Announcing LogStore

While it probably is a bit early for a “real” announcement,  I wanted to tell a bit about the project I have been working on the past days, a dedicated storage for (sys)log messages. It will be available as part of LogTools, the actual project I am working with. A key feature of the LogStore format will be its tamper-proofness. I wanted to write such an improved storage system for quite a while. However, I have to admit that the recent journald proposal brought more life to it. While the journald proposal aims at building a kind of Window Event Log for Linux, the LogTools effort is more interested in traditional text log files (but I won’t outrule going beyond that in the future).

It is important to note that LogTools, and their storage format LogStore, can protect any kind of text file. Of course, it is great for syslog logs, but you may also secure things like Apache http logs or whatever else you have in text format.

You may probably remember that I was – and still am – very skeptic about the way journald tries to secure logs via hash chains (be sure to read the comments as well!). While the journald propsal has some technical deficits, I learned that many folks we interested in this kind of hash-chaining, even though they knew it would not be truly tamperproof (I followed a lot of forum posts). Seeing this, I thought it may be useful to provide this level of protection inside some simple to use tools, what gave birth to LogTools. Still, my assesment in regard to journald holds to the current LogStore format as well: it is far from being real cryptography, it is insecure and it may be counter-productive if it generates a false sense of security. However, if one knows the limits, it can provide some useful function. So be sure to know what you do when you use these tools!

For LogStore, I have also planned to employ some real cryptography and cryptographically sign the hash chain. This will actually make the log tamperproof in a very strong sense as long as the signing key is not compromised. That functionality will be addressed once the initial release is out.

The LogStore format itself is deliberately defined in a text-tool friendly way and well documented. For your initial review, I have included its man page below.

The LogTools project is currently available only via the LogTools git. I plan to finish the remaining man pages soon (at latest this week), and then create distribution tarballs (and hopefully some simple packages, but this needs to be seen).

Feedback on this effort is appreciated.


logstore – enhanced log message storage  


The logstore is an enhanced log message storage. It can be used to store log messages in a way that secures their integrity. Currently, all data is stored in sequential files.
The logstore provides integrity checks by chaining of SHA1-checksums. Each log record (except the first) is hashed, together with the hash of the previous record. As such, manipulations inside the log store can be detected, as long as the checksums of all records are not also recomputed. Sequential logstore files are pure text files.


A sequential logstore is a text file containing variable-length records. Within each record, there is recordtype, cryptodesignator and content.

A single character designating the type of record. Currently only “m” is defined, which specified original message text.
Variable length, terminated by a colon. The is printable data that has some cryptographic function. For example, for “m”-type recrods it is the message’s chained SHA1 hash.
Variable length content terminated by a LF (. For obvious reasons, LF is not permitted within the message. Also, the US-ASCII NUL character ( ) is forbidden in order to prevent trouble with text based tools. It is suggested that only printable characters are used inside the message, but this is currently not enforced.
The structure is recordtype Rsyslog has a modular design. Consequently, there is a growing number of modules. See the html documentation for their full description.



In its current form, logstore provides limited security. While it is possible to verify the correctness of the hash chain, an attacker may simply rewrite the complete file, computing new hashes. However, protection can be (manually) gained by saving the last hash inside the file to a separate location. If so, one can compare the last hash with this previously saved information and check if it is still valid. If someone mangeled the store, this will not be the case. Once the authenticy of the last hash has been proven, it is easy to verify the rest ot the file. The logreader (1) tool can be used to do that.
In the future, cryptographic signatures based on public key cryptography will be used to protect the hash chain.


The following is a minimalistic 4-line sample of a sequential logstore file.

m04e3324670626451755aa2257a9b92395e26c2e4:line 1
m347d4500ea11fa41800a58972699e57e0c0d7cd7:line 2
m3c329d40e37ae20c475c06bfaab892892ef4579d:line 3
m807cc61d7b04cbc1f048810df9d3a652988d745e:line 4

Note that all records have “m” in the first postion, designating them as message records. The cryptographic hash in line one is a SHA1 hash of just line one’s content, whereas the hashes for lines n (with n>1) are taken after the concatenation of hash(n-1) and line(n), without the colon. So in order to obtain line two’s hash, the following string is hashed:


logreader(1), logwriter(1), liblogtools(3)


Rainer Gerhards (