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Merge pull request #201 from elichai/2020-03-ecdh
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Simplify callback logic to returning raw coordinates
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@apoelstra
apoelstra committed Apr 6, 2020
2 parents 3fd0897 + 2eff118 commit 86751b2
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Showing 3 changed files with 49 additions and 115 deletions.
4 changes: 2 additions & 2 deletions no_std_test/src/main.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -105,10 +105,10 @@ fn start(_argc: isize, _argv: *const *const u8) -> isize {

let _ = SharedSecret::new(&public_key, &secret_key);
let mut x_arr = [0u8; 32];
let y_arr = unsafe { SharedSecret::new_with_hash_no_panic(&public_key, &secret_key, |x,y| {
let y_arr = SharedSecret::new_with_hash(&public_key, &secret_key, |x,y| {
x_arr = x;
y.into()
})}.unwrap();
});
assert_ne!(x_arr, [0u8; 32]);
assert_ne!(&y_arr[..], &[0u8; 32][..]);

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157 changes: 47 additions & 110 deletions src/ecdh.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -22,7 +22,6 @@ use core::ops::{FnMut, Deref};
use key::{SecretKey, PublicKey};
use ffi::{self, CPtr};
use secp256k1_sys::types::{c_int, c_uchar, c_void};
use Error;

/// A tag used for recovering the public key from a compact signature
#[derive(Copy, Clone)]
Expand Down Expand Up @@ -89,39 +88,12 @@ impl Deref for SharedSecret {
}


unsafe fn callback_logic<F>(output: *mut c_uchar, x: *const c_uchar, y: *const c_uchar, data: *mut c_void) -> c_int
where F: FnMut([u8; 32], [u8; 32]) -> SharedSecret {
let callback: &mut F = &mut *(data as *mut F);

let mut x_arr = [0; 32];
let mut y_arr = [0; 32];
ptr::copy_nonoverlapping(x, x_arr.as_mut_ptr(), 32);
ptr::copy_nonoverlapping(y, y_arr.as_mut_ptr(), 32);

let secret = callback(x_arr, y_arr);
ptr::copy_nonoverlapping(secret.as_ptr(), output as *mut u8, secret.len());

secret.len() as c_int
}

#[cfg(feature = "std")]
unsafe extern "C" fn hash_callback_catch_unwind<F>(output: *mut c_uchar, x: *const c_uchar, y: *const c_uchar, data: *mut c_void) -> c_int
where F: FnMut([u8; 32], [u8; 32]) -> SharedSecret {

let res = ::std::panic::catch_unwind(||callback_logic::<F>(output, x, y, data));
if let Ok(len) = res {
len
} else {
-1
}
unsafe extern "C" fn c_callback(output: *mut c_uchar, x: *const c_uchar, y: *const c_uchar, _data: *mut c_void) -> c_int {
ptr::copy_nonoverlapping(x, output, 32);
ptr::copy_nonoverlapping(y, output.offset(32), 32);
1
}

unsafe extern "C" fn hash_callback_unsafe<F>(output: *mut c_uchar, x: *const c_uchar, y: *const c_uchar, data: *mut c_void) -> c_int
where F: FnMut([u8; 32], [u8; 32]) -> SharedSecret {
callback_logic::<F>(output, x, y, data)
}


impl SharedSecret {
/// Creates a new shared secret from a pubkey and secret key
#[inline]
Expand All @@ -137,35 +109,17 @@ impl SharedSecret {
ptr::null_mut(),
)
};
debug_assert_eq!(res, 1); // The default `secp256k1_ecdh_hash_function_default` should always return 1.
// The default `secp256k1_ecdh_hash_function_default` should always return 1.
// and the scalar was verified to be valid(0 > scalar > group_order) via the type system
debug_assert_eq!(res, 1);
ss.set_len(32); // The default hash function is SHA256, which is 32 bytes long.
ss
}

