qpdf/libqpdf/QPDF_encryption.cc

// This file implements methods from the QPDF class that involve
// encryption.

#include <qpdf/assert_debug.h>

#include <qpdf/QPDF.hh>

#include <qpdf/QPDFExc.hh>

#include <qpdf/MD5.hh>
#include <qpdf/Pl_AES_PDF.hh>
#include <qpdf/Pl_Buffer.hh>
#include <qpdf/Pl_RC4.hh>
#include <qpdf/Pl_SHA2.hh>
#include <qpdf/QTC.hh>
#include <qpdf/QUtil.hh>
#include <qpdf/RC4.hh>

#include <algorithm>
#include <string.h>

static unsigned char const padding_string[] = {
    0x28, 0xbf, 0x4e, 0x5e, 0x4e, 0x75, 0x8a, 0x41, 0x64, 0x00, 0x4e,
    0x56, 0xff, 0xfa, 0x01, 0x08, 0x2e, 0x2e, 0x00, 0xb6, 0xd0, 0x68,
    0x3e, 0x80, 0x2f, 0x0c, 0xa9, 0xfe, 0x64, 0x53, 0x69, 0x7a};

static unsigned int const key_bytes = 32;

// V4 key lengths apply to V <= 4
static unsigned int const OU_key_bytes_V4 = sizeof(MD5::Digest);

static unsigned int const OU_key_bytes_V5 = 48;
static unsigned int const OUE_key_bytes_V5 = 32;
static unsigned int const Perms_key_bytes_V5 = 16;

int
QPDF::EncryptionData::getV() const
{
    return this->V;
}

int
QPDF::EncryptionData::getR() const
{
    return this->R;
}

int
QPDF::EncryptionData::getLengthBytes() const
{
    return this->Length_bytes;
}

int
QPDF::EncryptionData::getP() const
{
    return this->P;
}

std::string const&
QPDF::EncryptionData::getO() const
{
    return this->O;
}

std::string const&
QPDF::EncryptionData::getU() const
{
    return this->U;
}

std::string const&
QPDF::EncryptionData::getOE() const
{
    return this->OE;
}

std::string const&
QPDF::EncryptionData::getUE() const
{
    return this->UE;
}

std::string const&
QPDF::EncryptionData::getPerms() const
{
    return this->Perms;
}

std::string const&
QPDF::EncryptionData::getId1() const
{
    return this->id1;
}

bool
QPDF::EncryptionData::getEncryptMetadata() const
{
    return this->encrypt_metadata;
}

void
QPDF::EncryptionData::setO(std::string const& O)
{
    this->O = O;
}

void
QPDF::EncryptionData::setU(std::string const& U)
{
    this->U = U;
}

void
QPDF::EncryptionData::setV5EncryptionParameters(
    std::string const& O,
    std::string const& OE,
    std::string const& U,
    std::string const& UE,
    std::string const& Perms)
{
    this->O = O;
    this->OE = OE;
    this->U = U;
    this->UE = UE;
    this->Perms = Perms;
}

static void
pad_or_truncate_password_V4(std::string const& password, char k1[key_bytes])
{
    size_t password_bytes =
        std::min(QIntC::to_size(key_bytes), password.length());
    size_t pad_bytes = key_bytes - password_bytes;
    memcpy(k1, password.c_str(), password_bytes);
    memcpy(k1 + password_bytes, padding_string, pad_bytes);
}

void
QPDF::trim_user_password(std::string& user_password)
{
    // Although unnecessary, this routine trims the padding string
    // from the end of a user password.  Its only purpose is for
    // recovery of user passwords which is done in the test suite.
    char const* cstr = user_password.c_str();
    size_t len = user_password.length();
    if (len < key_bytes) {
        return;
    }

    char const* p1 = cstr;
    char const* p2 = nullptr;
    while ((p2 = strchr(p1, '\x28')) != nullptr) {
        size_t idx = toS(p2 - cstr);
        if (memcmp(p2, padding_string, len - idx) == 0) {
            user_password = user_password.substr(0, idx);
            return;
        } else {
            QTC::TC("qpdf", "QPDF_encryption skip 0x28");
            p1 = p2 + 1;
        }
    }
}

