  /* asn.cpp                                
 *
 * Copyright (C) 2003 Sawtooth Consulting Ltd.
 *
 * This file is part of yaSSL.
 *
 * yaSSL is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 2 of the License, or
 * (at your option) any later version.
 *
 * yaSSL is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write to the Free Software
 * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA
 */

/* asn.cpp implements ASN1 BER, PublicKey, and x509v3 decoding 
*/

#include "runtime.hpp"
#include "asn.hpp"
#include "file.hpp"
#include "integer.hpp"
#include "rsa.hpp"
#include "dsa.hpp"
#include "dh.hpp"
#include "md5.hpp"
#include "md2.hpp"
#include "sha.hpp"
#include "coding.hpp"
#include <time.h>     // gmtime();
#include "memory.hpp"   // mySTL::auto_ptr

namespace TaoCrypt {

namespace { // locals


// to the minute
bool operator>(tm& a, tm& b)
{
    if (a.tm_year > b.tm_year)
        return true;

    if (a.tm_year == b.tm_year && a.tm_mon > b.tm_mon)
        return true;
    
    if (a.tm_year == b.tm_year && a.tm_mon == b.tm_mon && a.tm_mday >b.tm_mday)
        return true;

    if (a.tm_year == b.tm_year && a.tm_mon == b.tm_mon &&
        a.tm_mday == b.tm_mday && a.tm_hour > b.tm_hour)
        return true;

    if (a.tm_year == b.tm_year && a.tm_mon == b.tm_mon &&
        a.tm_mday == b.tm_mday && a.tm_hour == b.tm_hour &&
        a.tm_min > b.tm_min)
        return true;

    return false;
}


bool operator<(tm& a, tm&b)
{
    return !(a>b);
}


// like atoi but only use first byte
word32 btoi(byte b)
{
    return b - 0x30;
}


// two byte date/time, add to value
void GetTime(int& value, const byte* date, int& i)
{
    value += btoi(date[i++]) * 10;
    value += btoi(date[i++]);
}


// Make sure before and after dates are valid
bool ValidateDate(const byte* date, byte format, CertDecoder::DateType dt)
{
    tm certTime;
    memset(&certTime, 0, sizeof(certTime));
    int i = 0;

    if (format == UTC_TIME) {
        if (btoi(date[0]) >= 5)
            certTime.tm_year = 1900;
        else
            certTime.tm_year = 2000;
    }
    else  { // format == GENERALIZED_TIME
        certTime.tm_year += btoi(date[i++]) * 1000;
        certTime.tm_year += btoi(date[i++]) * 100;
    }

    GetTime(certTime.tm_year, date, i);     certTime.tm_year -= 1900; // adjust
    GetTime(certTime.tm_mon,  date, i);     certTime.tm_mon  -= 1;    // adjust
    GetTime(certTime.tm_mday, date, i);
    GetTime(certTime.tm_hour, date, i); 
    GetTime(certTime.tm_min,  date, i); 
    GetTime(certTime.tm_sec,  date, i); 

    assert(date[i] == 'Z');     // only Zulu supported for this profile

    time_t ltime = time(0);
    tm* localTime = gmtime(&ltime);

    if (dt == CertDecoder::BEFORE) {
        if (*localTime < certTime)
            return false;
    }
    else
        if (*localTime > certTime)
            return false;

    return true;
}


class BadCertificate {};

} // local namespace



// used by Integer as well
word32 GetLength(Source& source)
{
    word32 length = 0;

    byte b = source.next();
    if (b >= LONG_LENGTH) {        
        word32 bytes = b & 0x7F;

        while (bytes--) {
            b = source.next();
            length = (length << 8) | b;
        }
    }
    else
        length = b;

    return length;
}


word32 SetLength(word32 length, byte* output)
{
    word32 i = 0;

    if (length < LONG_LENGTH)
        output[i++] = length;
    else {
        output[i++] = BytePrecision(length) | 0x80;
      
        for (int j = BytePrecision(length); j; --j) {
            output[i] = length >> (j - 1) * 8;
            i++;
        }
    }
    return i;
}


