What is the Vault ciphertext format (in case I want to parse it)?

I’m planning to use the Vault transit secrets engine to do encryption using an aes256-gcm96 key.

https://learn.hashicorp.com/vault/encryption-as-a-service/eaas-transit shows some example ciphertext that might be returned by an encryption call:
vault:v1:cZNHVx+sxdMErXRSuDa1q/pz49fXTn1PScKfhf+PIZPvy8xKfkytpwKcbC0fF2U=

Is the format of this ciphertext documented somewhere? Is it possible to parse this ciphertext into separate fields? Specifically, I’m interested in parsing out the initialization vector and the tag, but I would like to know what all of the fields are.

(I understand that the ciphertext from that example might not have been encrypted with an AES key. If the format of the ciphertext varies by key type, I’m specifically interested in the ciphertext format for AES keys. But I imagine that others might be interested in the ciphertext format for other key types.)

The first prefix identifies it as having been wrapped by Vault (and is configurable). The “v1” indicates that it’s key version 1, so that when you rotate keys we know which version to use for decryption. The last part is a base64’d concatenation of the IV and ciphertext. Assuming you’re using AES-GCM, after de-base-64ing, the first 96 bits will be the IV, and the rest the ciphertext.

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Won’t change the parsing, but the ciphertext includes padding and some kind of MAC (like a SHA256 hash of the plain text) to detect cipher text attacks:

$ vault write --field=plaintext transit/decrypt/mykey ciphertext=vault:v1:VLiFIPvGDnBcRj+luztNV5OFiK/4vpRxQb59hNurwgQRb+f471LmU4A=
Error writing data to transit/decrypt/guillaume: Error making API request.

URL: PUT https://localhost:8200/v1/transit/decrypt/mykey
Code: 400. Errors:

* invalid ciphertext: unable to decrypt

So if after parsing you intend to decrypt by some other means (assuming you have the key), you will have to take this into account.

I will definitely want to be able to decrypt the ciphertext using a different means. How can I determine which parts are padding or the MAC?

@mikewertheim There is no MAC. It uses AES-GCM so it is authenticated but that’s built-in. There is also no padding outside of the normal AES algorithm.

Ah yes, GCM! I spotted this in Transit’s source code:

ciphertext, err := p.Encrypt(item.KeyVersion, item.DecodedContext, item.DecodedNonce, item.Plaintext)

That’s where I would start investiguating the source to find the format of the actual bytes that are encrypted. With that knowledge, it is just a matter of cherry picking the GCM data from the ciphertext and decrypting it.

I don’t like when I get questions about my use case from someone who doesn’t know the history behind it, so I’ll resist the urge to question yours… Still an interesting challenge. I might give it a shot!

The authentication data is part of the cipher text. There is nothing to cherry pick and decrypt.

The correct answer is still What is the Vault ciphertext format (in case I want to parse it)?

Thanks @jeff, still a little confused about the authenticated data… I’ll read up on that. I was expecting the AAD part of GCM to be separate, like the IV.

Anyway, to answer @mikewertheim, here is a roundtrip decryption of transit ciphertext. Works on my machine, ymmv!

Make the key exportable (they are not by default)

$ vault write transit/keys/guillaume/config exportable=true

Export the key

$ vault read transit/export/encryption-key/guillaume
Key     Value
---     -----
keys    map[1:VxJWkOYm2F5z1nF1th9zreS6ZAZMFkCq0c/Ik460ayw=]
name    guillaume
type    aes256-gcm96

Encrypt some data

$ vault write transit/encrypt/guillaume plaintext=$(base64 <<< "Hello, World!")
Key           Value
---           -----
ciphertext    vault:v1:gLyuEdMSHkbx1APz4Rs/rNuLDxSUWOUxdWEW5Tt5LK6Iv8tyJLMMYAN+

Decrypt the cipher text. My OpenSSL does not support aes-256-gcm on the command line, so here it is in Python (pip install cryptography)

import base64
from cryptography.hazmat.primitives.ciphers.aead import AESGCM

ciphertext = base64.b64decode('gLyuEdMSHkbx1APz4Rs/rNuLDxSUWOUxdWEW5Tt5LK6Iv8tyJLMMYAN+')

iv = ciphertext[:int(96/8)]  #First 96 bits
actual_ciphertext = ciphertext[int(96/8):]  #Remaining bits
aad = None
key = base64.b64decode('VxJWkOYm2F5z1nF1th9zreS6ZAZMFkCq0c/Ik460ayw=')

plaintext = AESGCM(key).decrypt(iv, actual_ciphertext, aad)

print(plaintext)

You will get back the original “Hello, World!\n”

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Thanks @ixe013 this works for AES256-GCM with non convergent encryption. I’m struggling to make it work with convergent encryption activated and Python.

I got a persistent cryptography.exceptions.InvalidTag which reveals the nonce/IV or the key is incorrect. Have you tried or successfuly got his working?

Appreciated

Jeff

OK got, I did not used the correct derivation algorithm. Find it was HKDF and not PBKDF2HMAC in: https://github.com/hashicorp/vault/blob/47a54ac8bc6c423805bb3be5c462a4676a62ae3c/sdk/helper/keysutil/encrypted_key_storage_test.go

import base64
from cryptography.hazmat.primitives.ciphers.aead import AESGCM
from cryptography.hazmat.primitives.kdf.hkdf import HKDF
from cryptography.hazmat.primitives import hashes
from cryptography.hazmat.backends import default_backend

backend = default_backend()

ciphertext = base64.b64decode('72CrTbu2j0++F1SfDkr2JVSqlK5lU0frqm0qWROOcvzv8V1K4nc6P62mCw==')

iv = ciphertext[:int(96/8)]  #First 96 bits
actual_ciphertext = ciphertext[int(96/8):] #Remaining bits
aad = None
password_bytes = base64.b64decode('/FVNsIbURbaNDxBEXxK4HRwCuT7xHqZ07Ji0cZwcPT0=')

hkdf = HKDF(
	algorithm=hashes.SHA256(),
	length=32,
	salt=b'', #Even if salt is recommended to be set, it is Null. An acceptable option
	info=b'1234', #Here is the context for the convergent encryption
	backend=backend
	)

key = hkdf.derive(password_bytes)

plaintext = AESGCM(key).decrypt(iv, actual_ciphertext, aad)

print(plaintext) 

will return b'This is a test!'

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