fn new_with_callback_internal<F>(point: &PublicKey, scalar: &SecretKey, mut closure: F, callback: ffi::EcdhHashFn) -> Result<SharedSecret, Error>
where F: FnMut([u8; 32], [u8; 32]) -> SharedSecret {
let mut ss = SharedSecret::empty();

let res = unsafe {
ffi::secp256k1_ecdh(
ffi::secp256k1_context_no_precomp,
ss.get_data_mut_ptr(),
point.as_ptr(),
scalar.as_ptr(),
callback,
&mut closure as *mut F as *mut c_void,
)
};
if res == -1 {
return Err(Error::CallbackPanicked);
}
debug_assert!(res >= 16); // 128 bit is the minimum for a secure hash function and the minimum we let users.
ss.set_len(res as usize);
Ok(ss)

}

/// Creates a new shared secret from a pubkey and secret key with applied custom hash function
/// The custom hash function must be in the form of `fn(x: [u8;32], y: [u8;32]) -> SharedSecret`
/// `SharedSecret` can be easily created via the `From` impl from arrays.
/// # Examples
/// ```
/// # use secp256k1::ecdh::SharedSecret;
Expand All @@ -182,43 +136,29 @@ impl SharedSecret {
/// });
///
/// ```
#[cfg(feature = "std")]
pub fn new_with_hash<F>(point: &PublicKey, scalar: &SecretKey, hash_function: F) -> Result<SharedSecret, Error>
pub fn new_with_hash<F>(point: &PublicKey, scalar: &SecretKey, mut hash_function: F) -> SharedSecret
where F: FnMut([u8; 32], [u8; 32]) -> SharedSecret {
Self::new_with_callback_internal(point, scalar, hash_function, hash_callback_catch_unwind::<F>)
}
let mut xy = [0u8; 64];

/// Creates a new shared secret from a pubkey and secret key with applied custom hash function
/// Note that this function is the same as [`new_with_hash`]
///
/// # Safety
/// The function doesn't wrap the callback with [`catch_unwind`]
/// so if the callback panics it will panic through an FFI boundray which is [`Undefined Behavior`]
/// If possible you should use [`new_with_hash`] which does wrap the callback with [`catch_unwind`] so is safe to use.
///
/// [`catch_unwind`]: https://doc.rust-lang.org/std/panic/fn.catch_unwind.html
/// [`Undefined Behavior`]: https://doc.rust-lang.org/nomicon/ffi.html#ffi-and-panics
/// [`new_with_hash`]: #method.new_with_hash
/// # Examples
/// ```
/// # use secp256k1::ecdh::SharedSecret;
/// # use secp256k1::{Secp256k1, PublicKey, SecretKey};
/// # fn sha2(_a: &[u8], _b: &[u8]) -> [u8; 32] {[0u8; 32]}
/// # let secp = Secp256k1::signing_only();
/// # let secret_key = SecretKey::from_slice(&[3u8; 32]).unwrap();
/// # let secret_key2 = SecretKey::from_slice(&[7u8; 32]).unwrap();
/// # let public_key = PublicKey::from_secret_key(&secp, &secret_key2);
//
/// let secret = unsafe { SharedSecret::new_with_hash_no_panic(&public_key, &secret_key, |x,y| {
/// let hash: [u8; 32] = sha2(&x,&y);
/// hash.into()
/// })};
///
///
/// ```
pub unsafe fn new_with_hash_no_panic<F>(point: &PublicKey, scalar: &SecretKey, hash_function: F) -> Result<SharedSecret, Error>
where F: FnMut([u8; 32], [u8; 32]) -> SharedSecret {
Self::new_with_callback_internal(point, scalar, hash_function, hash_callback_unsafe::<F>)
let res = unsafe {
ffi::secp256k1_ecdh(
ffi::secp256k1_context_no_precomp,
xy.as_mut_ptr(),
point.as_ptr(),
scalar.as_ptr(),
c_callback,
ptr::null_mut(),
)
};
// Our callback *always* returns 1.
// and the scalar was verified to be valid(0 > scalar > group_order) via the type system
debug_assert_eq!(res, 1);

let mut x = [0u8; 32];
let mut y = [0u8; 32];
x.copy_from_slice(&xy[..32]);
y.copy_from_slice(&xy[32..]);
hash_function(x, y)
}
}

