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use crate::keccak::{keccakf_u8, AlignedKeccakState, KECCAK_BLOCK_BITLEN_STR, KECCAK_BLOCK_SIZE};
use bitflags::bitflags;
use subtle::{self, ConstantTimeEq};
use zeroize::{Zeroize, ZeroizeOnDrop};
// With this feature on, a user can serialize and deserialize the state of a STROBE session
#[cfg(feature = "serialize_secret_state")]
use serde::{Deserialize, Serialize};
/// Version of Strobe that this crate implements.
pub const STROBE_VERSION: &[u8] = b"1.0.2";
/// A placeholder for STROBE version strings. This is the length of the real version strings, for
/// Keccak-f[1600]
const TEMPLATE_VERSION_STR: [u8; 29] = *b"Strobe-Keccak-sss/bbbb-vX.Y.Z";
bitflags! {
/// Operation flags defined in the Strobe paper. This is defined as a bitflags struct.
#[cfg_attr(feature = "serialize_secret_state", derive(Serialize, Deserialize))]
pub(crate) struct OpFlags: u8 {
/// Is data being moved inbound
const I = 1<<0;
/// Is data being sent to the application
const A = 1<<1;
/// Does this operation use cipher output
const C = 1<<2;
/// Is data being sent for transport
const T = 1<<3;
/// Use exclusively for metadata operations
const M = 1<<4;
/// Reserved and currently unimplemented. Using this will cause a panic.
const K = 1<<5;
}
}
impl Zeroize for OpFlags {
fn zeroize(&mut self) {
self.bits.zeroize()
}
}
/// Security parameter. Choice of 128 or 256 bits.
#[derive(Clone, Copy)]
#[cfg_attr(feature = "serialize_secret_state", derive(Serialize, Deserialize))]
#[repr(usize)]
pub enum SecParam {
B128 = 128,
B256 = 256,
}
/// An empty struct that just indicates that MAC verification failed
#[derive(Clone, Copy, Debug, PartialEq, Eq)]
pub struct AuthError;
impl core::fmt::Display for AuthError {
fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
f.write_str("MAC verification failed")
}
}
/// The main Strobe object. This is currently limited to using Keccak-f\[1600\] (the highest
/// security level) as the internal permutation function. For more information on this object, the
/// [protocol specification][spec] is a great resource.
///
/// [spec]: https://strobe.sourceforge.io/specs/
///
/// Description of method input
/// ---------------------------
/// Most operations exposed by `Strobe` take the same set of inputs. The arguments are
///
/// * `data` - The input data to the operation.
/// * `more` - For streaming purposes. Specifies whether you're trying to add more input / get more
/// output to/from the previous operation. For example:
///
/// ```rust
/// # extern crate strobe_rs;
/// # use strobe_rs::{SecParam, Strobe};
/// # fn main() {
/// # let mut s = Strobe::new(b"example-of-more", SecParam::B128);
/// s.ad(b"hello world", false);
/// # }
/// ```
/// is equivalent to
/// ```rust
/// # extern crate strobe_rs;
/// # use strobe_rs::{SecParam, Strobe};
/// # fn main() {
/// # let mut s = Strobe::new(b"example-of-more", SecParam::B128);
/// s.ad(b"hello ", false);
/// s.ad(b"world", true);
/// # }
/// ```
///
/// **NOTE:** If you try to set the `more` flag for an operation that is not preceded by the same
/// operation (e.g., if you try `ad` followed by `send_enc` with `more=true`), then **the function
/// will panic**, since that is an invalid use of the `more` flag.
///
/// Finally, `ratchet` and `meta_ratchet` take a `usize` argument instead of bytes. These functions
/// are individually commented below.
