225 lines
8.6 KiB
Markdown
225 lines
8.6 KiB
Markdown
page_title: TarSum checksum specification
|
|
page_description: Documentation for algorithms used in the TarSum checksum calculation
|
|
page_keywords: docker, checksum, validation, tarsum
|
|
|
|
# TarSum Checksum Specification
|
|
|
|
## Abstract
|
|
|
|
This document describes the algorithms used in performing the TarSum checksum
|
|
calculation on filesystem layers, the need for this method over existing
|
|
methods, and the versioning of this calculation.
|
|
|
|
|
|
## Introduction
|
|
|
|
The transportation of filesystems, regarding Docker, is done with tar(1)
|
|
archives. There are a variety of tar serialization formats [2], and a key
|
|
concern here is ensuring a repeatable checksum given a set of inputs from a
|
|
generic tar archive. Types of transportation include distribution to and from a
|
|
registry endpoint, saving and loading through commands or Docker daemon APIs,
|
|
transferring the build context from client to Docker daemon, and committing the
|
|
filesystem of a container to become an image.
|
|
|
|
As tar archives are used for transit, but not preserved in many situations, the
|
|
focus of the algorithm is to ensure the integrity of the preserved filesystem,
|
|
while maintaining a deterministic accountability. This includes neither
|
|
constraining the ordering or manipulation of the files during the creation or
|
|
unpacking of the archive, nor include additional metadata state about the file
|
|
system attributes.
|
|
|
|
## Intended Audience
|
|
|
|
This document is outlining the methods used for consistent checksum calculation
|
|
for filesystems transported via tar archives.
|
|
|
|
Auditing these methodologies is an open and iterative process. This document
|
|
should accommodate the review of source code. Ultimately, this document should
|
|
be the starting point of further refinements to the algorithm and its future
|
|
versions.
|
|
|
|
## Concept
|
|
|
|
The checksum mechanism must ensure the integrity and assurance of the
|
|
filesystem payload.
|
|
|
|
## Checksum Algorithm Profile
|
|
|
|
A checksum mechanism must define the following operations and attributes:
|
|
|
|
* Associated hashing cipher - used to checksum each file payload and attribute
|
|
information.
|
|
* Checksum list - each file of the filesystem archive has its checksum
|
|
calculated from the payload and attributes of the file. The final checksum is
|
|
calculated from this list, with specific ordering.
|
|
* Version - as the algorithm adapts to requirements, there are behaviors of the
|
|
algorithm to manage by versioning.
|
|
* Archive being calculated - the tar archive having its checksum calculated
|
|
|
|
## Elements of TarSum checksum
|
|
|
|
The calculated sum output is a text string. The elements included in the output
|
|
of the calculated sum comprise the information needed for validation of the sum
|
|
(TarSum version and hashing cipher used) and the expected checksum in hexadecimal
|
|
form.
|
|
|
|
There are two delimiters used:
|
|
* '+' separates TarSum version from hashing cipher
|
|
* ':' separates calculation mechanics from expected hash
|
|
|
|
Example:
|
|
|
|
```
|
|
"tarsum.v1+sha256:220a60ecd4a3c32c282622a625a54db9ba0ff55b5ba9c29c7064a2bc358b6a3e"
|
|
| | \ |
|
|
| | \ |
|
|
|_version_|_cipher__|__ |
|
|
| \ |
|
|
|_calculation_mechanics_|______________________expected_sum_______________________|
|
|
```
|
|
|
|
## Versioning
|
|
|
|
Versioning was introduced [0] to accommodate differences in calculation needed,
|
|
and ability to maintain reverse compatibility.
|
|
|
|
The general algorithm will be describe further in the 'Calculation'.
|
|
|
|
### Version0
|
|
|
|
This is the initial version of TarSum.
|
|
|
|
Its element in the TarSum checksum string is `tarsum`.
|
|
|
|
### Version1
|
|
|
|
Its element in the TarSum checksum is `tarsum.v1`.
|
|
|
|
The notable changes in this version:
|
|
* Exclusion of file `mtime` from the file information headers, in each file
|
|
checksum calculation
|
|
* Inclusion of extended attributes (`xattrs`. Also seen as `SCHILY.xattr.` prefixed Pax
|
|
tar file info headers) keys and values in each file checksum calculation
|
|
|
|
### VersionDev
|
|
|
|
*Do not use unless validating refinements to the checksum algorithm*
|
|
|
|
Its element in the TarSum checksum is `tarsum.dev`.
|
|
|
|
This is a floating place holder for a next version and grounds for testing
|
|
changes. The methods used for calculation are subject to change without notice,
|
|
and this version is for testing and not for production use.
|
|
|
|
## Ciphers
|
|
|
|
The official default and standard hashing cipher used in the calculation mechanic
|
|
is `sha256`. This refers to SHA256 hash algorithm as defined in FIPS 180-4.
