Merge bitcoin#19931: Change CSipHasher's count variable to uint8_t

812037c Change CSipHasher's count variable to uint8_t (Pieter Wuille)

Pull request description:

  SipHash technically supports arbitrarily long inputs (at least, I couldn't find a limit in the [paper](https://eprint.iacr.org/2012/351.pdf)), but only the low 8 bits of the length matter. Because of that we should use an unsigned type to track the length (as any signed type could overflow, which is UB). `uint8_t` is sufficient, however.

  Fixes bitcoin#19930.

ACKs for top commit:
  laanwj:
    anyhow re-ACK 812037c
  elichai:
    utACK 812037c
  practicalswift:
    ACK 812037c
  theStack:
    ACK 812037c

Tree-SHA512: 5b1440c9e4591460da198991fb421ad47d2d96def2014e761726ce361aa9575752f2c4085656e7e9badee3660ff005cc76fbd1afe4848faefe4502f3412bd896

Author: fanquake

Diff for: src/crypto/siphash.cpp

@@ -49,7 +49,7 @@ CSipHasher& CSipHasher::Write(const unsigned char* data, size_t size)
 {
     uint64_t v0 = v[0], v1 = v[1], v2 = v[2], v3 = v[3];
     uint64_t t = tmp;
-    int c = count;
+    uint8_t c = count;
 
     while (size--) {
         t |= ((uint64_t)(*(data++))) << (8 * (c % 8));

Diff for: src/crypto/siphash.h

@@ -15,7 +15,7 @@ class CSipHasher
 private:
     uint64_t v[4];
     uint64_t tmp;
-    int count;
+    uint8_t count; // Only the low 8 bits of the input size matter.
 
 public:
     /** Construct a SipHash calculator initialized with 128-bit key (k0, k1) */