# Cryptography

* Often data is just encoded in base64 or hex. Other times it's just compressed (gzip):
	- text 32 characters long --> md5 hash.
	- 40 characters long --> SHA1 hash.
	- equal signs spread --> base64 encoded string.
	- text only letters, without numbers or special characters --> Caesar, Vigenere, or other type of cipher.
	- hints about keys and signing --> likely RSA.




## MD5

- The MD5 hashing algorithm always returns 128 bit values, so the chance that two randomly chosen objects have the same hash is 1:2**128.



### Scripts

- Hash length extension attack
- Brute force hex digest chars


### Command Line
```
$ echo -n password | md5sum
5f4dcc3b5aa765d61d8327deb882cf99
```

- 32 chars

```
7e1321b3c8423b30c1cb077a2e3ac4f0a2a551a6458a8de22446cc76d639a9e98fc42c6cddf9966db3b09e843650343578b04d5e377d298e78455efc5ca404d5f4c9385f1902f7334b00b9b4ecd164de8bf8854bebe108183caeb845c7676ae48fc42c6ddf9966db3b09e84365034357327a6c4304ad5938eaf0efb6cc3e53dc7ff9ea9a069bd793691c422fb818
```

- Use Python's md5.md5().digest()

- md5 hashes: [here](http://hash-killer.com/), [here](http://www.md5this.com/), [here](http://www.hashkiller.co.uk/).

- [md5sum](http://linux.about.com/library/cmd/blcmdl1_md5sum.htm)
- [md5 creator](http://www.md5-creator.com/)

------

## SHA

- SHA-1 has output size of 160 bits, so chances of collisions are 2**160.

- [Hash maker](http://ratfactor.com/sha1).

### Scripts
- SHA-256 brute force


### Command Line

- Brute force:
```
import hashlib, itertools
hash = '6307c5441ebac07051e3b90d53c3106230dd9aa128601dcd5f63efcf824ce1ba'
ch = 'abcdef0123456789'
for a, b, c, d, e, f in itertools.product(ch, ch, ch, ch, ch, ch):
    if hashlib.sha256('ASIS_a9%s00f497f2eaa4372a7fc21f0d' % (a + b + c + d + e + f)).hexdigest() == hash:
        print 'ASIS_a9%s00f497f2eaa4372a7fc21f0d' % (a + b + c + d + e + f)
```




--------

## Rotation Ciphers


### Scripts
- Caesar
- Brute force rotation
- Pygenere
- Frequency analysis


### Online tools:

- Frequency analysis: [here](http://www.simonsingh.net/The_Black_Chamber/hintsandtips.html) and [here](http://www.xarg.org/tools/caesar-cipher)

- [Cesar Cipher decryption](http://www.xarg.org/tools/caesar-cipher/) and [here](http://tools.zenverse.net/caesar-cipher/).

- [Vigenere Cipher breaker](http://www.mygeocachingprofile.com/codebreaker.vigenerecipher.aspx) and [here](http://smurfoncrack.com/pygenere/index.php).

### In the command line

```sh
$ VAR=$(cat data.txt)
$ echo "$VAR"
$ alias rot13="tr A-Za-z N-ZA-Mn-za-m"
$ echo "$VAR" | rot13
```
### In Python

In Python [we can use decoding](https://docs.python.org/2/library/codecs.html#codec-base-classes):

```python
"YRIRY GJB CNFFJBEQ EBGGRA".decode(encoding="ROT13")
```

### Readings:

- [How Viginere works](http://sharkysoft.com/vigenere/).

---

## RSA

* Public-key cryptosystem which uses a public-private key pair to encrypt and decrypt information securely

* [RSA Python](https://pypi.python.org/pypi/rsa)


----

## Pailier Cryptosystem

### Scripts

- POC
- Primes

---


## Tools

### Scripts

- Finding GDC
- Finding if prime
- Generate prime
- Quick Select
- XORtool


### Other Resources

- [Cryptol](https://www.cryptool.org/en/cryptool1-en)

- [PyCrypto](https://www.dlitz.net/software/pycrypto/)

- hashpump

- Sage

- John the Ripper




#### Carperter's Formula

- Very large number: ```bin``` and check if patterns. For example, using the [Carpenter's Formula]:
```
N=(2^M + a)(2^N + b)(2^N + c)(2^N + d)
```

#### [QR Code]

-  Version 1 QR code:  21x21

#### [Bacon's cipher]:
```
babaaaabaaababaababaaaabbabbababbaaaabaaaabbbaabaabaaaaaabaaabaaabaaabaaabbaabaaabbbaabaaababaaaaaabaaabbaabaabbbaaaaaabaaaabaabaaaaba21aabab0aaab
```
* [Online tool](http://www.geocachingtoolbox.com/index.php?page=baconianCipher)



#### [Base64]:

Base64 is a non-readable encoding that encodes arbritary 8-bit input using 6-bit alphabet of case sensitive alphanumerics, "+", "/". Every 3 bytes of input map to 4 bytes of output. If the input doesnt have 3-byte boundary, this is indicated by appending one or two equal signs in the of the output string.