#[derive(Clone, Zeroize, ZeroizeOnDrop)]
#[cfg_attr(feature = "serialize_secret_state", derive(Serialize, Deserialize))]
pub struct Strobe {
/// Internal Keccak state
pub(crate) st: AlignedKeccakState,
/// Security parameter (128 or 256)
#[zeroize(skip)]
sec: SecParam,
/// This is the `R` parameter in the Strobe spec
rate: usize,
/// Index into `st`
pos: usize,
/// Index into `st`
pos_begin: usize,
/// Represents whether we're a sender or a receiver or uninitialized
is_receiver: Option<bool>,
/// The last operation performed. This is to verify that the `more` flag is only used across
/// identical operations.
prev_flags: Option<OpFlags>,
}
// This defines an operation and meta-operation that mutates its input
macro_rules! def_op_mut {
($name:ident, $meta_name:ident, $flags:expr, $doc_str:expr) => {
#[doc = $doc_str]
pub fn $name(&mut self, data: &mut [u8], more: bool) {
let flags = $flags;
self.operate(flags, data, more);
}
#[doc = $doc_str]
pub fn $meta_name(&mut self, data: &mut [u8], more: bool) {
let flags = $flags | OpFlags::M;
self.operate(flags, data, more);
}
};
}
// This defines an operation and meta-operation that does not mutate its input
macro_rules! def_op_no_mut {
($name:ident, $meta_name:ident, $flags:expr, $doc_str:expr) => {
#[doc = $doc_str]
pub fn $name(&mut self, data: &[u8], more: bool) {
let flags = $flags;
self.operate_no_mutate(flags, data, more);
}
#[doc = $doc_str]
pub fn $meta_name(&mut self, data: &[u8], more: bool) {
let flags = $flags | OpFlags::M;
self.operate_no_mutate(flags, data, more);
}
};
}
impl Strobe {
/// Makes a new `Strobe` object with a given protocol byte string and security parameter.
pub fn new(proto: &[u8], sec: SecParam) -> Strobe {
let rate = KECCAK_BLOCK_SIZE * 8 - (sec as usize) / 4 - 2;
assert!(rate >= 1);
assert!(rate < 254);
// Initialize state: st = F([0x01, R+2, 0x01, 0x00, 0x01, 0x60] + b"STROBEvX.Y.Z")
let mut st_buf = [0u8; KECCAK_BLOCK_SIZE * 8];
st_buf[0..6].copy_from_slice(&[0x01, (rate as u8) + 2, 0x01, 0x00, 0x01, 0x60]);
st_buf[6..13].copy_from_slice(b"STROBEv");
st_buf[13..18].copy_from_slice(STROBE_VERSION);
let mut st = AlignedKeccakState(st_buf);
keccakf_u8(&mut st);
let mut strobe = Strobe {
st,
sec,
rate,
pos: 0,
pos_begin: 0,
is_receiver: None,
prev_flags: None,
};
// Mix the protocol into the state
strobe.meta_ad(proto, false);
strobe
}
/// Returns a bytestring of the form `Strobe-Keccak-SEC/B-vVER` where `SEC` is the bits of
/// security (128 or 256), `B` is the block size (in bits) of the Keccak permutation function,
/// and `VER` is the protocol version.
pub fn version_str(&self) -> [u8; TEMPLATE_VERSION_STR.len()] {
let mut buf = TEMPLATE_VERSION_STR;
match self.sec {
SecParam::B128 => buf[14..17].copy_from_slice(b"128"),
SecParam::B256 => buf[14..17].copy_from_slice(b"256"),
}
buf[18..22].copy_from_slice(KECCAK_BLOCK_BITLEN_STR);
buf[24..29].copy_from_slice(STROBE_VERSION);
buf
}
/// Validates that the `more` flag is being used correctly. Panics when validation fails.
fn validate_streaming(&mut self, flags: OpFlags, more: bool) {
// Streaming only makes sense if this operation is the same as last. For example you can do
// s.ad("hello", false);
// s.ad(" world", true).
