sops 0.8.1

Secrets OPerationS (sops) is an editor of encrypted files

sop is an editor of encrypted files that supports YAML, JSON and TEXT formats and encrypts with AWS KMS and PGP (via GnuPG). Watch the demo.

Table of Contents

• 1 Up and running in 60 seconds

• 2 Usage

  • 2.1 Adding and removing keys

  • 2.2 Using KMS master keys in various AWS accounts

  • 2.3 Key Rotation

• 3 Note on YAML

• 4 Cryptographic details

  • 4.1 Message Authentication Code

• 5 Threat Model

  • 5.1 1. Compromised AWS credentials grant access to KMS master key

  • 5.2 2. Compromised PGP key

  • 5.3 3. Factorized RSA key

  • 5.4 4. Weak AES cryptography

• 6 License

• 7 Authors

• 8 Credits

1 Up and running in 60 seconds

First install some libraries from your package manager:

• RHEL family:

  sudo yum install libyaml-devel python-devel libffi-devel pip

• Debian family:

  sudo apt-get install libyaml-dev python-dev libffi-dev python-pip

• MacOS:

  brew install libffi libyaml
  sudo easy_install pip

Then install sops from pip:

sudo pip install --upgrade sops

Clone the repository, load the test PGP key and open the test files:

$ git clone https://github.com/mozilla/sops.git
$ cd sops
$ gpg --import tests/sops_functional_tests_key.asc
$ sops example.yaml

This last step will decrypt example.yaml using the test private key. To create your own secrets files using keys under your control, keep reading.

2 Usage

If you’re using AWS KMS, create one or multiple master keys in the IAM console and export them, comma separated, in the SOPS_KMS_ARN env variable. It is recommended to use at least two master keys in different regions.

export SOPS_KMS_ARN="arn:aws:kms:us-east-1:656532927350:key/920aff2e-c5f1-4040-943a-047fa387b27e,arn:aws:kms:ap-southeast-1:656532927350:key/9006a8aa-0fa6-4c14-930e-a2dfb916de1d"

Your AWS credentials must be present in ~/.aws/credentials. sops uses boto3.

$ cat ~/.aws/credentials
[default]
aws_access_key_id = AKI.....
aws_secret_access_key = mw......

If you want to use PGP, export the fingerprints of the public keys, comma separated, in the SOPS_PGP_FP env variable.

export SOPS_PGP_FP="85D77543B3D624B63CEA9E6DBC17301B491B3F21,E60892BB9BD89A69F759A1A0A3D652173B763E8F"

Note: you can use both PGP and KMS simultaneously.

Then simply call sops with a file path as argument. It will handle the encryption/decryption transparently and open the cleartext file in an editor.

$ sops mynewtestfile.yaml
mynewtestfile.yaml doesn't exist, creating it.
please wait while an encryption key is being generated and stored in a secure fashion
[... editing happens in vim, or whatever $EDITOR is set to ...]
file written to mynewtestfile.yaml

The resulting encrypted file looks like this:

myapp1: ENC[AES256_GCM,data:Tr7o=,iv:1=,aad:No=,tag:k=]
app2:
    db:
        user: ENC[AES256_GCM,data:CwE4O1s=,iv:2k=,aad:o=,tag:w==]
        password: ENC[AES256_GCM,data:p673w==,iv:YY=,aad:UQ=,tag:A=]
    # private key for secret operations in app2
    key: |-
        ENC[AES256_GCM,data:Ea3kL5O5U8=,iv:DM=,aad:FKA=,tag:EA==]
an_array:
- ENC[AES256_GCM,data:v8jQ=,iv:HBE=,aad:21c=,tag:gA==]
- ENC[AES256_GCM,data:X10=,iv:o8=,aad:CQ=,tag:Hw==]
- ENC[AES256_GCM,data:KN=,iv:160=,aad:fI4=,tag:tNw==]
sops:
    kms:
    -   created_at: 1441570389.775376
        enc: CiC....Pm1Hm
        arn: arn:aws:kms:us-east-1:656532927350:key/920aff2e-c5f1-4040-943a-047fa387b27e
    -   created_at: 1441570391.925734
        enc: Ci...awNx
        arn: arn:aws:kms:ap-southeast-1:656532927350:key/9006a8aa-0fa6-4c14-930e-a2dfb916de1d
    pgp:
    -   fp: 85D77543B3D624B63CEA9E6DBC17301B491B3F21
        created_at: 1441570391.930042
        enc: |
            -----BEGIN PGP MESSAGE-----
            hQIMA0t4uZHfl9qgAQ//UvGAwGePyHuf2/zayWcloGaDs0MzI+zw6CmXvMRNPUsA
                            ...=oJgS
            -----END PGP MESSAGE-----

A copy of the encryption/decryption key is stored securely in each KMS and PGP block. As long as one of the KMS or PGP method is still usable, you will be able to access you data.

To decrypt a file in a cat fashion, use the -d flag:

$ sops -d mynewtestfile.yaml

sops encrypted files contain the necessary information to decrypt their content. All a user of sops need is valid AWS credentials and the necessary permissions on KMS keys.

