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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
},
/// Simply hashes the password using MD5
///
/// This mechanism uses neither iteration to increase the time it
/// takes to derive the key from the password nor does it salt the
/// password, as well as using a very weak and fast hash
/// algorithm. This makes dictionary attacks more feasible.
///
/// This mechanism has been deprecated in RFC 2440. Do not use
/// this variant.
#[deprecated(note = "Use `S2K::Iterated`.")]
Implicit,
/// 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);
}
&S2K::Salted { ref salt, .. } => {
hash.update(salt);
hash.update(string);
}
&S2K::Iterated { ref salt, hash_bytes, .. }
if (hash_bytes as usize) < salt.len() + string.len() =>
{
// Independent of what the hash count is, we
// always hash the whole salt and password once.
hash.update(&salt[..]);
hash.update(string);
},
&S2K::Iterated { ref salt, hash_bytes, .. } => {
// Unroll the processing loop N times.
const N: usize = 16;
let data_len = salt.len() + string.len();
let octs_per_iter = N * data_len;
let mut data: SessionKey =
vec![0u8; octs_per_iter].into();
let full = hash_bytes as usize / octs_per_iter;
let tail = hash_bytes as usize - (full * octs_per_iter);
for i in 0..N {
let o = data_len * i;
data[o..o + salt.len()]
.clone_from_slice(salt);
data[o + salt.len()..o + data_len]
.clone_from_slice(string);
}
for _ in 0..full {
hash.update(&data);
}
if tail != 0 {
hash.update(&data[0..tail]);
}
}
S2K::Implicit |
S2K::Unknown { .. } | &S2K::Private { .. } =>
unreachable!(),
}
let _ = hash.digest(data);
zeros.push(0);
}
Ok(ret.into())
}),
S2K::Implicit => S2K::Simple {
hash: HashAlgorithm::MD5,
}.derive_key(password, key_size),
S2K::Unknown { tag, .. } | S2K::Private { tag, .. } =>
Err(Error::MalformedPacket(
format!("Unknown S2K type {:#x}", tag)).into()),
}
}
/// Returns whether this S2K mechanism is supported.
pub fn is_supported(&self) -> bool {
use self::S2K::*;
#[allow(deprecated)]
match self {
Simple { .. }
| Salted { .. }
| Iterated { .. }
| Implicit
=> true,
S2K::Private { .. }
| S2K::Unknown { .. }
=> false,
}
}
/// This function returns an encodable iteration count.
///
/// Not all iteration counts are encodable as *Iterated and Salted
/// S2K*. The largest encodable hash count is `0x3e00000`.
///
/// The returned value is larger or equal `hash_bytes`, or
/// `0x3e00000` if `hash_bytes` is larger than or equal
/// `0x3e00000`.
fn nearest_hash_count(hash_bytes: usize) -> u32 {
use std::usize;
match hash_bytes {
0..=1024 => 1024,
0x3e00001..=usize::MAX => 0x3e00000,
hash_bytes => {
for i in 0..256 {
let n = Self::decode_count(i as u8);
if n as usize >= hash_bytes {
return n;
}
}
0x3e00000
}
}
}
/// Decodes the OpenPGP encoding of the number of bytes to hash.
pub(crate) fn decode_count(coded: u8) -> u32 {
use std::cmp;
let mantissa = 16 + (coded as u32 & 15);
let exp = (coded as u32 >> 4) + 6;
mantissa << cmp::min(32 - 5, exp)
}
/// Converts `hash_bytes` into coded count representation.
///
/// # Errors
///
/// Fails with `Error::InvalidArgument` if `hash_bytes` cannot be
/// encoded. See also [`S2K::nearest_hash_count()`].
