diff --git a/skeletons/BIT_STRING.c b/skeletons/BIT_STRING.c
index b5ab945..fed5974 100644
--- a/skeletons/BIT_STRING.c
+++ b/skeletons/BIT_STRING.c
@@ -36,8 +36,8 @@
 	0,
 	0,
 #else
-	OCTET_STRING_decode_uper,	/* Unaligned PER decoder */
-	OCTET_STRING_encode_uper,	/* Unaligned PER encoder */
+	BIT_STRING_decode_uper,	/* Unaligned PER decoder */
+	BIT_STRING_encode_uper,	/* Unaligned PER encoder */
 #endif  /* ASN_DISABLE_PER_SUPPORT */
 	BIT_STRING_random_fill,
 	0	/* Use generic outmost tag fetcher */
@@ -197,20 +197,77 @@
 			return -1;
 	}
 
+    if(st->bits_unused) {
+        int ret = snprintf(scratch, sizeof(scratch), " (%d bit%s unused)",
+                           st->bits_unused, st->bits_unused == 1 ? "" : "s");
+        assert(ret > 0 && ret < (ssize_t)sizeof(scratch));
+        if(ret > 0 && ret < (ssize_t)sizeof(scratch)
+           && cb(scratch, ret, app_key) < 0)
+            return -1;
+    }
+
 	return 0;
 }
 
 /*
+ * Non-destructively remove the trailing 0-bits from the given bit string.
+ */
+static const BIT_STRING_t *
+BIT_STRING__compactify(const BIT_STRING_t *st, BIT_STRING_t *tmp) {
+    const uint8_t *b;
+    union {
+        const uint8_t *c_buf;
+        uint8_t *nc_buf;
+    } unconst;
+
+    if(st->size == 0) {
+        assert(st->bits_unused == 0);
+        return st;
+    } else {
+        for(b = &st->buf[st->size - 1]; b > st->buf && *b == 0; b--) {
+            ;
+        }
+        /* b points to the last byte which may contain data */
+        if(*b) {
+            int unused = 7;
+            uint8_t v = *b;
+            v &= -(int8_t)v;
+            if(v & 0x0F) unused -= 4;
+            if(v & 0x33) unused -= 2;
+            if(v & 0x55) unused -= 1;
+            tmp->size = b-st->buf + 1;
+            tmp->bits_unused = unused;
+        } else {
+            tmp->size = b-st->buf;
+            tmp->bits_unused = 0;
+        }
+
+        assert(b >= st->buf);
+    }
+
+    unconst.c_buf = st->buf;
+    tmp->buf = unconst.nc_buf;
+    return tmp;
+}
+
+/*
  * Lexicographically compare the common prefix of both strings,
  * and if it is the same return -1 for the smallest string.
  */
 int
 BIT_STRING_compare(const asn_TYPE_descriptor_t *td, const void *aptr,
                    const void *bptr) {
-    const BIT_STRING_t *a = aptr;
-    const BIT_STRING_t *b = bptr;
+    /*
+     * Remove information about trailing bits, since
+     * X.680 (08/2015) #22.7 "ensure that different semantics are not"
+     * "associated with [values that differ only in] the trailing 0 bits."
