netutils/dropbear: back hmac-sha2-256 with NuttX /dev/crypto

[FelipeMdeO]

Summary

Replace Dropbear's bundled libtomcrypt HMAC modules with an adapter that
computes the hmac-sha2-256 packet MAC through the NuttX crypto device
(/dev/crypto) using CRYPTO_SHA2_256_HMAC sessions: hmac_init opens the
session with the MAC key, hmac_process feeds data with COP_FLAG_UPDATE
and hmac_done reads the tag and frees the session.

The upstream Dropbear hmac_state is reused as-is, so no source patch is
needed: the existing hash field keeps the hash index and the /dev/crypto
session id is stored in the otherwise unused md buffer. The bundled
hmac_init.c/hmac_process.c/hmac_done.c are dropped from the build and
NETUTILS_DROPBEAR depends on CRYPTO_CRYPTODEV and
CRYPTO_CRYPTODEV_SOFTWARE_CRYPTO.

Only the HMAC is offloaded, not the plain SHA-256 hash: Dropbear's key
derivation (hashkeys() in common-kex.c) memcpy-clones a
partially-updated hash_state, which a kernel crypto session cannot
represent, so the SHA-256 descriptor stays on libtomcrypt.

Impact

• Users: none unless NETUTILS_DROPBEAR is enabled;

Testing

sim:dropbear on the NuttX simulator. Dropbear is started by NSH, a user is
created, and a host OpenSSH client connects forcing hmac-sha2-256 as the
MAC (with aes128-ctr, so the HMAC runs on every packet):

console Board side:

➜  nuttx git:(fix/dropbear-cryptodev-defconfigs) ./nuttx

dropbear [6:100]

NuttShell (NSH) NuttX-13.0.0
nsh> [6] Jun 01 00:00:00 using NuttX passwd auth at /tmp/passwd
dropbear: listening on port 2222
useradd felipe Testpass123
nsh> 
nsh> [6] Jun 01 00:01:22 connection from 192.168.15.8:47198
[6] Jun 01 00:01:30 Password auth succeeded for 'felipe' from 192.168.15.8:47198
[6] Jun 01 00:01:30 NSH PTY session started

console PC side:

➜  ~ ssh-keygen -R '[10.0.1.2]:2222'   # host key nova a cada boot do sim
ssh -p 2222 -c aes128-ctr -m hmac-sha2-256 felipe@10.0.1.2

# Host [10.0.1.2]:2222 found: line 9
/home/felipe-moura/.ssh/known_hosts updated.
Original contents retained as /home/felipe-moura/.ssh/known_hosts.old
The authenticity of host '[10.0.1.2]:2222 ([10.0.1.2]:2222)' can't be established.
ECDSA key fingerprint is <*************>
This key is not known by any other names.
Are you sure you want to continue connecting (yes/no/[fingerprint])? yes
Warning: Permanently added '[10.0.1.2]:2222' (ECDSA) to the list of known hosts.
felipe@10.0.1.2's password: 
nsh> ls
/:
 bin/
 dev/
 etc/
 proc/
 tmp/
 var/
nsh> ps
  TID   PID  PPID PRI POLICY   TYPE    NPX STATE    EVENT     SIGMASK            STACK COMMAND
    0     0     0   0 FIFO     Kthread   - Ready              0000000000000000 0069584 Idle_Task
    1     0     0 224 FIFO     Kthread   - Waiting  Semaphore 0000000000000000 0067456 sim_loop_wq 0x7879368003f0 0x787936800470
    2     0     0 224 FIFO     Kthread   - Waiting  Semaphore 0000000000000000 0067464 hpwork 0x40109020 0x401090a0
    3     0     0 100 FIFO     Kthread   - Waiting  Semaphore 0000000000000000 0067464 lpwork 0x401090e0 0x40109160
    5     5     0 100 FIFO     Task      - Waiting  Semaphore 0000000000000000 0067496 nsh_main
    6     6     5 100 FIFO     Task      - Ready              0000000000000000 0130984 dropbear
    7     7     6 100 FIFO     Task      - Running            0000000000000000 0073648 dropbear_nsh
    8     6     5 100 FIFO     pthread   - Waiting  Signal    0000000000000000 0067528 dropbear 0x40095843 0x7879369398b0
nsh> 

Test using STM32F7 DISCO:

Board Side:

nsh: �: command not found
nsh> ls /data
/data:
 .
 ..
 dropbear_ecdsa_host_key
 passwd
nsh> [4] Jan 14 00:00:18 connection from 192.168.15.8:45388
[4] Jan 14 00:00:24 passwd_verify failed for 'felipe': -14
[4] Jan 14 00:00:24 Bad password attempt for 'felipe' from 192.168.15.8:45388
[4] Jan 14 00:00:32 Password auth succeeded for 'felipe' from 192.168.15.8:45388
[4] Jan 14 00:00:32 NSH PTY session started
[4] Jan 14 00:01:57 Exit (felipe) from <192.168.15.8:45388>: Disconnect received

PC side:

➜  ~ ssh -p 2222 -c aes128-ctr -m hmac-sha2-256 felipe@192.168.15.155

felipe@192.168.15.155's password: 
Permission denied, please try again.
felipe@192.168.15.155's password: 
nsh> ls
/:
 data/
 dev/
 proc/
 var/
nsh> exit
Connection to 192.168.15.155 closed.
➜  ~