diff --git a/library/constant_time_internal.h b/library/constant_time_internal.h
index c6f6f0a..053cf12 100644
--- a/library/constant_time_internal.h
+++ b/library/constant_time_internal.h
@@ -169,8 +169,29 @@
 
 #if defined(MBEDTLS_BASE64_C)
 
+/** Given a value in the range 0..63, return the corresponding Base64 digit.
+ *
+ * The implementation assumes that letters are consecutive (e.g. ASCII
+ * but not EBCDIC).
+ *
+ * \param value     A value in the range 0..63.
+ *
+ * \return          A base64 digit converted from \p value.
+ */
 unsigned char mbedtls_ct_base64_enc_char( unsigned char value );
 
+/** 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).
+ *
+ * \param c     A base64 digit.
+ *
+ * \return      The value of the base64 digit \p c.
+ */
 signed char mbedtls_ct_base64_dec_value( unsigned char c );
 
 #endif /* MBEDTLS_BASE64_C */