Elliptic Curve host key and -R argument bug

Bruno Thomsen bth at kamstrup.dk
Tue Jan 28 22:18:44 WST 2014


Hi,

I have observed some strange dropbear behavior with the -R argument (Create hostkeys as required) and Elliptic Curve host keys.
The result is an extremely large temporary host key file (multiple megabytes).

SSH client: Google Chrome extension: Secure Shell 0.8.25

1) Connect to dropbear running without arguments and a single host key (ecdsa-sha2-nistp521) in /etc/dropbear/dropbear_ecdsa_host_key
2) Stop dropbear
3) Remove old host key and generate a single new host key (ecdsa-sha2-nistp256) in /etc/dropbear/dropbear_ecdsa_host_key
4) Start dropbear with -R argument
5) Reconnect to dropbear and it generate an extremely large host key (process never ends).



Result:
root at target:~ ll /etc/dropbear/
total 54508
dr--------    2 root     root          1024 Jan 28 13:38 .
drwxrwxr-x   19 root     root          1024 Jan 28 12:09 ..
-r--------    1 root     root           140 Jan 28 13:35 dropbear_ecdsa_host_key
-rw-------    1 root     root      55593054 Jan 28 13:39 dropbear_ecdsa_host_key.tmp377
root at target:~ ll /etc/dropbear/
total 66001
dr--------    2 root     root          1024 Jan 28 13:38 .
drwxrwxr-x   19 root     root          1024 Jan 28 12:09 ..
-r--------    1 root     root           140 Jan 28 13:35 dropbear_ecdsa_host_key
-rw-------    1 root     root      67316589 Jan 28 13:40 dropbear_ecdsa_host_key.tmp377
root at target:~ ll /etc/dropbear/
total 70657
dr--------    2 root     root          1024 Jan 28 13:38 .
drwxrwxr-x   19 root     root          1024 Jan 28 12:09 ..
-r--------    1 root     root           140 Jan 28 13:35 dropbear_ecdsa_host_key
-rw-------    1 root     root      72064080 Jan 28 13:40 dropbear_ecdsa_host_key.tmp377
root at target:~ ll /etc/dropbear/
total 84631
dr--------    2 root     root          1024 Jan 28 13:38 .
drwxrwxr-x   19 root     root          1024 Jan 28 12:09 ..
-r--------    1 root     root           140 Jan 28 13:35 dropbear_ecdsa_host_key
-rw-------    1 root     root      86317002 Jan 28 13:40 dropbear_ecdsa_host_key.tmp377
root at target:~ ll /etc/dropbear/
total 129200
dr--------    2 root     root          1024 Jan 28 13:38 .
drwxrwxr-x   19 root     root          1024 Jan 28 12:09 ..
-r--------    1 root     root           140 Jan 28 13:35 dropbear_ecdsa_host_key
-rw-------    1 root     root     131778657 Jan 28 13:41 dropbear_ecdsa_host_key.tmp377
root at target:~ ll /etc/dropbear/
total 130372
dr--------    2 root     root          1024 Jan 28 13:38 .
drwxrwxr-x   19 root     root          1024 Jan 28 12:09 ..
-r--------    1 root     root           140 Jan 28 13:35 dropbear_ecdsa_host_key
-rw-------    1 root     root     132973731 Jan 28 13:41 dropbear_ecdsa_host_key.tmp377
root at target:~ ll /etc/dropbear/
total 131219
dr--------    2 root     root          1024 Jan 28 13:38 .
drwxrwxr-x   19 root     root          1024 Jan 28 12:09 ..
-r--------    1 root     root           140 Jan 28 13:35 dropbear_ecdsa_host_key
-rw-------    1 root     root     133837839 Jan 28 13:41 dropbear_ecdsa_host_key.tmp377
root at target:~ top
Mem: 121580K used, 3376K free, 0K shrd, 10172K buff, 91900K cached
CPU: 20.0% usr 80.0% sys  0.0% nic  0.0% idle  0.0% io  0.0% irq  0.0% sirq
Load average: 2.30 1.04 0.52 2/55 1416
  PID  PPID USER     STAT   VSZ %MEM CPU %CPU COMMAND
  377 31162 root     R     2292  1.8   0 66.5 dropbear -R



Maybe it's a collision in /etc/dropbear/dropbear_ecdsa_host_key that holds an ecdsa-sha2-nistp256 and when dropbear tries to generate a new ecdsa-sha2-nistp521.


Venlig hilsen / Best regards

Kamstrup A/S <http://www.kamstrup.dk> 
Bruno Thomsen
Development engineer
Technology

Kamstrup A/S
Industrivej 28
DK-8660 Skanderborg
Tel:	 +45 89 93 10 00	 
Fax:	 +45 89 93 10 01	 
Dir:	 +45 89 93 13 94	 
E-mail:	 bth at kamstrup.dk	 
Web:	 www.kamstrup.dk	 



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