Secret Sharing with SSH Key

SSH - This article is part of a series.

Part 8: This Article

Technically, we can encrypt our content using an SSH key pair, but only if the key pair uses RSA algorithm.

I’ll be sharing some less commonnly used feature/topic in SSH starting from part 8 (SSH series).

Asymmetric encryption (public/private keys) is slow and cannot handle large content in nature. Thus, standard cryptographic practice requires a two-step process: encrypt the data with a fast symmetric key, then encrypt that symmetric key with a SSH public key.

Encryption
#

Even with an RSA key, SSH keys are not natively designed for file encryption. And we must convert the key format so that standard tools like openssl can read it.

PKCS8 Format
#

First, we generate a new RSA SSH key pair for testing. Then, we export the public SSH key to a standard PKCS8 format (rsa4096_public_key.pem).

% ssh-keygen -t rsa -b 4096 -C "rsa4k@testing"
...
Your identification has been saved in /home/xx/.ssh/id_rsa
Your public key has been saved in /home/xx/.ssh/id_rsa.pub

% ssh-keygen -e -f ~/.ssh/id_rsa.pub -m PKCS8 > rsa4096_public_key.pem 
% cp ~/.ssh/id_rsa ./rsa4096_private_key.pem
% ssh-keygen -p -f rsa4096_private_key.pem -m PEM

We also need the SSH private key for decryption later.

So we make a copy of the OpenSSH private key rsa4096_private_key.pem from id_rsa. And we convert it from OpenSSH private key format to RSA private key format.

Secret Key
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Generate a temporary random secret key called temp_secret.key.

% openssl rand -out temp_secret.key 64

Content Encryption
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Now, we encrypt our content (content.txt) and produce ciphertext.enc using the secret key (temp_secret.key).

% openssl aes-256-cbc -in content.txt -out ciphertext.enc \
   -pass file:temp_secret.key
% file ciphertext.enc
ciphertext.enc: openssl enc'd data with salted password

Public Key
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We also encrypt the secret key (temp_secret.key) with exported SSH public key (rsa4096_public_key.pem).

% openssl pkeyutl -encrypt -pubin -inkey rsa4096_public_key.pem \
   -in temp_secret.key -out secret.key.enc
% file secret.key.enc 
secret.key.enc: data
% ls *.enc
ciphertext.enc  secret.key.enc

Decryption
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The decryption involves 2 steps.

Secret Key
#

To extract the temporary secret key from secret.key.enc with RSA private key (rsa4096_private_key.pem).

% openssl pkeyutl -decrypt -inkey rsa4096_private_key.pem \
   -in secret.key.enc -out temp2_secret.key
% diff temp_secret.key temp2_secret.key

In this case, temp2_secret.key is the same as the temp_secret.key.

Content Decryption
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% openssl aes-256-cbc -d -in ciphertext.enc -out plain.txt \
    -pass file:temp2_secret.key
% diff plain.txt content.txt 

In this case, plain.txt is the same as the content.txt. And we just successfully decrypt the file (ciphertext.enc) back to plain text document.

Modern SSH key formats like ED25519 or ECDSA cannot be natively used for direct file encryption because they are mathematically designed strictly for digital signatures, not encryption.

Short POC
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Preparation and SSH keys conversion to PEM.

% lorem > content.txt
% ssh-keygen -f id_rsa.pub -e -m PKCS8 > rsa_pub.pem
% cp ~/.ssh/id_rsa .
% ssh-keygen -f id_rsa -m PEM -p -N ""    
% mv id_rsa rsa_pvt.pem

Encrypt the content.txt.

% openssl rsautl -pubin -inkey rsa_pub.pem -encrypt -pkcs \
   -in content.txt -out cipher.enc
The command rsautl was deprecated in version 3.0. Use 'pkeyutl' instead.

Decrypt the cipher.enc to plain.txt.

% openssl rsautl -inkey rsa_pvt.pem -decrypt -in cipher.enc -out plain.txt
The command rsautl was deprecated in version 3.0. Use 'pkeyutl' instead.
% diff plain.txt content.txt   
% ls 
cipher.enc  content.txt  id_rsa.pub  plain.txt  rsa_pub.pem  rsa_pvt.pem

Tools
#

age
age is a simple, modern and secure file encryption tool, format and Go library.
It features small explicit keys, post-quantum support, no config options, and UNIX-style composability.
jass
jass let us encrypt/decrypt data using SSH keys.