|
|
|
|
Though the TarSum algorithm itself is not exclusively bound to the single
|
|
hashing cipher `sha256`, support for alternate hashing ciphers was later added
|
|
[1]. Use cases for alternate cipher could include future-proofing TarSum
|
|
checksum format and using faster cipher hashes for tar filesystem checksums.
|
|
|
|
## Calculation
|
|
|
|
### Requirement
|
|
|
|
As mentioned earlier, the calculation is such that it takes into consideration
|
|
the lifecycle of the tar archive. In that the tar archive is not an immutable,
|
|
permanent artifact. Otherwise options like relying on a known hashing cipher
|
|
checksum of the archive itself would be reliable enough. The tar archive of the
|
|
filesystem is used as a transportation medium for Docker images, and the
|
|
archive is discarded once its contents are extracted. Therefore, for consistent
|
|
validation items such as order of files in the tar archive and time stamps are
|
|
subject to change once an image is received.
|
|
|
|
### Process
|
|
|
|
The method is typically iterative due to reading tar info headers from the
|
|
archive stream, though this is not a strict requirement.
|
|
|
|
#### Files
|
|
|
|
Each file in the tar archive have their contents (headers and body) checksummed
|
|
individually using the designated associated hashing cipher. The ordered
|
|
headers of the file are written to the checksum calculation first, and then the
|
|
payload of the file body.
|
|
|
|
The resulting checksum of the file is appended to the list of file sums. The
|
|
sum is encoded as a string of the hexadecimal digest. Additionally, the file
|
|
name and position in the archive is kept as reference for special ordering.
|
|
|
|
#### Headers
|
|
|
|
The following headers are read, in this
|
|
order ( and the corresponding representation of its value):
|
|
* 'name' - string
|
|
* 'mode' - string of the base10 integer
|
|
* 'uid' - string of the integer
|
|
* 'gid' - string of the integer
|
|
* 'size' - string of the integer
|
|
* 'mtime' (_Version0 only_) - string of integer of the seconds since 1970-01-01 00:00:00 UTC
|
|
* 'typeflag' - string of the char
|
|
* 'linkname' - string
|
|
* 'uname' - string
|
|
* 'gname' - string
|
|
* 'devmajor' - string of the integer
|
|
* 'devminor' - string of the integer
|
|
|
|
For >= Version1, the extented attribute headers ("SCHILY.xattr." prefixed pax
|
|
headers) included after the above list. These xattrs key/values are first
|
|
sorted by the keys.
|
|
|
|
#### Header Format
|
|
|
|
The ordered headers are written to the hash in the format of
|
|
|
|
"{.key}{.value}"
|
|
|
|
with no newline.
|
|
|
|
#### Body
|
|
|
|
After the order headers of the file have been added to the checksum for the
|
|
file, the body of the file is written to the hash.
|
|
|
|
#### List of file sums
|
|
|
|
The list of file sums is sorted by the string of the hexadecimal digest.
|
|
|
|
If there are two files in the tar with matching paths, the order of occurrence
|
|
for that path is reflected for the sums of the corresponding file header and
|
|
body.
|
|
|
|
#### Final Checksum
|
|
|
|
Begin with a fresh or initial state of the associated hash cipher. If there is
|
|
additional payload to include in the TarSum calculation for the archive, it is
|
|
written first. Then each checksum from the ordered list of file sums is written
|
|
to the hash.
|
|
|
|
The resulting digest is formatted per the Elements of TarSum checksum,
|
|
including the TarSum version, the associated hash cipher and the hexadecimal
|
|
encoded checksum digest.
|
|
|
|
## Security Considerations
|
|
|
|
The initial version of TarSum has undergone one update that could invalidate
|
|
handcrafted tar archives. The tar archive format supports appending of files
|
|
with same names as prior files in the archive. The latter file will clobber the
|
|
prior file of the same path. Due to this the algorithm now accounts for files
|
|
with matching paths, and orders the list of file sums accordingly [3].
|
|
|
|
## Footnotes
|
|
|
|
* [0] Versioning https://github.com/docker/docker/commit/747f89cd327db9d50251b17797c4d825162226d0
|
|
* [1] Alternate ciphers https://github.com/docker/docker/commit/4e9925d780665149b8bc940d5ba242ada1973c4e
|
|
* [2] Tar http://en.wikipedia.org/wiki/Tar_%28computing%29
|
|
* [3] Name collision https://github.com/docker/docker/commit/c5e6362c53cbbc09ddbabd5a7323e04438b57d31
|
|
|
|
## Acknowledgements
|
|
|
|
Joffrey F (shin-) and Guillaume J. Charmes (creack) on the initial work of the
|
|
TarSum calculation.
|
|
|