[SSHD-1107] Diffie Hellmann group exchange minimum key size should be configurable for security reasons

• Type: Improvement
• Status: Resolved
• Priority: Major
• Resolution: Fixed
• Affects Version/s: 2.5.1
• Fix Version/s: 2.6.0
• Assignee: Lyor Goldstein
• Reporter: Achim Hügen
• Created: 01/Dec/20 07:39
• Updated: 03/Dec/20 15:26
• Resolved: 03/Dec/20 14:49

Description

Currently the minimum key size for DH group exchange is 1024 bit (for example when using diffie-hellman-group-exchange-sha256 for KEX)

This is below the current recommendations of 2048 bit (see NIST, Table 2-1 and https://tools.ietf.org/id/draft-ietf-curdle-ssh-kex-sha2-09.html#rfc.section.3.4).

For compatibility reasons it is still essential to support diffie-hellman-group-exchange-sha256, but the minimum key size should be configurable, so that a server operator can harden its server.

The minimum size is hardcoded in org.apache.sshd.common.util.security.SecurityUtils#MIN_DHGEX_KEY_SIZE

This could be made configurable via a system property like one already exists for the maximum (see MAX_DHGEX_KEY_SIZE_PROP)