static std::string
pad_or_truncate_password_V4(std::string const& password)
{
    char k1[key_bytes];
    pad_or_truncate_password_V4(password, k1);
    return std::string(k1, key_bytes);
}

static std::string
truncate_password_V5(std::string const& password)
{
    return password.substr(
        0, std::min(static_cast<size_t>(127), password.length()));
}

static void
iterate_md5_digest(MD5& md5, MD5::Digest& digest, int iterations, int key_len)
{
    md5.digest(digest);

    for (int i = 0; i < iterations; ++i) {
        MD5 m;
        m.encodeDataIncrementally(
            reinterpret_cast<char*>(digest), QIntC::to_size(key_len));
        m.digest(digest);
    }
}

static void
iterate_rc4(
    unsigned char* data,
    size_t data_len,
    unsigned char* okey,
    int key_len,
    int iterations,
    bool reverse)
{
    auto key_ph = std::make_unique<unsigned char[]>(QIntC::to_size(key_len));
    unsigned char* key = key_ph.get();
    for (int i = 0; i < iterations; ++i) {
        int const xor_value = (reverse ? iterations - 1 - i : i);
        for (int j = 0; j < key_len; ++j) {
            key[j] = static_cast<unsigned char>(okey[j] ^ xor_value);
        }
        RC4 rc4(key, QIntC::to_int(key_len));
        rc4.process(data, data_len, data);
    }
}

static std::string
process_with_aes(
    std::string const& key,
    bool encrypt,
    std::string const& data,
    size_t outlength = 0,
    unsigned int repetitions = 1,
    unsigned char const* iv = nullptr,
    size_t iv_length = 0)
{
    Pl_Buffer buffer("buffer");
    Pl_AES_PDF aes(
        "aes",
        &buffer,
        encrypt,
        QUtil::unsigned_char_pointer(key),
        QIntC::to_uint(key.length()));
    if (iv) {
        aes.setIV(iv, iv_length);
    } else {
        aes.useZeroIV();
    }
    aes.disablePadding();
    for (unsigned int i = 0; i < repetitions; ++i) {
        aes.writeString(data);
    }
    aes.finish();
    auto bufp = buffer.getBufferSharedPointer();
    if (outlength == 0) {
        outlength = bufp->getSize();
    } else {
        outlength = std::min(outlength, bufp->getSize());
    }
    return std::string(reinterpret_cast<char*>(bufp->getBuffer()), outlength);
}

static std::string
hash_V5(
    std::string const& password,
    std::string const& salt,
    std::string const& udata,
    QPDF::EncryptionData const& data)
{
    Pl_SHA2 hash(256);
    hash.writeString(password);
    hash.writeString(salt);
    hash.writeString(udata);
    hash.finish();
    std::string K = hash.getRawDigest();

    std::string result;
    if (data.getR() < 6) {
        result = K;
    } else {
        // Algorithm 2.B from ISO 32000-1 chapter 7: Computing a hash

        int round_number = 0;
        bool done = false;
        while (!done) {
            // The hash algorithm has us setting K initially to the R5
            // value and then repeating a series of steps 64 times
            // before starting with the termination case testing.  The
            // wording of the specification is very unclear as to the
            // exact number of times it should be run since the
            // wording about whether the initial setup counts as round
            // 0 or not is ambiguous.  This code counts the initial
            // setup (R5) value as round 0, which appears to be
            // correct.  This was determined to be correct by
            // increasing or decreasing the number of rounds by 1 or 2
            // from this value and generating 20 test files.  In this
            // interpretation, all the test files worked with Adobe
            // Reader X.  In the other configurations, many of the
            // files did not work, and we were accurately able to
            // predict which files didn't work by looking at the
            // conditions under which we terminated repetition.

