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|
//! String-to-Key (S2K) specifiers.
//!
//! String-to-key (S2K) specifiers are used to convert password
//! strings into symmetric-key encryption/decryption keys. See
//! [Section 3.7 of RFC 4880].
//!
//! [Section 3.7 of RFC 4880]: https://tools.ietf.org/html/rfc4880#section-3.7
use crate::Error;
use crate::Result;
use crate::HashAlgorithm;
use crate::crypto::Password;
use crate::crypto::SessionKey;
use crate::crypto::hash::Digest;
use std::fmt;
#[cfg(test)]
use quickcheck::{Arbitrary, Gen};
/// String-to-Key (S2K) specifiers.
///
/// String-to-key (S2K) specifiers are used to convert password
/// strings into symmetric-key encryption/decryption keys. See
/// [Section 3.7 of RFC 4880]. This is used to encrypt messages with
/// a password (see [`SKESK`]), and to protect secret keys (see
/// [`key::Encrypted`]).
///
/// [Section 3.7 of RFC 4880]: https://tools.ietf.org/html/rfc4880#section-3.7
/// [`SKESK`]: crate::packet::SKESK
/// [`key::Encrypted`]: crate::packet::key::Encrypted
///
/// Note: This enum cannot be exhaustively matched to allow future
/// extensions.
#[non_exhaustive]
#[derive(Clone, PartialEq, Eq, Hash, Debug)]
pub enum S2K {
/// Repeatently hashes the password with a public `salt` value.
Iterated {
/// Hash used for key derivation.
hash: HashAlgorithm,
/// Public salt value mixed into the password.
salt: [u8; 8],
/// Number of bytes to hash.
///
/// This parameter increases the workload for an attacker
/// doing a dictionary attack. Note that not all values are
/// representable. See [`S2K::new_iterated`].
///
/// [`S2K::new_iterated`]: S2K::new_iterated()
hash_bytes: u32,
},
/// Hashes the password with a public `salt` value.
///
/// This mechanism does not use iteration to increase the time it
/// takes to derive the key from the password. This makes
/// dictionary attacks more feasible. Do not use this variant.
#[deprecated(note = "Use `S2K::Iterated`.")]
Salted {
/// Hash used for key derivation.
hash: HashAlgorithm,
/// Public salt value mixed into the password.
salt: [u8; 8],
},
/// Simply hashes the password.
///
/// This mechanism uses neither iteration to increase the time it
/// takes to derive the key from the password nor does it salt the
/// password. This makes dictionary attacks more feasible.
///
/// This mechanism has been deprecated in RFC 4880. Do not use this
/// variant.
#[deprecated(note = "Use `S2K::Iterated`.")]
Simple {
/// Hash used for key derivation.
hash: HashAlgorithm
},
/// Private S2K algorithm.
Private {
/// Tag identifying the private algorithm.
///
/// Tags 100 to 110 are reserved for private use.
tag: u8,
/// The parameters for the private algorithm.
///
/// This is optional, because when we parse a packet
/// containing an unknown S2K algorithm, we do not know how
/// many octets to attribute to the S2K's parameters. In this
/// case, `parameters` is set to `None`. Note that the
/// information is not lost, but stored in the packet. If the
/// packet is serialized again, it is written out.
parameters: Option<Box<[u8]>>,
},
/// Unknown S2K algorithm.
Unknown {
/// Tag identifying the unknown algorithm.
tag: u8,
/// The parameters for the unknown algorithm.
///
/// This is optional, because when we parse a packet
/// containing an unknown S2K algorithm, we do not know how
/// many octets to attribute to the S2K's parameters. In this
/// case, `parameters` is set to `None`. Note that the
/// information is not lost, but stored in the packet. If the
/// packet is serialized again, it is written out.
parameters: Option<Box<[u8]>>,
},
}
assert_send_and_sync!(S2K);
impl Default for S2K {
fn default() -> Self {
S2K::new_iterated(
// SHA2-256, being optimized for implementations on
// architectures with a word size of 32 bit, has a more
// consistent runtime across different architectures than
// SHA2-512. Furthermore, the digest size is large enough
// for every cipher algorithm currently in use.
HashAlgorithm::SHA256,
// This is the largest count that OpenPGP can represent.
// On moderate machines, like my Intel(R) Core(TM) i5-2400
// CPU @ 3.10GHz, it takes ~354ms to derive a key.
0x3e00000,
).expect("0x3e00000 is representable")
}
}
impl S2K {
/// Creates a new iterated `S2K` object.
///
/// Usually, you should use `S2K`s [`Default`] implementation to
/// create `S2K` objects with sane default parameters. The
/// parameters are chosen with contemporary machines in mind, and
/// should also be usable on lower-end devices like smart phones.
///
/// [`Default`]: std::default::Default
///
/// Using this method, you can tune the parameters for embedded
/// devices. Note, however, that this also decreases the work
/// factor for attackers doing dictionary attacks.
pub fn new_iterated(hash: HashAlgorithm, approx_hash_bytes: u32)
-> Result<Self> {
if approx_hash_bytes > 0x3e00000 {
Err(Error::InvalidArgument(format!(
"Number of bytes to hash not representable: {}",
approx_hash_bytes)).into())
} else {
let mut salt = [0u8; 8];
crate::crypto::random(&mut salt);
Ok(S2K::Iterated {
hash,
salt,
hash_bytes:
Self::nearest_hash_count(approx_hash_bytes as usize),
})
}
}
/// Derives a key of the given size from a password.
pub fn derive_key(&self, password: &Password, key_size: usize)
-> Result<SessionKey> {
#[allow(deprecated)]
match self {
&S2K::Simple { hash } | &S2K::Salted { hash, .. }
| &S2K::Iterated { hash, .. } => password.map(|string| {
let mut hash = hash.context()?;
// If the digest length is shorter than the key length,
// then we need to concatenate multiple hashes, each
// preloaded with i 0s.
let hash_sz = hash.digest_size();
let num_contexts = (key_size + hash_sz - 1) / hash_sz;
let mut zeros = Vec::with_capacity(num_contexts + 1);
let mut ret = vec![0u8; key_size];
for data in ret.chunks_mut(hash_sz) {
hash.update(&zeros[..]);
match self {
&S2K::Simple { .. } => {
hash.update(string);
}
|