* Shorten SHA-256 hashes used in store path name generation to 160

  bits, then encode them in a radix-32 representation (using digits
  and letters except e, o, u, and t).  This produces store paths like
  /nix/store/4i0zb0z7f88mwghjirkz702a71dcfivn-aterm-2.3.1.  The nice
  thing about this is that the hash part of the file name is still 32
  characters, as before with MD5.

  (Of course, shortening SHA-256 to 160 bits makes it no better than
  SHA-160 in theory, but hopefully it's a bit more resistant to
  attacks; it's certainly a lot slower.)

1 files changed, 19 insertions, 5 deletions

diff --git a/src/libutil/hash.hh b/src/libutil/hash.hh
index 4490d2ff7a..0c9d7b9cb9 100644
--- a/src/libutil/hash.hh
+++ b/src/libutil/hash.hh
@@ -8,7 +8,7 @@
 using namespace std;
 
 
-typedef enum { htMD5, htSHA1, htSHA256 } HashType;
+typedef enum { htUnknown, htMD5, htSHA1, htSHA256 } HashType;
 
 
 const int md5HashSize = 16;
@@ -24,7 +24,10 @@ struct Hash
 
     HashType type;
 
-    /* Create a zeroed hash object. */
+    /* Create an unusable hash object. */
+    Hash();
+
+    /* Create a zero-filled hash object. */
     Hash(HashType type);
 
     /* Check whether two hash are equal. */
@@ -36,13 +39,20 @@ struct Hash
     /* For sorting. */
     bool operator < (const Hash & h) const;
 
-    /* Convert a hash code into a hexadecimal representation. */
-    operator string() const;
 };
 
 
+/* Convert a hash to a hexadecimal representation. */
+string printHash(const Hash & hash);
+
 /* Parse a hexadecimal representation of a hash code. */
-Hash parseHash(const string & s);
+Hash parseHash(HashType ht, const string & s);
+
+/* Convert a hash to a base-32 representation. */
+string printHash32(const Hash & hash);
+
+/* Parse a base-32 representation of a hash code. */
+Hash parseHash32(HashType ht, const string & s);
 
 /* Verify that the given string is a valid hash code. */
 bool isHash(const string & s);
@@ -57,5 +67,9 @@ Hash hashFile(const Path & path, HashType ht);
    md5(dump(path)). */
 Hash hashPath(const Path & path, HashType ht);
 
+/* Compress a hash to the specified number of bytes by cyclically
+   XORing bytes together. */
+Hash compressHash(const Hash & hash, unsigned int newSize);
+
 
 #endif /* !__HASH_H */