* Removed the `id' attribute hack.

* Formalise the notion of fixed-output derivations, i.e., derivations
  for which a cryptographic hash of the output is known in advance.
  Changes to such derivations should not propagate upwards through the
  dependency graph.  Previously this was done by specifying the hash
  component of the output path through the `id' attribute, but this is
  insecure since you can lie about it (i.e., you can specify any hash
  and then produce a completely different output).  Now the
  responsibility for checking the output is moved from the builder to
  Nix itself.

  A fixed-output derivation can be created by specifying the
  `outputHash' and `outputHashAlgo' attributes, the latter taking
  values `md5', `sha1', and `sha256', and the former specifying the
  actual hash in hexadecimal or in base-32 (auto-detected by looking
  at the length of the attribute value).  MD5 is included for
  compatibility but should be considered deprecated.

* Removed the `drvPath' pseudo-attribute in derivation results.  It's
  no longer necessary.

* Cleaned up the support for multiple output paths in derivation store
  expressions.  Each output now has a unique identifier (e.g., `out',
  `devel', `docs').  Previously there was no way to tell output paths
  apart at the store expression level.

* `nix-hash' now has a flag `--base32' to specify that the hash should
  be printed in base-32 notation.

* `fetchurl' accepts parameters `sha256' and `sha1' in addition to
  `md5'.

* `nix-prefetch-url' now prints out a SHA-1 hash in base-32.  (TODO: a
  flag to specify the hash.)

17 files changed, 292 insertions, 101 deletions

diff --git a/corepkgs/fetchurl/builder.sh.in b/corepkgs/fetchurl/builder.sh.in
index c1a735d68fef..aaba65d6d26c 100644
--- a/corepkgs/fetchurl/builder.sh.in
+++ b/corepkgs/fetchurl/builder.sh.in
@@ -4,16 +4,10 @@ export PATH=/bin:/usr/bin
 
 echo "downloading $url into $out"
 
-prefetch=@storedir@/nix-prefetch-url-$md5
+prefetch=@storedir@/nix-prefetch-url-$outputHash
 if test -f "$prefetch"; then
     echo "using prefetched $prefetch";
     mv $prefetch $out
 else
     @curl@ --fail --location --max-redirs 20 "$url" > "$out"
 fi
-
-actual=$(@bindir@/nix-hash --flat $out)
-if test "$actual" != "$md5"; then
-    echo "hash is $actual, expected $md5"
-    exit 1
-fi
diff --git a/corepkgs/fetchurl/default.nix b/corepkgs/fetchurl/default.nix
index 8957662eccab..37f01b55eeb0 100644
--- a/corepkgs/fetchurl/default.nix
+++ b/corepkgs/fetchurl/default.nix
@@ -1,8 +1,23 @@
-{system, url, md5}:
+# Argh, this thing is duplicated (more-or-less) in Nixpkgs.  Need to
+# find a way to combine them.
+
+{system, url, outputHash ? "", outputHashAlgo ? "", md5 ? "", sha1 ? "", sha256 ? ""}:
+
+assert (outputHash != "" && outputHashAlgo != "")
+    || md5 != "" || sha1 != "" || sha256 != "";
 
 derivation {
   name = baseNameOf (toString url);
   builder = ./builder.sh;
+
+  # Compatibility with Nix <= 0.7.
   id = md5;
-  inherit system url md5;
+
+  # New-style output content requirements.
+  outputHashAlgo = if outputHashAlgo != "" then outputHashAlgo else
+      if sha256 != "" then "sha256" else if sha1 != "" then "sha1" else "md5";
+  outputHash = if outputHash != "" then outputHash else
+      if sha256 != "" then sha256 else if sha1 != "" then sha1 else md5;
+  
+  inherit system url;
 }
diff --git a/scripts/nix-prefetch-url.in b/scripts/nix-prefetch-url.in
index 9a67ca1417f2..c4731f3f627b 100644
--- a/scripts/nix-prefetch-url.in
+++ b/scripts/nix-prefetch-url.in
@@ -3,6 +3,8 @@
 url=$1
 hash=$2
 
+hashType="sha1"
+
 if test -z "$url"; then
     echo "syntax: nix-prefetch-url URL" >&2
     exit 1
@@ -27,7 +29,7 @@ if test -z "$hash"; then
     @curl@ --fail --location --max-redirs 20 "$url" > $tmpPath1
 
     # Compute the hash.
-    hash=$(@bindir@/nix-hash --flat $tmpPath1)
+    hash=$(@bindir@/nix-hash --base32 --type "$hashType" --flat $tmpPath1)
     if ! test -n "$QUIET"; then echo "hash is $hash" >&2; fi
 
