Subject: Re: [Bitcoin-development] [softfork proposal] Strict DER signatures

From: Matt Whitlock <[email-masked]>
Date: Wed, 21 Jan 2015 23:18:07 -0500

To be more in the C++ spirit, I would suggest changing the (const std::vector<unsigned char> &sig, size_t &off) parameters to (std::vector<unsigned char>::const_iterator &itr, std::vector<unsigned char>::const_iterator end).

Example:

bool ConsumeNumber(std::vector<unsigned char>::const_iterator &itr, std::vector<unsigned char>::const_iterator end, unsigned int len)
{
	// Length of number should be within signature.
	if (itr + len >= end) return false;
 
	// Negative numbers are not allowed.
	if (*itr & 0x80) return false;
 
	// Zero bytes at the start are not allowed, unless it would
	// otherwise be interpreted as a negative number.
	if (len > 1 && (*itr == 0x00) && !(*(itr + 1) & 0x80)) return false;
 
	// Consume number itself.
	itr += len;
	return true;
}

On Thursday, 22 January 2015, at 11:02 am, Rusty Russell wrote:
> Pieter Wuille <[email-masked]> writes:
> > Hello everyone,
> >
> > We've been aware of the risk of depending on OpenSSL for consensus
> > rules for a while, and were trying to get rid of this as part of BIP
> > 62 (malleability protection), which was however postponed due to
> > unforeseen complexities. The recent evens (see the thread titled
> > "OpenSSL 1.0.0p / 1.0.1k incompatible, causes blockchain rejection."
> > on this mailing list) have made it clear that the problem is very
> > real, however, and I would prefer to have a fundamental solution for
> > it sooner rather than later.
> 
> OK, I worked up a clearer (but more verbose) version with fewer
> magic numbers.  More importantly, feel free to steal the test cases.
> 
> One weirdness is the restriction on maximum total length, rather than a
> 32 byte (33 with 0-prepad) limit on signatures themselves.
> 
> Apologies for my babytalk C++.  Am sure there's a neater way.
> 
> /* Licensed under Creative Commons zero (public domain). */
> #include <vector>
> #include <cstdlib>
> #include <cassert>
> 
> #ifdef CLARIFY
> bool ConsumeByte(const std::vector<unsigned char> &sig, size_t &off,
>                  unsigned int &val)
> {
>     if (off >= sig.size()) return false;
> 
>     val = sig[off++];
>     return true;
> }
> 
> bool ConsumeTypeByte(const std::vector<unsigned char> &sig, size_t &off,
>                      unsigned int t)
> {
>     unsigned int type;
>     if (!ConsumeByte(sig, off, type)) return false;
> 
>     return (type == t);
> }
> 
> bool ConsumeNonZeroLength(const std::vector<unsigned char> &sig, size_t &off,
>                           unsigned int &len)
> {
>     if (!ConsumeByte(sig, off, len)) return false;
> 
>     // Zero-length integers are not allowed.
>     return (len != 0);
> }
> 
> bool ConsumeNumber(const std::vector<unsigned char> &sig, size_t &off,
>                    unsigned int len)
> {
>     // Length of number should be within signature.
>     if (off + len > sig.size()) return false;
> 
>     // Negative numbers are not allowed.
>     if (sig[off] & 0x80) return false;
> 
>     // Zero bytes at the start are not allowed, unless it would
>     // otherwise be interpreted as a negative number.
>     if (len > 1 && (sig[off] == 0x00) && !(sig[off+1] & 0x80)) return false;
> 
>     // Consume number itself.
>     off += len;
>     return true;
> }
> 
> // Consume a DER encoded integer, update off if successful.
> bool ConsumeDERInteger(const std::vector<unsigned char> &sig, size_t &off) {
>     unsigned int len;
> 
>     // Type byte must be "integer"
>     if (!ConsumeTypeByte(sig, off, 0x02)) return false;
>     if (!ConsumeNonZeroLength(sig, off, len)) return false;
>     // Now the BE encoded value itself.
