diff --git a/crypto/bytestring/CMakeLists.txt b/crypto/bytestring/CMakeLists.txt
index 33d3c21..362e702 100644
--- a/crypto/bytestring/CMakeLists.txt
+++ b/crypto/bytestring/CMakeLists.txt
@@ -5,6 +5,7 @@
 
   OBJECT
 
+  asn1_compat.c
   ber.c
   cbs.c
   cbb.c
diff --git a/crypto/bytestring/asn1_compat.c b/crypto/bytestring/asn1_compat.c
new file mode 100644
index 0000000..b17d2d1
--- /dev/null
+++ b/crypto/bytestring/asn1_compat.c
@@ -0,0 +1,51 @@
+/* Copyright (c) 2016, Google Inc.
+ *
+ * Permission to use, copy, modify, and/or distribute this software for any
+ * purpose with or without fee is hereby granted, provided that the above
+ * copyright notice and this permission notice appear in all copies.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+ * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+ * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY
+ * SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+ * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION
+ * OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN
+ * CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE. */
+
+
+#include <openssl/bytestring.h>
+
+#include <assert.h>
+#include <limits.h>
+#include <string.h>
+
+#include <openssl/mem.h>
+
+#include "internal.h"
+
+
+int CBB_finish_i2d(CBB *cbb, uint8_t **outp) {
+  assert(cbb->base->can_resize);
+
+  uint8_t *der;
+  size_t der_len;
+  if (!CBB_finish(cbb, &der, &der_len)) {
+    CBB_cleanup(cbb);
+    return -1;
+  }
+  if (der_len > INT_MAX) {
+    OPENSSL_free(der);
+    return -1;
+  }
+  if (outp != NULL) {
+    if (*outp == NULL) {
+      *outp = der;
+      der = NULL;
+    } else {
+      memcpy(*outp, der, der_len);
+      *outp += der_len;
+    }
+  }
+  OPENSSL_free(der);
+  return (int)der_len;
+}
diff --git a/crypto/bytestring/internal.h b/crypto/bytestring/internal.h
index f4f4b9c..2fed413 100644
--- a/crypto/bytestring/internal.h
+++ b/crypto/bytestring/internal.h
@@ -58,6 +58,15 @@
                                                 unsigned outer_tag,
                                                 unsigned inner_tag);
 
+/* CBB_finish_i2d calls |CBB_finish| on |cbb| which must have been initialized
+ * with |CBB_init|. If |outp| is not NULL then the result is written to |*outp|
+ * and |*outp| is advanced just past the output. It returns the number of bytes
+ * in the result, whether written or not, or a negative value on error. On
+ * error, it calls |CBB_cleanup| on |cbb|.
+ *
+ * This function may be used to help implement legacy i2d ASN.1 functions. */
+int CBB_finish_i2d(CBB *cbb, uint8_t **outp);
+
 
 #if defined(__cplusplus)
 }  /* extern C */
diff --git a/crypto/ecdsa/ecdsa_asn1.c b/crypto/ecdsa/ecdsa_asn1.c
index 61f5d62..3a47257 100644
--- a/crypto/ecdsa/ecdsa_asn1.c
+++ b/crypto/ecdsa/ecdsa_asn1.c
@@ -61,6 +61,7 @@
 #include <openssl/ec_key.h>
 #include <openssl/mem.h>
 
+#include "../bytestring/internal.h"
 #include "../ec/internal.h"
 
 
@@ -211,30 +212,15 @@
     ECDSA_SIG_free(*out);
     *out = ret;
   }
-  *inp += (size_t)len - CBS_len(&cbs);
+  *inp = CBS_data(&cbs);
   return ret;
 }
 
 int i2d_ECDSA_SIG(const ECDSA_SIG *sig, uint8_t **outp) {
-  uint8_t *der;
-  size_t der_len;
-  if (!ECDSA_SIG_to_bytes(&der, &der_len, sig)) {
+  CBB cbb;
+  if (!CBB_init(&cbb, 0) ||
+      !ECDSA_SIG_marshal(&cbb, sig)) {
     return -1;
   }
-  if (der_len > INT_MAX) {
-    OPENSSL_PUT_ERROR(ECDSA, ERR_R_OVERFLOW);
-    OPENSSL_free(der);
-    return -1;
-  }
-  if (outp != NULL) {
-    if (*outp == NULL) {
-      *outp = der;
-      der = NULL;
-    } else {
-      memcpy(*outp, der, der_len);
-      *outp += der_len;
-    }
-  }
-  OPENSSL_free(der);
-  return (int)der_len;
+  return CBB_finish_i2d(&cbb, outp);
 }
diff --git a/crypto/rsa/rsa_asn1.c b/crypto/rsa/rsa_asn1.c
index 2970717..83bae4d 100644
--- a/crypto/rsa/rsa_asn1.c
+++ b/crypto/rsa/rsa_asn1.c
@@ -67,6 +67,7 @@
 #include <openssl/mem.h>
 
 #include "internal.h"
+#include "../bytestring/internal.h"
 
 
 static int parse_integer_buggy(CBS *cbs, BIGNUM **out, int buggy) {
@@ -379,32 +380,17 @@
     RSA_free(*out);
     *out = ret;
   }
-  *inp += (size_t)len - CBS_len(&cbs);
+  *inp = CBS_data(&cbs);
   return ret;
 }
 
 int i2d_RSAPublicKey(const RSA *in, uint8_t **outp) {
-  uint8_t *der;
-  size_t der_len;
-  if (!RSA_public_key_to_bytes(&der, &der_len, in)) {
+  CBB cbb;
+  if (!CBB_init(&cbb, 0) ||
+      !RSA_marshal_public_key(&cbb, in)) {
     return -1;
   }
-  if (der_len > INT_MAX) {
-    OPENSSL_PUT_ERROR(RSA, ERR_R_OVERFLOW);
-    OPENSSL_free(der);
-    return -1;
-  }
-  if (outp != NULL) {
-    if (*outp == NULL) {
-      *outp = der;
-      der = NULL;
-    } else {
-      memcpy(*outp, der, der_len);
-      *outp += der_len;
-    }
-  }
-  OPENSSL_free(der);
-  return (int)der_len;
+  return CBB_finish_i2d(&cbb, outp);
 }
 
 RSA *d2i_RSAPrivateKey(RSA **out, const uint8_t **inp, long len) {
@@ -421,32 +407,17 @@
     RSA_free(*out);
     *out = ret;
   }
-  *inp += (size_t)len - CBS_len(&cbs);
+  *inp = CBS_data(&cbs);
   return ret;
 }
 
 int i2d_RSAPrivateKey(const RSA *in, uint8_t **outp) {
-  uint8_t *der;
-  size_t der_len;
-  if (!RSA_private_key_to_bytes(&der, &der_len, in)) {
+  CBB cbb;
+  if (!CBB_init(&cbb, 0) ||
+      !RSA_marshal_private_key(&cbb, in)) {
     return -1;
   }
-  if (der_len > INT_MAX) {
-    OPENSSL_PUT_ERROR(RSA, ERR_R_OVERFLOW);
-    OPENSSL_free(der);
-    return -1;
-  }
-  if (outp != NULL) {
-    if (*outp == NULL) {
-      *outp = der;
-      der = NULL;
-    } else {
-      memcpy(*outp, der, der_len);
-      *outp += der_len;
-    }
-  }
-  OPENSSL_free(der);
-  return (int)der_len;
+  return CBB_finish_i2d(&cbb, outp);
 }
 
 ASN1_SEQUENCE(RSA_PSS_PARAMS) = {