// But you can't do
// s.ad("hello", false);
// s.key(" world", true).
if more {
assert_eq!(
self.prev_flags,
Some(flags),
"`more` can only be used when this operation is the same as the previous operation"
);
}
// Update the last-performed operation (i.e., the one we're about to perform)
self.prev_flags = Some(flags);
}
// Runs the permutation function on the internal state
fn run_f(&mut self) {
self.st.0[self.pos] ^= self.pos_begin as u8;
self.st.0[self.pos + 1] ^= 0x04;
self.st.0[self.rate + 1] ^= 0x80;
keccakf_u8(&mut self.st);
self.pos = 0;
self.pos_begin = 0;
}
/// XORs the given data into the state. This is a special case of the `duplex` code in the
/// STROBE paper.
fn absorb(&mut self, data: &[u8]) {
for b in data {
self.st.0[self.pos] ^= *b;
self.pos += 1;
if self.pos == self.rate {
self.run_f();
}
}
}
/// XORs the given data into the state, then sets the data equal the state. This is a special
/// case of the `duplex` code in the STROBE paper.
fn absorb_and_set(&mut self, data: &mut [u8]) {
for b in data {
let state_byte = self.st.0.get_mut(self.pos).unwrap();
*state_byte ^= *b;
*b = *state_byte;
self.pos += 1;
if self.pos == self.rate {
self.run_f();
}
}
}
/// Copies the internal state into the given buffer. This is a special case of `absorb_and_set`
/// where `data` is all zeros.
fn copy_state(&mut self, data: &mut [u8]) {
for b in data {
*b = self.st.0[self.pos];
self.pos += 1;
if self.pos == self.rate {
self.run_f();
}
}
}
/// Overwrites the state with the given data while XORing the given data with the old state.
/// This is a special case of the `duplex` code in the STROBE paper.
fn exchange(&mut self, data: &mut [u8]) {
for b in data {
let state_byte = self.st.0.get_mut(self.pos).unwrap();
*b ^= *state_byte;
*state_byte ^= *b;
self.pos += 1;
if self.pos == self.rate {
self.run_f();
}
}
}
/// Overwrites the state with the given data. This is a special case of `Strobe::exchange`,
/// where we do not want to mutate the input data.
fn overwrite(&mut self, data: &[u8]) {
for b in data {
self.st.0[self.pos] = *b;
self.pos += 1;
if self.pos == self.rate {
self.run_f();
}
}
}
/// Copies the state into the given buffer and sets the state to 0. This is a special case of
/// `Strobe::exchange`, where `data` is assumed to be the all-zeros string. This is precisely
/// the case when the current operation is PRF.
fn squeeze(&mut self, data: &mut [u8]) {
for b in data {
let state_byte = self.st.0.get_mut(self.pos).unwrap();
*b = *state_byte;
*state_byte = 0;
self.pos += 1;
if self.pos == self.rate {
self.run_f();
}
}
}
/// Overwrites the state with a specified number of zeros. This is a special case of
/// `Strobe::exchange`. More specifically, it's a special case of `Strobe::overwrite` and
/// `Strobe::squeeze`. It's like `squeeze` in that we assume we've been given all zeros as
/// input, and like `overwrite` in that we do not mutate (or take) any input.
fn zero_state(&mut self, mut bytes_to_zero: usize) {
static ZEROS: [u8; 8 * KECCAK_BLOCK_SIZE] = [0u8; 8 * KECCAK_BLOCK_SIZE];
// Do the zero-writing in chunks
while bytes_to_zero > 0 {
let slice_len = core::cmp::min(self.rate - self.pos, bytes_to_zero);
self.st.0[self.pos..(self.pos + slice_len)].copy_from_slice(&ZEROS[..slice_len]);
self.pos += slice_len;
bytes_to_zero -= slice_len;
if self.pos == self.rate {
self.run_f();
}
}
}
/// Mixes the current state index and flags into the state, accounting for whether we are
/// sending or receiving
fn begin_op(&mut self, mut flags: OpFlags) {
if flags.contains(OpFlags::T) {
let is_op_receiving = flags.contains(OpFlags::I);
// If uninitialized, take on the direction of the first directional operation we get
if self.is_receiver.is_none() {
self.is_receiver = Some(is_op_receiving);
}
// So that the sender and receiver agree, toggle the I flag as necessary
// This is equivalent to flags ^= is_receiver
flags.set(OpFlags::I, self.is_receiver.unwrap() != is_op_receiving);
}
let old_pos_begin = self.pos_begin;
self.pos_begin = self.pos + 1;
// Mix in the position and flags
let to_mix = &mut [old_pos_begin as u8, flags.bits()];
self.absorb(&to_mix[..]);
let force_f = flags.contains(OpFlags::C) || flags.contains(OpFlags::K);
if force_f && self.pos != 0 {
self.run_f();
}
}
/// Performs the state / data transformation that corresponds to the given flags. If `more` is
/// given, this will treat `data` as a continuation of the data given in the previous
/// call to `operate`.