+     */
+    BIT_STRING_t compact_a, compact_b;
+    const BIT_STRING_t *a = BIT_STRING__compactify(aptr, &compact_a);
+    const BIT_STRING_t *b = BIT_STRING__compactify(bptr, &compact_b);
+    const asn_OCTET_STRING_specifics_t *specs = td->specifics;
 
-    (void)td;
+    assert(specs && specs->subvariant == ASN_OSUBV_BIT);
 
     if(a && b) {
         size_t common_prefix_size = a->size <= b->size ? a->size : b->size;
@@ -223,9 +280,9 @@
                 return 1;
             } else {
                 /* Figure out how many unused bits */
-                if(a->bits_unused < b->bits_unused) {
+                if(a->bits_unused > b->bits_unused) {
                     return -1;
-                } else if(a->bits_unused > b->bits_unused) {
+                } else if(a->bits_unused < b->bits_unused) {
                     return 1;
                 } else {
                     return 0;
@@ -243,6 +300,241 @@
     }
 }
 
+#ifndef  ASN_DISABLE_PER_SUPPORT
+
+#undef  RETURN
+#define RETURN(_code)                       \
+    do {                                    \
+        asn_dec_rval_t tmprval;             \
+        tmprval.code = _code;               \
+        tmprval.consumed = consumed_myself; \
+        return tmprval;                     \
+    } while(0)
+
+static asn_per_constraint_t asn_DEF_BIT_STRING_constraint_size = {
+    APC_SEMI_CONSTRAINED, -1, -1, 0, 0};
+
+asn_dec_rval_t
+BIT_STRING_decode_uper(const asn_codec_ctx_t *opt_codec_ctx,
+                         asn_TYPE_descriptor_t *td,
+                         const asn_per_constraints_t *constraints, void **sptr,
+                         asn_per_data_t *pd) {
+    asn_OCTET_STRING_specifics_t *specs = td->specifics
+		? (asn_OCTET_STRING_specifics_t *)td->specifics
+		: &asn_SPC_BIT_STRING_specs;
+    const asn_per_constraints_t *pc =
+        constraints ? constraints : td->encoding_constraints.per_constraints;
+	const asn_per_constraint_t *csiz;
+	asn_dec_rval_t rval = { RC_OK, 0 };
+	BIT_STRING_t *st = (BIT_STRING_t *)*sptr;
+	ssize_t consumed_myself = 0;
+	int repeat;
+
+	(void)opt_codec_ctx;
+
+	if(pc) {
+		csiz = &pc->size;
+	} else {
+		csiz = &asn_DEF_BIT_STRING_constraint_size;
+	}
+
+	if(specs->subvariant != ASN_OSUBV_BIT) {
+		ASN_DEBUG("Subvariant %d is not BIT OSUBV_BIT", specs->subvariant);
+		RETURN(RC_FAIL);
+    }
+
+	/*
+	 * Allocate the string.
+	 */
+	if(!st) {
+		st = (BIT_STRING_t *)(*sptr = CALLOC(1, specs->struct_size));
+		if(!st) RETURN(RC_FAIL);
+	}
+
+	ASN_DEBUG("PER Decoding %s size %ld .. %ld bits %d",
+		csiz->flags & APC_EXTENSIBLE ? "extensible" : "non-extensible",
+		csiz->lower_bound, csiz->upper_bound, csiz->effective_bits);
+
+	if(csiz->flags & APC_EXTENSIBLE) {
+		int inext = per_get_few_bits(pd, 1);
+		if(inext < 0) RETURN(RC_WMORE);
+		if(inext) {
+			csiz = &asn_DEF_BIT_STRING_constraint_size;
+		}
+	}
+
+	if(csiz->effective_bits >= 0) {
+		FREEMEM(st->buf);
+        st->size = (csiz->upper_bound + 7) >> 3;
+        st->buf = (uint8_t *)MALLOC(st->size + 1);
+		if(!st->buf) { st->size = 0; RETURN(RC_FAIL); }
+	}
+
+	/* X.691, #16.5: zero-length encoding */
+	/* X.691, #16.6: short fixed length encoding (up to 2 octets) */
+	/* X.691, #16.7: long fixed length encoding (up to 64K octets) */
+	if(csiz->effective_bits == 0) {
+		int ret;
+        ASN_DEBUG("Encoding BIT STRING size %ld", csiz->upper_bound);
+        ret = per_get_many_bits(pd, st->buf, 0, csiz->upper_bound);
+		if(ret < 0) RETURN(RC_WMORE);
+		consumed_myself += csiz->upper_bound;
+		st->buf[st->size] = 0;
