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Cryptographic Vulnerability: Insecure mode

Cryptographic Vulnerability: Insecure mode


A block cipher processes the data blocks of fixed size. Usually, the size of a message is larger than the block size. Hence, the long message is divided into a series of sequential message blocks, and the cipher operates on these blocks one at a time.

Common block ciphers are:

  • ECB: (Electronic Code Book) is the most straightforward way of processing a series of sequentially listed message blocks. The user takes the first block of plaintext and encrypts it with the key to produce the first block of ciphertext. He then takes the second block of plaintext and follows the same process with same key and so on so forth. This mode is insecure and must not be used.
  • CBC: (Cipher Block Chaining) provides message dependence for generating ciphertext and makes the system non-deterministic. In CBC mode, the current plaintext block is added to the previous ciphertext block, and then the result is encrypted with the key. Decryption is thus the reverse process, which involves decrypting the current ciphertext and then adding the previous ciphertext block to the result.
  • CFB: (Cipher Feedback) each ciphertext block gets ‘fed back’ into the encryption process in order to encrypt the next plaintext block. The CFB mode requires an initialization vector (IV) as the initial random n-bit input block. The IV need not be secret. CFB mode differs significantly from ECB mode, the ciphertext corresponding to a given plaintext block depends not just on that plaintext block and the key, but also on the previous ciphertext block. In other words, the ciphertext block is dependent of message.
  • OFB: (Output Feedback) involves feeding the successive output blocks from the underlying block cipher back to it. These feedback blocks provide string of bits to feed the encryption algorithm which act as the key-stream generator as in case of CFB mode.The key stream generated is XOR-ed with the plaintext blocks. The OFB mode requires an IV as the initial random n-bit input block. The IV need not be secret.
  • CTR: (Counter) It can be considered as a counter-based version of CFB mode without the feedback. In this mode, both the sender and receiver need to access to a reliable counter, which computes a new shared value each time a ciphertext block is exchanged. This shared counter is not necessarily a secret value, but challenge is that both sides must keep the counter synchronized.

Most implementation will default to using the insecure ECB mode if it is not explicitly specified. The mode of operation used to encrypt the data is vulnerable and should be replaced with a secure one.


When using AES encryption, it is important to choose the correct mode of operation to ensure the confidentiality and integrity of the encrypted data. The choice of mode can depend on the specific use case and security requirements. Here are some common encryption modes to consider:

  • Electronic Codebook (ECB): This is the simplest mode and involves dividing the plaintext into blocks and encrypting each block separately using the same key. However, this mode is vulnerable to attacks as identical plaintext blocks will produce identical ciphertext blocks.
  • Cipher Block Chaining (CBC): In this mode, each plaintext block is XORed with the previous ciphertext block before encryption. This adds randomness and makes it harder to identify patterns in the encrypted data. However, this mode is vulnerable to attacks if the same IV (initialization vector) is used multiple times or initialized to non-random value.
  • Counter (CTR): This mode involves encrypting a counter value and XORing it with the plaintext to generate the ciphertext. This is a popular mode for streaming applications as it allows for parallel encryption and decryption. However, it requires unique counter values and can be vulnerable if an attacker can guess the counter values.
  • Galois/Counter Mode (GCM): This mode provides both confidentiality and integrity protection by using a combination of CTR mode and a message authentication code (MAC). It uses a unique IV and incorporates additional data such as a sequence number to prevent replay attacks. This mode is commonly used in applications that require both confidentiality and integrity protection.

  • In general, it is recommended to use GCM mode for AES encryption as it provides both confidentiality and integrity protection. However, it is important to properly manage the key and IV to ensure security. It is also recommended to use authenticated encryption modes such as GCM to prevent attacks such as padding oracle attacks.


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