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Cipher for remote containers derived from ssh-agent signed challenge data

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sagecipher (ssh agent crypt) is a very small module which provides a cipher keyed through ssh-agent. The key is obtained by generating random challenge data, passing this to ssh-agent to obtain a signature via the users private ssh-key for use as key material, and finally passing this through PBKDF2.


pip install sagecipher


Before using, ssh-agent must be running with at least one ssh-key available for producing cipher key material:

$ source <(ssh-agent)
Agent pid 3710

$ ssh-add
Enter passphrase for /home/somebody/.ssh/id_rsa:
Identity added: /home/somebody/.ssh/id_rsa (/home/somebody/.ssh/id_rsa)
If ssh-agent is not available or does not have any keys available, expect to see a
sagecipher.cipher.SignError Exception:
>>> from sagecipher import *
>>> cfail = Cipher()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "sagecipher/", line 101, in __init__
    signature = sign_via_agent(self.challenge, self.fingerprint)
  File "sagecipher/", line 230, in sign_via_agent
    raise SignError(SignError.E_NO_KEYS)
sagecipher.cipher.SignError: SSH agent is not running or no keys are available

Using the cli tool to provide on-demand decryption to other tools

Check sagecipher --help for usage…

$ sagecipher encrypt - encfile
secret sauce
$ sagecipher decrypt encfile -
secret sauce
$ mkfifo decfile
$ sagecipher decrypt encfile decfile &
[1] 16753
$ cat decfile
secret sauce

Using sagecipher in a Python program

>>> from sagecipher import *
>>> # Create a new cipher using first key available from ssh-agent
>>> c1 = Cipher()
>>> # Use the cipher header to create a second/future instance
>>> c2 = Cipher(c1.header())
>>> # Display fingerprint of the SSH key used for derivation of cipher key
>>> to_hex(c2.fingerprint)
>>> # Encrypt with the first instance, decrypt back with the second...
>>> unpad(c2.decrypt_string(c1.encrypt(pad("Hello, cryptic world!"))))
'Hello, cryptic world!'
>>> # Encrypt and decrypt using helper methods.  The cipher header is
>>> # stored in the encrypted ciphertext.
>>> ciphertext = encrypt_string("Alice, I think someone is listening!")
>>> plaintext = decrypt_string(ciphertext)
>>> plaintext
'Alice, I think someone is listening!'

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