pub(crate) fn operate(&mut self, flags: OpFlags, data: &mut [u8], more: bool) {
// Make sure the K opflag isn't being used, and that the `more` flag isn't being misused
assert!(!flags.contains(OpFlags::K), "Op flag K not implemented");
self.validate_streaming(flags, more);
// If `more` isn't set, this is a new operation. Do the begin_op sequence
if !more {
self.begin_op(flags);
}
// Meta-ness is only relevant for `begin_op`. Remove it to simplify the below logic.
let flags = flags & !OpFlags::M;
// TODO?: Assert that input is empty under some flag conditions
if flags.contains(OpFlags::C) && flags.contains(OpFlags::T) && !flags.contains(OpFlags::I) {
// This is equivalent to the `duplex` operation in the Python implementation, with
// `cafter = True`
if flags == OpFlags::C | OpFlags::T {
// This is `send_mac`. Pretend the input is all zeros
self.copy_state(data)
} else {
self.absorb_and_set(data);
}
} else if flags == OpFlags::I | OpFlags::A | OpFlags::C {
// Special case of case below. This is PRF. Use `squeeze` instead of `exchange`.
self.squeeze(data);
} else if flags.contains(OpFlags::C) {
// This is equivalent to the `duplex` operation in the Python implementation, with
// `cbefore = True`
self.exchange(data);
} else {
// This should normally call `absorb`, but `absorb` does not mutate, so the implementor
// should have used operate_no_mutate instead
panic!("operate should not be called for operations that do not require mutation");
}
}
/// Performs the state transformation that corresponds to the given flags. If `more` is given,
/// this will treat `data` as a continuation of the data given in the previous call to
/// `operate`. This uses non-mutating variants of the specializations of the `duplex` function.
pub(crate) fn operate_no_mutate(&mut self, flags: OpFlags, data: &[u8], more: bool) {
// Make sure the K opflag isn't being used, and that the `more` flag isn't being misused
assert!(!flags.contains(OpFlags::K), "Op flag K not implemented");
self.validate_streaming(flags, more);
// If `more` isn't set, this is a new operation. Do the begin_op sequence
if !more {
self.begin_op(flags);
}
// There are no non-mutating variants of things with flags & (C | T | I) == C | T
if flags.contains(OpFlags::C) && flags.contains(OpFlags::T) && !flags.contains(OpFlags::I) {
panic!("operate_no_mutate called on something that requires mutation");
} else if flags.contains(OpFlags::C) {
// This is equivalent to a non-mutating form of the `duplex` operation in the Python
// implementation, with `cbefore = True`
self.overwrite(data);
} else {
// This is equivalent to the `duplex` operation in the Python implementation, with
// `cbefore = cafter = False`
self.absorb(data);
};
}
// This is separately defined because it's the only method that can return a `Result`. See docs
// for recv_mac and meta_recv_mac.
fn generalized_recv_mac<const N: usize>(
&mut self,
mac: &[u8; N],
is_meta: bool,
) -> Result<(), AuthError> {
// Make a temp buffer for the MAC. This is because operate() mutates the buffer
let mut mac_copy = *mac;
// These are the (meta_)recv_mac flags
let flags = if is_meta {
OpFlags::I | OpFlags::C | OpFlags::T | OpFlags::M
} else {
OpFlags::I | OpFlags::C | OpFlags::T
};
// recv_mac can never be streamed
self.operate(flags, &mut mac_copy, /* more */ false);
// Constant-time MAC check. This accumulates the truth values of byte == 0
let mut all_zero = subtle::Choice::from(1u8);
for b in mac_copy {
all_zero &= b.ct_eq(&0u8);
}
// Zeroize the temp buffer
mac_copy.zeroize();
// If the buffer isn't all zeros, that's an invalid MAC
if !bool::from(all_zero) {
Err(AuthError)
} else {
Ok(())
}
}
/// Attempts to authenticate the current state against the given MAC. On failure, it returns an
/// `AuthError`.
