add support for kms key aliases

[snstanton]

It would be convenient be able to use a key alias as in:
aws kms encrypt --key-id alias/my-key ...
rather than having to provide the full arn.

[autrilla]

Duplicate of #366

[snstanton]

This isn't actually a duplicate of 366. I was talking about the AWS key alias mechanism (e.g. aws kms list-aliases) not just adding a string label in the .sops file. In some aws apis, you can specify either the ARN for the key or an alias like alias/mykey.

[autrilla]

Oh, cool, I didn't know that even existed! Reopening

[rosscdh]

This does not seem to be fixed in 3.3.0 as documented in changelog not in HEAD of master

func (key MasterKey) createSession() (*session.Session, error) {
	re := regexp.MustCompile(`^arn:aws[\w-]*:kms:(.+):[0-9]+:(key|alias)/.+$`)

requires the full arn (which defies the point of an alias?)

[autrilla]

I have no idea how that AWS feature works, but #415 shows you don't need to provide the full ARN. You need the account ID and the alias name. We still have to tell the AWS SDK that it's an alias somehow.

I'm not sure I understand why this is an issue for you. My guess (but it's just that, a guess) is that you're specifying the key through command line flags, which to me feels like doing it wrong. It is convenient for one-off operations and examples, but I don't think it really is how you should actually use sops. Using a .sops.yaml file is a better approach in my opinion.

[rosscdh]

Hey there thanks for the reply.

I'll check the sops yaml setup (am using it via the kustomize sops plugin FYI) but i suspect based on the code i pasted above the full arn will still be necessary

Also re the purpose of a kms alias: its point appears to be to not require the region nor acc id as per https://docs.aws.amazon.com/cli/latest/reference/kms/encrypt.html

--key-id (string)

A unique identifier for the customer master key (CMK).

To specify a CMK, use its key ID, Amazon Resource Name (ARN), alias name, or alias ARN. When using an alias name, prefix it with "alias/" . To specify a CMK in a different AWS account, you must use the key ARN or alias ARN.

For example:

Key ID: 1234abcd-12ab-34cd-56ef-1234567890ab
Key ARN: arn:aws:kms:us-east-2:111122223333:key/1234abcd-12ab-34cd-56ef-1234567890ab
Alias name: alias/ExampleAlias
Alias ARN: arn:aws:kms:us-east-2:111122223333:alias/ExampleAlias

as you can see

aws kms encrypt \
    --key-id alias/ExampleAlias \
    --plaintext fileb://ExamplePlaintextFile \
    --output text \
    --query CiphertextBlob > C:\Temp\ExampleEncryptedFile.base64

is valid and then allows keys to be used independently of region and acc id

[autrilla]

I'll check the sops yaml setup (am using it via the kustomize sops plugin FYI) but i suspect based on the code i pasted above the full arn will still be necessary

I'm afraid so.

Also re the purpose of a kms alias: its point appears to be to not require the region nor acc id as per https://docs.aws.amazon.com/cli/latest/reference/kms/encrypt.html

--key-id (string)

A unique identifier for the customer master key (CMK).

To specify a CMK, use its key ID, Amazon Resource Name (ARN), alias name, or alias ARN. When using an alias name, prefix it with "alias/" . To specify a CMK in a different AWS account, you must use the key ARN or alias ARN.

For example:

Key ID: 1234abcd-12ab-34cd-56ef-1234567890ab
Key ARN: arn:aws:kms:us-east-2:111122223333:key/1234abcd-12ab-34cd-56ef-1234567890ab
Alias name: alias/ExampleAlias
Alias ARN: arn:aws:kms:us-east-2:111122223333:alias/ExampleAlias

I see. I think it would indeed be nice to be able to provide the key in the same formats the AWS CLI takes, which we currently don't support.

