Support multi-block SHA256 transforms

Extracted from a patch by Wladimir van der Laan.

src/crypto/sha256.cpp

@@ -43,9 +43,10 @@ void inline Initialize(uint32_t* s)
     s[7] = 0x5be0cd19ul;
 }
 
-/** Perform one SHA-256 transformation, processing a 64-byte chunk. */
-void Transform(uint32_t* s, const unsigned char* chunk)
+/** Perform a number of SHA-256 transformations, processing 64-byte chunks. */
+void Transform(uint32_t* s, const unsigned char* chunk, size_t blocks)
 {
+    while (blocks--) {
         uint32_t a = s[0], b = s[1], c = s[2], d = s[3], e = s[4], f = s[5], g = s[6], h = s[7];
         uint32_t w0, w1, w2, w3, w4, w5, w6, w7, w8, w9, w10, w11, w12, w13, w14, w15;
 
@@ -125,6 +126,8 @@ void Transform(uint32_t* s, const unsigned char* chunk)
         s[5] += f;
         s[6] += g;
         s[7] += h;
+        chunk += 64;
+    }
 }
 
 } // namespace sha256
@@ -147,14 +150,14 @@ CSHA256& CSHA256::Write(const unsigned char* data, size_t len)
         memcpy(buf + bufsize, data, 64 - bufsize);
         bytes += 64 - bufsize;
         data += 64 - bufsize;
-        sha256::Transform(s, buf);
+        sha256::Transform(s, buf, 1);
         bufsize = 0;
     }
-    while (end >= data + 64) {
-        // Process full chunks directly from the source.
-        sha256::Transform(s, data);
-        bytes += 64;
-        data += 64;
+    if (end - data >= 64) {
+        size_t blocks = (end - data) / 64;
+        sha256::Transform(s, data, blocks);
+        data += 64 * blocks;
+        bytes += 64 * blocks;
     }
     if (end > data) {
         // Fill the buffer with what remains.