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71 lines
2.6 KiB
Markdown
71 lines
2.6 KiB
Markdown
---
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title: "AES"
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aliases:
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- firmwareapi/pycom/aes.html
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- firmwareapi/pycom/aes.md
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- chapter/firmwareapi/pycom/aes
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---
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AES (Advanced Encryption Standard) is a symmetric block cipher standardised by NIST. It has a fixed data block size of 16 bytes. Its keys can be 128, 192, or 256 bits long.
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>AES is implemented using the ESP32 hardware module.
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## Quick Usage Example
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```python
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from crypto import AES
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import crypto
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key = b'notsuchsecretkey' # 128 bit (16 bytes) key
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iv = crypto.getrandbits(128) # hardware generated random IV (never reuse it)
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cipher = AES(key, AES.MODE_CFB, iv)
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msg = iv + cipher.encrypt(b'Attack at dawn')
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# ... after properly sent the encrypted message somewhere ...
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cipher = AES(key, AES.MODE_CFB, msg[:16]) # on the decryption side
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original = cipher.decrypt(msg[16:])
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print(original)
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```
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## Constructors
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### class ucrypto.AES(key, [mode=AES.MODE_ECB, IV, counter, segment_size])
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Create an AES object that will let you encrypt and decrypt messages.
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The arguments are:
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* `key` (byte string) is the secret key to use. It must be 16 (AES-128), 24 (AES-192), or 32 (AES-256) bytes long.
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* `mode` is the chaining mode to use for encryption and decryption. Can be the following values:
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* `AES.MODE_ECB`: Electronic Code Book. Simplest encryption mode.
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* `AES.MODE_CBC`: Cipher-Block Chaining. An Initialisation Vector (IV) is required.
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* `AES.MODE_CFB`: Cipher feedback. `plaintext` and `ciphertext` are processed in segments of `segment_size` bits. Works as a stream cipher.
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* `AES.MODE_CTR`: Counter mode. Each message block is associated to a counter which must be unique across all messages that get encrypted with the same key.
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* `IV` (byte string) initialisation vector. Should be 16 bytes long. It is ignored in modes `AES.MODE_ECB` and `AES.MODE_CRT`.
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> To avoid security issues, IV should always be a random number and should never be reused to encrypt two different messages. The same applies to the counter in CTR mode. You can use `crypto.getrandbits()` for this purpose.
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* `counter` (byte string) used only for `AES.MODE_CTR`. Should be 16 bytes long. Should not be reused.
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* `segment_size` is the number of bits `plaintext` and `ciphertext` are segmented in. Is only used in `AES.MODE_CFB`. Supported values are:
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* `AES.SEGMENT_8`
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* `AES.SEGMENT_128`
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## Methods
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### ucrypto.encrypt()
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Encrypt data with the key and the parameters set at initialisation.
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### ucrypto.decrypt()
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Decrypt data with the key and the parameters set at initialisation.
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## Constants
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* `AES.MODE_ECB`, `AES.MODE_CBC`, `AES.MODE_CFB`, `AES.MODE_CTR`
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* `AES.SEGMENT_8`, `AES.SEGMENT_128`
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