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doc: add security documentation to provide some background info (#7028)

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This commit adds some documentation about the design of the
SSE-C and SSE-S3 implementation. It describes how the Minio server
encrypt objects and manages keys.
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Andreas Auernhammer 6 years ago committed by kannappanr
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      docs/security/README.md

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# Minio Security Overview [![Slack](https://slack.minio.io/slack?type=svg)](https://slack.minio.io)
## Server-Side Encryption
Minio supports two different types of server-side encryption ([SSE](#sse)):
- **SSE-C**: The Minio server en/decrypts an object with a secret key provided by the S3 client
as part of the HTTP request headers. Therefore, [SSE-C](#ssec) requires TLS/HTTPS.
- **SSE-S3**: The Minio server en/decrypts an object with a secret key managed by a KMS.
Therefore, Minio requires a valid KMS configuration for [SSE-S3](#sses3).
### Server-Side Encryption - Preliminaries
#### Secret Keys
The Minio server uses an unique, randomly generated secret key per object also known as,
Object Encryption Key ([OEK](#oek)). Neither the client-provided SSE-C key nor the KMS-managed
key is directly used to en/decrypt an object. Instead, the OEK is stored as part of the object
metadata next to the object in an encrypted form. To en/decrypt the OEK another secret key is
needed also known as, Key Encryption Key ([KEK](#kek)).
The Minio server runs a key-derivation algorithm to generate the KEK using a pseudo-random
function ([PRF](#prf)):
`KEK := PRF(EK, IV, context_values)` where
- [EK](#ek): is the external key. In case of SSE-C this is the client-provided key. In case of SSE-S3
this is secret key generated by the KMS. For further details see
[SSE-C](#Server-Side-Encryption-with-client-provided-Keys) or
[SSE-S3](#Server-Side-Encryption-with-a-KMS).
- [IV](#iv): is a randomly generated initialization vector. It is public and part of the
object metadata.
- `context_values`: are values like the bucket and object name and other information which should be
cryptographically bound to the KEK.
To summarize for any encrypted object there exists (at least) three different keys:
1. [OEK](#oek): A secret and unique key used to encrypted the object, stored in an encrypted form as
part of the object metadata and only loaded to RAM in plaintext during en/decrypting the object.
2. [KEK](#kek): A secret and unique key used to en/decrypt the OEK and never stored anywhere.
It is(re-)generated whenever en/decrypting an object using an external secret key and public
parameters.
3. [EK](#ek): An external secret key - either the SSE-C client-provided key or a secret key
generated by the KMS.
#### Content Encryption
The Minio server uses an authenticated encryption scheme ([AEAD](#aead)) to en/decrypt and
authenticate the object content. The AEAD is combined with some state to build a
*Secure Channel*. A *Secure Channel* is a cryptographic construction that ensures confidentiality
and integrity of the processed data. In particular the *Secure Channel* splits the plaintext content
into fixed size chunks and en/decrypts each chunk separately using an unique key-nonce combination.
```
plaintext := chunk_0 || chunk_1 || chunk_2 || ...
| | |
| | |
AEAD <- key, nonce + 0 AEAD <- key, nonce + 1 AEAD <- key, nonce + 2 ...
| | |
| | |
ciphertext := sealed_chunk_0 || sealed_chunk_1 || sealed_chunk_2 || ...
```
<center>Figure 1 - Secure Channel construction</center>
In case of a S3 multi-part operation each part is en/decrypted with the scheme shown in
Figure 1. However, for each part an unique secret key is derived from the OEK and the part
number using a PRF. So in case of multi-part not the OEK but the output of `PRF(OEK, part_id)`
is used as secret key.
#### Cryptographic Primitives
The SSE schemes described in [Secret Keys](#Secret-Keys) and [Content Encryption](#Content-Encryption)
are generic over the cryptographic primitives. However, the Minio server uses the following
cryptographic primitive implementations:
- [PRF](#prf): HMAC-SHA-256
- [AEAD](#aead): AES-256-GCM if the CPU supports AES-NI, ChaCha20-Poly1305 otherwise.
