Gregory Maxwell on Jan 10 2015: OpenSSL 1.0.0p / 1.0.1k was recently released and is being pushed out by various operating system maintainers. My review determined that this update is incompatible with the Bitcoin system and could lead to consensus forks. Bitcoin Core released binaries from Bitcoin.org are unaffected, as are any built with the gitian deterministic build system. If you are running third-party or self-compiled Bitcoin Core or an alternative implementation using OpenSSL you must not update OpenSSL or must run a Bitcoin software containing a workaround: https://github.com/bitcoin/bitcoin/commit/488ed32f2ada1d1dd108fc245d025c4d5f252783 (versions of this will be backported to other stable branches soon) The tests included with Bitcoin Core in the test_bitcoin utility already detect this condition and fail. (_Do not ignore or disable the tests in order to run or distribute software which fails_) The incompatibility is due to the OpenSSL update changing the behavior of ECDSA validation to reject any signature which is not encoded in a very rigid manner. This was a result of OpenSSL's change for CVE-2014-8275 "Certificate fingerprints can be modified". While for most applications it is generally acceptable to eagerly reject some signatures, Bitcoin is a consensus system where all participants must generally agree on the exact validity or invalidity of the input data. In a sense, consistency is more important than "correctness". As a result, an uncontrolled 'fix' can constitute a security vulnerability for the Bitcoin system. The Bitcoin Core developers have been aware of this class of risk for a long time and have taken measures to mitigate it generally; e.g., shipping static binaries, internalizing the Leveldb library... etc. It was somewhat surprising, however, to see this kind of change show up as a "low" priority fix in a security update and pushed out live onto large numbers of systems within hours. We were specifically aware of potential hard-forks due to signature encoding handling and had been hoping to close them via BIP62 in 0.10. BIP62's purpose is to improve transaction malleability handling and as a side effect rigidly defines the encoding for signatures, but the overall scope of BIP62 has made it take longer than we'd like to deploy. (Coincidentally, I wrote about this concern and our unique demands on cryptographic software as part of a comment on Reddit shortly before discovering that part of this OpenSSL update was actually incompatible with Bitcoin: https://www.reddit.com/Bitcoin/comments/2rrxq7/on_why_010s_release_notes_say_we_have_reason_to/cnitbz3 ) The patches above, however, only fix one symptom of the general problem: relying on software not designed or distributed for consensus use (in particular OpenSSL) for consensus-normative behavior. Therefore, as an incremental improvement, I propose a targeted soft-fork to enforce strict DER compliance soon, utilizing a subset of BIP62. Adding a blockchain rule for strict DER will reduce the risk of consensus inconsistencies from alternative implementations of signature parsing or signature verification, simplify BIP62, and better isolate the cryptographic validation code from the consensus algorithm. A failure to do so will likely leave us in this situation, or possibly worse, again in the future. The relevant incompatible transactions are already non-standard on the network since 0.8.0's release in February 2013, although there was seemingly a single miner still mining incompatible transactions. That miner has been contacted and has fixed their software, so a soft-fork with no chain forking should be possible. original: http://lists.linuxfoundation.org/pipermail/bitcoin-dev/2015-January/007097.html
Pieter Wuille on Jul 28 2015: -----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 Hello all, I'd like to disclose a vulnerability I discovered in September 2014, which became unexploitable when BIP66's 95% threshold was reached earlier this month.
A specially-crafted transaction could have forked the blockchain between nodes:
using OpenSSL on a 32-bit systems and on 64-bit Windows systems
using OpenSSL on non-Windows 64-bit systems (Linux, OSX, ...)
using some non-OpenSSL codebases for parsing signatures
None. Transactions that could trigger this problem have become invalid on the network since BIP66's deployment of version 3 blocks reached 95% on July 4th, 2015.