PublicKey::PublicKey(const byte* k, word32 s) : key_(0), sz_(0)
{
    if (s) {
        SetSize(s);
        SetKey(k);
    }
}


void PublicKey::SetSize(word32 s)
{
    sz_ = s;
    key_ = new (tc) byte[sz_];
}


void PublicKey::SetKey(const byte* k)
{
    memcpy(key_, k, sz_);
}


void PublicKey::AddToEnd(const byte* data, word32 len)
{
    mySTL::auto_ptr<byte> tmp(new (tc) byte[sz_ + len], tcArrayDelete);

    memcpy(tmp.get(), key_, sz_);
    memcpy(tmp.get() + sz_, data, len);

    byte* del = 0;
    mySTL::swap(del, key_);
    tcArrayDelete(del);

    key_ = tmp.release();
    sz_ += len;
}


Signer::Signer(const byte* k, word32 kSz, const char* n, const byte* h)
    : key_(k, kSz), name_(0)
{
    if (n) {
        int sz = strlen(n);
        name_ = new (tc) char[sz + 1];
        memcpy(name_, n, sz);
        name_[sz] = 0;
    }

    memcpy(hash_, h, SHA::DIGEST_SIZE);
}

Signer::~Signer()
{
    tcArrayDelete(name_);
}


Error BER_Decoder::GetError()
{ 
    return source_.GetError(); 
}


Integer& BER_Decoder::GetInteger(Integer& integer)
{
    if (!source_.GetError().What())
        integer.Decode(source_);
    return integer;
}

  
// Read a Sequence, return length
word32 BER_Decoder::GetSequence()
{
    if (source_.GetError().What()) return 0;

    byte b = source_.next();
    if (b != (SEQUENCE | CONSTRUCTED)) {
        source_.SetError(SEQUENCE_E);
        return 0;
    }

    return GetLength(source_);
}


// Read a Sequence, return length
word32 BER_Decoder::GetSet()
{
    if (source_.GetError().What()) return 0;

    byte b = source_.next();
    if (b != (SET | CONSTRUCTED)) {
        source_.SetError(SET_E);
        return 0;
    }

    return GetLength(source_);
}


// Read Version, return it
word32 BER_Decoder::GetVersion()
{
    if (source_.GetError().What()) return 0;

    byte b = source_.next();
    if (b != INTEGER) {
        source_.SetError(INTEGER_E);
        return 0;
    }

    b = source_.next();
    if (b != 0x01) {
        source_.SetError(VERSION_E);
        return 0;
    }

    return source_.next();
}


// Read ExplicitVersion, return it or 0 if not there (not an error)
word32 BER_Decoder::GetExplicitVersion()
{
    if (source_.GetError().What()) return 0;

    byte b = source_.next();

    if (b == (CONTEXT_SPECIFIC | CONSTRUCTED)) { // not an error if not here
        source_.next();
        return GetVersion();
    }
    else 
        source_.prev(); // put back
  
    return 0;
}


// Decode a BER encoded RSA Private Key
void RSA_Private_Decoder::Decode(RSA_PrivateKey& key)
{
    ReadHeader();
    if (source_.GetError().What()) return;
    // public
    key.SetModulus(GetInteger(Integer().Ref()));
    key.SetPublicExponent(GetInteger(Integer().Ref()));

    // private
    key.SetPrivateExponent(GetInteger(Integer().Ref()));
    key.SetPrime1(GetInteger(Integer().Ref()));
    key.SetPrime2(GetInteger(Integer().Ref()));
    key.SetModPrime1PrivateExponent(GetInteger(Integer().Ref()));
    key.SetModPrime2PrivateExponent(GetInteger(Integer().Ref()));
    key.SetMultiplicativeInverseOfPrime2ModPrime1(GetInteger(Integer().Ref()));
}


void RSA_Private_Decoder::ReadHeader()
{
    GetSequence();
    GetVersion();
}


// Decode a BER encoded DSA Private Key
void DSA_Private_Decoder::Decode(DSA_PrivateKey& key)
{
    ReadHeader();
    if (source_.GetError().What()) return;
    // group parameters
    key.SetModulus(GetInteger(Integer().Ref()));
    key.SetSubGroupOrder(GetInteger(Integer().Ref()));
    key.SetSubGroupGenerator(GetInteger(Integer().Ref()));

    // key
    key.SetPublicPart(GetInteger(Integer().Ref()));
    key.SetPrivatePart(GetInteger(Integer().Ref()));   
}


void DSA_Private_Decoder::ReadHeader()
{
    GetSequence();
    GetVersion();
}


// Decode a BER encoded RSA Public Key
void RSA_Public_Decoder::Decode(RSA_PublicKey& key)
{
    ReadHeader();
    if (source_.GetError().What()) return;