     # Rename it so that the fetchurl builder can find it.
@@ -41,9 +43,11 @@ fi
 # Create a Nix expression that does a fetchurl.
 storeExpr=$( \
     echo "(import @datadir@/nix/corepkgs/fetchurl) \
-        {url = $url; md5 = \"$hash\"; system = \"@system@\";}" \
+        {url = $url; outputHashAlgo = \"$hashType\"; outputHash = \"$hash\"; system = \"@system@\";}" \
     | @bindir@/nix-instantiate -)
 
+echo "$storeExpr"    
+
 # Realise it.
 finalPath=$(@bindir@/nix-store -qnB --force-realise $storeExpr)
     
diff --git a/src/aterm-helper.pl b/src/aterm-helper.pl
index 376691c9a210..5067b0a9be8d 100755
--- a/src/aterm-helper.pl
+++ b/src/aterm-helper.pl
@@ -56,6 +56,7 @@ while (<STDIN>) {
         my $unpack = "";
         my $n = 1;
         foreach my $type (@types) {
+            my $realType = $type;
             $args .= ", ";
             if ($type eq "string") {
 #                $args .= "(ATerm) ATmakeAppl0(ATmakeAFun((char *) e$n, 0, ATtrue))";
@@ -83,6 +84,9 @@ while (<STDIN>) {
                 $unpack .= "    e$n = (ATermList) ATgetArgument(e, $m);\n";
             } elsif ($type eq "ATermBlob") {
                 $unpack .= "    e$n = (ATermBlob) ATgetArgument(e, $m);\n";
+            } elsif ($realType eq "string") {
+                $unpack .= "    e$n = ATgetArgument(e, $m);\n";
+                $unpack .= "    if (ATgetType(e$n) != AT_APPL) return false;\n";
             } else {
                 $unpack .= "    e$n = ATgetArgument(e, $m);\n";
             }
diff --git a/src/libexpr/primops.cc b/src/libexpr/primops.cc
index b59232f2cea3..9208f0b247a8 100644
--- a/src/libexpr/primops.cc
+++ b/src/libexpr/primops.cc
@@ -29,20 +29,60 @@ static PathSet storeExprRootsCached(EvalState & state, const Path & nePath)
 }
 
 
-static Hash hashDerivation(EvalState & state, StoreExpr ne)
+/* Returns the hash of a derivation modulo fixed-output
+   subderivations.  A fixed-output derivation is a derivation with one
+   output (`out') for which an expected hash and hash algorithm are
+   specified (using the `outputHash' and `outputHashAlgo'
+   attributes).  We don't want changes to such derivations to
+   propagate upwards through the dependency graph, changing output
+   paths everywhere.
+
+   For instance, if we change the url in a call to the `fetchurl'
+   function, we do not want to rebuild everything depending on it
+   (after all, (the hash of) the file being downloaded is unchanged).
+   So the *output paths* should not change.  On the other hand, the
+   *derivation store expression paths* should change to reflect the
+   new dependency graph.
+
+   That's what this function does: it returns a hash which is just the
+   of the derivation ATerm, except that any input store expression
+   paths have been replaced by the result of a recursive call to this
+   function, and that for fixed-output derivations we return
+   (basically) its outputHash. */
+static Hash hashDerivationModulo(EvalState & state, StoreExpr ne)
 {
     if (ne.type == StoreExpr::neDerivation) {
+
+        /* Return a fixed hash for fixed-output derivations. */
+        if (ne.derivation.outputs.size() == 1) {
+            DerivationOutputs::iterator i = ne.derivation.outputs.begin();
+            if (i->first == "out" &&
+                i->second.hash != "")
+            {
+                return hashString(htSHA256, "fixed:out:"
+                    + i->second.hashAlgo + ":"
+                    + i->second.hash + ":"
+                    + i->second.path);
+            }
+        }
+
+        /* For other derivations, replace the inputs paths with
+           recursive calls to this function.*/
 	PathSet inputs2;
         for (PathSet::iterator i = ne.derivation.inputs.begin();
-             i != ne.derivation.inputs.end(); i++)
+             i != ne.derivation.inputs.end(); ++i)
         {
-            DrvHashes::iterator j = state.drvHashes.find(*i);
-            if (j == state.drvHashes.end())
-                throw Error(format("don't know expression `%1%'") % (string) *i);
-            inputs2.insert(printHash(j->second));
+            Hash h = state.drvHashes[*i];
+            if (h.type == htUnknown) {
+                StoreExpr ne2 = storeExprFromPath(*i);
+                h = hashDerivationModulo(state, ne2);
+                state.drvHashes[*i] = h;
+            }
+            inputs2.insert(printHash(h));
         }
 	ne.derivation.inputs = inputs2;
     }
+    
     return hashTerm(unparseStoreExpr(ne));
 }
 