+        st->bits_unused = (8 - (csiz->upper_bound & 0x7)) & 0x7;
+        RETURN(RC_OK);
+	}
+
+	st->size = 0;
+	do {
+		ssize_t raw_len;
+		ssize_t len_bytes;
+		ssize_t len_bits;
+		void *p;
+		int ret;
+
+		/* Get the PER length */
+		raw_len = uper_get_length(pd, csiz->effective_bits, csiz->lower_bound,
+		                          &repeat);
+		if(raw_len < 0) RETURN(RC_WMORE);
+
+		ASN_DEBUG("Got PER length eb %ld, len %ld, %s (%s)",
+			(long)csiz->effective_bits, (long)raw_len,
+			repeat ? "repeat" : "once", td->name);
+        len_bits = raw_len;
+        len_bytes = (len_bits + 7) >> 3;
+        if(len_bits & 0x7) st->bits_unused = 8 - (len_bits & 0x7);
+        /* len_bits be multiple of 16K if repeat is set */
+        p = REALLOC(st->buf, st->size + len_bytes + 1);
+		if(!p) RETURN(RC_FAIL);
+		st->buf = (uint8_t *)p;
+
+        ret = per_get_many_bits(pd, &st->buf[st->size], 0, len_bits);
+        if(ret < 0) RETURN(RC_WMORE);
+		st->size += len_bytes;
+	} while(repeat);
+	st->buf[st->size] = 0;	/* nul-terminate */
+
+	return rval;
+}
+
+asn_enc_rval_t
+BIT_STRING_encode_uper(asn_TYPE_descriptor_t *td,
+                         const asn_per_constraints_t *constraints, void *sptr,
+                         asn_per_outp_t *po) {
+    asn_OCTET_STRING_specifics_t *specs =
+        td->specifics ? (asn_OCTET_STRING_specifics_t *)td->specifics
+                      : &asn_SPC_BIT_STRING_specs;
+    const asn_per_constraints_t *pc =
+        constraints ? constraints : td->encoding_constraints.per_constraints;
+	const asn_per_constraint_t *csiz;
+	const BIT_STRING_t *st = (const BIT_STRING_t *)sptr;
+	BIT_STRING_t compact_bstr;  /* Do not modify this directly! */
+	asn_enc_rval_t er = { 0, 0, 0 };
+	int inext = 0;		/* Lies not within extension root */
+	size_t size_in_bits;
+	const uint8_t *buf;
+	int ret;
+	int ct_extensible;
+
+	if(!st || (!st->buf && st->size))
+		ASN__ENCODE_FAILED;
+
+	if(specs->subvariant == ASN_OSUBV_BIT) {
+        if((st->size == 0 && st->bits_unused) || (st->bits_unused & ~7))
+            ASN__ENCODE_FAILED;
+        ASN_DEBUG("BIT STRING of %zu bytes, %d bits unused", size_in_bits,
+                  st->bits_unused);
+    } else {
+		ASN__ENCODE_FAILED;
+    }
+
+	if(pc) {
+        csiz = &pc->size;
+    } else {
+		csiz = &asn_DEF_BIT_STRING_constraint_size;
+	}
+	ct_extensible = csiz->flags & APC_EXTENSIBLE;
+
+    /* Figure out the size without the trailing bits */
+    st = BIT_STRING__compactify(st, &compact_bstr);
+    size_in_bits = 8 * st->size - st->bits_unused;
+
+    ASN_DEBUG(
+        "Encoding %s into %zu bits"
+        " (%ld..%ld, effective %d)%s",
+        td->name, size_in_bits, csiz->lower_bound, csiz->upper_bound,
+        csiz->effective_bits, ct_extensible ? " EXT" : "");
+
+    /* Figure out whether size lies within PER visible constraint */
+
+    if(csiz->effective_bits >= 0) {
+        if((ssize_t)size_in_bits > csiz->upper_bound) {
+            if(ct_extensible) {
+                csiz = &asn_DEF_BIT_STRING_constraint_size;
+                inext = 1;
+            } else {
+                ASN__ENCODE_FAILED;
+            }
+        }
+    } else {
+        inext = 0;
+    }
+
+    if(ct_extensible) {
+		/* Declare whether length is [not] within extension root */
+		if(per_put_few_bits(po, inext, 1))
+			ASN__ENCODE_FAILED;
+	}
+
+    if(csiz->effective_bits >= 0 && !inext) {
+        ASN_DEBUG("Encoding %zu bytes (%ld), length in %d bits", st->size,
+                  size_in_bits - csiz->lower_bound, csiz->effective_bits);
