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51-435: NTLM passwords are considered weak because they can be brute-forced very easily with modern hardware. NTLM is a challenge–response authentication protocol which uses three messages to authenticate a client in a connection-oriented environment (connectionless is similar), and a fourth additional message if integrity is desired. The NTLM protocol uses one or both of two hashed password values, both of which are also stored on

102-728: A brute-force attack, with 'anti-hammering' for countermeasures. Brute-force attacks work by calculating every possible combination that could make up a password and testing it to see if it is the correct password. As the password's length increases, the amount of time, on average, to find the correct password increases exponentially. The resources required for a brute-force attack grow exponentially with increasing key size , not linearly. Although U.S. export regulations historically restricted key lengths to 56-bit symmetric keys (e.g. Data Encryption Standard ), these restrictions are no longer in place, so modern symmetric algorithms typically use computationally stronger 128- to 256-bit keys. There

153-559: A hashcat compatible cracking format. With hashcat and sufficient GPU power the NTLM hash can be derived using a known plaintext attack by cracking the DES keys with hashcat mode 14000 as demonstrated by atom on the hashcat forums. Note that the password-equivalent hashes used in pass-the-hash attacks and password cracking must first be "stolen" (such as by compromising a system with permissions sufficient to access hashes). Also, these hashes are not

204-553: A key from a password is as specified in RFC1320 and FIPS46-2. Therefore, applications are generally advised not to use NTLM. Despite these recommendations, NTLM is still widely deployed on systems. A major reason is to maintain compatibility with older systems. However, it can be avoided in some circumstances. Microsoft has added the NTLM hash to its implementation of the Kerberos protocol to improve interoperability (in particular,

255-405: A multiword passphrase , using a password manager program or manually typing a password. It is possible to achieve a time–space tradeoff by pre-computing a list of hashes of dictionary words and storing these in a database using the hash as the key . This requires a considerable amount of preparation time, but this allows the actual attack to be executed faster. The storage requirements for

306-485: A select few passwords. In such a strategy, the attacker is not targeting a specific user. Dictionary attack In cryptanalysis and computer security , a dictionary attack is an attack using a restricted subset of a keyspace to defeat a cipher or authentication mechanism by trying to determine its decryption key or passphrase , sometimes trying thousands or millions of likely possibilities often obtained from lists of past security breaches. A dictionary attack

357-465: A server to authenticate to the client. NTLMv2 sends two responses to an 8-byte server challenge . Each response contains a 16-byte HMAC - MD5 hash of the server challenge, a fully/partially randomly generated client challenge , and an HMAC-MD5 hash of the user's password and other identifying information. The two responses differ in the format of the client challenge. The shorter response uses an 8-byte random value for this challenge. In order to verify

408-563: A system, as was done by the Venona project , generally relies not on pure cryptography, but upon mistakes in its implementation, such as the key pads not being truly random, intercepted keypads, or operators making mistakes. In case of an offline attack where the attacker has gained access to the encrypted material, one can try key combinations without the risk of discovery or interference. In case of online attacks, database and directory administrators can deploy countermeasures such as limiting

459-427: A tendency to choose short passwords that are ordinary words or common passwords; or variants obtained, for example, by appending a digit or punctuation character. Dictionary attacks are often successful, since many commonly used password creation techniques are covered by the available lists, combined with cracking software pattern generation. A safer approach is to randomly generate a long password (15 letters or more) or

510-469: A variable-length client challenge which includes (1) the current time in NT Time format, (2) an 8-byte random value (CC2 in the box below), (3) the domain name and (4) some standard format stuff. The response must include a copy of this client challenge, and is therefore variable length. In non-official documentation, this response is termed NTv2. Both LMv2 and NTv2 hash the client and server challenge with

561-400: Is one-time pad cryptography, where every cleartext bit has a corresponding key from a truly random sequence of key bits. A 140 character one-time-pad-encoded string subjected to a brute-force attack would eventually reveal every 140 character string possible, including the correct answer – but of all the answers given, there would be no way of knowing which was the correct one. Defeating such

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612-477: Is a cryptanalytic attack that can, in theory, be used to attempt to decrypt any encrypted data (except for data encrypted in an information-theoretically secure manner). Such an attack might be used when it is not possible to take advantage of other weaknesses in an encryption system (if any exist) that would make the task easier. When password-guessing, this method is very fast when used to check all short passwords, but for longer passwords other methods such as

663-481: Is a physical argument that a 128-bit symmetric key is computationally secure against brute-force attack. The Landauer limit implied by the laws of physics sets a lower limit on the energy required to perform a computation of kT   · ln 2 per bit erased in a computation, where T is the temperature of the computing device in kelvins , k is the Boltzmann constant , and the natural logarithm of 2

