diff --git a/sexp.c b/sexp.c
index ca0ad79..46d8635 100644
--- a/sexp.c
+++ b/sexp.c
@@ -130,3 +130,18 @@ cmsg_bytes_t sexp_data(sexp_t *x) {
       die("Unknown sexp kind %d in data accessor\n", x->kind);
   }
 }
+
+sexp_t *sexp_assoc(sexp_t *list, cmsg_bytes_t key) {
+  while (list != NULL) {
+    sexp_t *candidate = sexp_head(list);
+    if (sexp_stringp(candidate)) {
+      cmsg_bytes_t candidate_data = sexp_data(candidate);
+      if ((candidate_data.len == key.len)
+	  && (!memcmp(candidate_data.bytes, key.bytes, key.len))) {
+	return candidate;
+      }
+    }
+    list = sexp_tail(list);
+  }
+  return NULL;
+}
diff --git a/sexp.h b/sexp.h
index c6d41f1..cfcbc40 100644
--- a/sexp.h
+++ b/sexp.h
@@ -98,4 +98,6 @@ extern cmsg_bytes_t sexp_data(sexp_t *x);
     __val;						\
   })
 
+extern sexp_t *sexp_assoc(sexp_t *list, cmsg_bytes_t key);
+
 #endif