74 lines
4.0 KiB
Markdown
74 lines
4.0 KiB
Markdown
# Electronic Code Book \(ECB\)
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## ECB
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\(ECB\) Electronic Code Book - symmetric encryption scheme which **replaces each block of the clear text** by the **block of ciphertext**. It is the **simplest** encryption scheme. The main idea is to **split** the clear text into **blocks of N bits** \(depends on the size of the block of input data, encryption algorithm\) and then to encrypt \(decrypt\) each block of clear text using the only key.
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Using ECB has multiple security implications:
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* **Blocks from encrypted message can be removed**
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* **Blocks from encrypted message can be moved around**
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## Detection of the vulnerability
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Imagine you login into an application several times and you **always get the same cookie**. This is because the cookie of the application is **`<username>|<password>`**.
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Then, you generate to new users, both of them with the **same long password** and **almost** the **same** **username**.
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You find out that the **blocks of 8B** where the **info of both users** is the same are **equals**. Then, you imagine that this might be because **ECB is being used**.
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Like in the following example. Observe how these **2 decoded cookies** has several times the block **`\x23U\xE45K\xCB\x21\xC8`**
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```text
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\x23U\xE45K\xCB\x21\xC8\x23U\xE45K\xCB\x21\xC8\x04\xB6\xE1H\xD1\x1E \xB6\x23U\xE45K\xCB\x21\xC8\x23U\xE45K\xCB\x21\xC8+=\xD4F\xF7\x99\xD9\xA9
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\x23U\xE45K\xCB\x21\xC8\x23U\xE45K\xCB\x21\xC8\x04\xB6\xE1H\xD1\x1E \xB6\x23U\xE45K\xCB\x21\xC8\x23U\xE45K\xCB\x21\xC8+=\xD4F\xF7\x99\xD9\xA9
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```
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This is because the **username and password of those cookies contained several times the letter "a"** \(for example\). The **blocks** that are **different** are blocks that contained **at least 1 different character** \(maybe the delimiter "\|" or some necessary difference in the username\).
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Now, the attacker just need to discover if the format is `<username><delimiter><password>` or `<password><delimiter><username>`. For doing that, he can just **generate several usernames** with s**imilar and long usernames and passwords until he find the format and the length of the delimiter:**
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| Username length: | Password length: | Username+Password length: | Cookie's length \(after decoding\): |
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| :--- | :--- | :--- | :--- |
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| 2 | 2 | 4 | 8 |
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| 3 | 3 | 6 | 8 |
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| 3 | 4 | 7 | 8 |
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| 4 | 4 | 8 | 16 |
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| 7 | 7 | 14 | 16 |
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## Exploitation of the vulnerability
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### Removing entire blocks
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Knowing the format of the cookie \(`<username>|<password>`\), in order to impersonate the username `admin` create a new user called `aaaaaaaaadmin` and get the cookie and decode it:
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```text
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\x23U\xE45K\xCB\x21\xC8\xE0Vd8oE\x123\aO\x43T\x32\xD5U\xD4
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```
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We can see the pattern `\x23U\xE45K\xCB\x21\xC8` created previously with the username that contained only `a`.
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Then, you can remove the first block of 8B and you will et a valid cookie for the username `admin`:
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```text
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\xE0Vd8oE\x123\aO\x43T\x32\xD5U\xD4
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```
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### Moving blocks
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In many databases it is the same to search for `WHERE username='admin';` or for `WHERE username='admin ';` _\(Note the extra spaces\)_
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So, another way to impersonate the user `admin` would be to:
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* Generate a username that: `len(<username>) + len(<delimiter) % len(block)`. With a block size of `8B` you can generate username called: `username` , with the delimiter `|` the chunk `<username><delimiter>` will generate 2 blocks of 8Bs.
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* Then, generate a password that will fill an exact number of blocks containing the username we want to impersonate and spaces, like: `admin`
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The cookie of this user is going to be composed by 3 blocks: the first 2 is the blocks of the username + delimiter and the third one of the password \(which is faking the username\): `username |admin`
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**Then, just replace the first block with the last time and will be impersonating the user `admin`: `admin |username`**
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## References
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* [http://cryptowiki.net/index.php?title=Electronic\_Code\_Book\_\(ECB\)](http://cryptowiki.net/index.php?title=Electronic_Code_Book_%28ECB%29)
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