Birthday attacks against TLS ciphers with 64bit (Sweet32)
Understanding the Sweet32 Vulnerability: CVE-2016-2183
The advent of the internet and digital communications has fostered a significant need for robust encryption mechanisms to secure data transmission. Over the years, several encryption algorithms and protocols have been developed, each with its strengths and weaknesses. One such vulnerability that emerged in the landscape of cyber threats is known as Sweet32, which specifically targets 64-bit block ciphers used in the TLS and SSL protocols.
What is Sweet32?
Sweet32, designated as CVE-2016-2183, is a vulnerability that affects 64-bit block ciphers, particularly the Triple DES (3DES) algorithm. The name “Sweet32” is a nod to the Sweet32 birthday attack, which takes advantage of the mathematical properties of birthday collisions. In cryptography, a birthday collision occurs when two pieces of data share the same cryptographic hash value, thereby compromising the security of the encrypted data.
Vulnerability Details
The Sweet32 vulnerability leverages the fact that 64-bit block ciphers, such as 3DES, have a relatively small block size. In a long-lived TLS or SSL connection, where the same key and initialization vector (IV) are used to encrypt multiple blocks of data, the probability of block collision increases as more data is encrypted. This collision of blocks can allow an attacker, who is eavesdropping on the network, to decrypt portions of the encrypted data. The core principle behind Sweet32 is that with sufficient eavesdropped data, the attacker can potentially reconstruct the original plaintext.
Severity Rating
The severity of the Sweet32 vulnerability is considered moderate to high, depending on the specific use case and the presence of long-lived connections using 3DES. In scenarios where 3DES is heavily relied upon for encryption, such as in VPNs or certain web applications, the risk of exposure to sensitive information is significant. The vulnerability becomes particularly severe in environments where long-lived connections are common, as the prolonged use of the same encryption key and IV increases the likelihood of block collisions.
Comparison with RC4 Attacks
The attack scenario for Sweet32 is notably similar to the recent attacks on the RC4 stream cipher in HTTPS. Both attacks utilize a man-in-the-browser setting to generate a large number of HTTP requests, thereby creating ample data for analysis. The data complexity of the Sweet32 attack is comparable to that of RC4 attacks, requiring approximately 229.1 short queries of 512 bytes or 227.6 longer queries of 4 kB. While these figures represent the ideal case, they highlight the substantial amount of data needed to mount the attack effectively.
Identifying Vulnerable Systems
To determine if a device or system is vulnerable to the Sweet32 attack, it is crucial to examine the cipher suites and encryption algorithms used in the TLS or SSL configuration. Specifically, it is necessary to check if 3DES cipher suites are enabled and actively used. Network scanning tools such as Nmap, ZenMap, or security assessment tools like IISCrypto from Nartac Software and Nessus can assist in identifying vulnerable systems. For example, using Nmap, the following command can be employed to enumerate SSL/TLS cipher suites:
bash
nmap -sV –script ssl-enum-ciphers -p 443
This command helps in identifying whether 3DES cipher suites are enabled on the target system.
Applying a Fix
Before applying a fix for the Sweet32 vulnerability, it is essential to consider potential compatibility issues with older devices or software that rely solely on 3DES encryption. Older browsers, operating systems, and general software may face challenges if 3DES is disabled without proper support for alternative cipher suites like AES. To mitigate the vulnerability, the recommended approach is to disable or deprecate 3DES cipher suites in the TLS or SSL configuration and adopt stronger encryption algorithms, such as AES.
Disabling 3DES through Group Policy (GPO)
Open the Group Policy Management console and create a new Group Policy Object.
Edit the Group Policy Object and navigate to the following section:
Computer Configuration -> Policies -> Administrative Templates -> Network -> SSL Configuration Settings
Enable the policy “SSL Cipher Suite Order” and configure the value to prioritize secure cipher suites like AES, excluding 3DES. An example configuration value could be:
TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384,
TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256,
TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA384,
TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA256
Apply the GPO to the relevant Organizational Units (OU) or groups of computers in the Active Directory domain.
Perform a group policy update on the target computers to apply the changes immediately by running the following command on client machines:
bash
gpupdate /force
Disabling 3DES through a Script
Alternatively, a script can be used to disable 3DES cipher suites. The following PowerShell script demonstrates how to achieve this:
powershell
$protocols = Get-ItemProperty -Path “HKLM:\SYSTEM\CurrentControlSet\Control\SecurityProviders\SCHANNEL\Protocols”
$protocols | ForEach-Object {
$protocolName = $_.PSChildName
if ($_.Enabled -eq 1) {
Write-Host “Disabling 3DES Cipher Suites for $protocolName”
Set-ItemProperty -Path “HKLM:\SYSTEM\CurrentControlSet\Control\SecurityProviders\SCHANNEL\Protocols\$protocolName\Client” -Name “Enabled” -Value 0
Set-ItemProperty -Path “HKLM:\SYSTEM\CurrentControlSet\Control\SecurityProviders\SCHANNEL\Protocols\$protocolName\Server” -Name “Enabled” -Value 0
}
}
Write-Host “3DES Cipher Suites disabled successfully.”
Special care must be taken when executing these scripts, especially on older systems like Windows Server 2016 and earlier.
Rollback Plan
In the event that applying the fix causes unexpected issues or compatibility problems, it is crucial to have a rollback plan. This involves ensuring that backups or snapshots of the system’s configuration and state are available, allowing for a quick restoration to the previous working state if necessary.