Flag to enable "legacy" algos on initialization?

[Hr0bar]

Hi, I see a lot of users are expecing full in-place compatibility with old Jsch, but that does not happen due to the retirement of some now insecure algos:

#48
#47
#45
#40
#37

I propose adding a flag that would enable all algos that were previously enabled in the legacy Jsch, something like:

new com.jcraft.jsch.JSch(enable_insecure = "true");

Of course, they would be the least preferred/offered option in the negotiation list.

Reason:
we use Jsch to connect to all kinds of versions (some 10+ years old) of below OSes, and its simply impossible to live without these legacy algos. I know we can enable them via configuration as in the example existing issues, but a simple flag that would enable all of them at once is definitely a cleaner way.

• 3Com
• A10 ACOS
• Allot
• Arista EOS
• Aruba AOS
• BlueCat
• BlueCoat CAS
• BlueCoat PacketShaper
• BlueCoat ProxyAV
• BlueCoat ProxySG
• Brocade Vyatta NetworkOS
• CheckPoint GAIA
• CheckPoint IPSO
• CheckPoint SPLAT
• Cisco ACE
• Cisco ACS
• Cisco APIC
• Cisco ASA
• Cisco AsyncOS
• Cisco CatOS
• Cisco CSS
• Cisco Express
• Cisco FTD
• Cisco FWSM
• Cisco FXOS
• Cisco IOS
• Cisco IOS XR
• Cisco IPS
• Cisco ISE
• Cisco NXOS
• Cisco Prime
• Cisco WAAS
• Cisco WLC
• Citrix NetScaler
• Dell iDRAC
• Dell NetworkingOS
• Enterasys
• F5
• Forcepoint Stonesoft
• Fortinet FortiOS
• Gigamon GigaVUEOS
• HP
• Huawei
• IBM AlteonOS/BladeOS/NetworkingOS
• IBM ISCLI
• Infoblox NIOS
• Juniper JUNOS
• Juniper ScreenOS
• Lenovo CNOS
• Linux
• Nokia VitalQIP
• Nortel WSS
• Oracle Acme PacketOS
• Palo Alto PANOS
• PulseSecure
• Radware Alteon
• Riverbed RiOS
• Spectracom
• TrendMicro IPS TOS
• TrendMicro TPS TOS

[mwiede]

in general, I would be open to support such a flag, the only thing is, that we should keep the API of original Jsch, so that exchanging the library does not break anything. Feel free to send in a PR.

[norrisjeremy]

I'm not sure if I see adding a new constructor flag as especially valuable, as it still requires users to actively change the code to their applications using the JSch library. If they are already in a position to make such a code change, they can just as easily adjust their code to use a different set of crypto algorithms that are compatible with the SSH server with which they are interacting. Additionally, such a change would no longer be backwards compatible at an API level with the original JSch library, since it would not have a similarly defined constructor to the JSch class.

As an alternate approach, what if we instead added some system properties that users could utilize to adjust the various crypto algorithms? This would allow users to do something like java -Djsch.kex="..." -Djsch.server_host_key="..." -Djsch.cipher="..." -Djsch.mac="..." when launching their applications. The value in this approach to the issue is that a user would no longer need to make changes to their application code: they could just define system properties containing the set of crypto algorithms that are compatible with their SSH servers.

[norrisjeremy]

FYI, #61 includes proposal to allow usage of system properties to override the various crypto algorithm defaults.

[norrisjeremy]

Hi @Hr0bar,

In the recent 0.1.65 release we added several System Properties that can be administratively set by users to manipulate the default crypto algorithms used. This should allow users to set them without necessarily requiring them to modify their application code as we have had to suggest in the prior issues that were raised. Hopefully this will make it a bit easier to support older OS's that only support legacy/insecure crypto algorithms.

Thanks,
Jeremy

[mendhak]

A 'legacy or insecure = true' flag is very useful where application developers are allowing users to supply both the server details as well as keys to connect. In other words, where the server details are not known during the time of development.

As this library gets future updates, the list of algorithms deprecated will change, and many applications using this library will miss out on what flags and algorithms to allow for their usages to 'keep up' with the times. So the legacy=true flag becomes a clean convenience function, instead of having to wait for users to say something isn't working which causes friction and a poor application experience. It also it serves as a little reminder to them that what they're doing is not a good practice. As OP has said, although things gets deprecated, deprecated setups still exist, and will continue to exist.

In the case of my app, it's on Android so there aren't any application flags that can be set. I'd probably provide a UI toggle which if enabled, sets this legacy variable to true.