            ++round_number;
            std::string K1 = password + K + udata;
            qpdf_assert_debug(K.length() >= 32);
            std::string E = process_with_aes(
                K.substr(0, 16),
                true,
                K1,
                0,
                64,
                QUtil::unsigned_char_pointer(K.substr(16, 16)),
                16);

            // E_mod_3 is supposed to be mod 3 of the first 16 bytes
            // of E taken as as a (128-bit) big-endian number.  Since
            // (xy mod n) is equal to ((x mod n) + (y mod n)) mod n
            // and since 256 mod n is 1, we can just take the sums of
            // the the mod 3s of each byte to get the same result.
            int E_mod_3 = 0;
            for (unsigned int i = 0; i < 16; ++i) {
                E_mod_3 += static_cast<unsigned char>(E.at(i));
            }
            E_mod_3 %= 3;
            int next_hash = ((E_mod_3 == 0) ? 256 : (E_mod_3 == 1) ? 384 : 512);
            Pl_SHA2 sha2(next_hash);
            sha2.writeString(E);
            sha2.finish();
            K = sha2.getRawDigest();

            if (round_number >= 64) {
                unsigned int ch = static_cast<unsigned char>(*(E.rbegin()));

                if (ch <= QIntC::to_uint(round_number - 32)) {
                    done = true;
                }
            }
        }
        result = K.substr(0, 32);
    }

    return result;
}

static void
pad_short_parameter(std::string& param, size_t max_len)
{
    if (param.length() < max_len) {
        QTC::TC("qpdf", "QPDF_encryption pad short parameter");
        param.append(max_len - param.length(), '\0');
    }
}

std::string
QPDF::compute_data_key(
    std::string const& encryption_key,
    int objid,
    int generation,
    bool use_aes,
    int encryption_V,
    int encryption_R)
{
    // Algorithm 3.1 from the PDF 1.7 Reference Manual

    std::string result = encryption_key;

    if (encryption_V >= 5) {
        // Algorithm 3.1a (PDF 1.7 extension level 3): just use
        // encryption key straight.
        return result;
    }

    // Append low three bytes of object ID and low two bytes of generation
    result.append(1, static_cast<char>(objid & 0xff));
    result.append(1, static_cast<char>((objid >> 8) & 0xff));
    result.append(1, static_cast<char>((objid >> 16) & 0xff));
    result.append(1, static_cast<char>(generation & 0xff));
    result.append(1, static_cast<char>((generation >> 8) & 0xff));
    if (use_aes) {
        result += "sAlT";
    }

    MD5 md5;
    md5.encodeDataIncrementally(result.c_str(), result.length());
    MD5::Digest digest;
    md5.digest(digest);
    return std::string(
        reinterpret_cast<char*>(digest), std::min(result.length(), toS(16)));
}

std::string
QPDF::compute_encryption_key(
    std::string const& password, EncryptionData const& data)
{
    if (data.getV() >= 5) {
        // For V >= 5, the encryption key is generated and stored in
        // the file, encrypted separately with both user and owner
        // passwords.
        return recover_encryption_key_with_password(password, data);
    } else {
        // For V < 5, the encryption key is derived from the user
        // password.
        return compute_encryption_key_from_password(password, data);
    }
}

std::string
QPDF::compute_encryption_key_from_password(
    std::string const& password, EncryptionData const& data)
{
    // Algorithm 3.2 from the PDF 1.7 Reference Manual

    // This code does not properly handle Unicode passwords.
    // Passwords are supposed to be converted from OS codepage
    // characters to PDFDocEncoding.  Unicode passwords are supposed
    // to be converted to OS codepage before converting to
    // PDFDocEncoding.  We instead require the password to be
    // presented in its final form.