@@ -58,9 +98,7 @@ static Path copyAtom(EvalState & state, const Path & srcPath)
     ne.closure.roots.insert(dstPath);
     ne.closure.elems[dstPath] = elem;
 
-    Hash drvHash = hashDerivation(state, ne);
     Path drvPath = writeTerm(unparseStoreExpr(ne), "c");
-    state.drvHashes[drvPath] = drvHash;
 
     state.drvRoots[drvPath] = ne.closure.roots;
 
@@ -109,16 +147,11 @@ static void processBinding(EvalState & state, Expr e, StoreExpr & ne,
             if (!a) throw Error("derivation name missing");
             Path drvPath = evalPath(state, a);
 
-            a = queryAttr(e, "drvHash");
-            if (!a) throw Error("derivation hash missing");
-            Hash drvHash = parseHash(htMD5, evalString(state, a));
-
             a = queryAttr(e, "outPath");
             if (!a) throw Error("output path missing");
             PathSet drvRoots;
             drvRoots.insert(evalPath(state, a));
             
-            state.drvHashes[drvPath] = drvHash;
             state.drvRoots[drvPath] = drvRoots;
 
             ss.push_back(addInput(state, drvPath, ne));
@@ -188,8 +221,9 @@ static Expr primDerivation(EvalState & state, const ATermVector & _args)
     ne.type = StoreExpr::neDerivation;
 
     string drvName;
-    Hash outHash;
-    bool outHashGiven = false;
+    
+    string outputHash;
+    string outputHashAlgo;
 
     for (ATermIterator i(attrs.keys()); i; ++i) {
         string key = aterm2String(*i);
@@ -222,10 +256,8 @@ static Expr primDerivation(EvalState & state, const ATermVector & _args)
             if (key == "builder") ne.derivation.builder = s;
             else if (key == "system") ne.derivation.platform = s;
             else if (key == "name") drvName = s;
-            else if (key == "id") { 
-                outHash = parseHash(htMD5, s);
-                outHashGiven = true;
-            }
+            else if (key == "outputHash") outputHash = s;
+            else if (key == "outputHashAlgo") outputHashAlgo = s;
         }
     }
     
@@ -237,6 +269,24 @@ static Expr primDerivation(EvalState & state, const ATermVector & _args)
     if (drvName == "")
         throw Error("required attribute `name' missing");
 
+    /* If an output hash was given, check it. */
+    if (outputHash == "")
+        outputHashAlgo = "";
+    else {
+        HashType ht = parseHashType(outputHashAlgo);
+        if (ht == htUnknown)
+            throw Error(format("unknown hash algorithm `%1%'") % outputHashAlgo);
+        Hash h;
+        if (outputHash.size() == Hash(ht).hashSize * 2)
+            /* hexadecimal representation */
+            h = parseHash(ht, outputHash);
+        else
+            /* base-32 representation */
+            h = parseHash32(ht, outputHash);
+        string s = outputHash;
+        outputHash = printHash(h);
+    }
+
     /* Check the derivation name.  It shouldn't contain whitespace,
        but we are conservative here: we check whether only
        alphanumerics and some other characters appear. */
@@ -252,38 +302,33 @@ static Expr primDerivation(EvalState & state, const ATermVector & _args)
         }
 
     /* Construct the "masked" derivation store expression, which is
-       the final one except that the list of output paths is set to
-       the set of output names, and the corresponding environment
-       variables have an empty value.  This ensures that changes in
-       the set of output names do get reflected in the hash. */
+       the final one except that in the list of outputs, the output
+       paths are empty, and the corresponding environment variables
+       have an empty value.  This ensures that changes in the set of
+       output names do get reflected in the hash. */
     ne.derivation.env["out"] = "";
-    ne.derivation.outputs.insert("out");
+    ne.derivation.outputs["out"] =
+        DerivationOutput("", outputHashAlgo, outputHash);
         
-    /* Determine the output path by hashing the Nix expression with no
-       outputs to produce a unique but deterministic path name for
-       this derivation. */
-    if (!outHashGiven) outHash = hashDerivation(state, ne);
+    /* Use the masked derivation expression to compute the output
+       path. */
     Path outPath = makeStorePath("output:out",
-        outHash, drvName);
+        hashDerivationModulo(state, ne), drvName);
 
     /* Construct the final derivation store expression. */
     ne.derivation.env["out"] = outPath;
-    ne.derivation.outputs.clear();
-    ne.derivation.outputs.insert(outPath);
+    ne.derivation.outputs["out"] =
+        DerivationOutput(outPath, outputHashAlgo, outputHash);
 
     /* Write the resulting term into the Nix store directory. */
-    Hash drvHash = outHashGiven
-        ? hashString(printHash(outHash) + outPath, htMD5)
-        : hashDerivation(state, ne);
     Path drvPath = writeTerm(unparseStoreExpr(ne), "d-" + drvName);
 
     printMsg(lvlChatty, format("instantiated `%1%' -> `%2%'")
         % drvName % drvPath);
 