Given that, the only command a sops user need is:

$ sops <file>

<file> will be opened, decrypted, passed to a text editor (vim by default), encrypted if modified, and saved back to its original location. All of these steps, apart from the actual editing, are transparent to the user.

2.1 Adding and removing keys

When creating a new files, sops uses the PGP and KMS defined in the command line arguments –kms and –pgp, or from the environment variables SOPS_KMS_ARN and SOPS_PGP_FP. That information is stored in the file under the sops section. When editing a file, it is trivial to add or remove keys: invoke sops with the flag -s to display the master keys while editing, and add or remove kms or pgp keys under the sops section.

For example, to add a KMS master key to a file, we would add the following entry:

sops:
    kms:
    - arn: arn:aws:kms:us-east-1:656532927350:key/920aff2e-c5f1-4040-943a-047fa387b27e

And, similarly, to add a PGP master key, we add its fingerprint:

sops:
    pgp:
    - fp: 85D77543B3D624B63CEA9E6DBC17301B491B3F21

When the file is saved, sops will update its metadata and encrypt the data key with the freshly added master keys. The removed entries are simply deleted from the file.

2.2 Using KMS master keys in various AWS accounts

You can use keys in various accounts by tying each KMS master key to a role that the user is allowed to assume in each account. The IAM roles documentation has full details on how this needs to be configured on AWS’s side. From the point of view of sops, you only need to specify the role a KMS key must assume alongside its ARN, as follows:

sops:
    kms:
    -   arn: arn:aws:kms:us-east-1:656532927350:key/920aff2e-c5f1-4040-943a-047fa387b27e
        role: arn:aws:iam::927034868273:role/sops-dev-xyz

The role must have permission to call Encrypt and Decrypt using KMS. An example policy is shown below.

{
  "Sid": "Allow use of the key",
  "Effect": "Allow",
  "Action": [
        "kms:Encrypt",
        "kms:Decrypt",
        "kms:ReEncrypt*",
        "kms:GenerateDataKey*",
        "kms:DescribeKey"
  ],
  "Resource": "*",
  "Principal": {
        "AWS": [
          "arn:aws:iam::927034868273:role/sops-dev-xyz"
        ]
  }
}

You can specify a role in the –kms flag and SOPS_KMS_ARN variable by appending it to the ARN of the master key, separated by a + sign:

<KMS ARN>+<ROLE ARN>
arn:aws:kms:us-west-2:927034868273:key/fe86dd69-4132-404c-ab86-4269956b4500+arn:aws:iam::927034868273:role/sops-dev-xyz

2.3 Key Rotation

It is recommend to renew the data key on a regular basis. sops supports key rotation via the -r flag. A simple approach is to decrypt and reencrypt all files in place with rotation enabled:

for file in $(find . -type f -name "*.yaml"); do
        sops -d -i $file
        sops -e -i -r $file
done

3 Note on YAML

sops is designed to encrypt files that contain secrets, which are most likely strings or numbers. It will not work on complex YAML files that use references or anchors.

4 Cryptographic details

When sops creates a file, it generates a random 256 bits data key and asks each KMS and PGP master key to encrypt the data key. The encrypted version of the data key is stored in the sops metadata under sops.kms and sops.pgp.

For KMS:

sops:
    kms:
    -   enc: CiC6yCOtzsnFhkfdIslYZ0bAf//gYLYCmIu87B3sy/5yYxKnAQEBAQB4usgjrc7JxYZH3SLJWGdGwH//4GC2ApiLvOwd7Mv+cmMAAAB+MHwGCSqGSIb3DQEHBqBvMG0CAQAwaAYJKoZIhvcNAQcBMB4GCWCGSAFlAwQBLjARBAyGdRODuYMHbA8Ozj8CARCAO7opMolPJUmBXd39Zlp0L2H9fzMKidHm1vvaF6nNFq0ClRY7FlIZmTm4JfnOebPseffiXFn9tG8cq7oi
        enc_ts: 1439568549.245995
        arn: arn:aws:kms:us-east-1:656532927350:key/920aff2e-c5f1-4040-943a-047fa387b27e

For PGP:

sops:
    pgp:
    -   fp: 85D77543B3D624B63CEA9E6DBC17301B491B3F21
        created_at: 1441570391.930042
        enc: |
            -----BEGIN PGP MESSAGE-----
            Version: GnuPG v1