More specifically AES-256-GCM is only selected for X86-64 CPUs with AES-NI extension.
Further any secret key (apart from the KMS-generated ones) is 256 bits long. The KMS-generated keys
may be 256 bits but this depends on the KMS capabilities and configuration.
The *Secure Channel* splits the object content into chunks of a fixed size of `65536` bytes. The last
chunk may be smaller to avoid adding additional overhead and is treated specially to prevent truncation
attacks. The nonce value is 96 bits long and generated randomly per object / multi-part part. The
*Secure Channel* supports plaintexts up to `65536 * 2^32 = 256 TiB`.
#### Randomness
The Minio server generates unique keys and other cryptographic values using a cryptographically
secure pseudo-random number generator ([CSPRNG](#csprng)). However, in the context of SSE,
the Minio server does not require that the CSPRNG generates values that are indistinguishable
from truly random bit strings. Instead, it is sufficient if the generated values are unique - which
is a weaker requirement. Nevertheless other parts - for example the TLS-stack - may require that
CSPRNG-generated values are indistinguishable from truly random bit strings.
### Server-Side Encryption with client-provided Keys
SSE-C allows an S3 client to en/decrypt an object at the Minio server. Therefore the S3 client
sends a secret key as part of the HTTP request. This secret key is **never** stored by the
Minio server and only resides in RAM during the en/decryption process.
Minio does not assume or require that the client-provided key is unique. It may be used for
multiple objects or buckets. Especially a single client-provided key may be used for all
objects - even though all objects must be treated as compromised if that key is ever compromised.
#### Key rotation
S3 clients can change the client-provided key of an existing object. Therefore an S3 client
must perform a S3 COPY operation where the copy source and destination are equal. Further the
COPY request headers must contain the current and the new client key:
- `X-Amz-Server-Side-Encryption-Customer-Key`: Base64 encoded new key.
- `X-Amz-Copy-Source-Server-Side-Encryption-Customer-Key`: Base64 encoded current key.
Such a special COPY request is also known as S3 SSE-C key rotation.
### Server-Side Encryption with a KMS
SSE-S3 allows an S3 client to en/decrypt an object at the Minio server using a KMS. The Minio
server only assumes that the KMS provides two services:
1. `GenerateKey`: Takes a key ID and generates a new data key from a master key referenced by
the key ID. It returns the new data key in two different forms: The plain data key
and the data key encrypted using the master key.
2. `DecryptKey`: Takes a key ID and an encrypted data key and returns the plain data key - the
decryption of the encrypted data key using the master key referenced by the key ID -
on success or an error otherwise.
More details about supported KMS implementations and configuration can be found at the [KMS guide](https://github.com/minio/minio/blob/master/docs/kms/README.md).
The Minio server requests a new data key from the KMS for each uploaded object and uses that data key
as EK. Additionally it stores the encrypted form of the data key and the master key ID as part
of the object metadata. The plain data only resides in RAM during the en/decryption process.
The Minio server does not store any SSE-related key at the KMS. Instead the KMS is treated as trusted
component that performs key sealing/unsealing operations to build a key hierarchy:
```
CMK (master key)
|
+-----------------------------------+-----------------------------------+
| | |
+-------+----------------+ +-------+----------------+ ...