+    /* !!! assumes a single output */
     attrs.set("outPath", makeAttrRHS(makePath(toATerm(outPath)), makeNoPos()));
     attrs.set("drvPath", makeAttrRHS(makePath(toATerm(drvPath)), makeNoPos()));
-    attrs.set("drvHash",
-        makeAttrRHS(makeStr(toATerm(printHash(drvHash))), makeNoPos()));
     attrs.set("type", makeAttrRHS(makeStr(toATerm("derivation")), makeNoPos()));
 
     return makeAttrs(attrs);
diff --git a/src/libstore/misc.cc b/src/libstore/misc.cc
index d7c32336e4cd..6dc054fb43d7 100644
--- a/src/libstore/misc.cc
+++ b/src/libstore/misc.cc
@@ -20,8 +20,9 @@ PathSet storeExprRoots(const Path & nePath)
     if (ne.type == StoreExpr::neClosure)
         paths.insert(ne.closure.roots.begin(), ne.closure.roots.end());
     else if (ne.type == StoreExpr::neDerivation)
-        paths.insert(ne.derivation.outputs.begin(),
-            ne.derivation.outputs.end());
+        for (DerivationOutputs::iterator i = ne.derivation.outputs.begin();
+             i != ne.derivation.outputs.end(); ++i)
+            paths.insert(i->second.path);
     else abort();
 
     return paths;
diff --git a/src/libstore/normalise.cc b/src/libstore/normalise.cc
index 7907325a97eb..b016d8d354e6 100644
--- a/src/libstore/normalise.cc
+++ b/src/libstore/normalise.cc
@@ -702,6 +702,29 @@ static void drain(int fd)
 }
 
 
+PathSet outputPaths(const DerivationOutputs & outputs)
+{
+    PathSet paths;
+    for (DerivationOutputs::const_iterator i = outputs.begin();
+         i != outputs.end(); ++i)
+        paths.insert(i->second.path);
+    return paths;
+}
+
+
+string showPaths(const PathSet & paths)
+{
+    string s;
+    for (PathSet::const_iterator i = paths.begin();
+         i != paths.end(); ++i)
+    {
+        if (s.size() != 0) s += ", ";
+        s += *i;
+    }
+    return s;
+}
+
+
 NormalisationGoal::HookReply NormalisationGoal::tryBuildHook()
 {
     Path buildHook = getEnv("NIX_BUILD_HOOK");
@@ -786,7 +809,7 @@ NormalisationGoal::HookReply NormalisationGoal::tryBuildHook()
         }
 
         printMsg(lvlInfo, format("running hook to build path `%1%'")
-            % *expr.derivation.outputs.begin());
+            % showPaths(outputPaths(expr.derivation.outputs)));
         
         /* Write the information that the hook needs to perform the
            build, i.e., the set of input paths (including closure
@@ -807,9 +830,9 @@ NormalisationGoal::HookReply NormalisationGoal::tryBuildHook()
         writeStringToFile(inputListFN, s);
         
         s = "";
-        for (PathSet::iterator i = expr.derivation.outputs.begin();
+        for (DerivationOutputs::iterator i = expr.derivation.outputs.begin();
              i != expr.derivation.outputs.end(); ++i)
-            s += *i + "\n";
+            s += i->second.path + "\n";
         writeStringToFile(outputListFN, s);
         
         s = "";
@@ -848,7 +871,7 @@ bool NormalisationGoal::prepareBuild()
     /* Obtain locks on all output paths.  The locks are automatically
        released when we exit this function or Nix crashes. */
     /* !!! BUG: this could block, which is not allowed. */
-    outputLocks.lockPaths(expr.derivation.outputs);
+    outputLocks.lockPaths(outputPaths(expr.derivation.outputs));
 
     /* Now check again whether there is a successor.  This is because
        another process may have started building in parallel.  After
@@ -870,11 +893,11 @@ bool NormalisationGoal::prepareBuild()
        running the build hook. */
     
     /* The outputs are referenceable paths. */
-    for (PathSet::iterator i = expr.derivation.outputs.begin();
+    for (DerivationOutputs::iterator i = expr.derivation.outputs.begin();
          i != expr.derivation.outputs.end(); ++i)
     {
-        debug(format("building path `%1%'") % *i);
-        allPaths.insert(*i);
+        debug(format("building path `%1%'") % i->second.path);
+        allPaths.insert(i->second.path);
     }
     
