CVE-2018-6594 in PyCrypto
Summary
by MITRE
lib/Crypto/PublicKey/ElGamal.py in PyCrypto through 2.6.1 generates weak ElGamal key parameters, which allows attackers to obtain sensitive information by reading ciphertext data (i.e., it does not have semantic security in face of a ciphertext-only attack). The Decisional Diffie-Hellman (DDH) assumption does not hold for PyCrypto's ElGamal implementation.
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Analysis
by VulDB Data Team • 02/03/2023
The vulnerability identified as CVE-2018-6594 affects the PyCrypto library version 2.6.1 and earlier, specifically within the ElGamal cryptographic implementation located in lib/Crypto/PublicKey/ElGamal.py. This flaw represents a critical weakness in the cryptographic security model that undermines the fundamental assumptions underlying the ElGamal encryption scheme. The vulnerability stems from the generation of weak key parameters that fail to meet the mathematical requirements necessary for secure cryptographic operations. According to the CWE catalog, this corresponds to CWE-327, which addresses the use of weak cryptographic algorithms and the failure to implement proper cryptographic key generation mechanisms.
The technical flaw manifests in the implementation's violation of the Decisional Diffie-Hellman (DDH) assumption, a critical mathematical foundation for many cryptographic protocols including ElGamal encryption. The DDH assumption states that given three elements g^a, g^b, and g^c in a cyclic group, it should be computationally infeasible to determine whether c equals a+b modulo the group order. When this assumption fails, as demonstrated in PyCrypto's implementation, attackers can exploit the mathematical weakness to perform ciphertext-only attacks that reveal sensitive information about the encrypted data. This weakness directly impacts the semantic security properties of the encryption scheme, making it vulnerable to attacks that would normally be computationally infeasible.
The operational impact of this vulnerability extends beyond simple information disclosure, as it fundamentally compromises the confidentiality guarantees that ElGamal encryption is designed to provide. Attackers who can analyze ciphertext data can potentially reconstruct plaintext information without possessing the corresponding private key, which represents a complete breakdown of the encryption security model. The vulnerability affects any system using PyCrypto's ElGamal implementation for encryption, particularly those handling sensitive data such as financial transactions, personal identification information, or classified communications. This weakness aligns with ATT&CK technique T1552.004, which covers unsecured credentials and cryptographic key exposure, as the flawed implementation creates a vector for credential and data compromise through cryptographic weakness.
The implications of this vulnerability are particularly severe given that PyCrypto was widely used in Python applications for cryptographic operations, making numerous systems potentially vulnerable to attacks that exploit this mathematical weakness. Organizations relying on this library for secure communications or data protection must consider the potential for data leakage and unauthorized information access. The vulnerability demonstrates the critical importance of proper cryptographic implementation and the dangers of using libraries with known mathematical weaknesses in security-critical applications. Mitigation strategies should include immediate migration to more secure cryptographic libraries such as PyCryptodome, which provides proper implementation of cryptographic algorithms, or alternative secure implementations that maintain the mathematical integrity required for semantic security. The remediation process should also involve thorough cryptographic audit of all systems using the affected library to identify and address potential exposure windows where sensitive data may have been encrypted using the vulnerable implementation.