    MD5 md5;
    md5.encodeDataIncrementally(
        pad_or_truncate_password_V4(password).c_str(), key_bytes);
    md5.encodeDataIncrementally(data.getO().c_str(), key_bytes);
    char pbytes[4];
    int P = data.getP();
    pbytes[0] = static_cast<char>(P & 0xff);
    pbytes[1] = static_cast<char>((P >> 8) & 0xff);
    pbytes[2] = static_cast<char>((P >> 16) & 0xff);
    pbytes[3] = static_cast<char>((P >> 24) & 0xff);
    md5.encodeDataIncrementally(pbytes, 4);
    md5.encodeDataIncrementally(data.getId1().c_str(), data.getId1().length());
    if ((data.getR() >= 4) && (!data.getEncryptMetadata())) {
        char bytes[4];
        memset(bytes, 0xff, 4);
        md5.encodeDataIncrementally(bytes, 4);
    }
    MD5::Digest digest;
    int key_len =
        std::min(QIntC::to_int(sizeof(digest)), data.getLengthBytes());
    iterate_md5_digest(md5, digest, ((data.getR() >= 3) ? 50 : 0), key_len);
    return std::string(
        reinterpret_cast<char*>(digest), QIntC::to_size(key_len));
}

static void
compute_O_rc4_key(
    std::string const& user_password,
    std::string const& owner_password,
    QPDF::EncryptionData const& data,
    unsigned char key[OU_key_bytes_V4])
{
    if (data.getV() >= 5) {
        throw std::logic_error("compute_O_rc4_key called for file with V >= 5");
    }
    std::string password = owner_password;
    if (password.empty()) {
        password = user_password;
    }
    MD5 md5;
    md5.encodeDataIncrementally(
        pad_or_truncate_password_V4(password).c_str(), key_bytes);
    MD5::Digest digest;
    int key_len =
        std::min(QIntC::to_int(sizeof(digest)), data.getLengthBytes());
    iterate_md5_digest(md5, digest, ((data.getR() >= 3) ? 50 : 0), key_len);
    memcpy(key, digest, OU_key_bytes_V4);
}

static std::string
compute_O_value(
    std::string const& user_password,
    std::string const& owner_password,
    QPDF::EncryptionData const& data)
{
    // Algorithm 3.3 from the PDF 1.7 Reference Manual

    unsigned char O_key[OU_key_bytes_V4];
    compute_O_rc4_key(user_password, owner_password, data, O_key);

    char upass[key_bytes];
    pad_or_truncate_password_V4(user_password, upass);
    std::string k1(reinterpret_cast<char*>(O_key), OU_key_bytes_V4);
    pad_short_parameter(k1, QIntC::to_size(data.getLengthBytes()));
    iterate_rc4(
        QUtil::unsigned_char_pointer(upass),
        key_bytes,
        O_key,
        data.getLengthBytes(),
        (data.getR() >= 3) ? 20 : 1,
        false);
    return std::string(upass, key_bytes);
}

static std::string
compute_U_value_R2(
    std::string const& user_password, QPDF::EncryptionData const& data)
{
    // Algorithm 3.4 from the PDF 1.7 Reference Manual

    std::string k1 = QPDF::compute_encryption_key(user_password, data);
    char udata[key_bytes];
    pad_or_truncate_password_V4("", udata);
    pad_short_parameter(k1, QIntC::to_size(data.getLengthBytes()));
    iterate_rc4(
        QUtil::unsigned_char_pointer(udata),
        key_bytes,
        QUtil::unsigned_char_pointer(k1),
        data.getLengthBytes(),
        1,
        false);
    return std::string(udata, key_bytes);
}

static std::string
compute_U_value_R3(
    std::string const& user_password, QPDF::EncryptionData const& data)
{
    // Algorithm 3.5 from the PDF 1.7 Reference Manual

