SSH Encryption

Secure Shell (SSH) is a cryptographic network protocol meant to secure communications over an insecure connection between network devices. One of the ways SSH does this is by using a hybrid approach between asymmetric public/private key- and symmetric cryptography. SSH is most commonly used as a means for secure remote login and command execution, often in the context of a client-server interaction, but is also often used for authentication and in file transfers protocols (SFTP / SCP).

This article will discuss and explore, among other things, the possible ways of creating SSH-keys, the underlying methods of encryption and some general best practices concerning interactions with servers and ssh key management. It is therefore complementary to the article: "SSH for beginners"

Introduction

To understand how communications are encrypted in SSH we first need to understand some basic terms and concepts. The difference between asymmetric and symmetric cryptography is a good place to start.

Symmetric vs Asymmetric

Symmetric cryptography is something probably most people are and have been familiar since their youth. An example: The alphabet has 26 characters and we assign each position a number (a='1', b='2' etc.), then we proceed to "shift" or "rotate" each character for n steps down this sequence. If we take n=1 for example, so that each letter gets "bumped up" one value and effectively taking the place of the character that was there before. This gives: z='1', a='2', b='3' etc). This algorithm is called ROTn where n would be the number of steps to rotate the characters. N would simultaneously be both the key to encrypt and to decrypt the message (hence the name 'symmetric'). An example message would be "Hello World", which we would encrypt with ROT13 to "Uryyb Jbeyq" and could decrypt with the same key back to "Hello World". Just as many people have experimented with this algorithm to encrypt messages during childhood, almost all people discover quite quickly how easy it is to break such encryption. Of course many more complicated versions exist which are not as easily solved by hand but suffer the same underlying weaknesses as this simplified algorithm does and can get cracked quite easily with modern computing power using methods like frequency analysis. The stress here is on keeping the key a secret, available only to the trusted parties and this is where the distinction with asymmetrical cryptography comes into play.

Asymmetric cryptography, using different keys to encrypt and decrypt, still works very similarly to what we have discussed before. There is an encryption algorithm available (previously the ROTn algorithm but in next parts we will discuss others like RSA and Ed25519) which "jumbles" up the message depending on the specific key that is provided (this key will be known as the "public key" from now on) but where it differs is the decryption. Technically speaking the public key used to encrypt still contains all of the information needed to decrypt the message but this information is obfuscated by complexity and a modern computer's inability to retrieve this information in an acceptable amount of time. The other key in the pair (known as the "private key" from now on) is created alongside with the public key and allows for quick decryption of the message. This difference in decryption speed allows for safe sharing of public keys so that anyone wanting to send private data can encrypt it easily but only the holder of the private key can decrypt easily.

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Author: Frank Korving

Last Modified: 21.02.2017