OpenSSL Security Advisory [10 July 2001] WEAKNESS OF THE OpenSSL PRNG IN VERSIONS UP TO OpenSSL 0.9.6a ------------------------------------------------------------- CONTENTS: - Synopsis - Detailed problem description - Solution - Impact - Source code patch [*] - Acknowledgement [*] OpenSSL 0.9.6b has been corrected and does not require this patch. The source code of OpenSSL 0.9.6b is available as file openssl-0.9.6b.tar.gz from . If you were previously using the "engine" release of OpenSSL 0.9.6 or 0.9.6a, obtain file openssl-engine-0.9.6b.tar.gz instead. MD5 checksums: openssl-0.9.6b.tar.gz bd8c4d8c5bafc7a4d55d152989fdb327 openssl-engine-0.9.6b.tar.gz ab5ca5b157459c49bdab06a7db8a5a47 OpenSSL source code can also be obtained from a number of mirror sites. For a list, see . If you are using a pre-compiled OpenSSL package, please look for update information from the respective software distributor. The OpenSSL group itself does not distribute OpenSSL binaries. SYNOPSIS -------- The pseudo-random number generator (PRNG) in SSLeay/OpenSSL versions up to 0.9.6a is weakened by a design error. Knowing the output of specific PRNG requests (including a number of consecutive very short PRNG requests) would allow an attacker to determine the PRNG's internal state and thus to predict future PRNG output. Typical applications (including applications using OpenSSL's SSL/TLS library) are not vulnerable to this attack because PRNG requests usually happen in larger chunks. However, we strongly recommend upgrading to OpenSSL 0.9.6b, which includes a fixed PRNG. If upgrading to 0.9.6b is not immediately possible, the source code patch contained at the end of this advisory should be applied. DETAILED PROBLEM DESCRIPTION ---------------------------- Recently a cryptographic flaw in OpenSSL's built-in pseudo-random number generator (PRNG) was pointed out to us by Markku-Juhani O. Saarinen , who showed how an attacker could reconstruct the PRNG's internal state from the output of a couple of hundred 1-byte PRNG requests. This problem dates back to SSLeay, which OpenSSL is based on, and was found in other SSLeay-based toolkits as well. While a number of enhancements have been done to the original PRNG during the development of OpenSSL, this design error was overlooked so far. The PRNG (implemented in source code file crypto/md_rand.c) uses a hash function, by default SHA-1, to update its internal secret state and to generate output. The secret state consists of two components: A chaining variable 'md', sized according to the hash function's output (160 bits for SHA-1), and a large buffer 'state'. 'md' is always replaced by a hash function output during the PRNG's operation. 'state' is accessed circularly and is used for storing additional entropy. When generating output bytes, OpenSSL versions up to 0.9.6a set 'md' to the hash of one half of its previous value and some other data, including bytes from 'state'. The design error was that the half of 'md' input to the hash function was the same half that was also used as PRNG output, meaning that it in general cannot be considered secret. Also the number of bytes used from 'state' depended on the number of bytes requested as PRNG output and could be as small as one, allowing for easy brute-force analysis of all possible cases. The combination of these effects made it possible to reconstruct the complete internal PRNG state from the output of one PRNG request appropriately sized to gain knowledge on 'md' followed by enough consecutive 1-byte PRNG requests to traverse all of 'state'. SOLUTION -------- OpenSSL 0.9.6b changes the PRNG implementation as follows to give the PRNG its intended strength: 1. When updating 'md' during PRNG output generation, all of the previous 'md' value is hashed, including the secret half. 2. Also, the number of bytes from 'state' included into the hash is now independent from the number of PRNG bytes requested. The first measure alone would be sufficient to solve the problem. The second measure makes sure that additional data from 'state' is never mixed in in small portions; this heuristically further strengthens the PRNG. IMPACT ------ It is unlikely for applications to request PRNG bytes in a pattern allowing for the attack against the OpenSSL PRNG. Typically, applications will request PRNG bytes in larger chunks. No applications is known to us which is actually vulnerable. However, the PRNG design flaw is a significant weakness: The PRNG does not provide the intended strength under all circumstances. Therefore, we strongly recommend that all users upgrade to OpenSSL 0.9.6b as soon as possible. SOURCE CODE PATCH ----------------- If upgrading to OpenSSL 0.9.6b is not immediately possible, the following patch should be applied to file crypto/rand/md_rand.c in the OpenSSL source code tree. (The patch is compatible with OpenSSL versions 0.9.5 up to 0.9.6a.) This changes the PRNG in two ways, as discussed above. --- md_rand.c +++ md_rand.c @@ -313,6 +313,7 @@ { static volatile int stirred_pool = 0; int i,j,k,st_num,st_idx; + int num_ceil; int ok; long md_c[2]; unsigned char local_md[MD_DIGEST_LENGTH]; @@ -333,6 +334,12 @@ } #endif + if (num <= 0) + return 1; + + /* round upwards to multiple of MD_DIGEST_LENGTH/2 */ + num_ceil = (1 + (num-1)/(MD_DIGEST_LENGTH/2)) * (MD_DIGEST_LENGTH/2); + /* * (Based on the rand(3) manpage:) * @@ -418,11 +425,11 @@ md_c[1] = md_count[1]; memcpy(local_md, md, sizeof md); - state_index+=num; + state_index+=num_ceil; if (state_index > state_num) state_index %= state_num; - /* state[st_idx], ..., state[(st_idx + num - 1) % st_num] + /* state[st_idx], ..., state[(st_idx + num_ceil - 1) % st_num] * are now ours (but other threads may use them too) */ md_count[0] += 1; @@ -434,6 +441,7 @@ while (num > 0) { + /* num_ceil -= MD_DIGEST_LENGTH/2 */ j=(num >= MD_DIGEST_LENGTH/2)?MD_DIGEST_LENGTH/2:num; num-=j; MD_Init(&m); @@ -444,27 +452,28 @@ curr_pid = 0; } #endif - MD_Update(&m,&(local_md[MD_DIGEST_LENGTH/2]),MD_DIGEST_LENGTH/2); + MD_Update(&m,local_md,MD_DIGEST_LENGTH); MD_Update(&m,(unsigned char *)&(md_c[0]),sizeof(md_c)); #ifndef PURIFY MD_Update(&m,buf,j); /* purify complains */ #endif - k=(st_idx+j)-st_num; + k=(st_idx+MD_DIGEST_LENGTH/2)-st_num; if (k > 0) { - MD_Update(&m,&(state[st_idx]),j-k); + MD_Update(&m,&(state[st_idx]),MD_DIGEST_LENGTH/2-k); MD_Update(&m,&(state[0]),k); } else - MD_Update(&m,&(state[st_idx]),j); + MD_Update(&m,&(state[st_idx]),MD_DIGEST_LENGTH/2); MD_Final(local_md,&m); - for (i=0; i= st_num) st_idx=0; + if (i < j) + *(buf++)=local_md[i+MD_DIGEST_LENGTH/2]; } } *** END OF PATCH *** ACKNOWLEDGEMENT --------------- We thank Markku-Juhani O. Saarinen for discovering the PRNG problem and bringing it to our attention. URL for this Security Advisory: https://www.openssl.org/news/secadv_prng.txt