As specified on the website, this image is explicitly "placed in the public domain".
Public domainPublic domainfalsefalse
This image has been released into the public domain by its creator and original copyright holder. This applies worldwide. As such you are entirely free to reproduce it, create derivative works, or make commercial use of it as you see fit, without any requirement to give the creator credit. However, as a courtesy, a link back to https://signal.org/docs/specifications/doubleratchet/#ipr website would be appreciated.
In case this is not legally possible:
The copyright holder grants any entity the right to use this work for any purpose, without any conditions, unless such conditions are required by law. However, as a courtesy, a link back to https://signal.org/docs/specifications/doubleratchet/#ipr website would be appreciated.
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Key Derivation Function. As denoted on the [https://signal.org/docs/specifications/doubleratchet/#double-ratchet website], this image is explicitly "placed in the public domain"