            hQIMA0t4uZHfl9qgAQ//UvGAwGePyHuf2/zayWcloGaDs0MzI+zw6CmXvMRNPUsA
            pAgRKczJmDu4+XzN+cxX5Iq9xEWIbny9B5rOjwTXT3qcUYZ4Gkzbq4MWkjuPp/Iv
            qO4MJaYzoH5YxC4YORQ2LvzhA2YGsCzYnljmatGEUNg01yJ6r5mwFwDxl4Nc80Cn
            RwnHuGExK8j1jYJZu/juK1qRbuBOAuruIPPWVdFB845PA7waacG1IdUW3ZtBkOy3
            O0BIfG2ekRg0Nik6sTOhDUA+l2bewCcECI8FYCEjwHm9Sg5cxmP2V5m1mby+uKAm
            kewaoOyjbmV1Mh3iI1b/AQMr+/6ZE9MT2KnsoWosYamFyjxV5r1ZZM7cWKnOT+tu
            KOvGhTV1TeOfVpajNTNwtV/Oyh3mMLQ0F0HgCTqomQVqw5+sj7OWAASuD3CU/dyo
            pcmY5Qe0TNL1JsMNEH8LJDqSh+E0hsUxdY1ouVsg3ysf6mdM8ciWb3WRGxih1Vmf
            unfLy8Ly3V7ZIC8EHV8aLJqh32jIZV4i2zXIoO4ZBKrudKcECY1C2+zb/TziVAL8
            qyPe47q8gi1rIyEv5uirLZjgpP+JkDUgoMnzlX334FZ9pWtQMYW4Y67urAI4xUq6
            /q1zBAeHoeeeQK+YKDB7Ak/Y22YsiqQbNp2n4CKSKAE4erZLWVtDvSp+49SWmS/S
            XgGi+13MaXIp0ecPKyNTBjF+NOw/I3muyKr8EbDHrd2XgIT06QXqjYLsCb1TZ0zm
            xgXsOTY3b+ONQ2zjhcovanDp7/k77B+gFitLYKg4BLZsl7gJB12T8MQnpfSmRT4=
            =oJgS
            -----END PGP MESSAGE-----

sops then opens a text editor on the newly created file. The user adds data to the file and saves it when done.

Upon save, sops browses the entire file as of a key/value tree. Every time sops encounters a leaf value (a value that does not have children), it encrypts the value with AES256_GCM using the data key and a 256 bits random initialization vector.

Each file uses a single data key to encrypt all values of a document, but each value receives a unique initialization vector and has unique authentication data.

Additional data is used to guarantee the integrity of the encrypted data and of the tree structure: when encrypting the tree, key names are concatenated into a byte string that is used as AEAD additional data (aad) when encrypting the value. The aad field is not stored with the value but reconstructed from the tree structure every time.

The result of AES256_GCM encryption is stored in the leaf of the tree using a base64 encoded string format:

ENC[AES256_GCM,
    data:CwE4O1s=,
    iv:S0fozGAOxNma/pWDUuk1iEaYw0wlba0VOLHjPxIok2k=,
    tag:XaGsYaL9LCkLWJI0uxnTYw==]

where:

• data is the encrypted value

• iv is the 256 bits initialization vector

• tag is the authentication tag

The encrypted file is written to disk with nested keys in cleartext and values encrypted. We expect that keys do not carry sensitive information, and keeping them in cleartext allows for better diff and overall readability.

Any valid KMS or PGP master key can later decrypt the data key and access the data.

Multiple master keys allow for sharing encrypted files without sharing master keys, and provide disaster recovery solution. The recommended way to use sops is to have two KMS master keys in different region and one PGP public key with the private key stored offline. If, by any chance, both KMS master keys are lost, you can always recover the encrypted data using the PGP private key.

4.1 Message Authentication Code

In addition to authenticating branches of the tree using keys as additional data, sops computes a MAC on all the values to ensure that no value has been added or removed fraudulently. The MAC is stored encrypted with AES_GCM and the data key under tree->`sops`->`mac`.

5 Threat Model

The security of the data stored using sops is as strong as the weakest cryptographic mechanism. Values are encrypted using AES256_GCM which is the strongest symetric encryption algorithm known today. Data keys are encrypted in either KMS, which also uses AES256_GCM, or PGP which uses either RSA or ECDSA keys.

Going from the most likely to the least likely, the threats are as follow:

5.1 1. Compromised AWS credentials grant access to KMS master key

An attacker with access to an AWS console can grant itself access to one of the KMS master key used to encrypt a sops data key. This threat should be mitigated by protecting AWS accesses with strong controls, such as multi-factor authentication, and also by performing regular audits of permissions granted to AWS users.

5.2 2. Compromised PGP key

PGP keys are routinely mishandled, either because owners copy them from machine to machine, or because the key is left forgotten on an unused machine an attacker gains access to. When using PGP encryption, sops users should take special care of PGP private keys, and store them on smart cards or offline as often as possible.

5.3 3. Factorized RSA key

sops doesn’t apply any restriction on the size or type of PGP keys. A weak PGP keys, for example 512 bits RSA, could be factorized by an attacker to gain access to the private key and decrypt the data key. Users of sops should rely on strong keys, such as 2048+ bits RSA keys, or 256+ bits ECDSA keys.

5.4 4. Weak AES cryptography

A vulnerability in AES256_GCM could potentially leak the data key or the KMS master key used by a sops encrypted file. While no such vulnerability exists today, we recommend that users keep their encrypted files reasonably private.

6 License

Mozilla Public License Version 2.0

7 Authors

• Julien Vehent <[e-mail]>

• Daniel Thornton <[e-mail]>

• Alexis Metaireau <[e-mail]>

• Rémy Hubscher <[e-mail]>

8 Credits

sops is inspired by hiera-eyaml, credstash , sneaker, password store and too many years managing PGP encrypted files by hand…