     /* Get information about the inputs (these all exist now). */
@@ -901,9 +924,9 @@ bool NormalisationGoal::prepareBuild()
     /* We can skip running the builder if all output paths are already
        valid. */
     bool fastBuild = true;
-    for (PathSet::iterator i = expr.derivation.outputs.begin();
+    for (DerivationOutputs::iterator i = expr.derivation.outputs.begin();
          i != expr.derivation.outputs.end(); ++i)
-        if (!isValidPath(*i)) { 
+        if (!isValidPath(i->second.path)) { 
             fastBuild = false;
             break;
         }
@@ -921,7 +944,7 @@ bool NormalisationGoal::prepareBuild()
 void NormalisationGoal::startBuilder()
 {
     startNest(nest, lvlInfo,
-        format("building path `%1%'") % *expr.derivation.outputs.begin());
+        format("building path `%1%'") % showPaths(outputPaths(expr.derivation.outputs)))
     
     /* Right platform? */
     if (expr.derivation.platform != thisSystem)
@@ -931,10 +954,10 @@ void NormalisationGoal::startBuilder()
 
     /* If any of the outputs already exist but are not registered,
        delete them. */
-    for (PathSet::iterator i = expr.derivation.outputs.begin(); 
+    for (DerivationOutputs::iterator i = expr.derivation.outputs.begin(); 
          i != expr.derivation.outputs.end(); ++i)
     {
-        Path path = *i;
+        Path path = i->second.path;
         if (isValidPath(path))
             throw Error(format("obstructed build: path `%1%' exists") % path);
         if (pathExists(path)) {
@@ -1054,10 +1077,10 @@ void NormalisationGoal::createClosure()
        output path to determine what other paths it references.  Also make all
        output paths read-only. */
     PathSet usedPaths;
-    for (PathSet::iterator i = expr.derivation.outputs.begin(); 
+    for (DerivationOutputs::iterator i = expr.derivation.outputs.begin(); 
          i != expr.derivation.outputs.end(); ++i)
     {
-        Path path = *i;
+        Path path = i->second.path;
         if (!pathExists(path)) {
             throw BuildError(
                 format("builder for `%1%' failed to produce output path `%2%'")
@@ -1084,6 +1107,7 @@ void NormalisationGoal::createClosure()
 	/* For each path referenced by this output path, add its id to the
 	   closure element and add the id to the `usedPaths' set (so that the
 	   elements referenced by *its* closure are added below). */
+        PathSet outputPaths = ::outputPaths(expr.derivation.outputs);
         for (Paths::iterator j = refPaths.begin();
 	     j != refPaths.end(); ++j)
 	{
@@ -1092,8 +1116,7 @@ void NormalisationGoal::createClosure()
 	    elem.refs.insert(path);
             if (inClosures.find(path) != inClosures.end())
                 usedPaths.insert(path);
-	    else if (expr.derivation.outputs.find(path) ==
-                expr.derivation.outputs.end())
+	    else if (outputPaths.find(path) == outputPaths.end())
 		abort();
         }
 
@@ -1147,9 +1170,9 @@ void NormalisationGoal::createClosure()
        by running the garbage collector. */
     Transaction txn;
     createStoreTransaction(txn);
-    for (PathSet::iterator i = expr.derivation.outputs.begin(); 
+    for (DerivationOutputs::iterator i = expr.derivation.outputs.begin(); 
          i != expr.derivation.outputs.end(); ++i)
-        registerValidPath(txn, *i);
+        registerValidPath(txn, i->second.path);
     registerSuccessor(txn, nePath, nfPath);
     txn.commit();
 
diff --git a/src/libstore/store.cc b/src/libstore/store.cc
index e490bf258195..0d89f7a5d7ae 100644
--- a/src/libstore/store.cc
+++ b/src/libstore/store.cc
@@ -412,14 +412,14 @@ static void invalidatePath(const Path & path, Transaction & txn)
 
 
 Path makeStorePath(const string & type,
-    Hash & hash, const string & suffix)
+    const Hash & hash, const string & suffix)
 {
     /* e.g., "source:sha256:1abc...:/nix/store:foo.tar.gz" */
     string s = type + ":sha256:" + printHash(hash) + ":"
         + nixStore + ":" + suffix;
 
     return nixStore + "/"
-        + printHash32(compressHash(hashString(s, htSHA256), 20))
+        + printHash32(compressHash(hashString(htSHA256, s), 20))
         + "-" + suffix;
 }
 
@@ -432,7 +432,7 @@ Path addToStore(const Path & _srcPath)
     Hash h(htSHA256);
     {
         SwitchToOriginalUser sw;
-        h = hashPath(srcPath, htSHA256);
+        h = hashPath(htSHA256, srcPath);
     }
 
     string baseName = baseNameOf(srcPath);
@@ -456,7 +456,7 @@ Path addToStore(const Path & _srcPath)
             
             copyPath(srcPath, dstPath);
 
