// Copyright (c) 2015-2021 MinIO, Inc.
//
// This file is part of MinIO Object Storage stack
//
// This program is free software: you can redistribute it and/or modify
// it under the terms of the GNU Affero General Public License as published by
// the Free Software Foundation, either version 3 of the License, or
// (at your option) any later version.
//
// This program is distributed in the hope that it will be useful
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
// GNU Affero General Public License for more details.
//
// You should have received a copy of the GNU Affero General Public License
// along with this program.  If not, see <http://www.gnu.org/licenses/>.

// Package crypto implements AWS S3 related cryptographic building blocks
// for implementing Server-Side-Encryption (SSE-S3) and Server-Side-Encryption
// with customer provided keys (SSE-C).
//
// All objects are encrypted with an unique and randomly generated 'ObjectKey'.
// The ObjectKey itself is never stored in plaintext. Instead it is only stored
// in a sealed from. The sealed 'ObjectKey' is created by encrypting the 'ObjectKey'
// with an unique key-encryption-key. Given the correct key-encryption-key the
// sealed 'ObjectKey' can be unsealed and the object can be decrypted.
//
// ## SSE-C
//
// SSE-C computes the key-encryption-key from the client-provided key, an
// initialization vector (IV) and the bucket/object path.
//
//  1. Encrypt:
//     Input: ClientKey, bucket, object, metadata, object_data
//     -              IV := Random({0,1}²⁵⁶)
//     -       ObjectKey := SHA256(ClientKey || Random({0,1}²⁵⁶))
//     -       KeyEncKey := HMAC-SHA256(ClientKey, IV || 'SSE-C' || 'DAREv2-HMAC-SHA256' || bucket || '/' || object)
//     -       SealedKey := DAREv2_Enc(KeyEncKey, ObjectKey)
//     - enc_object_data := DAREv2_Enc(ObjectKey, object_data)
//     -        metadata <- IV
//     -        metadata <- SealedKey
//     Output: enc_object_data, metadata
//
//  2. Decrypt:
//     Input: ClientKey, bucket, object, metadata, enc_object_data
//     -          IV <- metadata
//     -   SealedKey <- metadata
//     -   KeyEncKey := HMAC-SHA256(ClientKey, IV || 'SSE-C' || 'DAREv2-HMAC-SHA256' || bucket || '/' || object)
//     -   ObjectKey := DAREv2_Dec(KeyEncKey, SealedKey)
//     - object_data := DAREv2_Dec(ObjectKey, enc_object_data)
//     Output: object_data
//
// ## SSE-S3
//
// SSE-S3 can use either a master key or a KMS as root-of-trust.
// The en/decryption slightly depens upon which root-of-trust is used.
//
// ### SSE-S3 and single master key
//
// The master key is used to derive unique object- and key-encryption-keys.
// SSE-S3 with a single master key works as SSE-C where the master key is
// used as the client-provided key.
//
//  1. Encrypt:
//     Input: MasterKey, bucket, object, metadata, object_data
//     -              IV := Random({0,1}²⁵⁶)
//     -       ObjectKey := SHA256(MasterKey || Random({0,1}²⁵⁶))
//     -       KeyEncKey := HMAC-SHA256(MasterKey, IV || 'SSE-S3' || 'DAREv2-HMAC-SHA256' || bucket || '/' || object)
//     -       SealedKey := DAREv2_Enc(KeyEncKey, ObjectKey)
//     - enc_object_data := DAREv2_Enc(ObjectKey, object_data)
//     -        metadata <- IV
//     -        metadata <- SealedKey
//     Output: enc_object_data, metadata
//
//  2. Decrypt:
//     Input: MasterKey, bucket, object, metadata, enc_object_data
//     -          IV <- metadata
//     -   SealedKey <- metadata
//     -   KeyEncKey := HMAC-SHA256(MasterKey, IV || 'SSE-S3' || 'DAREv2-HMAC-SHA256' || bucket || '/' || object)
//     -   ObjectKey := DAREv2_Dec(KeyEncKey, SealedKey)
//     - object_data := DAREv2_Dec(ObjectKey, enc_object_data)
//     Output: object_data
//
// ### SSE-S3 and KMS
//
// SSE-S3 requires that the KMS provides two functions:
//
//  1. Generate(KeyID) -> (Key, EncKey)
//
//  2. Unseal(KeyID, EncKey) -> Key
//
//  1. Encrypt:
//     Input: KeyID, bucket, object, metadata, object_data
//     -     Key, EncKey := Generate(KeyID)
//     -              IV := Random({0,1}²⁵⁶)
//     -       ObjectKey := SHA256(Key, Random({0,1}²⁵⁶))
//     -       KeyEncKey := HMAC-SHA256(Key, IV || 'SSE-S3' || 'DAREv2-HMAC-SHA256' || bucket || '/' || object)
//     -       SealedKey := DAREv2_Enc(KeyEncKey, ObjectKey)
//     - enc_object_data := DAREv2_Enc(ObjectKey, object_data)
//     -        metadata <- IV
//     -        metadata <- KeyID
//     -        metadata <- EncKey
//     -        metadata <- SealedKey
//     Output: enc_object_data, metadata
//
//  2. Decrypt:
//     Input: bucket, object, metadata, enc_object_data
//     -      KeyID  <- metadata
//     -      EncKey <- metadata
//     -          IV <- metadata
//     -   SealedKey <- metadata
//     -         Key := Unseal(KeyID, EncKey)
//     -   KeyEncKey := HMAC-SHA256(Key, IV || 'SSE-S3' || 'DAREv2-HMAC-SHA256' || bucket || '/' || object)
//     -   ObjectKey := DAREv2_Dec(KeyEncKey, SealedKey)
//     - object_data := DAREv2_Dec(ObjectKey, enc_object_data)
//     Output: object_data
package crypto