>     if (!ConsumeNumber(sig, off, len)) return false;
> 
>     return true;
> }
> 
> bool IsValidSignatureEncoding(const std::vector<unsigned char> &sig) {
>     // Format: 0x30 [total-length] 0x02 [R-length] [R] 0x02 [S-length] [S] [sighash]
>     // * total-length: 1-byte length descriptor of everything that follows,
>     //     excluding the sighash byte.
>     // * R-length: 1-byte length descriptor of the R value that follows.
>     // * R: arbitrary-length big-endian encoded R value. It cannot start with any
>     //     null bytes, unless the first byte that follows is 0x80 or higher, in which
>     //     case a single null byte is required.
>     // * S-length: 1-byte length descriptor of the S value that follows.
>     // * S: arbitrary-length big-endian encoded S value. The same rules apply.
>     // * sighash: 1-byte value indicating what data is hashed.
> 
>     // Accept empty signature as correctly encoded (but invalid) signature,
>     // even though it is not strictly DER.
>     if (sig.size() == 0) return true;
> 
>     // Maximum size constraint.
>     if (sig.size() > 73) return false;
> 
>     size_t off = 0;
> 
>     // A signature is of type "compound".
>     if (!ConsumeTypeByte(sig, off, 0x30)) return false;
> 
>     unsigned int len;
>     if (!ConsumeNonZeroLength(sig, off, len)) return false;
> 
>     // Make sure the length covers the rest (except sighash).
>     if (len + 1 != sig.size() - off) return false;
> 
>     // Check R value.
>     if (!ConsumeDERInteger(sig, off)) return false;
> 
>     // Check S value.
>     if (!ConsumeDERInteger(sig, off)) return false;
> 
>     // There should exactly one byte left (the sighash).
>     return off + 1 == sig.size() ? true : false;
> }
> #else
> bool IsValidSignatureEncoding(const std::vector<unsigned char> &sig) {
>     // Format: 0x30 [total-length] 0x02 [R-length] [R] 0x02 [S-length] [S] [sighash]
>     // * total-length: 1-byte length descriptor of everything that follows,
>     //     excluding the sighash byte.
>     // * R-length: 1-byte length descriptor of the R value that follows.
>     // * R: arbitrary-length big-endian encoded R value. It must use the shortest
>     //     possible encoding for a positive integers (which means no null bytes at
>     //     the start, except a single one when the next byte has its highest bit set).
>     // * S-length: 1-byte length descriptor of the S value that follows.
>     // * S: arbitrary-length big-endian encoded S value. The same rules apply.
>     // * sighash: 1-byte value indicating what data is hashed (not part of the DER
>     //     signature)
> 
>     // Accept empty signature as correctly encoded (but invalid) signature,
>     // even though it is not strictly DER. This avoids needing full DER signatures
>     // in places where any invalid signature would do. Given that the empty string is
>     // always invalid as signature, this is safe.
>     if (sig.size() == 0) return true;
> 
>     // Minimum and maximum size constraints.
>     if (sig.size() < 9) return false;
>     if (sig.size() > 73) return false;
> 
>     // A signature is of type 0x30 (compound).
>     if (sig[0] != 0x30) return false;
> 
>     // Make sure the length covers the entire signature.
>     if (sig[1] != sig.size() - 3) return false;
> 
>     // Extract the length of the R element.
>     unsigned int lenR = sig[3];
> 
>     // Make sure the length of the S element is still inside the signature.
>     if (5 + lenR >= sig.size()) return false;
> 
>     // Extract the length of the S element.
>     unsigned int lenS = sig[5 + lenR];
> 
>     // Verify that the length of the signature matches the sum of the length
>     // of the elements.
>     if ((size_t)(lenR + lenS + 7) != sig.size()) return false;
>  
>     // Check whether the R element is an integer.
>     if (sig[2] != 0x02) return false;
> 
>     // Zero-length integers are not allowed for R.
>     if (lenR == 0) return false;
> 
>     // Negative numbers are not allowed for R.
>     if (sig[4] & 0x80) return false;
> 
>     // Null bytes at the start of R are not allowed, unless R would
>     // otherwise be interpreted as a negative number.
>     if (lenR > 1 && (sig[4] == 0x00) && !(sig[5] & 0x80)) return false;
> 
>     // Check whether the S element is an integer.