    std::string k1 = QPDF::compute_encryption_key(user_password, data);
    MD5 md5;
    md5.encodeDataIncrementally(
        pad_or_truncate_password_V4("").c_str(), key_bytes);
    md5.encodeDataIncrementally(data.getId1().c_str(), data.getId1().length());
    MD5::Digest digest;
    md5.digest(digest);
    pad_short_parameter(k1, QIntC::to_size(data.getLengthBytes()));
    iterate_rc4(
        digest,
        sizeof(MD5::Digest),
        QUtil::unsigned_char_pointer(k1),
        data.getLengthBytes(),
        20,
        false);
    char result[key_bytes];
    memcpy(result, digest, sizeof(MD5::Digest));
    // pad with arbitrary data -- make it consistent for the sake of
    // testing
    for (unsigned int i = sizeof(MD5::Digest); i < key_bytes; ++i) {
        result[i] = static_cast<char>((i * i) % 0xff);
    }
    return std::string(result, key_bytes);
}

static std::string
compute_U_value(
    std::string const& user_password, QPDF::EncryptionData const& data)
{
    if (data.getR() >= 3) {
        return compute_U_value_R3(user_password, data);
    }

    return compute_U_value_R2(user_password, data);
}

static bool
check_user_password_V4(
    std::string const& user_password, QPDF::EncryptionData const& data)
{
    // Algorithm 3.6 from the PDF 1.7 Reference Manual

    std::string u_value = compute_U_value(user_password, data);
    size_t to_compare = ((data.getR() >= 3) ? sizeof(MD5::Digest) : key_bytes);
    return (memcmp(data.getU().c_str(), u_value.c_str(), to_compare) == 0);
}

static bool
check_user_password_V5(
    std::string const& user_password, QPDF::EncryptionData const& data)
{
    // Algorithm 3.11 from the PDF 1.7 extension level 3

    std::string user_data = data.getU().substr(0, 32);
    std::string validation_salt = data.getU().substr(32, 8);
    std::string password = truncate_password_V5(user_password);
    return (hash_V5(password, validation_salt, "", data) == user_data);
}

static bool
check_user_password(
    std::string const& user_password, QPDF::EncryptionData const& data)
{
    if (data.getV() < 5) {
        return check_user_password_V4(user_password, data);
    } else {
        return check_user_password_V5(user_password, data);
    }
}

static bool
check_owner_password_V4(
    std::string& user_password,
    std::string const& owner_password,
    QPDF::EncryptionData const& data)
{
    // Algorithm 3.7 from the PDF 1.7 Reference Manual

    unsigned char key[OU_key_bytes_V4];
    compute_O_rc4_key(user_password, owner_password, data, key);
    unsigned char O_data[key_bytes];
    memcpy(O_data, QUtil::unsigned_char_pointer(data.getO()), key_bytes);
    std::string k1(reinterpret_cast<char*>(key), OU_key_bytes_V4);
    pad_short_parameter(k1, QIntC::to_size(data.getLengthBytes()));
    iterate_rc4(
        O_data,
        key_bytes,
        QUtil::unsigned_char_pointer(k1),
        data.getLengthBytes(),
        (data.getR() >= 3) ? 20 : 1,
        true);
    std::string new_user_password =
        std::string(reinterpret_cast<char*>(O_data), key_bytes);
    bool result = false;
    if (check_user_password(new_user_password, data)) {
        result = true;
        user_password = new_user_password;
    }
    return result;
}

static bool
check_owner_password_V5(
    std::string const& owner_password, QPDF::EncryptionData const& data)
{
    // Algorithm 3.12 from the PDF 1.7 extension level 3

    std::string user_data = data.getU().substr(0, 48);
    std::string owner_data = data.getO().substr(0, 32);
    std::string validation_salt = data.getO().substr(32, 8);
    std::string password = truncate_password_V5(owner_password);
    return (hash_V5(password, validation_salt, user_data, data) == owner_data);
}

static bool
check_owner_password(
    std::string& user_password,
    std::string const& owner_password,
    QPDF::EncryptionData const& data)
{
    if (data.getV() < 5) {
        return check_owner_password_V4(user_password, owner_password, data);
    } else {
        return check_owner_password_V5(owner_password, data);
    }
}

