monero_mlsag/
lib.rs

1#![cfg_attr(docsrs, feature(doc_auto_cfg))]
2#![doc = include_str!("../README.md")]
3#![deny(missing_docs)]
4#![cfg_attr(not(feature = "std"), no_std)]
5#![allow(non_snake_case)]
6
7use std_shims::{
8  vec,
9  vec::Vec,
10  io::{self, Read, Write},
11};
12
13use zeroize::Zeroize;
14
15use curve25519_dalek::{traits::IsIdentity, Scalar, EdwardsPoint};
16
17use monero_io::*;
18use monero_generators::{H, hash_to_point};
19use monero_primitives::keccak256_to_scalar;
20
21/// Errors when working with MLSAGs.
22#[derive(Clone, Copy, PartialEq, Eq, Debug, thiserror::Error)]
23pub enum MlsagError {
24  /// Invalid ring (such as too small or too large).
25  #[error("invalid ring")]
26  InvalidRing,
27  /// Invalid amount of key images.
28  #[error("invalid amount of key images")]
29  InvalidAmountOfKeyImages,
30  /// Invalid ss matrix.
31  #[error("invalid ss")]
32  InvalidSs,
33  /// Invalid key image.
34  #[error("invalid key image")]
35  InvalidKeyImage,
36  /// Invalid ci vector.
37  #[error("invalid ci")]
38  InvalidCi,
39}
40
41/// A vector of rings, forming a matrix, to verify the MLSAG with.
42#[derive(Clone, PartialEq, Eq, Debug, Zeroize)]
43pub struct RingMatrix {
44  matrix: Vec<Vec<EdwardsPoint>>,
45}
46
47impl RingMatrix {
48  /// Construct a ring matrix from an already formatted series of points.
49  fn new(matrix: Vec<Vec<EdwardsPoint>>) -> Result<Self, MlsagError> {
50    // Monero requires that there is more than one ring member for MLSAG signatures:
51    // https://github.com/monero-project/monero/blob/ac02af92867590ca80b2779a7bbeafa99ff94dcb/
52    // src/ringct/rctSigs.cpp#L462
53    if matrix.len() < 2 {
54      Err(MlsagError::InvalidRing)?;
55    }
56    for member in &matrix {
57      if member.is_empty() || (member.len() != matrix[0].len()) {
58        Err(MlsagError::InvalidRing)?;
59      }
60    }
61
62    Ok(RingMatrix { matrix })
63  }
64
65  /// Construct a ring matrix for an individual output.
66  pub fn individual(
67    ring: &[[EdwardsPoint; 2]],
68    pseudo_out: EdwardsPoint,
69  ) -> Result<Self, MlsagError> {
70    let mut matrix = Vec::with_capacity(ring.len());
71    for ring_member in ring {
72      matrix.push(vec![ring_member[0], ring_member[1] - pseudo_out]);
73    }
74    RingMatrix::new(matrix)
75  }
76
77  /// Iterate over the members of the matrix.
78  fn iter(&self) -> impl Iterator<Item = &[EdwardsPoint]> {
79    self.matrix.iter().map(AsRef::as_ref)
80  }
81
82  /// Get the amount of members in the ring.
83  pub fn members(&self) -> usize {
84    self.matrix.len()
85  }
86
87  /// Get the length of a ring member.
88  ///
89  /// A ring member is a vector of points for which the signer knows all of the discrete logarithms
90  /// of.
91  pub fn member_len(&self) -> usize {
92    // this is safe to do as the constructors don't allow empty rings
93    self.matrix[0].len()
94  }
95}
96
97/// The MLSAG linkable ring signature, as used in Monero.
98#[derive(Clone, PartialEq, Eq, Debug, Zeroize)]
99pub struct Mlsag {
100  ss: Vec<Vec<Scalar>>,
101  cc: Scalar,
102}
103
104impl Mlsag {
105  /// Write a MLSAG.
106  pub fn write<W: Write>(&self, w: &mut W) -> io::Result<()> {
107    for ss in &self.ss {
108      write_raw_vec(write_scalar, ss, w)?;
109    }
110    write_scalar(&self.cc, w)
111  }
112
113  /// Read a MLSAG.
114  pub fn read<R: Read>(mixins: usize, ss_2_elements: usize, r: &mut R) -> io::Result<Mlsag> {
115    Ok(Mlsag {
116      ss: (0 .. mixins)
117        .map(|_| read_raw_vec(read_scalar, ss_2_elements, r))
118        .collect::<Result<_, _>>()?,
119      cc: read_scalar(r)?,
120    })
121  }
122
123  /// Verify a MLSAG.
124  ///
125  /// WARNING: This follows the Fiat-Shamir transcript format used by the Monero protocol, which
126  /// makes assumptions on what has already been transcripted and bound to within `msg`. Do not use
127  /// this if you don't know what you're doing.