///
pub(crate) fn encode_count(hash_bytes: u32) -> Result<u8> {
// eeee.mmmm -> (16 + mmmm) * 2^(6 + e)
let msb = 32 - hash_bytes.leading_zeros();
let (mantissa_mask, tail_mask) = match msb {
0..=10 => {
return Err(Error::InvalidArgument(
format!("S2K: cannot encode iteration count of {}",
hash_bytes)).into());
}
11..=32 => {
let m = 0b11_1100_0000 << (msb - 11);
let t = 1 << (msb - 11);
(m, t - 1)
}
_ => unreachable!()
};
let exp = if msb < 11 { 0 } else { msb - 11 };
let mantissa = (hash_bytes & mantissa_mask) >> (msb - 5);
if tail_mask & hash_bytes != 0 {
return Err(Error::InvalidArgument(
format!("S2K: cannot encode iteration count of {}",
hash_bytes)).into());
}
Ok(mantissa as u8 | (exp as u8) << 4)
}
}
impl fmt::Display for S2K {
fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
#[allow(deprecated)]
match self {
S2K::Simple{ hash } =>
f.write_fmt(format_args!("Simple S2K with {}", hash)),
S2K::Salted{ hash, salt } => {
f.write_fmt(
format_args!("Salted S2K with {} and salt\
{:x}{:x}{:x}{:x}{:x}{:x}{:x}{:x}",
hash,
salt[0], salt[1], salt[2], salt[3],
salt[4], salt[5], salt[6], salt[7]))
}
S2K::Iterated{ hash, salt, hash_bytes, } => {
f.write_fmt(
format_args!("Iterated and Salted S2K with {}, \
salt {:x}{:x}{:x}{:x}{:x}{:x}{:x}{:x} and \
{} bytes to hash",
hash,
salt[0], salt[1], salt[2], salt[3],
salt[4], salt[5], salt[6], salt[7],
hash_bytes))
}
S2K::Implicit => f.write_str("Implicit S2K"),
S2K::Private { tag, parameters } =>
if let Some(p) = parameters.as_ref() {
write!(f, "Private/Experimental S2K {}:{:?}", tag, p)
} else {
write!(f, "Private/Experimental S2K {}", tag)
},
S2K::Unknown { tag, parameters } =>
if let Some(p) = parameters.as_ref() {
write!(f, "Unknown S2K {}:{:?}", tag, p)
} else {
write!(f, "Unknown S2K {}", tag)
},
}
}
}
#[cfg(test)]
impl Arbitrary for S2K {
fn arbitrary(g: &mut Gen) -> Self {
use crate::arbitrary_helper::*;
#[allow(deprecated)]
match gen_arbitrary_from_range(0..7, g) {
0 => S2K::Simple{ hash: HashAlgorithm::arbitrary(g) },
1 => S2K::Salted{
hash: HashAlgorithm::arbitrary(g),
salt: {
let mut salt = [0u8; 8];
arbitrary_slice(g, &mut salt);
salt
},
},
2 => S2K::Iterated{
hash: HashAlgorithm::arbitrary(g),
salt: {
let mut salt = [0u8; 8];
arbitrary_slice(g, &mut salt);
salt
},
hash_bytes: S2K::nearest_hash_count(Arbitrary::arbitrary(g)),
},
3 => S2K::Private {
tag: gen_arbitrary_from_range(100..111, g),
parameters: Some(arbitrary_bounded_vec(g, 200).into()),
},
4 => S2K::Unknown {
tag: 2,
parameters: Some(arbitrary_bounded_vec(g, 200).into()),
},
5 => S2K::Unknown {
tag: gen_arbitrary_from_range(4..100, g),
parameters: Some(arbitrary_bounded_vec(g, 200).into()),
},
6 => S2K::Unknown {
tag: gen_arbitrary_from_range(111..256, g) as u8,
parameters: Some(arbitrary_bounded_vec(g, 200).into()),
},
_ => unreachable!(),
}
}
}
#[cfg(test)]
mod tests {
use super::*;
use crate::fmt::to_hex;
use crate::SymmetricAlgorithm;
use crate::Packet;
use crate::parse::{Parse, PacketParser};
#[test]
fn s2k_parser_test() {
use crate::packet::SKESK;
struct Test<'a> {
filename: &'a str,
s2k: S2K,
cipher_algo: SymmetricAlgorithm,
password: Password,
key_hex: &'a str,
}
// Note: this test only works with SK-ESK packets that don't
// contain an encrypted session key, i.e., the session key is
// the result of the s2k function. gpg generates this type of
// SK-ESK packet when invoked with -c, but not -e. (When
// invoked with -c and -e, it generates SK-ESK packets that
// include an encrypted session key.)
#[allow(deprecated)]
let tests = [
Test {
filename: "mode-0-password-1234.gpg",
cipher_algo: SymmetricAlgorithm::AES256,
s2k: S2K::Simple{ hash: HashAlgorithm::SHA1, },
password: "1234".into(),
key_hex: "7110EDA4D09E062AA5E4A390B0A572AC0D2C0220F352B0D292B65164C2A67301",
},
Test {
filename: "mode-1-password-123456-1.gpg",
cipher_algo: SymmetricAlgorithm::AES256,
s2k: S2K::Salted{
hash: HashAlgorithm::SHA1,
salt: [0xa8, 0x42, 0xa7, 0xa9, 0x59, 0xfa, 0x42, 0x2a],
},
password: "123456".into(),
key_hex: "8B79077CA448F6FB3D3AD2A264D3B938D357C9FB3E41219FD962DF960A9AFA08",