>     if (sig[lenR + 4] != 0x02) return false;
> 
>     // Zero-length integers are not allowed for S.
>     if (lenS == 0) return false;
> 
>     // Negative numbers are not allowed for S.
>     if (sig[lenR + 6] & 0x80) return false;
> 
>     // Null bytes at the start of S are not allowed, unless S would otherwise be
>     // interpreted as a negative number.
>     if (lenS > 1 && (sig[lenR + 6] == 0x00) && !(sig[lenR + 7] & 0x80)) return false;
> 
>     return true;
> }
> #endif
> 
> #define COMPOUND 0x30
> #define NOT_COMPOUND 0x31
> 
> // Len gets adjusted by check() to be actual length with this offset.
> #define LEN_OK 0
> #define LEN_TOO_BIG 1
> #define LEN_TOO_SMALL 0xff
> 
> #define INT 0x02
> #define NOT_INT 0x03
> 
> #define MINIMAL_SIGLEN 1
> #define MINIMAL_SIGVAL 0x0
> 
> #define NORMAL_SIGLEN 32
> #define NORMAL_SIGVAL(S) S, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, \
>         0x08, 0x09, 0x0a, 0x0b, 0x0c, 0x0d, 0x0e, 0x0f,              \
>         0x10, 0x11, 0x12, 0x13, 0x14, 0x15, 0x16, 0x17,              \
>         0x18, 0x19, 0x1a, 0x1b, 0x1c, 0x1d, 0x1e, 0x1f
> 
> // 33 bytes is possible, with 0 prepended.
> #define MAXIMAL_SIGLEN 33
> #define MAXIMAL_SIGVAL(S) NORMAL_SIGVAL(S), 0x20
> 
> #define OVERSIZE_SIGLEN 34
> #define OVERSIZE_SIGVAL(S) MAXIMAL_SIGVAL(S), 0x21
> 
> #define ZEROPAD_SIGLEN (1 + NORMAL_SIGLEN)
> #define ZEROPAD_SIGVAL(S) 00, NORMAL_SIGVAL(S)
> 
> #define SIGHASH 0xf0
> 
> static bool check(const std::vector<unsigned char> &sig)
> {
>     std::vector<unsigned char> fixed = sig;
> 
>     // Fixup length
>     if (fixed.size() > 1)
>         fixed[1] += fixed.size() - 3;
>     return IsValidSignatureEncoding(fixed);
> }
> 
> #define good(arr) assert(check(std::vector<unsigned char>(arr, arr+sizeof(arr))))
> #define bad(arr) assert(!check(std::vector<unsigned char>(arr, arr+sizeof(arr))))
> 
> // The OK cases.
> static unsigned char zerolen[] = { };
> static unsigned char normal[] = { COMPOUND, LEN_OK,
>                                   INT, NORMAL_SIGLEN, NORMAL_SIGVAL(0x1),
>                                   INT, NORMAL_SIGLEN, NORMAL_SIGVAL(0x2),
>                                   SIGHASH };
> static unsigned char min_r[] = { COMPOUND, LEN_OK,
>                                  INT, MINIMAL_SIGLEN, MINIMAL_SIGVAL,
>                                  INT, NORMAL_SIGLEN, NORMAL_SIGVAL(0x2),
>                                  SIGHASH };
> static unsigned char min_s[] = { COMPOUND, LEN_OK,
>                                  INT, NORMAL_SIGLEN, NORMAL_SIGVAL(0x1),
>                                  INT, MINIMAL_SIGLEN, MINIMAL_SIGVAL,
>                                  SIGHASH };
> static unsigned char max_r[] = { COMPOUND, LEN_OK,
>                                  INT, MAXIMAL_SIGLEN, MAXIMAL_SIGVAL(0x1),
>                                  INT, NORMAL_SIGLEN, NORMAL_SIGVAL(0x2),
>                                  SIGHASH };
> static unsigned char max_s[] = { COMPOUND, LEN_OK,
>                                  INT, NORMAL_SIGLEN, NORMAL_SIGVAL(0x1),
>                                  INT, MAXIMAL_SIGLEN, MAXIMAL_SIGVAL(0x2),
>                                  SIGHASH };
> // As long as total size doesn't go over, a single sig is allowed > 33 bytes
> static unsigned char wierd_s_len[] = { COMPOUND, LEN_OK,
>                                        INT, OVERSIZE_SIGLEN, OVERSIZE_SIGVAL(0x1),
>                                        INT, NORMAL_SIGLEN, NORMAL_SIGVAL(0x2),
>                                        SIGHASH };
