In this paper, we have three contributions as follows:(1)We find a vulnerability of OpenSSL that the field “not before” in certificates leaks the time of generating certificates, which is the seed of generating the field “serial number,” so that it is possible to predict the value of “serial number.”(2)We give the predicting method for the field “serial number” and forge certificates based on the proposed method and Stevens’s method. certs ; crl; csr; intermediate; newcerts; pfx; private. The current time of the day in microseconds provides about 36 bits of entropy. Furthermore, the serial number depends on the time in seconds and in nanoseconds in OpenSSL (Figures 3 and 4). How to export CA certificate chain from PFX in PEM format without bag attributes, OpenSSL fetches different SSL certificate than the one obtained via a browser, Command to get ssl certificate pinning from certificate. The flow of the forging a certificate is in Figure 1. In addition, we grabbed 180,000+ certificates from Internet, while 5000+ certificates are based on MD5, in other words 2.8% certificates. We will be providing unlimited waivers of publication charges for accepted research articles as well as case reports and case series related to COVID-19. OpenSSL uses a pseudo random number generator (PRNG) to output random numbers. Certificate serial number file. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. The openssl ca command uses two serial number files:. The input parameter md0 of RAND_add is the IV of SHA1 algorithm. We test the parameter “tv” in Figure 4 in different operation systems. We can get “not before” of certificates easily, then know the seed of “SHA1PRNG,” and predict the serial number. Jizhi Wang, "The Prediction of Serial Number in OpenSSL’s X.509 Certificate", Security and Communication Networks, vol. How do we predict the value of the field “not valid before” that is in the unit of second? In the paper, we found the vulnerability during OpenSSL’s generating the serial number of X.509 certificates. It MUST be unique for each certificate issued by a given CA (i.e., the issuer name and serial number identify a unique certificate). In [4], authors reported that the validity period started exactly 6 seconds after a certification request was submitted. In addition, the super-malware Flame was discovered in 2012 [7], which uses the method to forge a Microsoft’s certificate [8]. We reviewed the file to find how the valid time and serial number of certificates are generated. This tool can generate up to 250,000 unique random codes at a time. According to the chosen-prefix collision, the prefixes p and of two message blocks are chosen. CAs MUST force the serialNumber to be a non-negative integer. Similarly, EJBCA and NSS have the same vulnerability among other 5 open source libraries. RAND_add() and RAND_bytes() are the most important random number functions in OpenSSL. The valid time and the serial number of certificates in Botan. We reviewed the source codes of Botan 2.6 to find the way that the valid time and serial number of certificated are generated. The serial number is a fixed length, it cuts off at 64 bits, but if one of those bits is necessarily a zero – you’ve just lost one bit of entropy. serial. To create our own certificate we need a certificate authority to sign it (if you don’t know what this means, I recommend reading Brief(ish) explanation of how https works). We give the predicting method for the field “serial number” and forge certificates based on the proposed method and Stevens’s method. What are the advantages and disadvantages of water bottles versus bladders? certs/ca.cert.pem. It is hard to predict the output of random number generators of operation systems so far. We have investigated other open source libraries generating certificates, EJBCA [21], CFSSL [22], NSS [23], Botan [24], and Fortify [25], to find whether similar problems exist when generating serial numbers of certificates. Use the "-set_serial n" option to specify a number each time. Although MD5 has been replaced by CAs now, with the development of technology, new attacks for current hash algorithm adopted by CAs, such as SHA-256, will probably occur in the future. ⇒ OpenSSL "req -x509 -md5" - MD5 Digest for Signing. So in Step 5, we select randomly a value of m; the success probability is 0.01; in other words, we submit the application more than 69 times; the success probability is more than 50%. Cool Tip: If your SSL certificate expires soon – you will need to generate a new CSR! # Optionally include a file that is generated by the OpenSSL fipsinstall # application. According to the chosen-prefix collision attack, the generating collision pair is like random number, while only the field “subject public key info” is the analogy with random number. In EJBCA, a tool called CertTool is provided to generate certificates, where is in . Comodo / Sectigo is changing its Root CAs 28-12-2018 11:23:52. Since the value of “not before” leaks the time of certificates’ generation, attackers can limit a narrow range of the seeds for generating serial numbers in OpenSSL. CFSSL is an open source PKI/TLS toolkit developed by CloudFlare. The testing result shows that the real serial number of the certificate is one of the candidate