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diff --git a/libtomcrypt/src/pk/asn1/der/bit/der_length_bit_string.c b/libtomcrypt/src/pk/asn1/der/bit/der_length_bit_string.c
new file mode 100644
index 0000000..b9c99fb
--- /dev/null
+++ b/libtomcrypt/src/pk/asn1/der/bit/der_length_bit_string.c
@@ -0,0 +1,52 @@
+/* LibTomCrypt, modular cryptographic library -- Tom St Denis
+ *
+ * LibTomCrypt is a library that provides various cryptographic
+ * algorithms in a highly modular and flexible manner.
+ *
+ * The library is free for all purposes without any express
+ * guarantee it works.
+ */
+#include "tomcrypt.h"
+
+/**
+  @file der_length_bit_string.c
+  ASN.1 DER, get length of BIT STRING, Tom St Denis
+*/
+
+#ifdef LTC_DER
+/**
+  Gets length of DER encoding of BIT STRING
+  @param nbits  The number of bits in the string to encode
+  @param outlen [out] The length of the DER encoding for the given string
+  @return CRYPT_OK if successful
+*/
+int der_length_bit_string(unsigned long nbits, unsigned long *outlen)
+{
+   unsigned long nbytes;
+   LTC_ARGCHK(outlen != NULL);
+
+   /* get the number of the bytes */
+   nbytes = (nbits >> 3) + ((nbits & 7) ? 1 : 0) + 1;
+
+   if (nbytes < 128) {
+      /* 03 LL PP DD DD DD ... */
+      *outlen = 2 + nbytes;
+   } else if (nbytes < 256) {
+      /* 03 81 LL PP DD DD DD ... */
+      *outlen = 3 + nbytes;
+   } else if (nbytes < 65536) {
+      /* 03 82 LL LL PP DD DD DD ... */
+      *outlen = 4 + nbytes;
+   } else {
+      return CRYPT_INVALID_ARG;
+   }
+
+   return CRYPT_OK;
+}
+
+#endif
+
+
+/* ref:         $Format:%D$ */
+/* git commit:  $Format:%H$ */
+/* commit time: $Format:%ai$ */