diff --git a/core/bitArray.js b/core/bitArray.js
index 40e4acd0..803b9c59 100644
--- a/core/bitArray.js
+++ b/core/bitArray.js
@@ -42,6 +42,27 @@ sjcl.bitArray = {
     return (bend === undefined) ? a : sjcl.bitArray.clamp(a, bend-bstart);
   },
 
+  /**
+   * Extract a number packed into a bit array.
+   * @param {bitArray} a The array to slice.
+   * @param {Number} bstart The offset to the start of the slice, in bits.
+   * @param {Number} length The length of the number to extract.
+   * @return {Number} The requested slice.
+   */
+  extract: function(a, bstart, blength) {
+    // FIXME: this Math.floor is not necessary at all, but for some reason
+    // seems to suppress a bug in the Chromium JIT.
+    var x, sh = Math.floor((-bstart-blength) & 31);
+    if ((bstart + blength - 1 ^ bstart) & -32) {
+      // it crosses a boundary
+      x = (a[bstart/32|0] << (32 - sh)) ^ (a[bstart/32+1|0] >>> sh);
+    } else {
+      // within a single word
+      x = a[bstart/32|0] >>> sh;
+    }
+    return x & ((1<<blength) - 1);
+  },
+
   /**
    * Concatenate two bit arrays.
    * @param {bitArray} a1 The first array.