Generation of Predictable IV with CBC Mode
The product generates and uses a predictable initialization Vector (IV) with Cipher Block Chaining (CBC) Mode, which causes algorithms to be susceptible to dictionary attacks when they are encrypted under the same key.
Description
CBC mode eliminates a weakness of Electronic Code Book (ECB) mode by allowing identical plaintext blocks to be encrypted to different ciphertext blocks. This is possible by the XOR-ing of an IV with the initial plaintext block so that every plaintext block in the chain is XOR'd with a different value before encryption. If IVs are reused, then identical plaintexts would be encrypted to identical ciphertexts. However, even if IVs are not identical but are predictable, then they still break the security of CBC mode against Chosen Plaintext Attacks (CPA).
Background
CBC mode is a commonly used mode of operation for a block cipher. It works by XOR-ing an IV with the initial block of a plaintext prior to encryption and then XOR-ing each successive block of plaintext with the previous block of ciphertext before encryption.
C_0 = IV
C_i = E_k{M_i XOR C_{i-1}}
When used properly, CBC mode provides security against chosen plaintext attacks. Having an unpredictable IV is a crucial underpinning of this. See [REF-1171].
Demonstrations
The following examples help to illustrate the nature of this weakness and describe methods or techniques which can be used to mitigate the risk.
Note that the examples here are by no means exhaustive and any given weakness may have many subtle varieties, each of which may require different detection methods or runtime controls.
Example One
In the following examples, CBC mode is used when encrypting data:
EVP_CIPHER_CTX ctx;
char key[EVP_MAX_KEY_LENGTH];
char iv[EVP_MAX_IV_LENGTH];
RAND_bytes(key, b);
memset(iv,0,EVP_MAX_IV_LENGTH);
EVP_EncryptInit(&ctx,EVP_bf_cbc(), key,iv);public class SymmetricCipherTest {
public static void main() {
byte[] text ="Secret".getBytes();
byte[] iv ={
0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00
};
KeyGenerator kg = KeyGenerator.getInstance("DES");
kg.init(56);
SecretKey key = kg.generateKey();
Cipher cipher = Cipher.getInstance("DES/CBC/PKCS5Padding");
IvParameterSpec ips = new IvParameterSpec(iv);
cipher.init(Cipher.ENCRYPT_MODE, key, ips);
return cipher.doFinal(inpBytes);
}
}In both of these examples, the initialization vector (IV) is always a block of zeros. This makes the resulting cipher text much more predictable and susceptible to a dictionary attack.
See Also
Weaknesses in this category are related to randomness.
Weaknesses in this category are related to the "Common Mode Frailties" category from the SEI ETF "Categories of Security Vulnerabilities in ICS" as published in March ...
Weaknesses in this category are related to the A02 category "Cryptographic Failures" in the OWASP Top Ten 2021.
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