128  pub fn verify(
129    &self,
130    msg: &[u8; 32],
131    ring: &RingMatrix,
132    key_images: &[EdwardsPoint],
133  ) -> Result<(), MlsagError> {
134    // Mlsag allows for layers to not need linkability, hence they don't need key images
135    // Monero requires that there is always only 1 non-linkable layer - the amount commitments.
136    if ring.member_len() != (key_images.len() + 1) {
137      Err(MlsagError::InvalidAmountOfKeyImages)?;
138    }
139
140    let mut buf = Vec::with_capacity(6 * 32);
141    buf.extend_from_slice(msg);
142
143    let mut ci = self.cc;
144
145    // This is an iterator over the key images as options with an added entry of `None` at the
146    // end for the non-linkable layer
147    let key_images_iter = key_images.iter().map(|ki| Some(*ki)).chain(core::iter::once(None));
148
149    if ring.matrix.len() != self.ss.len() {
150      Err(MlsagError::InvalidSs)?;
151    }
152
153    for (ring_member, ss) in ring.iter().zip(&self.ss) {
154      if ring_member.len() != ss.len() {
155        Err(MlsagError::InvalidSs)?;
156      }
157
158      for ((ring_member_entry, s), ki) in ring_member.iter().zip(ss).zip(key_images_iter.clone()) {
159        #[allow(non_snake_case)]
160        let L = EdwardsPoint::vartime_double_scalar_mul_basepoint(&ci, ring_member_entry, s);
161
162        let compressed_ring_member_entry = ring_member_entry.compress();
163        buf.extend_from_slice(compressed_ring_member_entry.as_bytes());
164        buf.extend_from_slice(L.compress().as_bytes());
165
166        // Not all dimensions need to be linkable, e.g. commitments, and only linkable layers need
167        // to have key images.
168        if let Some(ki) = ki {
169          if ki.is_identity() || (!ki.is_torsion_free()) {
170            Err(MlsagError::InvalidKeyImage)?;
171          }
172
173          #[allow(non_snake_case)]
174          let R = (s * hash_to_point(compressed_ring_member_entry.to_bytes())) + (ci * ki);
175          buf.extend_from_slice(R.compress().as_bytes());
176        }
177      }
178
179      ci = keccak256_to_scalar(&buf);
180      // keep the msg in the buffer.
181      buf.drain(msg.len() ..);
182    }
183
184    if ci != self.cc {
185      Err(MlsagError::InvalidCi)?
186    }
187    Ok(())
188  }
189}
190
191/// Builder for a RingMatrix when using an aggregate signature.
192///
193/// This handles the formatting as necessary.
194#[derive(Clone, PartialEq, Eq, Debug, Zeroize)]
195pub struct AggregateRingMatrixBuilder {
196  key_ring: Vec<Vec<EdwardsPoint>>,
197  amounts_ring: Vec<EdwardsPoint>,
198  sum_out: EdwardsPoint,
199}
200
201impl AggregateRingMatrixBuilder {
202  /// Create a new AggregateRingMatrixBuilder.
203  ///
204  /// This takes in the transaction's outputs' commitments and fee used.
205  pub fn new(commitments: &[EdwardsPoint], fee: u64) -> Self {
206    AggregateRingMatrixBuilder {
207      key_ring: vec![],
208      amounts_ring: vec![],
209      sum_out: commitments.iter().sum::<EdwardsPoint>() + (*H * Scalar::from(fee)),
210    }
211  }
212
213  /// Push a ring of [output key, commitment] to the matrix.
214  pub fn push_ring(&mut self, ring: &[[EdwardsPoint; 2]]) -> Result<(), MlsagError> {
215    if self.key_ring.is_empty() {
216      self.key_ring = vec![vec![]; ring.len()];
217      // Now that we know the length of the ring, fill the `amounts_ring`.
218      self.amounts_ring = vec![-self.sum_out; ring.len()];
219    }
220
221    if (self.amounts_ring.len() != ring.len()) || ring.is_empty() {
222      // All the rings in an aggregate matrix must be the same length.
223      return Err(MlsagError::InvalidRing);
224    }
225
226    for (i, ring_member) in ring.iter().enumerate() {
227      self.key_ring[i].push(ring_member[0]);
228      self.amounts_ring[i] += ring_member[1]
229    }
230
231    Ok(())
232  }
233
234  /// Build and return the [`RingMatrix`].
235  pub fn build(mut self) -> Result<RingMatrix, MlsagError> {
236    for (i, amount_commitment) in self.amounts_ring.drain(..).enumerate() {
237      self.key_ring[i].push(amount_commitment);
238    }
239    RingMatrix::new(self.key_ring)
240  }
241}
monero_mlsag/lib.rs

monero_mlsag/
lib.rs