-            Hash h2 = hashPath(dstPath, htSHA256);
+            Hash h2 = hashPath(htSHA256, dstPath);
             if (h != h2)
                 throw Error(format("contents of `%1%' changed while copying it to `%2%' (%3% -> %4%)")
                     % srcPath % dstPath % printHash(h) % printHash(h2));
@@ -477,7 +477,7 @@ Path addToStore(const Path & _srcPath)
 
 Path addTextToStore(const string & suffix, const string & s)
 {
-    Hash hash = hashString(s, htSHA256);
+    Hash hash = hashString(htSHA256, s);
 
     Path dstPath = makeStorePath("text", hash, suffix);
     
diff --git a/src/libstore/store.hh b/src/libstore/store.hh
index 25a6bc8b9669..c27918cbb4f9 100644
--- a/src/libstore/store.hh
+++ b/src/libstore/store.hh
@@ -83,7 +83,7 @@ bool isValidPath(const Path & path);
 
 /* Constructs a unique store path name. */
 Path makeStorePath(const string & type,
-    Hash & hash, const string & suffix);
+    const Hash & hash, const string & suffix);
     
 /* Copy the contents of a path to the store and register the validity
    the resulting path.  The resulting path is returned. */
diff --git a/src/libstore/storeexpr-ast.def b/src/libstore/storeexpr-ast.def
index 9d2433dbe4be..0c70948d4b6c 100644
--- a/src/libstore/storeexpr-ast.def
+++ b/src/libstore/storeexpr-ast.def
@@ -5,3 +5,4 @@ Derive | ATermList ATermList string string ATermList ATermList | ATerm |
 
 | string string | ATerm | EnvBinding |
 | string ATermList | ATerm | ClosureElem |
+| string string string string | ATerm | DerivationOutput |
diff --git a/src/libstore/storeexpr.cc b/src/libstore/storeexpr.cc
index de29959edf60..d8300a066a0e 100644
--- a/src/libstore/storeexpr.cc
+++ b/src/libstore/storeexpr.cc
@@ -8,7 +8,7 @@
 
 Hash hashTerm(ATerm t)
 {
-    return hashString(atPrint(t), htMD5);
+    return hashString(htSHA256, atPrint(t));
 }
 
 
@@ -20,14 +20,20 @@ Path writeTerm(ATerm t, const string & suffix)
 }
 
 
+void checkPath(const string & s)
+{
+    if (s.size() == 0 || s[0] != '/')
+        throw Error(format("bad path `%1%' in store expression") % s);
+}
+    
+
 static void parsePaths(ATermList paths, PathSet & out)
 {
     for (ATermIterator i(paths); i; ++i) {
         if (ATgetType(*i) != AT_APPL)
             throw badTerm("not a path", *i);
         string s = aterm2String(*i);
-        if (s.size() == 0 || s[0] != '/')
-            throw badTerm("not a path", *i);
+        checkPath(s);
         out.insert(s);
     }
 }
@@ -92,7 +98,18 @@ static bool parseDerivation(ATerm t, Derivation & derivation)
     if (!matchDerive(t, outs, ins, platform, builder, args, bnds))
         return false;
 
-    parsePaths(outs, derivation.outputs);
+    for (ATermIterator i(outs); i; ++i) {
+        ATerm id, path, hashAlgo, hash;
+        if (!matchDerivationOutput(*i, id, path, hashAlgo, hash))
+            return false;
+        DerivationOutput out;
+        out.path = aterm2String(path);
+        checkPath(out.path);
+        out.hashAlgo = aterm2String(hashAlgo);
+        out.hash = aterm2String(hash);
+        derivation.outputs[aterm2String(id)] = out;
+    }
+
     parsePaths(ins, derivation.inputs);
 
     derivation.builder = aterm2String(builder);
@@ -155,6 +172,16 @@ static ATerm unparseClosure(const Closure & closure)
 
 static ATerm unparseDerivation(const Derivation & derivation)
 {
+    ATermList outputs = ATempty;
+    for (DerivationOutputs::const_iterator i = derivation.outputs.begin();
+         i != derivation.outputs.end(); i++)
+        outputs = ATinsert(outputs,
+            makeDerivationOutput(
+                toATerm(i->first),
+                toATerm(i->second.path),
+                toATerm(i->second.hashAlgo),
+                toATerm(i->second.hash)));
+
     ATermList args = ATempty;
     for (Strings::const_iterator i = derivation.args.begin();
          i != derivation.args.end(); i++)
@@ -169,7 +196,7 @@ static ATerm unparseDerivation(const Derivation & derivation)
                 toATerm(i->second)));
 
     return makeDerive(
-        unparsePaths(derivation.outputs),
+        ATreverse(outputs),
         unparsePaths(derivation.inputs),
         toATerm(derivation.platform),
         toATerm(derivation.builder),
diff --git a/src/libstore/storeexpr.hh b/src/libstore/storeexpr.hh
index 07676c3ccedb..d8b8b2a96cbf 100644
--- a/src/libstore/storeexpr.hh
+++ b/src/libstore/storeexpr.hh
@@ -20,12 +20,30 @@ struct Closure
     ClosureElems elems;
 };
 
