3 files changed, 16 insertions, 4 deletions
diff --git a/src/libexpr/primops.cc b/src/libexpr/primops.cc
index 3f915fc232a9..c04d41bf4ddf 100644
--- a/src/libexpr/primops.cc
+++ b/src/libexpr/primops.cc
@@ -353,13 +353,16 @@ static Expr primDerivationStrict(EvalState & state, const ATermVector & args)
         HashType ht = parseHashType(outputHashAlgo);
         if (ht == htUnknown)
             throw EvalError(format("unknown hash algorithm `%1%'") % outputHashAlgo);
-        Hash h;
-        if (outputHash.size() == Hash(ht).hashSize * 2)
+        Hash h(ht);
+        if (outputHash.size() == h.hashSize * 2)
             /* hexadecimal representation */
             h = parseHash(ht, outputHash);
-        else
+        else if (outputHash.size() == hashLength32(h))
             /* base-32 representation */
             h = parseHash32(ht, outputHash);
+        else
+            throw Error(format("hash `%1%' has wrong length for hash type `%2%'")
+                % outputHash % outputHashAlgo);
         string s = outputHash;
         outputHash = printHash(h);
         if (outputHashRecursive) outputHashAlgo = "r:" + outputHashAlgo;
diff --git a/src/libutil/hash.cc b/src/libutil/hash.cc
index 16597fd47cbf..7381948f2180 100644
--- a/src/libutil/hash.cc
+++ b/src/libutil/hash.cc
@@ -120,6 +120,12 @@ static unsigned char divMod(unsigned char * bytes, unsigned char y)
 }
 
 
+unsigned int hashLength32(const Hash & hash)
+{
+    return (hash.hashSize * 8 - 1) / 5 + 1;
+}
+
+
 // omitted: E O U T
 const string base32Chars = "0123456789abcdfghijklmnpqrsvwxyz";
 
@@ -127,7 +133,7 @@ const string base32Chars = "0123456789abcdfghijklmnpqrsvwxyz";
 string printHash32(const Hash & hash)
 {
     Hash hash2(hash);
-    unsigned int len = (hash.hashSize * 8 - 1) / 5 + 1;
+    unsigned int len = hashLength32(hash);
 
     const char * chars = base32Chars.c_str();
     
diff --git a/src/libutil/hash.hh b/src/libutil/hash.hh
index 95629bc9e5ba..74ae51db3286 100644
--- a/src/libutil/hash.hh
+++ b/src/libutil/hash.hh
@@ -49,6 +49,9 @@ string printHash(const Hash & hash);
 /* Parse a hexadecimal representation of a hash code. */
 Hash parseHash(HashType ht, const string & s);
 
+/* Returns the length of a base-32 hash representation. */
+unsigned int hashLength32(const Hash & hash);
+
 /* Convert a hash to a base-32 representation. */
 string printHash32(const Hash & hash);