std::string
QPDF::recover_encryption_key_with_password(
    std::string const& password, EncryptionData const& data)
{
    // Disregard whether Perms is valid.
    bool disregard;
    return recover_encryption_key_with_password(password, data, disregard);
}

static void
compute_U_UE_value_V5(
    std::string const& user_password,
    std::string const& encryption_key,
    QPDF::EncryptionData const& data,
    std::string& U,
    std::string& UE)
{
    // Algorithm 3.8 from the PDF 1.7 extension level 3
    char k[16];
    QUtil::initializeWithRandomBytes(
        reinterpret_cast<unsigned char*>(k), sizeof(k));
    std::string validation_salt(k, 8);
    std::string key_salt(k + 8, 8);
    U = hash_V5(user_password, validation_salt, "", data) + validation_salt +
        key_salt;
    std::string intermediate_key = hash_V5(user_password, key_salt, "", data);
    UE = process_with_aes(intermediate_key, true, encryption_key);
}

static void
compute_O_OE_value_V5(
    std::string const& owner_password,
    std::string const& encryption_key,
    QPDF::EncryptionData const& data,
    std::string const& U,
    std::string& O,
    std::string& OE)
{
    // Algorithm 3.9 from the PDF 1.7 extension level 3
    char k[16];
    QUtil::initializeWithRandomBytes(
        reinterpret_cast<unsigned char*>(k), sizeof(k));
    std::string validation_salt(k, 8);
    std::string key_salt(k + 8, 8);
    O = hash_V5(owner_password, validation_salt, U, data) + validation_salt +
        key_salt;
    std::string intermediate_key = hash_V5(owner_password, key_salt, U, data);
    OE = process_with_aes(intermediate_key, true, encryption_key);
}

void
compute_Perms_value_V5_clear(
    std::string const& encryption_key,
    QPDF::EncryptionData const& data,
    unsigned char k[16])
{
    // From algorithm 3.10 from the PDF 1.7 extension level 3
    unsigned long long extended_perms =
        0xffffffff00000000LL | static_cast<unsigned long long>(data.getP());
    for (int i = 0; i < 8; ++i) {
        k[i] = static_cast<unsigned char>(extended_perms & 0xff);
        extended_perms >>= 8;
    }
    k[8] = data.getEncryptMetadata() ? 'T' : 'F';
    k[9] = 'a';
    k[10] = 'd';
    k[11] = 'b';
    QUtil::initializeWithRandomBytes(k + 12, 4);
}

static std::string
compute_Perms_value_V5(
    std::string const& encryption_key, QPDF::EncryptionData const& data)
{
    // Algorithm 3.10 from the PDF 1.7 extension level 3
    unsigned char k[16];
    compute_Perms_value_V5_clear(encryption_key, data, k);
    return process_with_aes(
        encryption_key,
        true,
        std::string(reinterpret_cast<char*>(k), sizeof(k)));
}

std::string
QPDF::recover_encryption_key_with_password(
    std::string const& password, EncryptionData const& data, bool& perms_valid)
{
    // Algorithm 3.2a from the PDF 1.7 extension level 3

    // This code does not handle Unicode passwords correctly.
    // Empirical evidence suggests that most viewers don't.  We are
    // supposed to process the input string with the SASLprep (RFC
    // 4013) profile of stringprep (RFC 3454) and then convert the
    // result to UTF-8.

    perms_valid = false;
    std::string key_password = truncate_password_V5(password);
    std::string key_salt;
    std::string user_data;
    std::string encrypted_file_key;
    if (check_owner_password_V5(key_password, data)) {
        key_salt = data.getO().substr(40, 8);
        user_data = data.getU().substr(0, 48);
        encrypted_file_key = data.getOE().substr(0, 32);
    } else if (check_user_password_V5(key_password, data)) {
        key_salt = data.getU().substr(40, 8);
        encrypted_file_key = data.getUE().substr(0, 32);
    }
    std::string intermediate_key =
        hash_V5(key_password, key_salt, user_data, data);
    std::string file_key =
        process_with_aes(intermediate_key, false, encrypted_file_key);

