SSH not working to C3 compute-optimized machines - amazon-web-services

Question

I cannot connect to any machine I create on EC2 that belongs to the C3 family.
I have no problem connecting with SSH to any other type of machine.
What do I need to do to further debug this?
The steps I've taken:
I create a t1.micro machine with the same image (an ubuntu 13.10 64bit AMI ami-2f252646), and the same key-pair, and hte same security group. It works fine.
I ssh to a t1.micro machine, and then ssh again from that machine to the C3 machine. That tells me the machine is up and running and my problem is connecting to the C3 machine from my PC directly (going through the office router).
I try to telnet to the t1.micro machine on port 22 - I get a connection.
I try to telnet to the C3 machine on port 22 - does not work.
I try to telnet to the C3 machine on port 22 from another PC, not from the office - I get a connection.
I tried this with several C3 type machines, all iwht the same result.
So:
The machine is up and running, and can accept connections.
There is obviously a problem in the coupling between my office connection and the C3 machine.
My office connection works fine with any other type of m1/c1/g1/m2 machine, so it's only the "3" family that has that problem.
I'm at a loss on how to solve this, or even debug this further. Right now I'm tunneling to my machine through a proxy t1.micro machine...
My operating system is itself Ubuntu 13.10
Here is a gist link to the output of my ssh -vvv command
It seems to get stuck at debug1: expecting SSH2_MSG_KEX_ECDH_REPLY

Answer 1

Solved it, with the help of AWS professional (paid) services.
The problem is with some of the authentication protocols.
I have a high (9000) MTU (minimal transfer unit) configured in my network connection, I need this to access big chunks of data.
The solution is either to lower the MTU to below 1400 (which is not good for me, because I need it), or to change the SSH config, which worked for me.
sudo vi /etc/ssh/ssh_config
and uncomment the lines starting with Ciphers and MACs
mine says:
Ciphers aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,aes128-cbc,3des-cbc
MACs hmac-md5,hmac-sha1,umac-64#openssh.com,hmac-ripemd160
This is a bug from Ubuntu 12.10 and onwards (it works in 12.04 and below)

Answer 2

There is another option besides using just the provided pem with an Ami that you've created yourself.
Go back and spin up the current image on an instance size that you know works. Ssh into the instance as the main user and then create a secondary user and add them to the sudoers group.
sudo useradd -d /home/myuser -m myuser
sudo usermod -a -G sudo myuser
Then change to the new user and go to their home directory and create a .ssh folder, change this to 0700. Go inside the .ssh directory an vi (or your text editor of choice) and create an authorized_keys file.
Insert your PUBLIC key contents into this file.
Change the permissions on this file to 0600.
sudo su myuser
cd ~
mkdir .ssh
chmod 0700 .ssh
cd .ssh
vi authorized_keys
chmod 0600 authorized_keys
Exit out of the user. Before you exit the box you probably want to edit (as sudo) /etc/passwd and change the users shell from sh to bash.
Exit out of the box and test connecting with your new user before returning your new Ami.
Now spin up the new ami as a C3 instance and connect in with your user.