    // public key
    key.SetModulus(GetInteger(Integer().Ref()));
    key.SetPublicExponent(GetInteger(Integer().Ref()));
}


void RSA_Public_Decoder::ReadHeader()
{
    GetSequence();
}


// Decode a BER encoded DSA Public Key
void DSA_Public_Decoder::Decode(DSA_PublicKey& key)
{
    ReadHeader();
    if (source_.GetError().What()) return;

    // group parameters
    key.SetModulus(GetInteger(Integer().Ref()));
    key.SetSubGroupOrder(GetInteger(Integer().Ref()));
    key.SetSubGroupGenerator(GetInteger(Integer().Ref()));

    // key
    key.SetPublicPart(GetInteger(Integer().Ref()));
}


void DSA_Public_Decoder::ReadHeader()
{
    GetSequence();
}


void DH_Decoder::ReadHeader()
{
    GetSequence();
}


// Decode a BER encoded Diffie-Hellman Key
void DH_Decoder::Decode(DH& key)
{
    ReadHeader();
    if (source_.GetError().What()) return;

    // group parms
    key.SetP(GetInteger(Integer().Ref()));
    key.SetG(GetInteger(Integer().Ref()));
}


CertDecoder::CertDecoder(Source& s, bool decode, SignerList* signers)
    : BER_Decoder(s), certBegin_(0), sigIndex_(0), sigLength_(0),
      signature_(0), issuer_(0), subject_(0)
{ 
    if (decode)
        Decode(signers); 
}


CertDecoder::~CertDecoder()
{
    tcArrayDelete(subject_);
    tcArrayDelete(issuer_);
    tcArrayDelete(signature_);
}


// process certificate header, set signature offset
void CertDecoder::ReadHeader()
{
    if (source_.GetError().What()) return;

    GetSequence();  // total
    certBegin_ = source_.get_index();

    sigIndex_ = GetSequence();  // this cert
    sigIndex_ += source_.get_index();

    GetExplicitVersion(); // version
    GetInteger(Integer().Ref());  // serial number
}


// Decode a x509v3 Certificate
void CertDecoder::Decode(SignerList* signers)
{
    if (source_.GetError().What()) return;
    DecodeToKey();
    if (source_.GetError().What()) return;

    if (source_.get_index() != sigIndex_)
        source_.set_index(sigIndex_);

    word32 confirmOID = GetAlgoId();
    GetSignature();
    if (source_.GetError().What()) return;

    if ( confirmOID != signatureOID_ ) {
        source_.SetError(SIG_OID_E);
        return;
    }

    if ( memcmp(issuerHash_, subjectHash_, SHA::DIGEST_SIZE) == 0 ) {
        if (!ValidateSelfSignature())
            source_.SetError(SIG_CONFIRM_E);
    }
    else
        if (!ValidateSignature(signers))
            source_.SetError(SIG_OTHER_E);
}


void CertDecoder::DecodeToKey()
{
    ReadHeader();
    signatureOID_ = GetAlgoId();
    GetName(ISSUER);   
    GetValidity();
    GetName(SUBJECT);   
    GetKey();
}


// Read public key
void CertDecoder::GetKey()
{
    if (source_.GetError().What()) return;

    GetSequence();    
    keyOID_ = GetAlgoId();

    if (keyOID_ == RSAk) {
        byte b = source_.next();
        if (b != BIT_STRING) {
            source_.SetError(BIT_STR_E);
            return;
        }
        b = source_.next();      // length, future
        b = source_.next(); 
        while(b != 0)
            b = source_.next();
    }
    else if (keyOID_ == DSAk)
        ;   // do nothing
    else {
        source_.SetError(UNKNOWN_OID_E);
        return;
    }

    StoreKey();
    if (keyOID_ == DSAk)
        AddDSA();
}


// Save public key
void CertDecoder::StoreKey()
{
    if (source_.GetError().What()) return;

    word32 read = source_.get_index();
    word32 length = GetSequence();

    read = source_.get_index() - read;
    length += read;

    while (read--) source_.prev();

    key_.SetSize(length);
    key_.SetKey(source_.get_current());
    source_.advance(length);
}


