diff --git a/library/constant_time.c b/library/constant_time.c
index cdaa7fa..5bf594f 100644
--- a/library/constant_time.c
+++ b/library/constant_time.c
@@ -344,6 +344,34 @@
     return( digit );
 }
 
+/* Given a Base64 digit, return its value.
+ * If c is not a Base64 digit ('A'..'Z', 'a'..'z', '0'..'9', '+' or '/'),
+ * return -1.
+ *
+ * The implementation assumes that letters are consecutive (e.g. ASCII
+ * but not EBCDIC).
+ *
+ * The implementation is constant-flow (no branch or memory access depending
+ * on the value of c) unless the compiler inlines and optimizes a specific
+ * access.
+ */
+signed char mbedtls_ct_base64_dec_value( unsigned char c )
+{
+    unsigned char val = 0;
+    /* For each range of digits, if c is in that range, mask val with
+     * the corresponding value. Since c can only be in a single range,
+     * only at most one masking will change val. Set val to one plus
+     * the desired value so that it stays 0 if c is in none of the ranges. */
+    val |= mbedtls_ct_uchar_mask_of_range( 'A', 'Z', c ) & ( c - 'A' +  0 + 1 );
+    val |= mbedtls_ct_uchar_mask_of_range( 'a', 'z', c ) & ( c - 'a' + 26 + 1 );
+    val |= mbedtls_ct_uchar_mask_of_range( '0', '9', c ) & ( c - '0' + 52 + 1 );
+    val |= mbedtls_ct_uchar_mask_of_range( '+', '+', c ) & ( c - '+' + 62 + 1 );
+    val |= mbedtls_ct_uchar_mask_of_range( '/', '/', c ) & ( c - '/' + 63 + 1 );
+    /* At this point, val is 0 if c is an invalid digit and v+1 if c is
+     * a digit with the value v. */
+    return( val - 1 );
+}
+
 #endif /* MBEDTLS_BASE64_C */
 
 #if defined(MBEDTLS_PKCS1_V15) && defined(MBEDTLS_RSA_C) && !defined(MBEDTLS_RSA_ALT)