Certain attacks (such as Logjam) leverage the weakness of "small" Diffie-Hellman (DH) keys.
To mitigate the risk of such attackers, users may either exclude vulnerable ciphers, or lengthen the DH keys used by Repose.
Instructions for the former are above.
For the latter, note the following:
Diffie-Hellman (DH) keys of sizes less than 1024 bits have been deprecated because of their insufficient strength.
In JDK 8, you can customize the ephemeral DH key size with the system property jdk.tls.ephemeralDHKeySize.
— Java Secure Socket Extension (JSSE) Reference Guide
In other words, the Java option -Djdk.tls.ephemeralDHKeySize=2048
can be passed when starting Repose to force the use of longer DH keys.