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Allgemeine Informationen findest du wie immer hier:
https://de.wikipedia.org/wiki/Digitale_Signatur

Die nutzen alle asymmetrische Kryptographie - RSA mit Schlüssellängen 2048-8192 bits. Ein neueres Verfahren sind elliptische Kurven, die aber eine problematische Standardisierung aufweisen, bei der Hintertüren existieren können. Eine alternative asymmetrische Kryptographie - GMR, das ausschliesslich signiert und nicht verschlüsselt, findest Du hier als OSS: GMR Digital Signature System

Wenn Du eMails signieren willst, dann empfehle ich den eM Client:
https://de.emclient.com/download

Interessant unter Ubuntu ist für mich: seahorse
https://wiki.ubuntuusers.de/Seahorse/

...und als GPG Alternative - frühes entwicklungsstadium:
https://github.com/rnpgp/rnp

gmr-digital-signature-system V2.0.0
===================================

brought to you by Gerrit Leder, Copyright 2001-2020 (gerrit.leder@gmail.com)

Homepage: https://gmr-digitalsignature.eu
Project repository: https://gitlab.com/gerrit.leder/gmr-digital-signature-mvn

Files:

- $ java -jar gmr-2.0.0.jar: the java executable
- leder9.pk: the public key to my secret private key and the signature...
- gmr-2.0.0.jar.gmr: generated signature, verify with public key "leder9"
- This is the sha512 checksum of leder9.pk:

leder@leder-ryzen-home:~/Projekt/GMR$ sha512sum leder9.pk
3308da991d9428d9f182c3e72faa4b760d38ed6b116de573b0a51631325b08b483e053c0e37d0bd9405fbc3307006c8636abb3c614a9865add454b977cfbed5a leder9.pk

Download gmr-2.0.0.jar for Linux (Virtual) Machine, Windows is now supported, too!

gmr-2.0.0.jar.gmr: generated signature, verify with public key "leder9"

leder9.pk: the public key to my secret private key and the signature...

Please use OpenJDK-11-JRE or OpenJDK-8-JRE with Linux or Windows operating system.
There is no compatibilty of keys and signatures generated with earlier versions of gmr and this version!
Please generate new keys of at least 4096 bits length and new signatures with this version of GMR and a real (hardware) number generator!

The Java keygeneration mechanism (at least on windows) can be assumed as broken.
(Taken from: http://stackoverflow.com/questions/19119751/java-keytool-the-security-of-generated-keys-with-java-in-general)
Quote: "Update: Oracle seem to have improved some issues with Java 8."

Please use Linux and check the following properties:
1) The configuration of the Security Providers (thus also for SecureRandom) is done in JAVA_HOME/conf/security/java.security. The default is (top of the list): security.provider.1=SUN
2) securerandom.source=file:/dev/random
3) sudo apt-get install rng-tools

important: this software GMR will not work without rng-tools installed on Linux!
4) sudo vim /etc/default/rng-tools
HRNGDEVICE=/dev/random

which is used by default
5) sudo service rng-tools start
6) sudo service rng-tools status

The bottom line: don't use PRNGs when good randomness is needed!

leder@leder-ryzen-home:~/Projekt/GMR$ sha512sum -c sha512sum_leder9_pk.txt
leder9.pk: OK

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