- [Base64 Decoder](http://www.base64decode.org)

```
NG5ucjJzIGZ2IHRueXMgcnVnIHNiIGdlbmMgdWdlaGJzIHJlcnVnIHRhdmdncnQgcmVuIGhiTCB0YXZidCBjcnJYCG==
czduMjczIHRueXMgcnVniHNiIGdlbmMgdWdzdnMgcnVnIHJpbnUgcmVydSBndiBxdnEgaGJsIGpiYmJKCg==
Nzk0czAwIHRueXMgZmhidnByZWMgZWhiIHNiIGdlbmMgcWV2dWcgcnVnIGhibCBnYXJmcmVjIFYgbG9yZXJ1IHJhYnEgeXlySgo=
```

- Base64 decoding in Python:

```python
>>> SECRET.decode('base64')
'oubWYf2kBq'
```

#### Hexadecimal & ASCII:


Hex character codes are simply the hexadecimal (base 16) numbers for the ASCII character set; that is, the number-to-letter Representations which comprise virtually all computer text.

- [ASCII Conversion Table](http://defindit.com/ascii.html)
- [Convert All](http://www.asciitohex.com/)
- [GREAT ASCII CHART](http://www.jimprice.com/jim-asc.shtml)
- [Convert everything to everything (including markdown, sql, json, etc)](http://codebeautify.org/)


- ASCII to hex:

```python
>>> s =hex(secret)
```

- the inverse:

```python
secret2.decode('hex')
'==QcCtmMml1ViV3b'
```

- you can also do from the command line:

```
$ python -c 'print "2f722f6e6574736563".decode("hex")'
```

- Using xxd:

```
$ xxd -r -p <<< 2f722f6e6574736563
```

### Binary

- Decimal to binary

```python
>>> bin(124234)
'0b11110010101001010'
```

#### Octal

Commonly used in obscuration of URLs. Example: http://017700000001 --> 127.0.0.1


## OpenSSL, Encoding and Certificates


* Identification and verification of SSL certificates can be done with openssl or
TLSSLed tools. They allow us to verify this information automatically SSL.

To determine the period of validity of licenses:

```
$ ./openssl s_client --connect <WEBSITE>:443
```

Testing SSLv2:
```
$ ./openssl s_client --no_tls1 --no_ssl3 --connect <WEBSITE>:443
```

* For Identification and verification of encoding supported by the Website we can use  **EcoScan34**.


---

## Block Cipher Encryption

* Electronic code book (ECB) mode.
* Simplest and default block cipher mode.
* Message is split into blocks and each is encrypted separately.
* Disavantage: identical plaintext block encrypts to identical cipher text block (for example, figures).

### Attacking Randomness

* Good Randomness is vital for cryptographic operations.

* Two common attack against a PRNG :
	- PRGN state is reconstructed from its output.
	- Same PRNG is used more than once.

* Statistically random is not secure random!
	- if a PRNG is seeded with a value the attacker can influence, the state of the PRNG is likely compromised.

* Seed race condition attacks:
	- System clock often used to seed PRNG
	- Submit 10's or 100's of rquests at a time. Seed a PRNG with the same system clock and the output will be the same.




----



[SHA]:http://en.wikipedia.org/wiki/Secure_Hash_Algorithm
[MD5]: http://en.wikipedia.org/wiki/MD5
[Base64]: http://en.wikipedia.org/wiki/Base64
[Bacon's cipher]:http://en.wikipedia.org/wiki/Bacon's_ciphe
[Carpenter's Formula]:http://security.cs.pub.ro/hexcellents/wiki/writeups/asis_rsang
[pngcheck]: http://www.libpng.org/pub/png/apps/pngcheck.html
[karmadecay]: http://karmadecay.com/
[tineye]:  https://www.tineye.com/
[images.google.com]: https://images.google.com/?gws_rd=ssl
[base64 decoding]: http://www.motobit.com/util/base64-decoder-encoder.asp
[pnginfo]: http://www.stillhq.com/pngtools/
[namechk]: http://namechk.com
[QR Code]: http://en.wikipedia.org/wiki/QR_code