Swift Static Initialization Vector

Description

Reusing an IV or nonce with the same key can reveal repeated plaintext prefixes in CBC and compromise the confidentiality of different messages in CTR and GCM. In GCM, reuse also weakens authentication-tag protection.

Potential impact

• Exposure of repeated plaintext patterns
• Increased opportunity for ciphertext analysis
• Greater risk of data recovery with the same key and weakened GCM authentication

Remediation

1. Generate a cryptographically secure random IV or nonce for each encryption operation.
2. IVs and nonces need not be secret, but must not be reused with the same key.
3. Prefer APIs that handle nonce generation, such as CryptoKit's AES-GCM.

Examples

Before

let aes = try AES(
    key: key,
    blockMode: CBC(iv: Array<UInt8>(repeating: 0, count: AES.blockSize)))

After

import Foundation
import CryptoKit

func encryptWithFreshNonce(
    data: Data,
    key: SymmetricKey
) throws -> AES.GCM.SealedBox {
    return try AES.GCM.seal(data, using: key)
}

Explanation:

• Before: A fixed IV can reveal relationships between ciphertexts when input repeats.
• After: Omitting the nonce from AES.GCM.seal lets the API generate a fresh random nonce.

References

• CWE-329
• CWE-1204
• CodeQL - Static initialization vector