This commit adds two functions for sealing/unsealing the
etag (a.k.a. content MD5) in case of SSE single-part upload.
Sealing the ETag is neccessary in case of SSE-S3 to preserve
the security guarantees. In case of SSE-S3 AWS returns the
content-MD5 of the plaintext object as ETag. However, we
must not store the MD5 of the plaintext for encrypted objects.
Otherwise it becomes possible for an attacker to detect
equal/non-equal encrypted objects. Therefore we encrypt
the ETag before storing on the backend. But we only need
to encrypt the ETag (content-MD5) if the client send it -
otherwise the client cannot verify it anyway.
This commit updates the key derivation to reflect the
latest change of crypto/doc.go. This includes handling
the insecure legacy KDF.
Since #6064 is fixed, the 3. test case for object key
generation is enabled again.
This commit introduces a new crypto package providing
AWS S3 related cryptographic building blocks to implement
SSE-S3 (master key or KMS) and SSE-C.
This change only adds some basic functionallity esp.
related to SSE-S3 and documents the general approach
for SSE-S3 and SSE-C.
