diff --git a/corepkgs/fetchurl/builder.sh.in b/corepkgs/fetchurl/builder.sh.in
index c1a735d68..aaba65d6d 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 8957662ec..37f01b55e 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 9a67ca141..c4731f3f6 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 376691c9a..5067b0a9b 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 b59232f2c..9208f0b24 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 d7c32336e..6dc054fb4 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 7907325a9..b016d8d35 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 e490bf258..0d89f7a5d 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 25a6bc8b9..c27918cbb 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 9d2433dbe..0c70948d4 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 de29959ed..d8300a066 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 07676c3cc..d8b8b2a96 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 324e2bf7f..5c93c41dc 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 0c9d7b9cb..398b17421 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 0143c1d24..0dd4efa30 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 a38c2ab9e..31fff1ffa 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 2cef7818e..78c6f4401 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)));
     }
 }