Expand Down Expand Up @@ -248,9 +188,9 @@ mod tests {
let (sk1, pk1) = s.generate_keypair(&mut thread_rng());
let (sk2, pk2) = s.generate_keypair(&mut thread_rng());

let sec1 = SharedSecret::new_with_hash(&pk1, &sk2, |x,_| x.into()).unwrap();
let sec2 = SharedSecret::new_with_hash(&pk2, &sk1, |x,_| x.into()).unwrap();
let sec_odd = SharedSecret::new_with_hash(&pk1, &sk1, |x,_| x.into()).unwrap();
let sec1 = SharedSecret::new_with_hash(&pk1, &sk2, |x,_| x.into());
let sec2 = SharedSecret::new_with_hash(&pk2, &sk1, |x,_| x.into());
let sec_odd = SharedSecret::new_with_hash(&pk1, &sk1, |x,_| x.into());
assert_eq!(sec1, sec2);
assert_ne!(sec_odd, sec2);
}
Expand All @@ -262,32 +202,29 @@ mod tests {
let expect_result: [u8; 64] = [123; 64];
let mut x_out = [0u8; 32];
let mut y_out = [0u8; 32];
let result = SharedSecret::new_with_hash(&pk1, &sk1, | x, y | {
let result = SharedSecret::new_with_hash(&pk1, &sk1, |x, y| {
x_out = x;
y_out = y;
expect_result.into()
}).unwrap();
let result_unsafe = unsafe {SharedSecret::new_with_hash_no_panic(&pk1, &sk1, | x, y | {
x_out = x;
y_out = y;
expect_result.into()
}).unwrap()};
});
assert_eq!(&expect_result[..], &result[..]);
assert_eq!(result, result_unsafe);
assert_ne!(x_out, [0u8; 32]);
assert_ne!(y_out, [0u8; 32]);
}

#[test]
fn ecdh_with_hash_callback_panic() {
let s = Secp256k1::signing_only();
let (sk1, pk1) = s.generate_keypair(&mut thread_rng());
let mut res = [0u8; 48];
let result = SharedSecret::new_with_hash(&pk1, &sk1, | x, _ | {
res.copy_from_slice(&x); // res.len() != x.len(). this will panic.
res.into()
});
assert_eq!(result, Err(Error::CallbackPanicked));
fn test_c_callback() {
let x = [5u8; 32];
let y = [7u8; 32];
let mut output = [0u8; 64];
let res = unsafe { super::c_callback(output.as_mut_ptr(), x.as_ptr(), y.as_ptr(), ::ptr::null_mut()) };
assert_eq!(res, 1);
let mut new_x = [0u8; 32];
let mut new_y = [0u8; 32];
new_x.copy_from_slice(&output[..32]);
new_y.copy_from_slice(&output[32..]);
assert_eq!(x, new_x);
assert_eq!(y, new_y);
}
}

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3 changes: 0 additions & 3 deletions src/lib.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -496,8 +496,6 @@ pub enum Error {
InvalidTweak,
/// Didn't pass enough memory to context creation with preallocated memory
NotEnoughMemory,
/// The callback has panicked.
CallbackPanicked,
}

impl Error {
Expand All @@ -511,7 +509,6 @@ impl Error {
Error::InvalidRecoveryId => "secp: bad recovery id",
Error::InvalidTweak => "secp: bad tweak",
Error::NotEnoughMemory => "secp: not enough memory allocated",
Error::CallbackPanicked => "secp: a callback passed has panicked",
}
}
}
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