1
  2
  3
  4
  5
  6
  7
  8
  9
 10
 11
 12
 13
 14
 15
 16
 17
 18
 19
 20
 21
 22
 23
 24
 25
 26
 27
 28
 29
 30
 31
 32
 33
 34
 35
 36
 37
 38
 39
 40
 41
 42
 43
 44
 45
 46
 47
 48
 49
 50
 51
 52
 53
 54
 55
 56
 57
 58
 59
 60
 61
 62
 63
 64
 65
 66
 67
 68
 69
 70
 71
 72
 73
 74
 75
 76
 77
 78
 79
 80
 81
 82
 83
 84
 85
 86
 87
 88
 89
 90
 91
 92
 93
 94
 95
 96
 97
 98
 99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146


use crypto::digest::Digest;

use tree::Tree;
use merkledigest::MerkleDigest;

/// An inclusion proof represent the fact that a `value` is a member
/// of a `MerkleTree` with root hash `root_hash`, and hash function `digest`.
#[derive(Debug)]
pub struct Proof<D, T> {

    /// The hash function used in the original `MerkleTree`
    pub digest: D,

    /// The hash of the root of the original `MerkleTree`
    pub root_hash: Vec<u8>,

    /// The first `Lemma` of the `Proof`
    pub lemma: Lemma,

    /// The value concerned by this `Proof`
    pub value: T
}

impl <D, T> Proof<D, T> {

    /// Constructs a new `Proof`
    pub fn new(digest: D, root_hash: Vec<u8>, lemma: Lemma, value: T) -> Self {
        Proof {
            digest: digest,
            root_hash: root_hash,
            lemma: lemma,
            value: value
        }
    }

    /// Checks whether this inclusion proof is well-formed,
    /// and whether its root hash matches the given `root_hash`.
    pub fn validate(&self, root_hash: &[u8]) -> bool where D: Digest + Clone {
        if self.root_hash != root_hash || self.lemma.node_hash != root_hash {
            return false
        }

        self.validate_lemma(&self.lemma, &mut self.digest.clone())
    }

    fn validate_lemma(&self, lemma: &Lemma, digest: &mut D) -> bool where D: Digest {
        match lemma.sub_lemma {

            None =>
                lemma.sibling_hash.is_none(),

            Some(ref sub) =>
                match lemma.sibling_hash {
                    None =>
                        false,

                    Some(Positioned::Left(ref hash)) => {
                        let hashes_match = digest.combine_hashes(hash, &sub.node_hash) == lemma.node_hash;
                        hashes_match && self.validate_lemma(sub, digest)
                    }

                    Some(Positioned::Right(ref hash)) => {
                        let hashes_match = digest.combine_hashes(&sub.node_hash, hash) == lemma.node_hash;
                        hashes_match && self.validate_lemma(sub, digest)
                    }

                }
        }
    }

}


/// A `Lemma` holds the hash of a node, the hash of its sibling node,
/// and a sub lemma, whose `node_hash`, when combined with this `sibling_hash`
/// must be equal to this `node_hash`.
#[derive(Debug)]
pub struct Lemma {
    pub node_hash: Vec<u8>,
    pub sibling_hash: Option<Positioned<Vec<u8>>>,
    pub sub_lemma: Option<Box<Lemma>>
}

impl Lemma {

    /// Attempts to generate a proof that the a value with hash `needle` is a member of the given `tree`.
    pub fn new<T>(tree: &Tree<T>, needle: &[u8]) -> Option<Lemma> where T: Into<Vec<u8>> + Clone {
        match *tree {
            Tree::Leaf { ref hash, .. } =>
                Lemma::new_leaf_proof(hash, needle),

            Tree::Node { ref hash, ref left, ref right } =>
                Lemma::new_tree_proof(hash, needle, left, right)
        }
    }

    fn new_leaf_proof(hash: &[u8], needle: &[u8]) -> Option<Lemma> {
        if *hash == *needle {
            Some(Lemma {
                node_hash: hash.into(),
                sibling_hash: None,
                sub_lemma: None
            })
        } else {
            None
        }
    }

    fn new_tree_proof<T>(hash: &[u8], needle: &[u8], left: &Tree<T>, right: &Tree<T>) -> Option<Lemma> where T: Into<Vec<u8>> + Clone {
        Lemma::new(left, needle)
            .map(|lemma| {
                let right_hash = right.hash().clone();
                let sub_lemma = Some(Positioned::Right(right_hash));
                (lemma, sub_lemma)
            })
            .or_else(|| {
                let sub_lemma = Lemma::new(right, needle);
                sub_lemma.map(|lemma| {
                    let left_hash = left.hash().clone();
                    let sub_lemma = Some(Positioned::Left(left_hash));
                    (lemma, sub_lemma)
                })
            })
            .map(|(sub_lemma, sibling_hash)| {
                Lemma {
                    node_hash: hash.into(),
                    sibling_hash: sibling_hash,
                    sub_lemma: Some(Box::new(sub_lemma))
                }
            })
    }

}

/// Tags a value so that we know from which branch of a `Tree` (if any) it was found.
#[derive(Debug, PartialEq)]
pub enum Positioned<T> {

    /// The value was found in the left branch
    Left(T),

    /// The value was found in the right branch
    Right(T)
}