+        int add_trailer = (ssize_t)size_in_bits < csiz->lower_bound;
+        ret = per_put_few_bits(
+            po,
+            add_trailer ? csiz->lower_bound : size_in_bits - csiz->lower_bound,
+            csiz->effective_bits);
+        if(ret) ASN__ENCODE_FAILED;
+        ret = per_put_many_bits(po, st->buf, size_in_bits);
+        if(ret) ASN__ENCODE_FAILED;
+        if(add_trailer) {
+            static uint8_t zeros[16];
+            size_t trailing_zero_bits = csiz->lower_bound - size_in_bits;
+            while(trailing_zero_bits > 0) {
+                if(trailing_zero_bits > 8 * sizeof(zeros)) {
+                    ret = per_put_many_bits(po, zeros, 8 * sizeof(zeros));
+                    trailing_zero_bits -= 8 * sizeof(zeros);
+                } else {
+                    ret = per_put_many_bits(po, zeros, trailing_zero_bits);
+                    trailing_zero_bits = 0;
+                }
+                if(ret) ASN__ENCODE_FAILED;
+            }
+        }
+        ASN__ENCODED_OK(er);
+    }
+
+    ASN_DEBUG("Encoding %zu bytes", st->size);
+
+    buf = st->buf;
+    do {
+        ssize_t maySave = uper_put_length(po, size_in_bits);
+        if(maySave < 0) ASN__ENCODE_FAILED;
+
+        ASN_DEBUG("Encoding %zd of %zu", maySave, size_in_bits);
+
+        ret = per_put_many_bits(po, buf, maySave);
+        if(ret) ASN__ENCODE_FAILED;
+
+        buf += maySave >> 3;
+        size_in_bits -= maySave;
+        assert(!(maySave & 0x07) || !size_in_bits);
+    } while(size_in_bits);
+
+    ASN__ENCODED_OK(er);
+}
+
+#endif  /* ASN_DISABLE_PER_SUPPORT */
 
 asn_random_fill_result_t
 BIT_STRING_random_fill(const asn_TYPE_descriptor_t *td, void **sptr,
@@ -277,14 +569,42 @@
     /* Figure out how far we should go */
     rnd_bits = lengths[asn_random_between(
         0, sizeof(lengths) / sizeof(lengths[0]) - 1)];
+    if(!constraints) constraints = &td->encoding_constraints;
     if(constraints->per_constraints) {
         const asn_per_constraint_t *pc =
             &td->encoding_constraints.per_constraints->size;
         if(pc->flags & APC_CONSTRAINED) {
+            long suggested_upper_bound = pc->upper_bound < (ssize_t)max_length
+                                             ? pc->upper_bound
+                                             : max_length;
             if(max_length < (size_t)pc->lower_bound) {
                 return result_skipped;
             }
-            rnd_bits = asn_random_between(pc->lower_bound, pc->upper_bound);
+            if(pc->flags & APC_EXTENSIBLE) {
+                switch(asn_random_between(0, 5)) {
+                case 0:
+                    if(pc->lower_bound > 0) {
+                        rnd_bits = pc->lower_bound - 1;
+                        break;
+                    }
+                    /* Fall through */
+                case 1:
+                    rnd_bits = pc->upper_bound + 1;
+                    break;
+                case 2:
+                    /* Keep rnd_bits from the table */
+                    if(rnd_bits < max_length) {
+                        break;
+                    }
+                    /* Fall through */
+                default:
+                    rnd_bits = asn_random_between(pc->lower_bound,
+                                                  suggested_upper_bound);
+                }
+            } else {
+                rnd_bits =
+                    asn_random_between(pc->lower_bound, suggested_upper_bound);
+            }
         } else {
             rnd_bits = asn_random_between(0, max_length - 1);
         }