// DSA has public key after group
void CertDecoder::AddDSA()
{
    if (source_.GetError().What()) return;

    byte b = source_.next();
    if (b != BIT_STRING) {
        source_.SetError(BIT_STR_E);
        return;
    }
    b = source_.next();      // length, future
    b = source_.next(); 
    while(b != 0)
        b = source_.next();

    word32 idx = source_.get_index();
    b = source_.next();
    if (b != INTEGER) {
        source_.SetError(INTEGER_E);
        return;
    }

    word32 length = GetLength(source_);
    length += source_.get_index() - idx;

    key_.AddToEnd(source_.get_buffer() + idx, length);    
}


// process algo OID by summing, return it
word32 CertDecoder::GetAlgoId()
{
    if (source_.GetError().What()) return 0;
    word32 length = GetSequence();
    
    byte b = source_.next();
    if (b != OBJECT_IDENTIFIER) {
        source_.SetError(OBJECT_ID_E);
        return 0;
    }

    length = GetLength(source_);
    word32 oid = 0;
    
    while(length--)
        oid += source_.next();        // just sum it up for now

    if (oid != SHAwDSA && oid != DSAk) {
        b = source_.next();               // should have NULL tag and 0

        if (b != TAG_NULL) {
            source_.SetError(TAG_NULL_E);
            return 0;
        }

        b = source_.next();
        if (b != 0) {
            source_.SetError(EXPECT_0_E);
            return 0;
        }
    }
 
    return oid;
}


// read cert signature, store in signature_
word32 CertDecoder::GetSignature()
{
    if (source_.GetError().What()) return 0;
    byte b = source_.next();

    if (b != BIT_STRING) {
        source_.SetError(BIT_STR_E);
        return 0;
    }

    sigLength_ = GetLength(source_);
  
    b = source_.next();
    if (b != 0) {
        source_.SetError(EXPECT_0_E);
        return 0;
    }
    sigLength_--;

    signature_ = new (tc) byte[sigLength_];
    memcpy(signature_, source_.get_current(), sigLength_);
    source_.advance(sigLength_);

    return sigLength_;
}


// read cert digest, store in signature_
word32 CertDecoder::GetDigest()
{
    if (source_.GetError().What()) return 0;
    byte b = source_.next();

    if (b != OCTET_STRING) {
        source_.SetError(OCTET_STR_E);
        return 0;
    }

    sigLength_ = GetLength(source_);

    signature_ = new (tc) byte[sigLength_];
    memcpy(signature_, source_.get_current(), sigLength_);
    source_.advance(sigLength_);

    return sigLength_;
}


// process NAME, either issuer or subject
void CertDecoder::GetName(NameType nt)
{
    if (source_.GetError().What()) return;

    SHA    sha;
    word32 length = GetSequence();  // length of all distinguished names
    length += source_.get_index();

    while (source_.get_index() < length) {
        GetSet();
        GetSequence();

        byte b = source_.next();
        if (b != OBJECT_IDENTIFIER) {
            source_.SetError(OBJECT_ID_E);
            return;
        }

        word32 oidSz = GetLength(source_);
        byte joint[2];
        memcpy(joint, source_.get_current(), sizeof(joint));

        // v1 name types
        if (joint[0] == 0x55 && joint[1] == 0x04) {
            source_.advance(2);
            byte   id      = source_.next();  
            b              = source_.next();    // strType
            word32 strLen  = GetLength(source_);

            if (id == COMMON_NAME) {
                char*& ptr = (nt == ISSUER) ? issuer_ : subject_;
                ptr = new (tc) char[strLen + 1];
                memcpy(ptr, source_.get_current(), strLen);
                ptr[strLen] = 0;
            }
            sha.Update(source_.get_current(), strLen);
            source_.advance(strLen);
        }
        else {
            // skip
            source_.advance(oidSz + 1);
            word32 length = GetLength(source_);
            source_.advance(length);
        }
    }
    if (nt == ISSUER)
        sha.Final(issuerHash_);
    else
        sha.Final(subjectHash_);
}


// process a Date, either BEFORE or AFTER
void CertDecoder::GetDate(DateType dt)
{
    if (source_.GetError().What()) return;

    byte b = source_.next();
    if (b != UTC_TIME && b != GENERALIZED_TIME) {
        source_.SetError(TIME_E);
        return;
    }

    word32 length = GetLength(source_);
    byte date[MAX_DATE_SZ];
    if (length > MAX_DATE_SZ || length < MIN_DATE_SZ) {
        source_.SetError(DATE_SZ_E);
        return;
    }

    memcpy(date, source_.get_current(), length);
    source_.advance(length);

    if (!ValidateDate(date, b, dt))
        if (dt == BEFORE)
            source_.SetError(BEFORE_DATE_E);
        else
            source_.SetError(AFTER_DATE_E);
}


void CertDecoder::GetValidity()
{
    if (source_.GetError().What()) return;