+
+struct DerivationOutput
+{
+    Path path;
+    string hashAlgo; /* hash used for expected hash computation */
+    string hash; /* expected hash, may be null */
+    DerivationOutput()
+    {
+    }
+    DerivationOutput(Path path, string hashAlgo, string hash)
+    {
+        this->path = path;
+        this->hashAlgo = hashAlgo;
+        this->hash = hash;
+    }
+};
+
+typedef map<string, DerivationOutput> DerivationOutputs;
 typedef map<string, string> StringPairs;
 
 struct Derivation
 {
-    PathSet outputs;
-    PathSet inputs; /* Store expressions, not actual inputs */
+    DerivationOutputs outputs; /* keyed on symbolic IDs */
+    PathSet inputs; /* store expressions, not actual inputs */
     string platform;
     Path builder;
     Strings args;
diff --git a/src/libutil/hash.cc b/src/libutil/hash.cc
index 324e2bf7f4c2..5c93c41dc613 100644
--- a/src/libutil/hash.cc
+++ b/src/libutil/hash.cc
@@ -132,6 +132,55 @@ string printHash32(const Hash & hash)
 }
 
 
+static bool mul(uint16_t * words, unsigned short y, int maxSize)
+{
+    unsigned short carry = 0;
+
+    for (int pos = 0; pos < maxSize; ++pos) {
+        unsigned int m = words[pos] * y + carry;
+        words[pos] = m & 0xffff;
+        carry = m >> 16;
+    }
+
+    return carry;
+}
+
+
+static bool add(uint16_t * words, unsigned short y, int maxSize)
+{
+    unsigned short carry = y;
+
+    for (int pos = 0; pos < maxSize; ++pos) {
+        unsigned int m = words[pos] + carry;
+        words[pos] = m & 0xffff;
+        carry = m >> 16;
+        if (carry == 0) break;
+    }
+
+    return carry;
+}
+
+
+Hash parseHash32(HashType ht, const string & s)
+{
+    Hash hash(ht);
+
+    for (unsigned int i = 0; i < s.length(); ++i) {
+        char c = s[i];
+        unsigned char digit;
+        for (digit = 0; digit < sizeof(chars); ++digit) /* !!! slow */
+            if (chars[digit] == c) break;
+        if (digit >= 32)
+            throw Error(format("invalid base-32 hash `%1%'") % s);
+        if (mul((uint16_t *) hash.hash, 32, hash.hashSize / 2) ||
+            add((uint16_t *) hash.hash, digit, hash.hashSize / 2))
+            throw Error(format("base-32 hash `%1%' is too large") % s);
+    }
+
+    return hash;
+}
+
+
 bool isHash(const string & s)
 {
     if (s.length() != 32) return false;
@@ -181,7 +230,7 @@ static void finish(HashType ht, Ctx & ctx, unsigned char * hash)
 }
 
 
-Hash hashString(const string & s, HashType ht)
+Hash hashString(HashType ht, const string & s)
 {
     Ctx ctx;
     Hash hash(ht);
@@ -192,7 +241,7 @@ Hash hashString(const string & s, HashType ht)
 }
 
 
-Hash hashFile(const Path & path, HashType ht)
+Hash hashFile(HashType ht, const Path & path)
 {
     Ctx ctx;
     Hash hash(ht);
@@ -226,7 +275,7 @@ struct HashSink : DumpSink
 };
 
 
-Hash hashPath(const Path & path, HashType ht)
+Hash hashPath(HashType ht, const Path & path)
 {
     HashSink sink;
     sink.ht = ht;
@@ -246,3 +295,12 @@ Hash compressHash(const Hash & hash, unsigned int newSize)
         h.hash[i % newSize] ^= hash.hash[i];
     return h;
 }
+
+
+HashType parseHashType(const string & s)
+{
+    if (s == "md5") return htMD5;
+    else if (s == "sha1") return htSHA1;
+    else if (s == "sha256") return htSHA256;
+    else return htUnknown;
+}
diff --git a/src/libutil/hash.hh b/src/libutil/hash.hh
index 0c9d7b9cb936..398b17421644 100644
--- a/src/libutil/hash.hh
+++ b/src/libutil/hash.hh
@@ -58,18 +58,21 @@ Hash parseHash32(HashType ht, const string & s);
 bool isHash(const string & s);
 
 /* Compute the hash of the given string. */
-Hash hashString(const string & s, HashType ht);
+Hash hashString(HashType ht, const string & s);
 