714-413: Is about 0.693 (0.6931471805599453). No irreversible computing device can use less energy than this, even in principle. Thus, in order to simply flip through the possible values for a 128-bit symmetric key (ignoring doing the actual computing to check it) would, theoretically, require 2 − 1 bit flips on a conventional processor. If it is assumed that the calculation occurs near room temperature (≈300 K),

765-610: Is based on trying all the strings in a pre-arranged listing. Such attacks originally used words found in a dictionary (hence the phrase dictionary attack ); however, now there are much larger lists available on the open Internet containing hundreds of millions of passwords recovered from past data breaches. There is also cracking software that can use such lists and produce common variations, such as substituting numbers for similar-looking letters . A dictionary attack tries only those possibilities which are deemed most likely to succeed. Dictionary attacks often succeed because many people have

816-464: Is confusing. Any computer acting as server and authenticating a user fulfills the role of DC in this context, for example a Windows computer with a local account such as Administrator when that account is used during a network logon. Prior to Windows NT 4.0 Service Pack 4, the SSP would negotiate NTLMv1 and fall back to LM if the other machine did not support it. Starting with Windows NT 4.0 Service Pack 4,

867-726: Is simply the energy requirement for cycling through the key space; the actual time it takes to flip each bit is not considered, which is certainly greater than 0 (see Bremermann's limit ). However, this argument assumes that the register values are changed using conventional set and clear operations, which inevitably generate entropy . It has been shown that computational hardware can be designed not to encounter this theoretical obstruction (see reversible computing ), though no such computers are known to have been constructed. As commercial successors of governmental ASIC solutions have become available, also known as custom hardware attacks , two emerging technologies have proven their capability in

918-431: The dictionary attack are used because a brute-force search takes too long. Longer passwords, passphrases and keys have more possible values, making them exponentially more difficult to crack than shorter ones due to diversity of characters. Brute-force attacks can be made less effective by obfuscating the data to be encoded making it more difficult for an attacker to recognize when the code has been cracked or by making

969-414: The key space to search through was found to be much smaller than originally thought, because of a lack of entropy in their pseudorandom number generators . These include Netscape 's implementation of Secure Sockets Layer (SSL) (cracked by Ian Goldberg and David Wagner in 1995) and a Debian / Ubuntu edition of OpenSSL discovered in 2008 to be flawed. A similar lack of implemented entropy led to

1020-530: The little endian UTF-16 Unicode password). Both hash values are 16 bytes (128 bits) each. The NTLM protocol also uses one of two one-way functions , depending on the NTLM version; NT LanMan and NTLM version 1 use the DES-based LanMan one-way function (LMOWF), while NTLMv2 uses the NT MD4 based one-way function (NTOWF). The server authenticates the client by sending an 8-byte random number,

1071-413: The 256-bit key space. An underlying assumption of a brute-force attack is that the complete key space was used to generate keys, something that relies on an effective random number generator , and that there are no defects in the algorithm or its implementation. For example, a number of systems that were originally thought to be impossible to crack by brute force have nevertheless been cracked because

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1122-613: The LM hash and the NT hash are returned as the response, but this is configurable. NTLMv2, introduced in Windows NT 4.0 SP4 (and natively supported in Windows 2000), is a challenge-response authentication protocol. It is intended as a cryptographically strengthened replacement for NTLMv1, enhancing NTLM security by hardening the protocol against many spoofing attacks and adding the ability for

1173-501: The NT hash of the user's password and other identifying information. The exact formula is to begin with the NT hash, which is stored in the SAM or AD, and continue to hash in, using HMAC - MD5 , the username and domain name. In the box below, X stands for the fixed contents of a formatting field. The NTLM2 Session protocol is similar to MS-CHAPv2. It consists of authentication from NTLMv1 combined with session security from NTLMv2. Briefly,

1224-616: The NTLM Security Support Provider (SSP) directly. Your application should not access the NTLM security package directly; instead, it should use the Negotiate security package. Negotiate allows your application to take advantage of more advanced security protocols if they are supported by the systems involved in the authentication. Currently, the Negotiate security package selects between Kerberos and NTLM. Negotiate selects Kerberos unless it cannot be used by one of

1275-471: The NTLMv1 algorithm is applied, except that an 8-byte client challenge is appended to the 8-byte server challenge and MD5-hashed. The least 8-byte half of the hash result is the challenge utilized in the NTLMv1 protocol. The client challenge is returned in one 24-byte slot of the response message, the 24-byte calculated response is returned in the other slot. This is a strengthened form of NTLMv1 which maintains

1326-591: The RC4-HMAC encryption type). According to an independent researcher, this design decision allows Domain Controllers to be tricked into issuing an attacker with a Kerberos ticket if the NTLM hash is known. Microsoft adopted Kerberos as the preferred authentication protocol for Windows 2000 and subsequent Active Directory domains. Kerberos is typically used when a server belongs to a Windows Server domain . Microsoft recommends developers neither to use Kerberos nor

