+ if (EVP_DecodeUpdate(ctx, a, &outl2, (const unsigned char *)src, size) < 0) {
+ outl = -1;
+ goto err;
+ }
+ outl += outl2;
+ EVP_DecodeFinal(ctx, a + outl, &outl2);
+ outl += outl2;
+
+ /* Strip off the leading padding */
+ if (padsize != 0) {
+ if ((int)padsize >= outl) {
+ outl = -1;
+ goto err;
+ }
+
+ /*
+ * If we added 1 byte of padding prior to encoding then we have 2 bytes
+ * of "real" data which gets spread across 4 encoded bytes like this:
+ * (6 bits pad)(2 bits pad | 4 bits data)(6 bits data)(6 bits data)
+ * So 1 byte of pre-encoding padding results in 1 full byte of encoded
+ * padding.
+ * If we added 2 bytes of padding prior to encoding this gets encoded
+ * as:
+ * (6 bits pad)(6 bits pad)(4 bits pad | 2 bits data)(6 bits data)
+ * So 2 bytes of pre-encoding padding results in 2 full bytes of encoded
+ * padding, i.e. we have to strip the same number of bytes of padding
+ * from the encoded data as we added to the pre-encoded data.
+ */
+ memmove(a, a + padsize, outl - padsize);
+ outl -= padsize;
+ }
+
+ err:
+ EVP_ENCODE_CTX_free(ctx);
+
+ return outl;