 /* Compute the hash of the given file. */
-Hash hashFile(const Path & path, HashType ht);
+Hash hashFile(HashType ht, const Path & path);
 
 /* Compute the hash of the given path.  The hash is defined as
    md5(dump(path)). */
-Hash hashPath(const Path & path, HashType ht);
+Hash hashPath(HashType ht, const Path & path);
 
 /* Compress a hash to the specified number of bytes by cyclically
    XORing bytes together. */
 Hash compressHash(const Hash & hash, unsigned int newSize);
 
+/* Parse a string representing a hash type. */
+HashType parseHashType(const string & s);
+
 
 #endif /* !__HASH_H */
diff --git a/src/nix-env/main.cc b/src/nix-env/main.cc
index 0143c1d24e44..0dd4efa30f31 100644
--- a/src/nix-env/main.cc
+++ b/src/nix-env/main.cc
@@ -33,7 +33,6 @@ struct DrvInfo
     string system;
     Path drvPath;
     Path outPath;
-    Hash drvHash;
 };
 
 typedef map<Path, DrvInfo> DrvInfos;
@@ -68,10 +67,6 @@ bool parseDerivation(EvalState & state, Expr e, DrvInfo & drv)
     if (!a) throw badTerm("derivation path missing", e);
     drv.drvPath = evalPath(state, a);
 
-    a = queryAttr(e, "drvHash");
-    if (!a) throw badTerm("derivation hash missing", e);
-    drv.drvHash = parseHash(htMD5, evalString(state, a));
-
     a = queryAttr(e, "outPath");
     if (!a) throw badTerm("output path missing", e);
     drv.outPath = evalPath(state, a);
@@ -191,7 +186,7 @@ void createUserEnv(EvalState & state, const DrvInfos & drvs,
     for (DrvInfos::const_iterator i = drvs.begin(); 
          i != drvs.end(); ++i)
     {
-        ATerm t = makeAttrs(ATmakeList6(
+        ATerm t = makeAttrs(ATmakeList5(
             makeBind(toATerm("type"),
                 makeStr(toATerm("derivation")), makeNoPos()),
             makeBind(toATerm("name"),
@@ -200,8 +195,6 @@ void createUserEnv(EvalState & state, const DrvInfos & drvs,
                 makeStr(toATerm(i->second.system)), makeNoPos()),
             makeBind(toATerm("drvPath"),
                 makePath(toATerm(i->second.drvPath)), makeNoPos()),
-            makeBind(toATerm("drvHash"),
-                makeStr(toATerm(printHash(i->second.drvHash))), makeNoPos()),
             makeBind(toATerm("outPath"),
                 makePath(toATerm(i->second.outPath)), makeNoPos())
             ));
diff --git a/src/nix-hash/help.txt b/src/nix-hash/help.txt
index a38c2ab9ed06..31fff1ffabd6 100644
--- a/src/nix-hash/help.txt
+++ b/src/nix-hash/help.txt
@@ -4,3 +4,5 @@ nix-hash [OPTIONS...] [FILES...]
 files.
 
   --flat: compute hash of regular file contents, not metadata
+  --base32: print hash in base-32 instead of hexadecimal
+  --type HASH: use hash algorithm HASH ("md5" (default), "sha1", "sha256")
diff --git a/src/nix-hash/nix-hash.cc b/src/nix-hash/nix-hash.cc
index 2cef7818e857..78c6f44012b4 100644
--- a/src/nix-hash/nix-hash.cc
+++ b/src/nix-hash/nix-hash.cc
@@ -15,22 +15,25 @@ void run(Strings args)
 {
     HashType ht = htMD5;
     bool flat = false;
+    bool base32 = false;
     
     for (Strings::iterator i = args.begin();
          i != args.end(); i++)
     {
         if (*i == "--flat") flat = true;
+        else if (*i == "--base32") base32 = true;
         else if (*i == "--type") {
             ++i;
             if (i == args.end()) throw UsageError("`--type' requires an argument");
-            if (*i == "md5") ht = htMD5;
-            else if (*i == "sha1") ht = htSHA1;
-            else if (*i == "sha256") ht = htSHA256;
-            else throw UsageError(format("unknown hash type `%1%'") % *i);
+            ht = parseHashType(*i);
+            if (ht == htUnknown)
+                throw UsageError(format("unknown hash type `%1%'") % *i);
+        }
+        else {
+            Hash h = flat ? hashFile(ht, *i) : hashPath(ht, *i);
+            cout << format("%1%\n") %
+                (base32 ? printHash32(h) : printHash(h));
         }
-        else
-            cout << format("%1%\n") % printHash(
-                (flat ? hashFile(*i, ht) : hashPath(*i, ht)));
     }
 }