connectbot: ConnectBot crashes when connecting to SSH servers

Bug description

ConnectBot crashes when trying to connect to any of several SSH severs. This happens using password or public-key (tested with RSA) authentication.

Steps to reproduce

1. Open ConnectBot
2. Touch a host to start an SSH connection
3. Crash

Expected behavior

Should see a remote shell prompt instead of crash.

Android device

• OS: Android 7.0
• ConnectBot Version: 1.9.7 (beta from Play Store)

Server information

(one of them:)

• OS: Ubuntu Server 20.04 LTS
• SSH Software and Version: OpenSSH_8.2p1 Ubuntu-4ubuntu0.1, OpenSSL 1.1.1f 31 Mar 2020
• Pubkeys used (if applicable): Ed25519

Additional context

Everything worked fine before the last ConnectBot update; although I’m not sure which version was installed, it was up-to date with whichever version was available before on the Play Store (beta enabled).

Here’s the server log and stacktrace (from ADB) for the same connection:

• server.log

Jul 11 23:13:05 ubuntu sshd[3520]: message repeated 8 times: [ debug1: server_input_global_request: rtype keepalive@openssh.com want_reply 1]
Jul 11 23:13:06 ubuntu sshd[3419]: debug1: Forked child 3530.
Jul 11 23:13:06 ubuntu sshd[3530]: debug1: Set /proc/self/oom_score_adj to 0
Jul 11 23:13:06 ubuntu sshd[3530]: debug1: rexec start in 5 out 5 newsock 5 pipe 7 sock 8
Jul 11 23:13:06 ubuntu sshd[3530]: debug1: inetd sockets after dupping: 4, 4
Jul 11 23:13:06 ubuntu sshd[3530]: Connection from 10.0.1.11 port 42893 on 10.0.0.123 port 22 rdomain ""
Jul 11 23:13:06 ubuntu sshd[3530]: debug1: Local version string SSH-2.0-OpenSSH_8.2p1 Ubuntu-4ubuntu0.1
Jul 11 23:13:06 ubuntu sshd[3530]: debug1: Remote protocol version 2.0, remote software version TrileadSSH2Java_213
Jul 11 23:13:06 ubuntu sshd[3530]: debug1: no match: TrileadSSH2Java_213
Jul 11 23:13:06 ubuntu sshd[3530]: debug1: permanently_set_uid: 111/65534 [preauth]
Jul 11 23:13:06 ubuntu sshd[3530]: debug1: list_hostkey_types: rsa-sha2-512,rsa-sha2-256,ssh-rsa,ecdsa-sha2-nistp256,ssh-ed25519 [preauth]
Jul 11 23:13:06 ubuntu sshd[3530]: debug1: SSH2_MSG_KEXINIT sent [preauth]
Jul 11 23:13:06 ubuntu sshd[3530]: debug1: SSH2_MSG_KEXINIT received [preauth]
Jul 11 23:13:06 ubuntu sshd[3530]: debug1: kex: algorithm: curve25519-sha256 [preauth]
Jul 11 23:13:06 ubuntu sshd[3530]: debug1: kex: host key algorithm: ssh-ed25519 [preauth]
Jul 11 23:13:06 ubuntu sshd[3530]: debug1: kex: client->server cipher: aes256-ctr MAC: hmac-sha2-256-etm@openssh.com compression: none [preauth]
Jul 11 23:13:06 ubuntu sshd[3530]: debug1: kex: server->client cipher: aes256-ctr MAC: hmac-sha2-256-etm@openssh.com compression: none [preauth]
Jul 11 23:13:06 ubuntu sshd[3530]: debug1: expecting SSH2_MSG_KEX_ECDH_INIT [preauth]

• stacktrace.txt

07-12 00:13:06.185 15275 15321 E AndroidRuntime: FATAL EXCEPTION: Thread-4
07-12 00:13:06.185 15275 15321 E AndroidRuntime: Process: org.connectbot, PID: 15275
07-12 00:13:06.185 15275 15321 E AndroidRuntime: java.lang.IllegalStateException: Arithmetic error in curve multiplication with the public key: 0900000000000000000000000000000000000000000000000000000000000000
07-12 00:13:06.185 15275 15321 E AndroidRuntime: 	at com.google.crypto.tink.subtle.Curve25519.curveMult(Curve25519.java:332)
07-12 00:13:06.185 15275 15321 E AndroidRuntime: 	at com.google.crypto.tink.subtle.X25519.computeSharedSecret(X25519.java:103)
07-12 00:13:06.185 15275 15321 E AndroidRuntime: 	at com.google.crypto.tink.subtle.X25519.publicFromPrivate(X25519.java:121)
07-12 00:13:06.185 15275 15321 E AndroidRuntime: 	at com.trilead.ssh2.crypto.dh.Curve25519Exchange.init(Curve25519Exchange.java:43)
07-12 00:13:06.185 15275 15321 E AndroidRuntime: 	at com.trilead.ssh2.transport.KexManager.handleMessage(KexManager.java:574)
07-12 00:13:06.185 15275 15321 E AndroidRuntime: 	at com.trilead.ssh2.transport.TransportManager.receiveLoop(TransportManager.java:614)
07-12 00:13:06.185 15275 15321 E AndroidRuntime: 	at com.trilead.ssh2.transport.TransportManager$1.run(TransportManager.java:310)
07-12 00:13:06.185 15275 15321 E AndroidRuntime: 	at java.lang.Thread.run(Thread.java:761)

Regarding the “public key” seen on the stacktrace (0900000000000000000000000000000000000000000000000000000000000000): I’ve seen stacktraces with different, maybe random, values there before. But now every host I test crashes with that value there, everytime.