Merge pull request #26 from vin01/patch-1

Update crypto.md

cheatsheets/crypto.md

@@ -47,3 +47,31 @@
 ```
 0123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345
 ```
+
+**Length extension attack**
+
+In cryptography and computer security, a length extension attack is a type of attack where an attacker can use `Hash(message1)` and the length of `message1` to calculate `Hash(message1 ∥ message2)` for an attacker-controlled `message2`.
+
+In Summary: Given a hash that is composed of a string with an unknown prefix, an attacker can append to the string and produce a new hash that still has the unknown prefix.
+
+An example:
+
+```
+http://example.com/download?file=report.pdf&mac=563162c9c71a17367d44c165b84b85ab59d036f9
+```
+
+```
+http://example.com/download?file=report.pdf%80%00%00%00%00%00%00%00%00%00%00%00%00%00%00%00%00
+
+%00%00%00%00%00%00%00%00%00%00%00%00%00%00%00%00%00%00%00%00%00%00%00
+
+%00%00%A8/../../../../../../../etc/passwd&mac=ee40aa8ec0cfafb7e2ec4de20943b673968857a5
+```
+A related HackerOne report: https://hackerone.com/reports/251572
+
+Tool to extend a hash: https://github.com/iagox86/hash_extender
+
+More details about the attack:
+
+https://www.whitehatsec.com/blog/hash-length-extension-attacks/
+https://blog.skullsecurity.org/2012/everything-you-need-to-know-about-hash-length-extension-attacks