},
Test {
filename: "mode-1-password-foobar-2.gpg",
cipher_algo: SymmetricAlgorithm::AES256,
s2k: S2K::Salted{
hash: HashAlgorithm::SHA1,
salt: [0xbc, 0x95, 0x58, 0x45, 0x81, 0x3c, 0x7c, 0x37],
},
password: "foobar".into(),
key_hex: "B7D48AAE9B943B22A4D390083E8460B5EDFA118FE1688BF0C473B8094D1A8D10",
},
Test {
filename: "mode-3-password-qwerty-1.gpg",
cipher_algo: SymmetricAlgorithm::AES256,
s2k: S2K::Iterated {
hash: HashAlgorithm::SHA1,
salt: [0x78, 0x45, 0xf0, 0x5b, 0x55, 0xf7, 0xb4, 0x9e],
hash_bytes: S2K::decode_count(241),
},
password: "qwerty".into(),
key_hex: "575AD156187A3F8CEC11108309236EB499F1E682F0D1AFADFAC4ECF97613108A",
},
Test {
filename: "mode-3-password-9876-2.gpg",
cipher_algo: SymmetricAlgorithm::AES256,
s2k: S2K::Iterated {
hash: HashAlgorithm::SHA1,
salt: [0xb9, 0x67, 0xea, 0x96, 0x53, 0xdb, 0x6a, 0xc8],
hash_bytes: S2K::decode_count(43),
},
password: "9876".into(),
key_hex: "736C226B8C64E4E6D0325C6C552EF7C0738F98F48FED65FD8C93265103EFA23A",
},
Test {
filename: "mode-3-aes192-password-123.gpg",
cipher_algo: SymmetricAlgorithm::AES192,
s2k: S2K::Iterated {
hash: HashAlgorithm::SHA1,
salt: [0x8f, 0x81, 0x74, 0xc5, 0xd9, 0x61, 0xc7, 0x79],
hash_bytes: S2K::decode_count(238),
},
password: "123".into(),
key_hex: "915E96FC694E7F90A6850B740125EA005199C725F3BD27E3",
},
Test {
filename: "mode-3-twofish-password-13-times-0123456789.gpg",
cipher_algo: SymmetricAlgorithm::Twofish,
s2k: S2K::Iterated {
hash: HashAlgorithm::SHA1,
salt: [0x51, 0xed, 0xfc, 0x15, 0x45, 0x40, 0x65, 0xac],
hash_bytes: S2K::decode_count(238),
},
password: "0123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789".into(),
key_hex: "EA264FADA5A859C40D88A159B344ECF1F51FF327FDB3C558B0A7DC299777173E",
},
Test {
filename: "mode-3-aes128-password-13-times-0123456789.gpg",
cipher_algo: SymmetricAlgorithm::AES128,
s2k: S2K::Iterated {
hash: HashAlgorithm::SHA1,
salt: [0x06, 0xe4, 0x61, 0x5c, 0xa4, 0x48, 0xf9, 0xdd],
hash_bytes: S2K::decode_count(238),
},
password: "0123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789".into(),
key_hex: "F3D0CE52ED6143637443E3399437FD0F",
},
];
for test in tests.iter().filter(|t| t.cipher_algo.is_supported()) {
let path = crate::tests::message(&format!("s2k/{}", test.filename));
let pp = PacketParser::from_bytes(path).unwrap().unwrap();
if let Packet::SKESK(SKESK::V4(ref skesk)) = pp.packet {
assert_eq!(skesk.symmetric_algo(), test.cipher_algo);
assert_eq!(skesk.s2k(), &test.s2k);
let key = skesk.s2k().derive_key(
&test.password,
skesk.symmetric_algo().key_size().unwrap());
if let Ok(key) = key {
let key = to_hex(&key[..], false);
assert_eq!(key, test.key_hex);
} else {
panic!("Session key: None!");
}
} else {
panic!("Wrong packet!");
}
// Get the next packet.
let (_, ppr) = pp.next().unwrap();
assert!(ppr.is_eof());
}
}
quickcheck! {
fn s2k_display(s2k: S2K) -> bool {
let s = format!("{}", s2k);
!s.is_empty()
}
}
quickcheck! {
fn s2k_parse(s2k: S2K) -> bool {
match s2k {
S2K::Unknown { tag, .. } =>
(tag > 3 && tag < 100) || tag == 2 || tag > 110,
S2K::Private { tag, .. } =>
(100..=110).contains(&tag),
_ => true
}
}
}
#[test]
fn s2k_coded_count_roundtrip() {
for cc in 0..0x100usize {
let hash_bytes = S2K::decode_count(cc as u8);
assert!(hash_bytes >= 1024
&& S2K::encode_count(hash_bytes).unwrap() == cc as u8);
}
}
quickcheck!{
fn s2k_coded_count_approx(i: u32) -> bool {
let approx = S2K::nearest_hash_count(i as usize);
let cc = S2K::encode_count(approx).unwrap();
(approx >= i || i > 0x3e00000) && S2K::decode_count(cc) == approx
}
}
#[test]
fn s2k_coded_count_approx_1025() {
let i = 1025;
let approx = S2K::nearest_hash_count(i);
let cc = S2K::encode_count(approx).unwrap();
assert!(approx as usize >= i || i > 0x3e00000);
assert_eq!(S2K::decode_count(cc), approx);
}
#[test]
fn s2k_coded_count_approx_0x3e00000() {
let i = 0x3e00000;
let approx = S2K::nearest_hash_count(i);
let cc = S2K::encode_count(approx).unwrap();
assert!(approx as usize >= i || i > 0x3e00000);
assert_eq!(S2K::decode_count(cc), approx);
}
}