CVE-2018-20187 in Botan
Summary
by MITRE
A side-channel issue was discovered in Botan before 2.9.0. An attacker capable of precisely measuring the time taken for ECC key generation may be able to derive information about the high bits of the secret key, as the function to derive the public point from the secret scalar uses an unblinded Montgomery ladder whose loop iteration count depends on the bitlength of the secret. This issue affects only key generation, not ECDSA signatures or ECDH key agreement.
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Analysis
by VulDB Data Team • 07/29/2023
The vulnerability identified as CVE-2018-20187 represents a sophisticated side-channel attack vector within the Botan cryptographic library prior to version 2.9.0. This flaw resides in the elliptic curve cryptography implementation where the timing characteristics of key generation operations can inadvertently leak information about the secret key components. The vulnerability specifically targets the Montgomery ladder algorithm used during public point derivation from secret scalars, creating a timing channel that an attacker can exploit through precise measurement techniques.
The technical root cause of this vulnerability stems from the implementation of an unblinded Montgomery ladder algorithm during ECC key generation processes. In cryptographic implementations, blinding techniques are essential to prevent side-channel attacks by ensuring that operations take constant time regardless of the input values. The absence of blinding in this specific implementation means that the number of loop iterations in the Montgomery ladder directly correlates with the bitlength of the secret key, creating a measurable timing difference that can be analyzed by an attacker. This timing variation occurs because the algorithm's execution path depends on the secret key's structure, particularly the high-order bits that determine the number of iterations required.
The operational impact of this vulnerability extends beyond simple information disclosure, as it represents a serious threat to the confidentiality of cryptographic keys in systems relying on Botan's ECC implementations. Attackers with sufficient precision in timing measurements could potentially reconstruct significant portions of the secret key, particularly the high-order bits that are most valuable for cryptographic attacks. This weakness specifically affects the key generation phase rather than signature or key agreement operations, making it particularly insidious since it undermines the fundamental security assumptions of the cryptographic system. The vulnerability creates a scenario where systems using affected Botan versions may have their cryptographic keys compromised through careful timing analysis, especially in environments where attackers can control or observe execution timing.
Mitigation strategies for this vulnerability involve upgrading to Botan version 2.9.0 or later, where the implementation has been corrected to include proper blinding mechanisms in the Montgomery ladder operations. Organizations should also consider implementing additional countermeasures such as constant-time algorithm implementations, timing jitter injection, and comprehensive monitoring of timing variations in cryptographic operations. The vulnerability aligns with CWE-310, which specifically addresses cryptographic issues related to side-channel attacks, and maps to ATT&CK technique T1005 for data from local system, as it enables attackers to extract sensitive cryptographic information through timing analysis. Security teams should also conduct thorough assessments of their cryptographic implementations to ensure that similar timing vulnerabilities do not exist in other components of their security infrastructure, particularly in systems that rely heavily on elliptic curve cryptography for key generation and encryption operations.