Add support for hash chaining to detect modifications in postings

The following details of a posting contribute to its hash:

  fullname of account
  string representation of amount

Each posting hashes contributes to the transaction hash, which is compromised
of:

  previous transaction’s hash (as encountered in parsing order)
  actual date
  optional auxiliary date
  optional code
  payee
  hashes of all postings

Note that this means that changes in the “code” or any of the comments

1 files changed, 24 insertions, 0 deletions

diff --git a/src/post.cc b/src/post.cc
index e2063a36..9d799a9e 100644
--- a/src/post.cc
+++ b/src/post.cc
@@ -671,6 +671,30 @@ void post_t::set_reported_account(account_t * acct)
   acct->xdata().reported_posts.push_back(this);
 }
 
+extern "C" unsigned char *SHA512(
+  void *data, unsigned int data_len, unsigned char *digest);
+
+namespace {
+  std::string bufferToHex(const unsigned char* buffer, std::size_t size) {
+      std::ostringstream oss;
+      oss << std::hex << std::setfill('0');
+      for(std::size_t i = 0; i < size; ++i)
+          oss << std::setw(2) << static_cast<int>(buffer[i]);
+      return oss.str();
+  }
+}
+
+string post_t::hash(string nonce) const {
+  unsigned char data[128];
+  std::ostringstream repr;
+  repr << nonce;
+  repr << account->fullname();
+  repr << amount.to_string();
+  string repr_str(repr.str());
+  SHA512((void *)repr_str.c_str(), repr_str.length(), data);
+  return bufferToHex(data, 64 /*SHA512_DIGEST_LENGTH*/);
+}
+
 void extend_post(post_t& post, journal_t& journal)
 {
   commodity_t& comm(post.amount.commodity());