[rosscdh]

@autrilla submitted a PR for review (needs tests but want to see if imp is ok first

#541

[wwentland]

It would be amazing if this could be implemented. Especially now that KMS supports multi-regional keys:

• https://docs.aws.amazon.com/kms/latest/developerguide/multi-region-keys-overview.html

[haarchri]

we get the following error in decrypt process with alias

Group 0: FAILED
  arn:aws:kms:eu-central-1:123456789102:alias/sops-example: FAILED
    - | Error decrypting key: AccessDeniedException: The ciphertext
      | refers to a customer master key that does not exist, does
      | not exist in this region, or you are not allowed to access.
      | 	status code: 400, request id:
      | 12411fe7-14f1-4dcc-xxe3-190bf941a7xx

cloudtrail:

"errorMessage": "User: arn:aws:sts::123456789102:assumed-role/sops is not authorized to perform: kms:Decrypt on resource: arn:aws:kms:eu-central-1:123456789102:key/xx930d41-d9a4-462a-8b61-12be1d944c8x because no identity-based policy allows the kms:Decrypt action",

our solution:

encrypt:

sops --kms arn:aws:kms:eu-central-1:123456789102:alias/sops-example --encrypt --encrypted-regex '^(users)$' test.yaml

decrypt:

sops --kms arn:aws:kms:eu-central-1:123456789102:alias/sops-example --decrypt test.yaml

we need to use kms:ResourceAliases in iam policy because it seems sops changes the alias/sops-example in decrypt process to the key-id

The kms:ResourceAliases condition is a condition of the resource, not the request. As such, a request that doesn't specify the alias can still satisfy the condition.

{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Effect": "Allow",
      "Action": [
        "kms:Encrypt",
        "kms:Decrypt",
        "kms:ReEncrypt*",
        "kms:GenerateDataKey*",
        "kms:DescribeKey"
      ],
      "Resource": "*",
      "Condition": {
        "ForAnyValue:StringLike": {
          "kms:ResourceAliases": [
            "alias/sops-example"
          ]
        }
      }
    },
    {
      "Action": [
        "kms:Describe*",
        "kms:Get*",
        "kms:List*",
        "tag:GetResources"
      ],
      "Effect": "Allow",
      "Resource": "*"
    }
  ]
}

https://docs.amazonaws.cn/en_us/kms/latest/developerguide/alias-authorization.html

[typekpb]

as far as I've tested, this one is resolved in the meantime and should be closed.

[dudicoco]

@typekpb how did you test this?
Doesn't work for me:

$ sops --encrypt -in-place secrets.yaml
Could not generate data key: [failed to encrypt new data key with master key "alias/example": Failed to create session: No valid ARN found in "alias/example"]

The code still only checks for an ARN: https://github.com/mozilla/sops/blob/1bb30e28b484dd0f8611ee1807766c9bbdc941ad/kms/keysource.go#L196-L201

[haarchri]

@dudicoco check my last comment for IAM policy - this is working

[dudicoco]

@haarchri this issue is specifically about using the alias without the full ARN - alias/example.

[haarchri]

using the arn of the alias is not an option ?

[dudicoco]

@haarchri for some it is, for others it's preferable to not use the full ARN

[elovelan]

This would make using multi-region keys considerably easier, especially in a DR scenario (not sure if KMS has ever gone down in a region but it's feasible).

I'll take a look at the code in #541 and see if I can make it work for all scenarios supported by the Encrypt API (i.e. add support for Key ID without ARN as well) and open a new PR.

Just to be clear: this PR might be a month or so away; I don't want to set expectations too high!

[moshest]

I'm not sure this is needed. You can use the alias name with the following ARN:

arn:aws:kms:${region}:${account_id}:alias/${alias_name}`

For example:

arn:aws:kms:us-east-1:1234567890:alias/my/key

[dudicoco]

I'm not sure this is needed. You can use the alias name with the following ARN:

arn:aws:kms:${region}:${account_id}:alias/${alias_name}`

For example:

arn:aws:kms:us-east-1:1234567890:alias/my/key

We know the full ARN can be used, it's all over the comments and in the original comment as well...
We would like not to hardcode the ARN (especially the account ID) within the SOPS yaml file.