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// Copyright 2017-2018 int08h LLC
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
extern crate hex;
use std::io::{Cursor, Read, Write};
use std::str::FromStr;
use ring::aead::{open_in_place, seal_in_place, OpeningKey, SealingKey, AES_256_GCM};
use ring::rand::{SecureRandom, SystemRandom};
use super::super::MIN_SEED_LENGTH;
use byteorder::{LittleEndian, ReadBytesExt, WriteBytesExt};
use kms::{KmsError, KmsProvider, AD, DEK_SIZE_BYTES, NONCE_SIZE_BYTES, TAG_SIZE_BYTES};
const DEK_LEN_FIELD: usize = 2;
const NONCE_LEN_FIELD: usize = 2;
// 2 bytes - encrypted DEK length
// 2 bytes - nonce length
// n bytes - encrypted DEK
// n bytes - nonce
// n bytes - opaque (AEAD encrypted seed + tag)
const MIN_PAYLOAD_SIZE: usize = DEK_LEN_FIELD
+ NONCE_LEN_FIELD
+ DEK_SIZE_BYTES
+ NONCE_SIZE_BYTES
+ MIN_SEED_LENGTH as usize
+ TAG_SIZE_BYTES;
// No input prefix to skip, consume entire buffer
const IN_PREFIX_LEN: usize = 0;
// Convenience function to create zero-filled Vec of given size
fn vec_zero_filled(len: usize) -> Vec<u8> {
(0..len).into_iter().map(|_| 0).collect()
}
/// Envelope encryption of the long-term key seed value.
///
/// The seed is encrypted using AES-GCM-256 with:
///
/// * 32 byte (256 bit) random key
/// * 12 byte (96 bit) random nonce
/// * 16 byte (128 bit) authentication tag
///
/// Randomness obtained from
/// [`ring::rand::SecureRandom`](https://briansmith.org/rustdoc/ring/rand/trait.SecureRandom.html).
///
/// The key used to encrypt the seed is wrapped (encrypted) using a
/// [`KmsProvider`](trait.KmsProvider.html) implementation.
///
pub struct EnvelopeEncryption;
impl EnvelopeEncryption {
/// Decrypt a seed previously encrypted with `encrypt_seed()`
pub fn decrypt_seed(kms: &KmsProvider, ciphertext_blob: &[u8]) -> Result<Vec<u8>, KmsError> {
if ciphertext_blob.len() < MIN_PAYLOAD_SIZE {
return Err(KmsError::InvalidData(format!(
"ciphertext too short: min {}, found {}",
MIN_PAYLOAD_SIZE,
ciphertext_blob.len()
)));
}
let mut tmp = Cursor::new(ciphertext_blob);
// Read the lengths of the wrapped DEK and of the nonce
let dek_len = tmp.read_u16::<LittleEndian>()? as usize;
let nonce_len = tmp.read_u16::<LittleEndian>()? as usize;
if dek_len != DEK_SIZE_BYTES || nonce_len != NONCE_SIZE_BYTES {
return Err(KmsError::InvalidData(format!(
"invalid DEK ({}) or nonce ({}) length",
dek_len, nonce_len
)));
}
// Consume the wrapped DEK
let mut encrypted_dek = vec_zero_filled(dek_len);
tmp.read_exact(&mut encrypted_dek)?;
// Consume the nonce
let mut nonce = vec_zero_filled(nonce_len);
tmp.read_exact(&mut nonce)?;
// Consume the encrypted seed + tag
let mut encrypted_seed = Vec::new();
tmp.read_to_end(&mut encrypted_seed)?;
// Invoke KMS to decrypt the DEK
let dek = kms.decrypt_dek(&encrypted_dek)?;
// Decrypt the seed value using the DEK
let dek_open_key = OpeningKey::new(&AES_256_GCM, &dek)?;
match open_in_place(
&dek_open_key,
&nonce,
AD.as_bytes(),
IN_PREFIX_LEN,
&mut encrypted_seed,
) {
Ok(plaintext_seed) => Ok(plaintext_seed.to_vec()),
Err(_) => Err(KmsError::OperationFailed(
"failed to decrypt plaintext seed".to_string(),
)),
}
}
///
/// Encrypt the seed value and protect the seed's encryption key using a
/// [`KmsProvider`](trait.KmsProvider.html).
///
/// The returned encrypted byte blob is safe to store on unsecured media.
///
pub fn encrypt_seed(kms: &KmsProvider, plaintext_seed: &[u8]) -> Result<Vec<u8>, KmsError> {
// Generate random DEK and nonce
let rng = rand::SystemRandom::new();
let mut dek = [0u8; DEK_SIZE_BYTES];
let mut nonce = [0u8; NONCE_SIZE_BYTES];
rng.fill(&mut dek)?;
rng.fill(&mut nonce)?;
// Ring will overwrite plaintext with ciphertext in this buffer
let mut plaintext_buf = plaintext_seed.to_vec();
// Reserve space for the authentication tag which will be appended after the ciphertext
plaintext_buf.reserve(TAG_SIZE_BYTES);
for _ in 0..TAG_SIZE_BYTES {
plaintext_buf.push(0);
}
// Encrypt the plaintext seed using the DEK
let dek_seal_key = SealingKey::new(&AES_256_GCM, &dek)?;
let encrypted_seed = match seal_in_place(
&dek_seal_key,
&nonce,
AD.as_bytes(),
&mut plaintext_buf,
TAG_SIZE_BYTES,
) {
Ok(enc_len) => plaintext_buf[..enc_len].to_vec(),
Err(_) => {
return Err(KmsError::OperationFailed(
"failed to encrypt plaintext seed".to_string(),
))
}
};
// Use the KMS to wrap the DEK
let wrapped_dek = kms.encrypt_dek(&dek.to_vec())?;
// And coalesce everything together
let mut output = Vec::new();
output.write_u16::<LittleEndian>(wrapped_dek.len() as u16)?;
output.write_u16::<LittleEndian>(nonce.len() as u16)?;
output.write_all(&wrapped_dek)?;
output.write_all(&nonce)?;
output.write_all(&encrypted_seed)?;
Ok(output)
}
}
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