diff --git a/SM4-with-AESENCLAST.md b/SM4-with-AESENCLAST.md
index 0456935..ed013db 100644
--- a/SM4-with-AESENCLAST.md
+++ b/SM4-with-AESENCLAST.md
@@ -1,3 +1,14 @@
 This is the pure golang code to study SM4 implementation with AESENCLAST instruction.
 
-[sm4 with AESENCLAST](https://gist.github.com/emmansun/ae4677d71c75ff8407d5f5b3a884f5d2)
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+[sm4 with AESENCLAST](https://gist.github.com/emmansun/ae4677d71c75ff8407d5f5b3a884f5d2)
+
+We combine various linear operations into two affine transforms (one on 
+each side), A1 and A2. Here affine transform consists of a multiplication 
+with a 8x8 binary matrix M and addition of a 8-bit constant C.
+```
+SM4-S(x) = A2(AES-S(A1(x))
+A1(x) = M1*x + C1
+A2(x) = M2*x + C2
+```
+
+The combinations of (M1, C1, M2, C2) are not unique.
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