etag: add Format and Decrypt functions (#14659)

This commit adds two new functions to the
internal `etag` package:
 - `ETag.Format`
 - `Decrypt`

The `Decrypt` function decrypts an encrypted
ETag using a decryption key. It returns not
encrypted / multipart ETags unmodified.

The `Decrypt` function is mainly used when
handling SSE-S3 encrypted single-part objects.
In particular, the ETag of an SSE-S3 encrypted
single-part object needs to be decrypted since
S3 clients expect that this ETag is equal to the
content MD5.

The `ETag.Format` method also covers SSE ETag handling.
MinIO encrypts all ETags of SSE single part objects.
However, only the ETag of SSE-S3 encrypted single part
objects needs to be decrypted.
The ETag of an SSE-C or SSE-KMS single part object
does not correspond to its content MD5 and can be
a random value.
The `ETag.Format` function formats an ETag such that
it is an AWS S3 compliant ETag. In particular, it
returns non-encrypted ETags (single / multipart)
unmodified. However, for encrypted ETags it returns
the trailing 16 bytes as ETag. For encrypted ETags
the last 16 bytes will be a random value.

The main purpose of `Format` is to format ETags
such that clients accept them as well-formed AWS S3
ETags.
It differs from the `String` method since `String`
will return string representations for encrypted
ETags that are not AWS S3 compliant.

Signed-off-by: Andreas Auernhammer <hi@aead.dev>

internal/etag/etag.go

@@ -108,7 +108,9 @@ package etag
 
 import (
 	"bytes"
+	"crypto/hmac"
 	"crypto/md5"
+	"crypto/sha256"
 	"encoding/base64"
 	"encoding/hex"
 	"errors"
@@ -116,6 +118,9 @@ import (
 	"net/http"
 	"strconv"
 	"strings"
+
+	"github.com/minio/minio/internal/fips"
+	"github.com/minio/sio"
 )
 
 // ETag is a single S3 ETag.
@@ -188,6 +193,47 @@ func (e ETag) Parts() int {
 	return parts
 }
 
+// Format returns an ETag that is formatted as specified
+// by AWS S3.
+//
+// An AWS S3 ETag is 16 bytes long and, in case of a multipart
+// upload, has a `-N` suffix encoding the number of object parts.
+// An ETag is not AWS S3 compatible when encrypted. When sending
+// an ETag back to an S3 client it has to be formatted to be
+// AWS S3 compatible.
+//
+// Therefore, Format returns the last 16 bytes of an encrypted
+// ETag.
+//
+// In general, a caller has to distinguish the following cases:
+//  - The object is a multipart object. In this case,
+//    Format returns the ETag unmodified.
+//  - The object is a SSE-KMS or SSE-C encrypted single-
+//    part object. In this case, Format returns the last
+//    16 bytes of the encrypted ETag which will be a random
+//    value.
+//  - The object is a SSE-S3 encrypted single-part object.
+//    In this case, the caller has to decrypt the ETag first
+//    before calling Format.
+//    S3 clients expect that the ETag of an SSE-S3 encrypted
+//    single-part object is equal to the object's content MD5.
+//    Formatting the SSE-S3 ETag before decryption will result
+//    in a random-looking ETag which an S3 client will not accept.
+//
+// Hence, a caller has to check:
+//   if method == SSE-S3 {
+//      ETag, err := Decrypt(key, ETag)
+//      if err != nil {
+//      }
+//   }
+//   ETag = ETag.Format()
+func (e ETag) Format() ETag {
+	if !e.IsEncrypted() {
+		return e
+	}
+	return e[len(e)-16:]
+}
+
 var _ Tagger = ETag{} // compiler check
 
 // ETag returns the ETag itself.
@@ -277,6 +323,31 @@ func Get(h http.Header) (ETag, error) {
 // identical.
 func Equal(a, b ETag) bool { return bytes.Equal(a, b) }
 
+// Decrypt decrypts the ETag with the given key.
+//
+// If the ETag is not encrypted, Decrypt returns
+// the ETag unmodified.
+func Decrypt(key []byte, etag ETag) (ETag, error) {
+	const HMACContext = "SSE-etag"
+
+	if !etag.IsEncrypted() {
+		return etag, nil
+	}
+	mac := hmac.New(sha256.New, key)
+	mac.Write([]byte(HMACContext))
+	decryptionKey := mac.Sum(nil)
+
+	plaintext := make([]byte, 0, 16)
+	etag, err := sio.DecryptBuffer(plaintext, etag, sio.Config{
+		Key:          decryptionKey,
+		CipherSuites: fips.CipherSuitesDARE(),
+	})
+	if err != nil {
+		return nil, err
+	}
+	return etag, nil
+}
+
 // Parse parses s as an S3 ETag, returning the result.
 // The string can be an encrypted, singlepart
 // or multipart S3 ETag. It returns an error if s is