serial numbers that we predict (in Table 4). Although identical-prefix collision can be used to forge certificates, the kind of forgery is meaningless in practical attacks because the user’s identity is in the prefix and cannot be changed. You have to set an initial value like "1000" in the file. Configure openssl.cnf for Root CA Certificate. The generation algorithm of “serial number” is “SHA1PRNG” and the seed is set as “current time” (in millisenonds). The security of OpenSSL’s PRNG in Android and Debian has been reported in [10, 14]. After that I'd like to format the certificate in following format hexhex:hexhex:...:hexhex In the paper, we found the vulnerability during OpenSSL’s generating the serial number of X.509 certificates. site design / logo © 2021 Stack Exchange Inc; user contributions licensed under cc by-sa. If the chosen-prefix collision of som… After that, I used the certificate authority to re-issue a new certificate. Is it normal to need to replace my brakes every few months? In 2007, a real faked X.509 certificate based on the chosen-prefix collision of MD5 was presented by Marc Stevens. Generate Serial numbers. In the method, attackers needed to predict the serial number of X.509 certificates generated by CAs besides constructing the collision pairs of MD5. This is the simplest method to deal with the problem. In the wild, however, many valid certificates still use MD5 [9]. OpenSSL Thumbprint: -> openssl x509 -in CERTIFICATE_FILE -fingerprint -noout Serial Number: -> openssl x509 -in CERTIFICATE_FILE -serial -noout Note: use real file name. The submitting time was recorded and the value of “not before” was checked after receiving the certificate. Linux is a registered trademark of Linus Torvalds. Fixing this error is easy. NSS is a set of libraries supporting cross-platform network security services and developed by Mozilla. The overview of collision complexities is in Table 1. If the resulting outputs are equal to the outputs of the real random number generator, then the attacker knows the used seed of the real random number generator. SEE ALSO CAs MUST force the serialNumber to be a non-negative integer. Click Serial number or Thumbprint. After a serial of function calling, the functions “RAND_add(const void buf, int num, double add)” and “RAND_bytes(unsigned char buf, int num)” are called in bn_rand.c (Figure 5). The serial number will be incremented each time a new certificate is created. Although MD5 has been replaced by CAs now, with the development of technology, new attacks for current hash algorithm adopted by CAs, such as SHA-256, will probably occur in the future. And RFC 3280 has this to say: 4.1.2.2 Serial number The serial number MUST be a positive integer assigned by the CA to each certificate. Concluding the above analysis on OpenSSL, EJBCA, CFSSL, NSS, Botan, and Fortify, we can compare the way generating valid time and serial number of certificates in Table 5. Thus, for attackers, to predict the serial number of certificates, a natural idea is to brute force every 100 nanoseconds in the second according to Algorithms 1 and 2. Before guessing the serial number and validity period in certificates, they need to collect/apply for enough certificates issued by the CA and look for whether the two fields have any patterns. It is possible to forge certificates based on the method presented by Stevens. In Section 2, some preliminaries are introduced and the problems solved by the paper are defined. CRL over HTTPS: is it really a bad practice? Since the first real MD5 collision attack was presented by Wang [1, 2] in 2004, it is possible to construct forged certificates based on the collision attack of MD5. Thanks for contributing an answer to Unix & Linux Stack Exchange! Thus, an attack can try through all the possible seeds and generate the results according to his/her instance of the random number generator. allows you to override the serial number select process and thus control. To answer the two questions, we need to know how CAs generate the value of the two fields. If used in conjunction with the -CA option the serial number file (as specified by the -CAserial or -CAcreateserial options) is not used. The second part of the sed command (s/:$//) searches for a colon at the end of the output and replaces it with an empty string, resulting in the desired output. In this paper, we will focus on whether the randomness of some fields in certificates is enough to prevent attackers from predicting. The project is supported by Key Research and Development Plan of Shandong Province, China (NO.2017CXGC0704), and Fundamental Research Fund of Shandong Academy of Sciences, China (NO.2018:12-16). We reviewed the source codes of NSS 3.38 to find the way that the valid time and serial number of certificated are generated. The answers I've found are pointing to the lack of index file. That is sent to sed. Just including the Subject of the Issuer would be duplicating the Issuer DN already available in the certificate. Among attacks, collision of hash algorithms is one of the most serious threats. In that case, attackers still need to predict the value of fields controlled by CAs in order to construct forged certificates.
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