    GetSequence();
    GetDate(BEFORE);
    GetDate(AFTER);
}


bool CertDecoder::ValidateSelfSignature()
{
    Source pub(key_.GetKey(), key_.size());
    return ConfirmSignature(pub);
}


// extract compare signature hash from plain and place into digest
void CertDecoder::GetCompareHash(const byte* plain, word32 sz, byte* digest,
                                 word32 digSz)
{
    if (source_.GetError().What()) return;

    Source s(plain, sz);
    CertDecoder dec(s, false);

    dec.GetSequence();
    dec.GetAlgoId();
    dec.GetDigest();

    if (dec.sigLength_ > digSz) {
        source_.SetError(SIG_LEN_E);
        return;
    }

    memcpy(digest, dec.signature_, dec.sigLength_);
}


// validate signature signed by someone else
bool CertDecoder::ValidateSignature(SignerList* signers)
{
    assert(signers);

    SignerList::iterator first = signers->begin();
    SignerList::iterator last  = signers->end();

    while (first != last) {
        if ( memcmp(issuerHash_, (*first)->GetHash(), SHA::DIGEST_SIZE) == 0) {
      
            const PublicKey& iKey = (*first)->GetPublicKey();
            Source pub(iKey.GetKey(), iKey.size());
            return ConfirmSignature(pub);
        }   
        ++first;
    }
    return false;
}


// RSA confirm
bool CertDecoder::ConfirmSignature(Source& pub)
{
    HashType ht;
    mySTL::auto_ptr<HASH> hasher(tcDelete);

    if (signatureOID_ == MD5wRSA) {
        hasher.reset(new (tc) MD5);
        ht = MD5h;
    }
    else if (signatureOID_ == MD2wRSA) {
        hasher.reset(new (tc) MD2);
        ht = MD2h;
    }
    else if (signatureOID_ == SHAwRSA || signatureOID_ == SHAwDSA) {
        hasher.reset(new (tc) SHA);
        ht = SHAh;
    }
    else {
        source_.SetError(UNKOWN_SIG_E);
        return false;
    }

    byte digest[SHA::DIGEST_SIZE];      // largest size

    hasher->Update(source_.get_buffer() + certBegin_, sigIndex_ - certBegin_);
    hasher->Final(digest);

    if (keyOID_ == RSAk) {
        // put in ASN.1 signature format
        Source build;
        Signature_Encoder(digest, hasher->getDigestSize(), ht, build);

        RSA_PublicKey pubKey(pub);
        RSAES_Encryptor enc(pubKey);

        return enc.SSL_Verify(build.get_buffer(), build.size(), signature_);
    }
    else  { // DSA
        // extract r and s from sequence
        byte seqDecoded[DSA_SIG_SZ];
        DecodeDSA_Signature(seqDecoded, signature_, sigLength_);

        DSA_PublicKey pubKey(pub);
        DSA_Verifier  ver(pubKey);

        return ver.Verify(digest, seqDecoded);
    }
}


Signature_Encoder::Signature_Encoder(const byte* dig, word32 digSz,
                                     HashType digOID, Source& source)
{
    // build bottom up

    // Digest
    byte digArray[MAX_DIGEST_SZ];
    word32 digestSz = SetDigest(dig, digSz, digArray);

    // AlgoID
    byte algoArray[MAX_ALGO_SZ];
    word32 algoSz = SetAlgoID(digOID, algoArray);

    // Sequence
    byte seqArray[MAX_SEQ_SZ];
    word32 seqSz = SetSequence(digestSz + algoSz, seqArray);

    source.grow(seqSz + algoSz + digestSz);  // make sure enough room
    source.add(seqArray,  seqSz);
    source.add(algoArray, algoSz);
    source.add(digArray,  digestSz);
}



word32 Signature_Encoder::SetDigest(const byte* d, word32 dSz, byte* output)
{
    output[0] = OCTET_STRING;
    output[1] = dSz;
    memcpy(&output[2], d, dSz);
    
    return dSz + 2;
}



word32 DER_Encoder::SetAlgoID(HashType aOID, byte* output)
{
    // adding TAG_NULL and 0 to end
    static const byte shaAlgoID[] = { 0x2b, 0x0e, 0x03, 0x02, 0x1a,
                                      0x05, 0x00 };
    static const byte md5AlgoID[] = { 0x2a, 0x86, 0x48, 0x86, 0xf7, 0x0d,
                                      0x02, 0x05, 0x05, 0x00  };
    static const byte md2AlgoID[] = { 0x2a, 0x86, 0x48, 0x86, 0xf7, 0x0d,
                                      0x02, 0x02, 0x05, 0x00};

