Is it possible to access to a public image in an KMS encrypted bucket without signature

Question

Hi everyone,

we tried to encrypt our S3 bucket with SSE-KMS key provided by AWS and everything is working fine except when we try to access images uploaded with ACL public-read. We get this error:

<Error>
<Code>InvalidArgument</Code>
<Message>Requests specifying Server Side Encryption with AWS KMS managed keys require AWS Signature Version 4.</Message>
<ArgumentName>Authorization</ArgumentName>
<ArgumentValue>null</ArgumentValue>
<RequestId>7DTKMXMNF96AK54C</RequestId>
<HostId>h97Cnkid3HVkjoJ/iZR2FniRNz4Gm59eqUlnSU2KNyzwhC7FtDyVnA3NVS7nKx0CtGECI1YnlzI=</HostId>
</Error>

It looks like a nonsense to have public encrypted objects but we use only one bucket for our application and some objects can be public while some other are not. We do not know how to generate a link that make the image accessible to everyone and that will not expire.

Can anybody help on this ?

Regards Quentin

Answer

All object access to objects encrypted with SSE-KMS requires IAM permissions to decrypt, regardless of the ACL settings (which only controls the get/put actions, not encryption). You have two options, don't use [default bucket encryption](https://docs.aws.amazon.com/AmazonS3/latest/userguide/bucket-encryption.html) and only encrypt objects that are not going to be public. Or use two buckets, one with default bucket encryption for private objects, and one without for public objects. You might also try using SSE-S3, which doesn't require IAM permissions to decrypt, but may require the request to be signed with SigV4 (I haven't tested this).

Answer

All GET and PUT requests for an object protected by AWS KMS fail if they are not made via SSL or TLS, or if they are not made using SigV4.

https://docs.aws.amazon.com/AmazonS3/latest/userguide/UsingAWSSDK.html#specify-signature-version

Answer

Thanks for your quick answers. I'll try to encrypt only private files and disable default bucket encryption.
Too bad we cannot do the opposite and decide not to encrypt some objects

Is it possible to access to a public image in an KMS encrypted bucket without signature

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