Add EVP_EncodedLength.

Several callers of EVP_EncodeBlock are doing ad-hoc versions of this
function without any overflow checks.

Change-Id: I4d0cad2347ea8c44b42465e8b14b2783db69ee8f
Reviewed-on: https://boringssl-review.googlesource.com/1511
Reviewed-by: David Benjamin <davidben@chromium.org>
Reviewed-by: Adam Langley <agl@google.com>

include/openssl/base64.h

diff --git a/include/openssl/base64.h b/include/openssl/base64.h
index 38cb0ee..521004a 100644
--- a/include/openssl/base64.h
+++ b/include/openssl/base64.h
@@ -102,6 +102,12 @@
 OPENSSL_EXPORT size_t
     EVP_EncodeBlock(uint8_t *dst, const uint8_t *src, size_t src_len);
 
+/* EVP_EncodedLength sets |*out_len| to the number of bytes that will be needed
+ * to call |EVP_EncodeBlock| on an input of length |len|. This includes the
+ * final NUL that |EVP_EncodeBlock| writes. It returns one on success or zero
+ * on error. */
+int EVP_EncodedLength(size_t *out_len, size_t len);
+
 
 /* Decoding */
 

include/openssl/err.h

diff --git a/include/openssl/err.h b/include/openssl/err.h
index 8ebd73f..d8e7cd5 100644
--- a/include/openssl/err.h
+++ b/include/openssl/err.h
@@ -444,6 +444,7 @@
 #define ERR_R_SHOULD_NOT_HAVE_BEEN_CALLED (2 | ERR_R_FATAL)
 #define ERR_R_PASSED_NULL_PARAMETER (3 | ERR_R_FATAL)
 #define ERR_R_INTERNAL_ERROR (4 | ERR_R_FATAL)
+#define ERR_R_OVERFLOW (5 | ERR_R_FATAL)
 
 /* System error functions */
 #define SYS_F_fopen 100