> static unsigned char wierd_r_len[] = { COMPOUND, LEN_OK,
>                                        INT, NORMAL_SIGLEN, NORMAL_SIGVAL(0x1),
>                                        INT, OVERSIZE_SIGLEN, OVERSIZE_SIGVAL(0x2),
>                                        SIGHASH };
> static unsigned char zeropad_s[] = { COMPOUND, LEN_OK,
>                                      INT, ZEROPAD_SIGLEN, ZEROPAD_SIGVAL(0x81),
>                                      INT, NORMAL_SIGLEN, NORMAL_SIGVAL(0x2),
>                                      SIGHASH };
> static unsigned char zeropad_r[] = { COMPOUND, LEN_OK,
>                                      INT, NORMAL_SIGLEN, NORMAL_SIGVAL(0x1),
>                                      INT, ZEROPAD_SIGLEN, ZEROPAD_SIGVAL(0x82),
>                                      SIGHASH };
> 
> 
> // The fail cases.
> static unsigned char not_compound[] = { NOT_COMPOUND, LEN_OK,
>                                         INT, NORMAL_SIGLEN, NORMAL_SIGVAL(0x1),
>                                         INT, NORMAL_SIGLEN, NORMAL_SIGVAL(0x2),
>                                         SIGHASH };
> static unsigned char short_len[] = { COMPOUND, LEN_TOO_SMALL,
>                                      INT, NORMAL_SIGLEN, NORMAL_SIGVAL(0x1),
>                                      INT, NORMAL_SIGLEN, NORMAL_SIGVAL(0x2),
>                                      SIGHASH };
> static unsigned char long_len[] = { COMPOUND, LEN_TOO_BIG,
>                                     INT, NORMAL_SIGLEN, NORMAL_SIGVAL(0x1),
>                                     INT, NORMAL_SIGLEN, NORMAL_SIGVAL(0x2),
>                                     SIGHASH };
> static unsigned char r_notint[] = { COMPOUND, LEN_OK,
>                                     NOT_INT, NORMAL_SIGLEN, NORMAL_SIGVAL(0x1),
>                                     INT, NORMAL_SIGLEN, NORMAL_SIGVAL(0x2),
>                                     SIGHASH };
> static unsigned char s_notint[] = { COMPOUND, LEN_OK,
>                                     INT, NORMAL_SIGLEN, NORMAL_SIGVAL(0x1),
>                                     NOT_INT, NORMAL_SIGLEN, NORMAL_SIGVAL(0x2),
>                                     SIGHASH };
> static unsigned char s_oversig[] = { COMPOUND, LEN_OK,
>                                      INT, OVERSIZE_SIGLEN, OVERSIZE_SIGVAL(0x1),
>                                      INT, MAXIMAL_SIGLEN, MAXIMAL_SIGVAL(0x2),
>                                      SIGHASH };
> static unsigned char r_oversig[] = { COMPOUND, LEN_OK,
>                                      INT, MAXIMAL_SIGLEN, MAXIMAL_SIGVAL(0x1),
>                                      INT, OVERSIZE_SIGLEN, OVERSIZE_SIGVAL(0x2),
>                                      SIGHASH };
> static unsigned char s_negative[] = { COMPOUND, LEN_OK,
>                                       INT, NORMAL_SIGLEN, NORMAL_SIGVAL(0x81),
>                                       INT, NORMAL_SIGLEN, NORMAL_SIGVAL(0x2),
>                                       SIGHASH };
> static unsigned char r_negative[] = { COMPOUND, LEN_OK,
>                                       INT, NORMAL_SIGLEN, NORMAL_SIGVAL(0x1),
>                                       INT, NORMAL_SIGLEN, NORMAL_SIGVAL(0x82),
>                                       SIGHASH };
> static unsigned char zeropad_bad_s[] = { COMPOUND, LEN_OK,
>                                          INT, ZEROPAD_SIGLEN, ZEROPAD_SIGVAL(0x1),
>                                          INT, NORMAL_SIGLEN, NORMAL_SIGVAL(0x2),
