diff --git a/cid/Makefile b/cid/Makefile
index bb4c1ee..638d6c9 100644
--- a/cid/Makefile
+++ b/cid/Makefile
@@ -7,7 +7,7 @@ endif
 
 LFLAGS = 
 DEPS = 
-OBJS = cid.o
+OBJS = cid.o set.o
 
 %.o: %.c $(DEPS)
 	$(CC) -c -o $@ $< $(CFLAGS)
diff --git a/cid/set.c b/cid/set.c
new file mode 100644
index 0000000..837ce06
--- /dev/null
+++ b/cid/set.c
@@ -0,0 +1,181 @@
+#include <stdlib.h>
+#include <string.h>
+#include "ipfs/cid/cid.h"
+#include "ipfs/util/errs.h"
+
+struct CidSet *ipfs_cid_set_new ()
+{
+    return calloc(1, sizeof(struct CidSet));
+}
+
+void ipfs_cid_set_destroy (struct CidSet **set)
+{
+    struct CidSet *prev;
+
+    if (set) {
+        while (*set) {
+            prev = *set;
+            *set = *set->next;
+            if (prev->cid) {
+                free (prev->cid);
+            }
+            free (prev);
+        }
+    }
+}
+
+int ipfs_cid_set_add (struct CidSet *set, struct Cid *cid, int visit)
+{
+    if (!set || !cid) {
+        return ErrInvalidParam;
+    }
+    if (!set->cid) { // First item.
+        set->cid = malloc(sizeof (struct Cid));
+        if (!set->cid) {
+            return ErrAllocFailed;
+        }
+        memcpy(set->cid, cid, sizeof (struct Cid));
+        set->cid->hash = calloc(1, cid->hash_length);
+        if (!set->cid->hash) {
+            free (set->cid);
+            return ErrAllocFailed;
+        }
+        memcpy(set->cid->hash, cid->hash, cid->hash_length);
+        return 0;
+    }
+    for (;;) {
+        if ((set->cid->hash_length == cid->hash_length) &&
+            (memcmp(set->cid->hash, cid->hash, cid->hash_length)==0)) {
+            // Already added.
+            if (!visit) {
+                // update with new cid.
+                free(set->cid->hash);
+                memcpy(set->cid, cid, sizeof (struct Cid));
+                set->cid->hash = calloc(1, cid->hash_length);
+                if (!set->cid->hash) {
+                    return ErrAllocFailed;
+                }
+                memcpy(set->cid->hash, cid->hash, cid->hash_length);
+            }
+            return 0;
+        }
+        if (!set->next) {
+            set->next = ipfs_cid_set_new();
+            if (!set->next) {
+                return ErrAllocFailed;
+            }
+            set = set->next;
+            set->cid = malloc(sizeof (struct Cid));
+            if (!set->cid) {
+                return ErrAllocFailed;
+            }
+            memcpy(set->cid, cid, sizeof (struct Cid));
+            set->cid->hash = calloc(1, cid->hash_length);
+            if (!set->cid->hash) {
+                return ErrAllocFailed;
+            }
+            memcpy(set->cid->hash, cid->hash, cid->hash_length);
+            return 0;
+        }
+        set = set->next;
+    }
+}
+
+int ipfs_cid_set_has (struct CidSet *set, struct Cid *cid)
+{
+    if (!set || !cid || !set->cid) {
+        return 0;
+    }
+    for (;;) {
+        if ((set->cid->hash_length == cid->hash_length) &&
+            (memcmp(set->cid->hash, cid->hash, cid->hash_length)==0)) {
+            return 1; // has
+        }
+        if (!set->next) {
+            return 0; // end without found.
+        }
+        set = set->next;
+    }
+}
+
+int ipfs_cid_set_remove (struct CidSet *set, struct Cid *cid)
+{
+    struct CidSet *prev = set;
+
+    if (!set || !cid || !set->cid) {
+        return 0;
+    }
+    for (;;) {
+        if ((set->cid->hash_length == cid->hash_length) &&
+            (memcmp(set->cid->hash, cid->hash, cid->hash_length)==0)) {
+            free (set->cid);
+            if (prev == set) { // first item
+                set = set->next;
+                if (!set) {
+                    prev->cid = NULL;
+                    return 1;
+                }
+                prev->cid = set->cid;
+            } else {
+                prev->next = set->next;
+            }
+            free (set);
+            return 1; // removed
+        }
+        if (!set->next) {
+            return 0; // end without found.
+        }
+        prev = set;
+        set = set->next;
+    }
+}
+
+int ipfs_cid_set_len (struct CidSet *set)
+{
+    int len;
+
+    if (!set || !set->cid) {
+        return 0;
+    }
+
+    for (len = 0 ; set ; len++, set = set->next);
+
+    return len;
+}
+
+unsigned char **ipfs_cid_set_keys (struct CidSet *set)
+{
+    int i, len=ipfs_cid_set_len(set);
+    unsigned char **ret;
+
+    ret = calloc(len+1, sizeof(char*));
+    if (ret) {
+        for (i = 0 ; i<len ; len++) {
+            if (set && set->cid && set->cid->hash) {
+                ret[i] = calloc(1, set->cid->hash_length + 1);
+                if (ret[i]) {
+                    memcpy(ret[i], set->cid->hash, set->cid->hash_length);
+                }
+            }
+            set = set->next;
+        }
+    }
+    return ret;
+}
+
+int ipfs_cid_set_foreach (struct CidSet *set, int (*func)(struct Cid *))
+{
+    int err = 0;
+
+    while (set) {
+        if (set->cid) {
+            err = func (set->cid);
+            if (err) {
+                return err;
+            }
+        }
+        set = set->next;
+    }
+
+    return err;
+}
diff --git a/include/ipfs/cid/cid.h b/include/ipfs/cid/cid.h
index a145d7e..dfff375 100644
--- a/include/ipfs/cid/cid.h
+++ b/include/ipfs/cid/cid.h
@@ -26,6 +26,11 @@ struct Cid {
 	size_t hash_length;
 };
 
+struct CidSet {
+    struct Cid *cid;
+    struct CidSet *next;
+};
+
 /***
  * encode a Cid into a protobuf array of bytes
  * @param incoming the incoming Cid struct
@@ -95,4 +100,13 @@ int ipfs_cid_hash_to_base58(const unsigned char* hash, size_t hash_length, unsig
  */
 int ipfs_cid_cast(const unsigned char* incoming, size_t incoming_size, struct Cid* cid);
 
+struct CidSet *ipfs_cid_set_new ();
+void ipfs_cid_set_destroy (struct CidSet **set);
+int ipfs_cid_set_add (struct CidSet *set, struct Cid *cid, int visit);
+int ipfs_cid_set_has (struct CidSet *set, struct Cid *cid);
+int ipfs_cid_set_remove (struct CidSet *set, struct Cid *cid);
+int ipfs_cid_set_len (struct CidSet *set);
+unsigned char **ipfs_cid_set_keys (struct CidSet *set);
+int ipfs_cid_set_foreach (struct CidSet *set, int (*func)(struct Cid *));
+
 #endif