The problem is related to the signature encoding rules. Bitcoin's signatures are ASN.1 BER encoded. BER is a complex standard that allows many different encodings for the same data. Since Bitcoin Core 0.8, a standardness rule has been in effect that only allowed subset of encodings (DER) for relay and mining, even though any BER remained valid in the blockchain - at least in theory. In practice, BER has many weird edge cases, and I have not found a single cryptographic codebase that can parse all of them correctly. This includes OpenSSL, Crypto++, BouncyCastle, btcec, and our own libsecp256k1 library. This on itself would not be a problem, as full nodes on the network currently use OpenSSL. However, while researching what was needed to make libsecp256k1 compatible with it, I discovered that OpenSSL is even inconsistent with itself across different platforms. One of the features of BER is the ability for internal structures to have a length descriptor whose size itself is up to 126 bytes (see X.690-0207 220.127.116.11). A 1 terabyte data structure would for example use a 5-byte length descriptor. However, there is no requirement to use the shortest possible descriptor, so even a 70-byte ECDSA signature could use a 5-byte length descriptor and be valid. Unfortunately, OpenSSL supports length descriptors only as long as their size is at most that of a C 'long int', a type whose size depends on the platform (Windows and 32-bit Linux/OSX have a 4-byte long int, 64-bit Linux/OSX have an 8-byte long int). See https://github.com/openssl/openssl/blob/bfa34f551c2d38e826deb44a269cb0f720f9f63b/crypto/asn1/asn1_lib.c#L178. Some non-OpenSSL based signature validation systems don't support such length descriptors at all, resulting in an extra forking risk on top for them if used for blockchain validation. This effectively means that a block chain containing a transaction with a valid signature using such a 5-byte length descriptor would be accepted by some systems and not by others, resulting in a fork if it were mined into a block.
2013-Feb-19: Bitcoin Core 0.8.0 was released, which made non-DER
signatures non-standard. No release since then would relay or mine transactions that could trigger the vulnerability. However, such a transaction was still valid inside blocks.
2014-Feb-10: I proposed BIP62 to deal with transaction malleability.
The BIP62 draft includes a rule that would make version 2 transactions with non-DER signatures invalid.
2014-Jul-18: In order to make Bitcoin's signature encoding rules not
2015-Jan-10: OpenSSL releases versions 1.0.0p and 1.0.1k, with a fix
for CVE-2014-8275. The fix introduced a restriction on ECDSA signatures to be strict DER, which would have solved all problems related to signature encodings, except Bitcoin's consensus-critical nature requires bug-for-bug compatibility between nodes. Worse, it seemed that there was again a small (1%) number of blocks being created with non-DER signatures in it, resulting in actual forks. The only immediate solution that did not introduce more risk for forks was parsing and re-encoding signatures using OpenSSL itself before verification to bypass the restriction, making the problem persist. See http://lists.linuxfoundation.org/pipermail/bitcoin-dev/2015-January/007097.html.
2015-Jan-21: The new attention to signature encoding might have
revealed the vulnerability, and the presence of miners not enforcing strict DER might have made the vulnerability actually exploitable. BIP62 was still a moving target, so we wanted a faster means to solve this. Therefore, a new BIP was proposed with just the strict DER requirement, numbered BIP66. This would both allow non-OpenSSL verification, and solve the vulnerability, without needing to fix the less urgent malleability problem that BIP62 wanted to address. See http://lists.linuxfoundation.org/pipermail/bitcoin-dev/2015-January/007156.html.
2015-Feb-17: Bitcoin Core 0.10.0 was released, with support for
2015-Jul-04: BIP66's 95% threshold was reached, enabling a consensus
rule for strict DER signatures in the blockchain. This solved the vulnerability, and opens the door to using non-OpenSSL signature verification in the near future. Pieter Wuille -----BEGIN PGP SIGNATURE----- Version: GnuPG v1 iQGcBAEBAgAGBQJVt5FGAAoJEFeJbS/48LZX3ccMAJdPrpa8ggcYEyy8naqc7ewL 1Mwv24p/6Q8+T7Q6EWmgoApY1jljF+AzgSpfaf310QZf9yuC/JC++AmHfUaa9UQG Mq1+duX64uDWIeNKTfuCwZvU0ataARZKmFUpp60UF+VtiJyLo9tpHTVajM0lv9Oq OX40qHVC/iBogRLNREC1ggWH1JPMTbEch50YX1bgNi3gE5gtMggSQ2OXrGCCtrvR 7cVFlIyOhlLtvSAnxzmHyY8Iol+qVhIZi4mCmDgOoQKVaiYm1cODQ+nrMHx02DKC Wqstwb/mET/vbCX4qxSNQ2B+mQk0WO/gSrWiQkBLi/AfLBh/8A/kL1RpKxVQzoaP O165LbXye42w8Js/sE/zT6d4CIbYaW0GpF6m4agwDYgPLomhdk/elPRojKYsEab+ oFWPVagqKI9e/pjFBxqfIv3iyx1hHB6YIaX5TfFRVjsWzag5Qi2ssQYOQymyjg4J UHNxW0PMPAOg5KS/uFja4OWxstHhTW9G+rslEK9g== =7F3K -----END PGP SIGNATURE----- original: http://lists.linuxfoundation.org/pipermail/bitcoin-dev/2015-July/009697.html
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