>                                          SIGHASH };
> static unsigned char zeropad_bad_r[] = { COMPOUND, LEN_OK,
>                                          INT, NORMAL_SIGLEN, NORMAL_SIGVAL(0x1),
>                                          INT, ZEROPAD_SIGLEN, ZEROPAD_SIGVAL(0x2),
>                                          SIGHASH };
> static unsigned char missing_sighash[] = { COMPOUND, LEN_OK,
>                                            INT, NORMAL_SIGLEN, NORMAL_SIGVAL(0x1),
>                                            INT, NORMAL_SIGLEN, NORMAL_SIGVAL(0x2) };
> static unsigned char extra_byte[] = { COMPOUND, LEN_OK,
>                                       INT, NORMAL_SIGLEN, NORMAL_SIGVAL(0x1),
>                                       INT, NORMAL_SIGLEN, NORMAL_SIGVAL(0x2),
>                                       SIGHASH, 0 };
> 
> // Bad signature lengths
> static unsigned char zerolen_r[] = { COMPOUND, LEN_OK,
>                                      INT, 0,
>                                      INT, NORMAL_SIGLEN, NORMAL_SIGVAL(0x2),
>                                      SIGHASH };
> static unsigned char zerolen_s[] = { COMPOUND, LEN_OK,
>                                      INT, NORMAL_SIGLEN, NORMAL_SIGVAL(0x1),
>                                      INT, 0,
>                                      SIGHASH };
> static unsigned char overlen_r_by_1[] = { COMPOUND, LEN_OK,
>                                           INT, NORMAL_SIGLEN + 1 + 1 + NORMAL_SIGLEN + 1 + 1, NORMAL_SIGVAL(0x1),
>                                           INT, NORMAL_SIGLEN, NORMAL_SIGVAL(0x2),
>                                           SIGHASH };
> static unsigned char overlen_s_by_1[] = { COMPOUND, LEN_OK,
>                                           INT, NORMAL_SIGLEN, NORMAL_SIGVAL(0x1),
>                                           INT, NORMAL_SIGLEN+1+1, NORMAL_SIGVAL(0x2),
>                                           SIGHASH };
> static unsigned char underlen_r_by_1[] = { COMPOUND, LEN_OK,
>                                            INT, NORMAL_SIGLEN-1, NORMAL_SIGVAL(0x1),
>                                            INT, NORMAL_SIGLEN, NORMAL_SIGVAL(0x2),
>                                            SIGHASH };
> static unsigned char underlen_s_by_1[] = { COMPOUND, LEN_OK,
>                                            INT, NORMAL_SIGLEN, NORMAL_SIGVAL(0x1),
>                                            INT, NORMAL_SIGLEN-1, NORMAL_SIGVAL(0x2),
>                                            SIGHASH };
> 
> int main()
> {
>     good(zerolen);
>     good(normal);
>     good(min_r);
>     good(min_s);
>     good(max_r);
>     good(max_s);
>     good(wierd_s_len);
>     good(wierd_r_len);
>     good(zeropad_s);
>     good(zeropad_r);
> 
>     // Try different amounts of truncation.
>     for (size_t i = 1; i < sizeof(normal)-1; i++)
>         assert(!check(std::vector<unsigned char>(normal, normal+i)));
> 
>     bad(not_compound);
>     bad(short_len);
>     bad(long_len);
>     bad(r_notint);
>     bad(s_notint);
>     bad(s_oversig);
>     bad(r_oversig);
>     bad(s_negative);
>     bad(r_negative);
>     bad(s_negative);
>     bad(r_negative);
>     bad(zeropad_bad_s);
>     bad(zeropad_bad_r);
>     bad(zerolen_r);
>     bad(zerolen_s);
>     bad(overlen_r_by_1);
>     bad(overlen_s_by_1);
>     bad(underlen_r_by_1);
>     bad(underlen_s_by_1);
>     bad(missing_sighash);
>     bad(extra_byte);
> 
>     return 0;
> }
> 
> 
> 
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