the data at worst by 65536 + 7 = 65543 Bytes. Raw files that are smaller than 5 Mib SHOULD NOT contain any skippable frames and omit any indexing for performance reasons.

The resulting blocks consisting of compressed data MUST be encrypted in ChaCha20-Poly1305_ietf encrypted blocks as specified in RFC7539 with 65536 Bytes size, using a securely generated random encryption secret. All blocks SHOULD be preceeded by a per block random generated 12 byte Nonce and end with a 16 byte message authentication code (MAC). This results in a total blocksize of 65562 Bytes. The last encrypted block of the file CAN have a smaller size than this if the file has an uncompressed size of less than 5 Mib.

If the file is larger than 5 Mib the number of blocks that build 5 Mib of raw data SHOULD be summed up resulting in a 1 Byte unsigned integer between 1 and 81 (with last chunk +2 = 83). 81 is the maximum because 5 Mebibytes are exactly 80 x 65536 Bytes chunks and in the worst case with no compression the skippable frame could extend this by a maximum of one block. This index number is stored in the last one or two encrypted blocks of skippable frames in the file as index for fast access of data in order.


The primary header is identical to the header specified by the crypt4gh standard and contains the block and encryption information for a specific recipient. This header is generated ad-hoc and NOT stored with the data itself to avoid re-encrypting the first section multiple times.

The footer consists of one or two encrypted 65536 Byte sized blocks of skippable frames that contain 1 byte unsigned integers with index information about each block of 5 Mib raw uncompressed data in order. These blocks have the following structure in little-endian format.

  • Header with Magic_Number 0x184D2A51 for one block 0x184D2A52 if two blocks are attached. (4 Bytes)
  • Frame_Size = 65536 as unsigned 32 bit integer
  • Block_Total = 32 bit unsigned integer with the total number of 64Kib + 28 Byte blocks.
  • Block_List = The size of each 5 Mib segment in multiples of 64Kib + 28 Byte blocks as unsigned 8 Bit integer in order
  • Padding = 0x00 Bytes to fill the 64 Kib block.

If the footer contains two blocks (indicated by the Magic_Number 0x184D2A52) both blocks should repeat the Header / Frame_Size / Block_number sections with the same information.

Practical guidance

This section contains practical recommendations for building encryption logic that comply with this format.

Compression and Encryption

  • Split the file in 5 Mib chunks
  • If this results in only one chunk:
    • compress the whole chunk
    • Split the compressed data in 64Kib sections
    • encrypt the 64 Kib sections with a random nonce
    • concatenate Nonce + Encrypted data + MAC to one block
    • concatenate all blocks in order
  • If this results in multiple chunks:
    • compress each chunk
    • calculate the "missing" bytes to fill a 64 Kib section for each chunk with the formular: Compressed size % 65536 if the result is < 8 the skippable chunk will be result + 64 Kib otherwise the skippable frame should be of size result
    • Create a skippable frame of size result with a magic header of 0x184D2A50 to align the result to 64Kib
    • split the resulting compressed section in 64 Kib sections and remember the number of sections
    • encrypt the 64 Kib sections with a random nonce
    • concatenate Nonce + Encrypted data + MAC to one block
    • (if shared) Start with a crypt4gh header containing encryption information
    • concatenate all blocks and all chunks in order
    • create a skippable frame as described in the Footer section with all remembered number of sections for each compressed chunk
    • append the one or two footers to the file

Decryption and Decompression

This procedure has two options a simple single threaded one and a more parallelizable multi-threaded one. Multi-threading only gives a significant advantage for files that are larger than 10-20 Mib.

Option A (single-threaded):

  • This procedure should work with regular tools for crypt4gh and zstandard.
  • Read the file from the start, obtain encryption information via the header section
  • Decrypt each 64 Kib in order using the encryption key from the header and the prepended 12 Byte Nonce in each Block.
  • The resulting data can be piped directly in the zstd decompressor.
  • All padding information and the footer section should be skipped as skippable frame.

Option B (multi-threaded):

  • Obtain the content-length of the compressed and encrypted file
  • If the file is significantly smaller than 5 Mib -> Proceed with Option A
  • Read and decrypt the last two encrypted blocks 2x (64 Kib + 28 Bytes) and store them in separate variables.
  • Check the last block for its Magic_Number, if it is 0x184D2A51 discard the penultimate block, if the number is 0x184D2A52 begin with the penultimate.
  • Decide how many parallel decryption and decompression threads should be spawned.
  • Read and split the Block_Total described in the Footer section roughly in your number of parallel threads. This results in a number of 64 Kib blocks that should be handled by each thread.
  • Start iterating through the Block_List section of the Footer and sum up the number of blocks for each entry in the blocklist, remember the initial block beginning with 0. If the sum surpasses the determined block count from the previous step spawn a separate thread handling the section from your initial block up to the end of the current block. Set the initial block to the beginning of the next block and repeat the process.
    • Each thread gets a "initial" and an "up-to" block index to process
    • These number relate to byte offsets in the file via the formular: blocknumber * (65536 + 28)
    • Example 1: Handle all blocks from 0 to 222 -> Range: 0 - 14554764 Byte
    • Example 2: Handle all blocks from 222 to 444 -> Range: 14554765 (14554764 + 1) - 29109528 Byte
    • Because the data is aligned it can be handled equivalent to Option A
  • Afterwards concatenate all decrypted / decompressed parts from each thread in the correct order to get the full file.

Option C (specific Range):

If you want to get only a specific range from the file the procedure is as follows:

  • Get the Footer as described in Option B, if the file is smaller than 5 Mib decrypt / decompress the whole file and extract the requested range from the resulting raw data.
  • Determine the needed sections based on your Range request. The data is compressed in chunks of 5 Mib, so the range must first be converted to an index of 5 Mib Blocks. This can be done by integer dividing the index with 5Mib (5242880)
  • Example: Range: Begin: 5242111 - End: 20971320 -> Begin // 5 Mib = 0, End // 5 Mib = 3 -> The 5 Mib blocks with the indizes from 0 up to 3 are needed.
  • Iterate the Blocklist from the beginning, sum up all counts up to Begin index in Variable A and sum up the counts from Begin to end index separately in Variable B.
  • The resulting variables A and B indicate the the Range of compressed and decrypted bytes that needs to be decrypted and compressed to get all data of the requested Range. The formular to calculate the Ranges is: From: Variable A * (65536 + 28) to Variable A * (65536 + 28) + Variable B * (65536 + 28) This Range should contain only full encrypted / compressed blocks of 5 Mib size that are needed for the request.
  • The blocks can be decrypted and decompressed as in Option A or Option B.
  • To get the "true" requested range afterwards the first Begin Range % 5 Mib Bytes and the last 5 Mib - End Range % 5 Mib Bytes or must be discarded. This can also be done by first discarding the beginning and afterwards only returning the "size" of the requested range (Begin Range - End Range)


The Aruna file format considers multiple aspects like compresion ratio, access speed etc. and tries to create a balanced middle ground that is best suitable for a wide range of filetypes. By utilizing existing standard algorithms and procedures the resulting file is readable by existing tools and does not need specific software to be handled. However the full potential of this file format can only be established with customized software that uses the additional information stored in the skippable frames.


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