pub fn recv_mac<const N: usize>(&mut self, mac: &[u8; N]) -> Result<(), AuthError> {
self.generalized_recv_mac(mac, /* is_meta */ false)
}
/// Attempts to authenticate the current state against the given MAC. On failure, it returns an
/// `AuthError`.
pub fn meta_recv_mac<const N: usize>(&mut self, mac: &[u8; N]) -> Result<(), AuthError> {
self.generalized_recv_mac(mac, /* is_meta */ true)
}
// This is separately defined because it's the only method that takes an integer and mutates
// its input
fn generalized_ratchet(&mut self, num_bytes_to_zero: usize, more: bool, is_meta: bool) {
// These are the (meta_)ratchet flags
let flags = if is_meta {
OpFlags::C | OpFlags::M
} else {
OpFlags::C
};
// We don't make an `operate` call, since this is a super special case. That means we have
// to validate the flags and make the `begin_op` call manually.
self.validate_streaming(flags, more);
if !more {
self.begin_op(flags);
}
self.zero_state(num_bytes_to_zero);
}
/// Ratchets the internal state forward in an irreversible way by zeroing bytes.
///
/// Takes a `usize` argument specifying the number of bytes of public state to zero. If the
/// size exceeds `self.rate`, Keccak-f will be called before more bytes are zeroed.
pub fn ratchet(&mut self, num_bytes_to_zero: usize, more: bool) {
self.generalized_ratchet(num_bytes_to_zero, more, /* is_meta */ false)
}
/// Ratchets the internal state forward in an irreversible way by zeroing bytes.
///
/// Takes a `usize` argument specifying the number of bytes of public state to zero. If the
/// size exceeds `self.rate`, Keccak-f will be called before more bytes are zeroed.
pub fn meta_ratchet(&mut self, num_bytes_to_zero: usize, more: bool) {
self.generalized_ratchet(num_bytes_to_zero, more, /* is_meta */ true)
}
//
// These operations mutate their inputs
//
def_op_mut!(
send_enc,
meta_send_enc,
OpFlags::A | OpFlags::C | OpFlags::T,
"Sends an encrypted message."
);
def_op_mut!(
recv_enc,
meta_recv_enc,
OpFlags::I | OpFlags::A | OpFlags::C | OpFlags::T,
"Receives an encrypted message."
);
def_op_mut!(
send_mac,
meta_send_mac,
OpFlags::C | OpFlags::T,
"Sends a MAC of the internal state. \
The output is independent of the initial contents of the input buffer."
);
def_op_mut!(
prf,
meta_prf,
OpFlags::I | OpFlags::A | OpFlags::C,
"Extracts pseudorandom data as a function of the internal state. \
The output is independent of the initial contents of the input buffer."
);
//
// These operations do not mutate their inputs
//
def_op_no_mut!(
send_clr,
meta_send_clr,
OpFlags::A | OpFlags::T,
"Sends a plaintext message."
);
def_op_no_mut!(
recv_clr,
meta_recv_clr,
OpFlags::I | OpFlags::A | OpFlags::T,
"Receives a plaintext message."
);
def_op_no_mut!(
ad,
meta_ad,
OpFlags::A,
"Mixes associated data into the internal state."
);
def_op_no_mut!(
key,
meta_key,
OpFlags::A | OpFlags::C,
"Sets a symmetric cipher key."
);
}
#[test]
fn version_str() {
let s128 = Strobe::new(b"version_str test", SecParam::B128);
assert_eq!(&s128.version_str(), b"Strobe-Keccak-128/1600-v1.0.2");
let s256 = Strobe::new(b"version_str test", SecParam::B256);
assert_eq!(&s256.version_str(), b"Strobe-Keccak-256/1600-v1.0.2");
}