1 // Copyright (c) 2015-2016 The btcsuite developers
2 // Use of this source code is governed by an ISC
3 // license that can be found in the LICENSE file.
10 "github.com/btcsuite/btcd/btcec"
11 "github.com/btcsuite/btcd/chaincfg/chainhash"
14 // sigCacheEntry represents an entry in the SigCache. Entries within the
15 // SigCache are keyed according to the sigHash of the signature. In the
16 // scenario of a cache-hit (according to the sigHash), an additional comparison
17 // of the signature, and public key will be executed in order to ensure a complete
18 // match. In the occasion that two sigHashes collide, the newer sigHash will
19 // simply overwrite the existing entry.
20 type sigCacheEntry struct {
22 pubKey *btcec.PublicKey
25 // SigCache implements an ECDSA signature verification cache with a randomized
26 // entry eviction policy. Only valid signatures will be added to the cache. The
27 // benefits of SigCache are two fold. Firstly, usage of SigCache mitigates a DoS
28 // attack wherein an attack causes a victim's client to hang due to worst-case
29 // behavior triggered while processing attacker crafted invalid transactions. A
30 // detailed description of the mitigated DoS attack can be found here:
31 // https://bitslog.wordpress.com/2013/01/23/fixed-bitcoin-vulnerability-explanation-why-the-signature-cache-is-a-dos-protection/.
32 // Secondly, usage of the SigCache introduces a signature verification
33 // optimization which speeds up the validation of transactions within a block,
34 // if they've already been seen and verified within the mempool.
35 type SigCache struct {
37 validSigs map[chainhash.Hash]sigCacheEntry
41 // NewSigCache creates and initializes a new instance of SigCache. Its sole
42 // parameter 'maxEntries' represents the maximum number of entries allowed to
43 // exist in the SigCache at any particular moment. Random entries are evicted
44 // to make room for new entries that would cause the number of entries in the
45 // cache to exceed the max.
46 func NewSigCache(maxEntries uint) *SigCache {
48 validSigs: make(map[chainhash.Hash]sigCacheEntry, maxEntries),
49 maxEntries: maxEntries,
53 // Exists returns true if an existing entry of 'sig' over 'sigHash' for public
54 // key 'pubKey' is found within the SigCache. Otherwise, false is returned.
56 // NOTE: This function is safe for concurrent access. Readers won't be blocked
57 // unless there exists a writer, adding an entry to the SigCache.
58 func (s *SigCache) Exists(sigHash chainhash.Hash, sig *btcec.Signature, pubKey *btcec.PublicKey) bool {
60 entry, ok := s.validSigs[sigHash]
63 return ok && entry.pubKey.IsEqual(pubKey) && entry.sig.IsEqual(sig)
66 // Add adds an entry for a signature over 'sigHash' under public key 'pubKey'
67 // to the signature cache. In the event that the SigCache is 'full', an
68 // existing entry is randomly chosen to be evicted in order to make space for
71 // NOTE: This function is safe for concurrent access. Writers will block
72 // simultaneous readers until function execution has concluded.
73 func (s *SigCache) Add(sigHash chainhash.Hash, sig *btcec.Signature, pubKey *btcec.PublicKey) {
77 if s.maxEntries <= 0 {
81 // If adding this new entry will put us over the max number of allowed
82 // entries, then evict an entry.
83 if uint(len(s.validSigs)+1) > s.maxEntries {
84 // Remove a random entry from the map. Relying on the random
85 // starting point of Go's map iteration. It's worth noting that
86 // the random iteration starting point is not 100% guaranteed
87 // by the spec, however most Go compilers support it.
88 // Ultimately, the iteration order isn't important here because
89 // in order to manipulate which items are evicted, an adversary
90 // would need to be able to execute preimage attacks on the
91 // hashing function in order to start eviction at a specific
93 for sigEntry := range s.validSigs {
94 delete(s.validSigs, sigEntry)
98 s.validSigs[sigHash] = sigCacheEntry{sig, pubKey}