netutils/dropbear: back chacha20-poly1305 with NuttX /dev/crypto

[FelipeMdeO]

Summary

Replace Dropbear's bundled libtomcrypt chacha20-poly1305 implementation with
an adapter that drives the NuttX crypto device (/dev/crypto). The SSH
chacha20-poly1305@openssh.com construction maps onto CRYPTO_CHACHA20_DJB
(the original 64-bit counter/nonce ChaCha20 parameterization the protocol
uses) for the packet-length and payload keystreams, and onto CRYPTO_POLY1305
for the authentication tag. Plain (non-HMAC) MACs are driven in two steps
through /dev/crypto: COP_FLAG_UPDATE feeds the data, then a final call
retrieves the tag. The bundled chachapoly.c is dropped from the build.

This is the successor to the merged hmac-sha2-256 adapter (#3640).

Impact

• Users: none unless NETUTILS_DROPBEAR is enabled; then the
  chacha20-poly1305@openssh.com cipher runs through /dev/crypto instead of
  the bundled libtomcrypt code.
• Build: NETUTILS_DROPBEAR now depends on the cryptodev options above.

Testing

Tested on the NuttX simulator (sim:dropbear) and on real
STM32F746G-DISCO hardware. A host OpenSSH client connects forcing
chacha20-poly1305@openssh.com, so the new adapter (cipher + embedded Poly1305
tag) runs on every packet.

STM32F746G-DISCO (software /dev/crypto backend)

Board side (serial console):

NuttShell (NSH) NuttX-13.0.0
nsh> [4] Jan 14 00:00:05 using NuttX passwd auth at /data/passwd
dropbear: listening on port 2222

nsh> useradd admin Felipe123!
nsh> ifconfig
eth0	Link encap:Ethernet HWaddr 00:e0:de:ad:be:ef at RUNNING mtu 1486
	inet addr:192.168.15.155 DRaddr:192.168.15.1 Mask:255.255.255.0

nsh> [4] Jan 14 00:01:04 Password auth succeeded for 'admin' from 192.168.15.8:39594
[4] Jan 14 00:01:05 NSH PTY session started
[4] Jan 14 00:01:05 Exit (admin) from <192.168.15.8:39594>: Disconnect received

PC side (note cipher: chacha20-poly1305@openssh.com negotiated in both
directions):

$ ssh -v -c chacha20-poly1305@openssh.com -p 2222 admin@192.168.15.155
debug1: Remote protocol version 2.0, remote software version dropbear_2026.91
debug1: kex: algorithm: curve25519-sha256
debug1: kex: host key algorithm: ecdsa-sha2-nistp256
debug1: kex: server->client cipher: chacha20-poly1305@openssh.com MAC: <implicit> compression: none
debug1: kex: client->server cipher: chacha20-poly1305@openssh.com MAC: <implicit> compression: none
Authenticated to 192.168.15.155 ([192.168.15.155]:2222) using "password".

nsh> uname -a
NuttX stntest 13.0.0 2099ceda33 Jul 24 2026 20:05:23 arm stm32f746g-disco
nsh> free
      total       used       free    maxused    maxfree  nused  nfree name
     283788     145108     138680     145736     135800    117      8 Umem
nsh> uptime
00:01:05 up  0:01, load average: 0.00, 0.00, 0.00
nsh> exit
Connection to 192.168.15.155 closed.

[cederom]

Thank you @FelipeMdeO :-)

[FelipeMdeO]

Hello @simbit18 , do you know what is this pipeline issue?

[acassis]

Hello @simbit18 , do you know what is this pipeline issue?

@FelipeMdeO @simbit18 I issued Re-run task and it still failing

[acassis]

Since the issue is in CI and not in PR, I think it is safe to merge