| EK_1 | EK_1_encrypted | | EK_2 | EK_2_encrypted |
+---+----------+---------+ +---+----------+---------+
| | | |
| | | |
+---+---+ | +---+---+ |
| KEK_1 | | | KEK_2 | |
+---+---+ | +---+---+ |
| | | |
| | | |
+---+---+ | +---+---+ |
| OEK_1 | | | OEK_2 | |
+---+---+ | +---+---+ |
| |
| |
| |
+---------+---------+ +---------+---------+
| object_metadata_1 | | object_metadata_2 |
+-------------------+ +-------------------+
```
<center>Figure 2 - KMS key hierarchy</center>
#### Key rotation - Basic Operation
The Minio server supports key rotation for SSE-S3 encrypted objects. Therefore, an S3 client
must perform a S3 COPY operation where the copy source and destination are equal and the SSE-S3 HTTP
header is set. The minio server decrypts the OEK using the current encrypted data key and the
master key ID of the object metadata. If this succeeds, the server requests a new data key
from the KMS using the master key ID of the **current Minio KMS configuration** and re-wraps the
*OEK* with a new *KEK* derived from the new data key / EK:
```
object metadata KMS
| |
| +----------------+ 1a | +-------+
|-------------------->| EK_1_encrypted |-----------|->| CMK_1 |
| +----------------+ | +---+---+
| | |
| +---------------+ +------+ 1b | |
|------------->| OEK_encrypted | | EK_1 |<---|------+
| +-------+-------+ +------+ |
| \ / |
| \___ 2 ___/ |
| \___/ |
| | |
| +--+--+ |
| | OEK | | +-------+
| +--+--+ | | CMK_2 |
| | | +---+---+
| | | |
| 5 +----------------+ |4 +------+ 3a | |
|<------| OEK_encrypted' |<----+-------| EK_2 |<---|------+
| +----------------+ +------+ | |
| +----------------+ 3b | |
|<-------------------| EK_2_encrypted |<-----------|------+
| +----------------+ |
| |
1a) Send encrypted data key and master key ID to KMS.
1b) Receive decrypted data key.
2) Decrypt encrypted object key with the KEK derived from the data key.
3a) Receive new plain data key from the KMS using the master key ID of the server config.
3b) Receive encrypted form of the data key from the KMS.
4) Derive a new KEK from the new data key and re-encrypt the OEK with it.
5) Store the encrypted OEK encrypted data key and master key ID in object metadata.
```
<center>Figure 3 - KMS data key rotation</center>
#### Key rotation - Extensions
The basic SSE-S3 key rotation operation can be used to build more powerful key management
operations. The following options are possible to perform manually but do not have fully
functional API's at this time.
1. **Master key migration**: The [SSE-S3 key rotation](#Key-rotation---Basic-Operation) can be performed
on multiple/all objects to move them from one to another master key.
2. **Secure object erasure**: The [SSE-S3 key rotation](#Key-rotation---Basic-Operation) can be applied
to one/multiple objects with a randomly generated master key which is
not stored at the KMS. That leads to an encrypted data key which can
never be decrypted anymore.
3. **Periodical key migration**: The [SSE-S3 key rotation](#Key-rotation---Basic-Operation) can be
invoked after a certain time period to migrate one or more objects
from one master key to another.
#### Secure Erasure and Locking
The Minio server requires an available KMS to en/decrypt SSE-S3 encrypted objects. Therefore it
is possible to erase or lock some or all encrypted objects. For example in case of a detected attack
or other emergency situations the following actions can be taken:
- Seal the KMS such that it cannot be accessed by Minio server anymore. That will lock **all**
SSE-S3 encrypted objects protected by master keys stored on the KMS. All these objects
can not be decrypted as long as the KMS is sealed.
- Seal/Unmount one/some master keys. That will lock all SSE-S3 encrypted objects protected by
these master keys. All these objects can not be decrypted as long as the key(s) are sealed.
- Delete one/some master keys. From a security standpoint, this is equal to erasing all SSE-S3
encrypted objects protected by these master keys. All these objects are lost forever as they cannot
be decrypted. Especially deleting all master keys at the KMS is equivalent to secure erasing all
SSE-S3 encrypted objects.
## Acronyms
- <a name="aead"></a>**AEAD**: Authenticated Encryption with Associated Data
- <a name="csprng"></a>**CSPRNG**: Cryptographically Secure Pseudo Random Number Generator
- <a name="ek"></a>**EK**: External Key
- <a name="iv"></a>**IV**: Initialization Vector
- <a name="kek"></a>**KEK**: Key Encryption Key
- <a name="oek"></a>**OEK**: Object Encryption Key
- <a name="prf"></a>**PRF**: Pseudo Random Function
- <a name="sse"></a>**SSE**: Server-Side Encryption
- <a name="ssec"></a>**SSE-C**: Server-Side Encryption with client-provided Keys
- <a name="sses3"></a>**SSE-S3**: Server-Side Encryption with a KMS
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