1377-482: The SSP would negotiate NTLMv2 Session whenever both client and server would support it. Up to and including Windows XP, this used either 40- or 56-bit encryption on non-U.S. computers, since the United States had severe restrictions on the export of encryption technology at the time. Starting with Windows XP SP3, 128-bit encryption could be added by installing an update and on Windows 7, 128-bit encryption would be

1428-508: The Von Neumann-Landauer Limit can be applied to estimate the energy required as ≈10 joules , which is equivalent to consuming 30 gigawatts of power for one year. This is equal to 30×10 W×365×24×3600 s = 9.46×10 J or 262.7 TWh (about 0.1% of the yearly world energy production ). The full actual computation – checking each key to see if a solution has been found – would consume many times this amount. Furthermore, this

1479-544: The Windows implementation of the NTLM authentication mechanism which broke the security of the protocol allowing attackers to gain read/write access to files and remote code execution. One of the attacks presented included the ability to predict pseudo-random numbers and challenges/responses generated by the protocol. These flaws had been present in all versions of Windows for 17 years. The security advisory explaining these issues included fully working proof-of-concept exploits. All these flaws were fixed by MS10-012. In 2012, it

1530-521: The ability to use existing Domain Controller infrastructure yet avoids a dictionary attack by a rogue server. For a fixed X , the server computes a table where location Y has value K such that Y=DES_K(X) . Without the client participating in the choice of challenge, the server can send X , look up response Y in the table and get K . This attack can be made practical by using rainbow tables . However, existing NTLMv1 infrastructure allows that

1581-417: The attacker do more work to test each guess. One of the measures of the strength of an encryption system is how long it would theoretically take an attacker to mount a successful brute-force attack against it. Brute-force attacks are an application of brute-force search, the general problem-solving technique of enumerating all candidates and checking each one. The word 'hammering' is sometimes used to describe

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1632-444: The breaking of Enigma's code. Credential recycling is the hacking practice of re-using username and password combinations gathered in previous brute-force attacks. A special form of credential recycling is pass the hash , where unsalted hashed credentials are stolen and re-used without first being brute forced. Certain types of encryption, by their mathematical properties, cannot be defeated by brute force. An example of this

1683-440: The brute-force attack of certain ciphers. One is modern graphics processing unit (GPU) technology, the other is the field-programmable gate array (FPGA) technology. GPUs benefit from their wide availability and price-performance benefit, FPGAs from their energy efficiency per cryptographic operation. Both technologies try to transport the benefits of parallel processing to brute-force attacks. In case of GPUs some hundreds, in

1734-405: The case of FPGA some thousand processing units making them much better suited to cracking passwords than conventional processors. For instance in 2022, 8 Nvidia RTX 4090 GPU were linked together to test password strength by using the software Hashcat with results that showed 200 billion eight-character NTLM password combinations could be cycled through in 48 minutes. Various publications in

1785-449: The challenge. The client performs an operation involving the challenge and a secret shared between client and server, specifically one of the two password hashes described above. The client returns the 24-byte result of the computation. In fact, in NTLMv1 the computations are usually made using both hashes and both 24-byte results are sent. The server verifies that the client has computed the correct result, and from this infers possession of

1836-543: The challenge/response pair is not verified by the server, but sent to a Domain Controller for verification. Using NTLM2 Session, this infrastructure continues to work if the server substitutes for the challenge the hash of the server and client challenges. Since 2010, Microsoft no longer recommends NTLM in applications: Implementers should be aware that NTLM does not support any recent cryptographic methods, such as AES or SHA-256. It uses cyclic redundancy checks (CRC) or MD5 for integrity, and RC4 for encryption. Deriving

1887-692: The computer running Windows Vista acts as the server. Prior versions of Windows (back as far as Windows NT 4.0 Service Pack 4) could be configured to behave this way, but it was not the default. NTLM remains vulnerable to the pass the hash attack, which is a variant on the reflection attack which was addressed by Microsoft security update MS08-068. For example, Metasploit can be used in many cases to obtain credentials from one machine which can be used to gain control of another machine. The Squirtle toolkit can be used to leverage web site cross-site scripting attacks into attacks on nearby assets via NTLM. In February 2010, Amplia Security discovered several flaws in

1938-585: The default. In Windows Vista and above, LM has been disabled for inbound authentication. Windows NT-based operating systems up through and including Windows Server 2003 store two password hashes, the LAN Manager (LM) hash and the Windows NT hash. Starting in Windows Vista , the capability to store both is there, but one is turned off by default. This means that LM authentication no longer works if

