Encasha: Layer-2 Namecoin Verification of SSH Host Keys

Encasha enables SSH clients (e.g. OpenSSH) to automatically verify host keys for Namecoin domains, without putting SSHFP records on-chain. Encasha works by stapling a Namecoin message signature in an SSH certificate extension.

Building

1. Ensure you have the Go tools installed.

2. Clone encasha.

3. Run go install ./.... encasha will be built. The binaries will be at $GOPATH/bin/encasha.

Installing in OpenSSH

KnownHostsCommand /usr/bin/encasha --reason "%I" --host "%H" --key "%K"

Configuration

Encasha uses Namecoin Core cookie authentication by default. If you have a non-default Namecoin RPC port, cookie path, or username/password, these can be set via a TOML configuration file in $HOME/.config/encasha/encasha.conf.

Encasha uses "strict" mode by default; this enforces an STS-like policy for all Namecoin domains (i.e. any domains under the bit and bit.onion eTLD's). This unfortunately will also break Namecoin domains that use on-chain SSHFP records. If you must use on-chain SSHFP records, you can disable strict mode via TOML config.

TODO: document/improve this.

Certificate Generation

Generate a domain CA:

ssh-keygen -f ./example.bit_ca_key -C 'example.bit Domain CA'

Generate a host key:

ssh-keygen -f ./example.bit_host_key -C 'example.bit Host Key'

Get the base64-encoded key:

cat example.bit_ca_key.pub

Sign the following message with Namecoin:

Namecoin SSH Stapled Certification: {"address":"FILL IN NAMECOIN ADDRESS HERE BEFORE SIGNING","domain":"example.bit","sshpub":"FILL IN SSH CA BASE64 PUBKEY HERE BEFORE SIGNING"}

Sign the host key with the domain CA:

ssh-keygen -h -s ./example.bit_ca_key -I 'example.bit Host Certificate' -n example.bit -O 'extension:namecoin-ssh-stapled@ssh.x--nmc.bit=[{"blockchainaddress":"FILL IN NAMECOIN ADDRESS HERE BEFORE SIGNING","blockchainsig":"FILL IN NAMECOIN SIGNATURE HERE BEFORE SIGNING"}]' ./example.bit_host_key.pub

Inspect the cert:

ssh-keygen -L -f example.bit_host_key-cert.pub

Add the cert to the server:

HostCertificate ./example.bit_host_key-cert.pub

Certificate Revocation

Transfer your Namecoin name to a new address to revoke all SSH certifications. (As a corollary, if you want to update your name without revoking SSH certifications, you must reuse the same Namecoin address.)

Relationship to Encaya

Encasha does for SSH what Encaya's off-chain mode does for X.509. Encasha and Encaya share the same underlying codebase for validating stapled Namecoin certifications. Encaya's on-chain modes are not applicable to SSH because SSH implementations already support that functionality natively (via the SSHFP DNS record type).

Theoretical Background

See Namecoin's 38C3 talks.

Licence

Copyright (C) 2025 Namecoin Developers.

Encasha is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version.

Encasha is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.

You should have received a copy of the GNU General Public License along with Encasha. If not, see https://www.gnu.org/licenses/.