Use of insecure cipher modePTC-W1005

Use of insecure cipher mode such as ECB is not recommended for use in cryptographic protocols because it is semantically insecure.

A block cipher uses a symmetric key to encrypt data in groups (blocks) of a pre-determined size (128 bits, 256 bits, etc). When dealing with data of arbitrary length, the cipher must be combined with a mode of operation.

When ECB mode is used, a message is divided into blocks. Each block of plaintext is encrypted independently of any other block. ECB encrypts identical plaintext blocks into identical ciphertext blocks. This is a problem because it will reveal if the same messages blocks are encrypted multiple times.

Using ECB allows an attacker to:

• Detect if two encrypted messages are identical.
• Detect if a block contains repetitive data.
• Detect if encrypted messages share a common prefix.

It is recommended to use any other mode besides ECB.

Bad practice

from cryptography.hazmat.primitives.ciphers import Cipher, algorithms, modes
cipher = Cipher(algorithms.AES(key), modes.ECB())

Recommended

from cryptography.hazmat.primitives.ciphers import Cipher, algorithms, modes
cipher = Cipher(algorithms.AES(key), modes.CBC(iv))

References:

• ECB mode
• Read more about different cipher modes here -- https://cryptography.io/en/latest/hazmat/primitives/symmetric-encryption/#module-cryptography.hazmat.primitives.ciphers.modes
• Stack overflow - Why not to use ECB
• OWASP Top 10 2021 Category A02 - Cryptographic Failures
• OWASP Top 10 2021 Category A06 - Vulnerable and Outdated Components
• CWE 327 - Use of a Broken or Risky Cryptographic Algorithm