1989-565: The fields of cryptographic analysis have proved the energy efficiency of today's FPGA technology, for example, the COPACOBANA FPGA Cluster computer consumes the same energy as a single PC (600 W), but performs like 2,500 PCs for certain algorithms. A number of firms provide hardware-based FPGA cryptographic analysis solutions from a single FPGA PCI Express card up to dedicated FPGA computers. WPA and WPA2 encryption have successfully been brute-force attacked by reducing

2040-457: The hope of eventually guessing correctly. The attacker systematically checks all possible passwords and passphrases until the correct one is found. Alternatively, the attacker can attempt to guess the key which is typically created from the password using a key derivation function . This is known as an exhaustive key search . This approach doesn't depend on intellectual tactics; rather, it relies on making several attempts. A brute-force attack

2091-435: The number of attempts that a password can be tried, introducing time delays between successive attempts, increasing the answer's complexity (e.g., requiring a CAPTCHA answer or employing multi-factor authentication ), and/or locking accounts out after unsuccessful login attempts. Website administrators may prevent a particular IP address from trying more than a predetermined number of password attempts against any account on

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2142-450: The pre-computed tables were once a major cost, but now they are less of an issue because of the low cost of disk storage . Pre-computed dictionary attacks are particularly effective when a large number of passwords are to be cracked. The pre-computed dictionary needs be generated only once, and when it is completed, password hashes can be looked up almost instantly at any time to find the corresponding password. A more refined approach involves

2193-425: The response, the server must receive as part of the response the client challenge. For this shorter response, the 8-byte client challenge appended to the 16-byte response makes a 24-byte package which is consistent with the 24-byte response format of the previous NTLMv1 protocol. In certain non-official documentation (e.g. DCE/RPC Over SMB, Leighton) this response is termed LMv2. The second response sent by NTLMv2 uses

2244-533: The same as the NTLMSSP_AUTH "hash" transmitted over the network during a conventional NTLM authentication. NTLM implementations for Linux include Cntlm and winbind (part of Samba ) allow Linux applications to use NTLM proxies. FreeBSD also supports storing passwords via Crypt (C) in the insecure NT-Hash form. Brute-force attack In cryptography , a brute-force attack consists of an attacker submitting many passwords or passphrases with

2295-410: The secret, and hence the authenticity of the client. Both the hashes produce 16-byte quantities. Five bytes of zeros are appended to obtain 21 bytes. The 21 bytes are separated in three 7-byte (56-bit) quantities. Each of these 56-bit quantities is used as a key to DES encrypt the 64-bit challenge. The three encryptions of the challenge are reunited to form the 24-byte response. Both the response using

2346-454: The server (or domain controller), and which through a lack of salting are password equivalent , meaning that if you grab the hash value from the server, you can authenticate without knowing the actual password. The two are the LM hash (a DES -based function applied to the first 14 characters of the password converted to the traditional 8-bit PC charset for the language), and the NT hash ( MD4 of

2397-482: The site. Additionally, the MITRE D3FEND framework provides structured recommendations for defending against brute-force attacks by implementing strategies such as network traffic filtering, deploying decoy credentials, and invalidating authentication caches. In a reverse brute-force attack, a single (usually common) password is tested against multiple usernames or encrypted files. The process may be repeated for

2448-537: The systems involved in the authentication. The NTLM SSP is used in the following situations: After it has been decided either by the application developer or by the Negotiate SSP that the NTLM SSP be used for authentication, Group Policy dictates the ability to use each of the protocols that the NTLM SSP implements. There are five authentication levels. DC would mean Domain Controller, but use of that term

2499-437: The use of rainbow tables , which reduce storage requirements at the cost of slightly longer lookup-times. See LM hash for an example of an authentication system compromised by such an attack. Pre-computed dictionary attacks, or "rainbow table attacks", can be thwarted by the use of salt , a technique that forces the hash dictionary to be recomputed for each password sought, making precomputation infeasible, provided that

2550-517: The workload by a factor of 50 in comparison to conventional CPUs and some hundred in case of FPGAs. Advanced Encryption Standard (AES) permits the use of 256-bit keys. Breaking a symmetric 256-bit key by brute force requires 2 times more computational power than a 128-bit key. One of the fastest supercomputers in 2019 has a speed of 100 petaFLOPS which could theoretically check 100 trillion (10 ) AES keys per second (assuming 1000 operations per check), but would still require 3.67×10 years to exhaust

2601-445: Was demonstrated that every possible 8-character NTLM password hash permutation can be cracked in under 6 hours. In 2019, this time was reduced to roughly 2.5 hours by using more modern hardware. Also, Rainbow tables are available for eight- and nine-character NTLM passwords. Shorter passwords can be recovered by brute force methods. In 2019, EvilMog published a tool called the ntlmv1-multitool to format NTLMv1 challenge responses in

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