Re: [PATCH 3/3] OpenSSH: use safer cryptography defaults

["Peter Müller" <peter.mueller@link38.eu>]

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Hello *,

could somebody have a look at this sometimes?

If possible, I'd like to add this to the upcoming Core 121, too
- in case we are not becoming too big with the new kernel already.

Thanks, and best regards,
Peter Müller


> By default, OpenSSH uses crypto algorithms such as SHA1, which are
> considered insecure and should not be used anymore. This patch
> updates the used ciphers, message-digest algorithms and key exchange
> algorithms according https://stribika.github.io/2015/01/04/secure-secure-shell.html .
> 
> For the kex algo "diffie-hellman-group-exchange-sha256", an intact
> SSH moduli file is required. To make sure we are not falling back
> to insecure crypto here, its presence is checked at SSH startup.
> 
> On my machines, this file was already there, but it makes sense to
> me to double-check this. This patch should not make problems except
> for very outdated OpenSSH clients (older than 6.x) or PuTTY versions.
> 
> This partially addresses #11538 and requires patch 2/3.
> 
> Signed-off-by: Peter Müller <peter.mueller(a)link38.eu>
> ---
>  config/rootfiles/core/121/update.sh |  6 +++++-
>  lfs/openssh                         |  4 ++++
>  src/initscripts/system/sshd         | 12 ++++++++++++
>  3 files changed, 21 insertions(+), 1 deletion(-)
> 
> diff --git a/config/rootfiles/core/121/update.sh b/config/rootfiles/core/121/update.sh
> index 3ec251292..99c174156 100644
> --- a/config/rootfiles/core/121/update.sh
> +++ b/config/rootfiles/core/121/update.sh
> @@ -60,7 +60,11 @@ rm -rvf \
>  sed -i /etc/ssh/sshd_config \
>  	-e 's/^#SyslogFacility AUTH$/SyslogFacility AUTH/' \
>  	-e 's/^#LogLevel INFO$/LogLevel INFO/' \
> -	-e 's/^#StrictModes .*$/StrictModes yes/'
> +	-e 's/^#StrictModes .*$/StrictModes yes/' \
> +	-e 's/^#RekeyLimit default none$/Ciphers chacha20-poly1305(a)openssh.com,aes256-gcm(a)openssh.com,aes128-gcm(a)openssh.com,aes256-ctr,aes192-ctr,aes128-ctr\
> +			MACs hmac-sha2-512-etm(a)openssh.com,hmac-sha2-256-etm(a)openssh.com,umac-128-etm(a)openssh.com,hmac-sha2-512,hmac-sha2-256,umac-128(a)openssh.com\
> +			KexAlgorithms curve25519-sha256(a)libssh.org,diffie-hellman-group-exchange-sha256\
> +			#RekeyLimit default none/'
>  
>  # Start services
>  /etc/init.d/sshd restart
> diff --git a/lfs/openssh b/lfs/openssh
> index 7e8468ac9..3043501a2 100644
> --- a/lfs/openssh
> +++ b/lfs/openssh
> @@ -96,6 +96,10 @@ $(TARGET) : $(patsubst %,$(DIR_DL)/%,$(objects))
>  		-e 's/^#\?AllowTcpForwarding .*$$/AllowTcpForwarding no/' \
>  		-e 's/^#\?PermitRootLogin .*$$/PermitRootLogin yes/' \
>  		-e 's/^#StrictModes .*$/StrictModes yes/' \
> +		-e 's/^#RekeyLimit default none$/Ciphers chacha20-poly1305(a)openssh.com,aes256-gcm(a)openssh.com,aes128-gcm(a)openssh.com,aes256-ctr,aes192-ctr,aes128-ctr\
> +			MACs hmac-sha2-512-etm(a)openssh.com,hmac-sha2-256-etm(a)openssh.com,umac-128-etm(a)openssh.com,hmac-sha2-512,hmac-sha2-256,umac-128(a)openssh.com\
> +			KexAlgorithms curve25519-sha256(a)libssh.org,diffie-hellman-group-exchange-sha256\
> +			#RekeyLimit default none/' \
>  		-e 's|^#\?HostKey /etc/ssh/ssh_host_dsa_key$$||' \
>  		-e 's|^#\?HostKey /etc/ssh/ssh_host_ecdsa_key$$||' \
>  		-e 's|^#\?HostKey /etc/ssh/ssh_host_ed25519_key$$||' \
> diff --git a/src/initscripts/system/sshd b/src/initscripts/system/sshd
> index 7b4092d38..d7958e800 100644
> --- a/src/initscripts/system/sshd
> +++ b/src/initscripts/system/sshd
> @@ -23,6 +23,18 @@ case "$1" in
>  		evaluate_retval
>  	done
>  
> +	# Make sure moduli file is properly present
> +	# (https://stribika.github.io/2015/01/04/secure-secure-shell.html)
> +	modulifile="/etc/ssh/moduli"
> +	if [ ! -e "${modulifile}" ]; then
> +		boot_mesg "Generating SSH moduli file (this may take a while)..."
> +
> +		ssh-keygen -G /etc/ssh/moduli.all -b 4096
> +		ssh-keygen -T /etc/ssh/moduli.safe -f /etc/ssh/moduli.all
> +		mv /etc/ssh/moduli.safe /etc/ssh/moduli
> +		rm -f /etc/ssh/moduli.all
> +	fi
> +
>          [ -e "/var/ipfire/remote/enablessh" ] || exit 0 # SSH is not enabled
>          boot_mesg "Starting SSH Server..."
>          loadproc /usr/sbin/sshd 
> 


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