Merge pull request #20 from hdG8/flip-my-letters-20-points

Flip My Letters writeup.

Author: Michael Zhang

SUMMARY.md

@@ -6,6 +6,7 @@
   * [Simple ROP \[120 points\]](/binary-exploitation/simple-rop-120-points.md)
   * [Heaps of Knowledge \[420 points\]](/binary-exploitation/heaps-of-knowledge-420-points.md)
 * [Cryptography](cryptography.md)
+  * [Flip My Letters \[20 points\]](/cryptography/flip-my-letters-20-points.md)
   * [RSA 1 \[50 points\]](/cryptography/rsa-1-50-points.md)
   * [Hash on Hash \[100 points\]](/cryptography/hash-on-hash-100-points.md)
   * [Security Through Obscurity \[150 points\]](/cryptography/security-through-obscurity-150-points.md)

cryptography.md

@@ -3,7 +3,7 @@
 This category focuses on using advanced mathematical topics to encrypt data to prevent it from being intercepted/tampered with. Cryptography has many practical applications from HTTPS to cryptocurrency. The cryptography challenges in this contest include:
 
 * Clear and Concise Commentary on Caesar Cipher \[20 points\]
-* Flip My Letters \[20 points\]
+* [Flip My Letters \[20 points\]](/cryptography/flip-my-letters-20-points.md)
 * [RSA 1 \[50 points\]](/cryptography/rsa-1-50-points.md)
 * Let Me Be Frank \[75 points\]
 * RSA 2 \[80 points\]

cryptography/flip-my-letters-20-points.md

@@ -0,0 +1,24 @@
+# Flip My Letters - 20 points
+
+I dropped my alphabet on its head, can you help me reassemble it?
+```
+easyctf{r_wlmg_vevm_mvvw_zm_zhxrr_gzyov}
+```
+
+### Solution
+
+The hint suggests that it's a substitution cipher, where the key is the alphabet in reverse.
+It is easy to test the idea using the [tr](https://en.wikipedia.org/wiki/Tr_%28Unix%29) Unix command.
+Sure enough, it works.
+
+```
+$ echo "r_wlmg_vevm_mvvw_zm_zhxrr_gzyov" | tr abcdefghijklmnopqrstuvwxyz zyxwvutsrqponmlkjihgfedcba
+i_dont_even_need_an_ascii_table
+```
+
+Flag:
+```
+easyctf{i_dont_even_need_an_ascii_table}
+```
+
+### External Writeups