    int algoSz = 0;
    const byte* algoName = 0;

    switch (aOID) {
    case SHAh:
        algoSz = sizeof(shaAlgoID);
        algoName = shaAlgoID;
        break;

    case MD2h:
        algoSz = sizeof(md2AlgoID);
        algoName = md2AlgoID;
        break;

    case MD5h:
        algoSz = sizeof(md5AlgoID);
        algoName = md5AlgoID;
        break;

    default:
        error_.SetError(UNKOWN_HASH_E);
        return 0;
    }


    byte ID_Length[MAX_LENGTH_SZ];
    word32 idSz = SetLength(algoSz - 2, ID_Length); // don't include TAG_NULL/0

    byte seqArray[MAX_SEQ_SZ + 1];  // add object_id to end
    word32 seqSz = SetSequence(idSz + algoSz + 1, seqArray);
    seqArray[seqSz++] = OBJECT_IDENTIFIER;

    memcpy(output, seqArray, seqSz);
    memcpy(output + seqSz, ID_Length, idSz);
    memcpy(output + seqSz + idSz, algoName, algoSz);

    return seqSz + idSz + algoSz;
}


word32 SetSequence(word32 len, byte* output)
{
  
    output[0] = SEQUENCE | CONSTRUCTED;
    return SetLength(len, output + 1) + 1;
}


word32 EncodeDSA_Signature(const byte* signature, byte* output)
{
    Integer r(signature, 20);
    Integer s(signature + 20, 20);

    return EncodeDSA_Signature(r, s, output);
}


word32 EncodeDSA_Signature(const Integer& r, const Integer& s, byte* output)
{
    word32 rSz = r.ByteCount();
    word32 sSz = s.ByteCount();

    byte rLen[MAX_LENGTH_SZ + 1];
    byte sLen[MAX_LENGTH_SZ + 1];

    rLen[0] = INTEGER;
    sLen[0] = INTEGER;

    word32 rLenSz = SetLength(rSz, &rLen[1]) + 1;
    word32 sLenSz = SetLength(sSz, &sLen[1]) + 1;

    byte seqArray[MAX_SEQ_SZ];

    word32 seqSz = SetSequence(rLenSz + rSz + sLenSz + sSz, seqArray);
    
    // seq
    memcpy(output, seqArray, seqSz);
    // r
    memcpy(output + seqSz, rLen, rLenSz);
    r.Encode(output + seqSz + rLenSz, rSz);
    // s
    memcpy(output + seqSz + rLenSz + rSz, sLen, sLenSz);
    s.Encode(output + seqSz + rLenSz + rSz + sLenSz, sSz);

    return seqSz + rLenSz + rSz + sLenSz + sSz;
}


// put sequence encoded dsa signature into decoded in 2 20 byte integers
word32 DecodeDSA_Signature(byte* decoded, const byte* encoded, word32 sz)
{
    Source source(encoded, sz);

    if (source.next() != (SEQUENCE | CONSTRUCTED)) {
        source.SetError(SEQUENCE_E);
        return 0;
    }

    GetLength(source);  // total

    // r
    if (source.next() != INTEGER) {
        source.SetError(INTEGER_E);
        return 0;
    }
    word32 rLen = GetLength(source);
    if (rLen != 20)
        if (rLen == 21) {       // zero at front, eat
            source.next();
            --rLen;
        }
        else if (rLen == 19) {  // add zero to front so 20 bytes
            decoded[0] = 0;
            decoded++;
        }
        else {
            source.SetError(DSA_SZ_E);
            return 0;
        }
    memcpy(decoded, source.get_buffer() + source.get_index(), rLen);
    source.advance(rLen);

    // s
    if (source.next() != INTEGER) {
        source.SetError(INTEGER_E);
        return 0;
    }
    word32 sLen = GetLength(source);
    if (sLen != 20)
        if (sLen == 21) {
            source.next();          // zero at front, eat
            --sLen;
        }
        else if (sLen == 19) {
            decoded[rLen] = 0;      // add zero to front so 20 bytes
            decoded++;
        }
        else {
            source.SetError(DSA_SZ_E);
            return 0;
        }
    memcpy(decoded + rLen, source.get_buffer() + source.get_index(), sLen);
    source.advance(sLen);

    return 40;
}


} // namespace