    // Decrypt Perms and check against expected value
    std::string perms_check =
        process_with_aes(file_key, false, data.getPerms(), 12);
    unsigned char k[16];
    compute_Perms_value_V5_clear(file_key, data, k);
    perms_valid = (memcmp(perms_check.c_str(), k, 12) == 0);

    return file_key;
}

QPDF::encryption_method_e
QPDF::interpretCF(
    std::shared_ptr<EncryptionParameters> encp, QPDFObjectHandle cf)
{
    if (cf.isName()) {
        std::string filter = cf.getName();
        if (encp->crypt_filters.count(filter) != 0) {
            return encp->crypt_filters[filter];
        } else if (filter == "/Identity") {
            return e_none;
        } else {
            return e_unknown;
        }
    } else {
        // Default: /Identity
        return e_none;
    }
}

void
QPDF::initializeEncryption()
{
    if (this->m->encp->encryption_initialized) {
        return;
    }
    this->m->encp->encryption_initialized = true;

    // After we initialize encryption parameters, we must used stored
    // key information and never look at /Encrypt again.  Otherwise,
    // things could go wrong if someone mutates the encryption
    // dictionary.

    if (!this->m->trailer.hasKey("/Encrypt")) {
        return;
    }

    // Go ahead and set this->m->encrypted here.  That way, isEncrypted
    // will return true even if there were errors reading the
    // encryption dictionary.
    this->m->encp->encrypted = true;

    std::string id1;
    QPDFObjectHandle id_obj = this->m->trailer.getKey("/ID");
    if ((id_obj.isArray() && (id_obj.getArrayNItems() == 2) &&
         id_obj.getArrayItem(0).isString())) {
        id1 = id_obj.getArrayItem(0).getStringValue();
    } else {
        // Treating a missing ID as the empty string enables qpdf to
        // decrypt some invalid encrypted files with no /ID that
        // poppler can read but Adobe Reader can't.
        warn(damagedPDF("trailer", "invalid /ID in trailer dictionary"));
    }

    QPDFObjectHandle encryption_dict = this->m->trailer.getKey("/Encrypt");
    if (!encryption_dict.isDictionary()) {
        throw damagedPDF("/Encrypt in trailer dictionary is not a dictionary");
    }

    if (!(encryption_dict.getKey("/Filter").isName() &&
          (encryption_dict.getKey("/Filter").getName() == "/Standard"))) {
        throw QPDFExc(
            qpdf_e_unsupported,
            this->m->file->getName(),
            "encryption dictionary",
            this->m->file->getLastOffset(),
            "unsupported encryption filter");
    }
    if (!encryption_dict.getKey("/SubFilter").isNull()) {
        warn(
            qpdf_e_unsupported,
            "encryption dictionary",
            this->m->file->getLastOffset(),
            "file uses encryption SubFilters, which qpdf does not support");
    }

    if (!(encryption_dict.getKey("/V").isInteger() &&
          encryption_dict.getKey("/R").isInteger() &&
          encryption_dict.getKey("/O").isString() &&
          encryption_dict.getKey("/U").isString() &&
          encryption_dict.getKey("/P").isInteger())) {
        throw damagedPDF(
            "encryption dictionary",
            "some encryption dictionary parameters are missing or the wrong "
            "type");
    }

    int V = encryption_dict.getKey("/V").getIntValueAsInt();
    int R = encryption_dict.getKey("/R").getIntValueAsInt();
    std::string O = encryption_dict.getKey("/O").getStringValue();
    std::string U = encryption_dict.getKey("/U").getStringValue();
    int P = static_cast<int>(encryption_dict.getKey("/P").getIntValue());

    // If supporting new encryption R/V values, remember to update
    // error message inside this if statement.
    if (!(((R >= 2) && (R <= 6)) &&
          ((V == 1) || (V == 2) || (V == 4) || (V == 5)))) {
        throw QPDFExc(
            qpdf_e_unsupported,
            this->m->file->getName(),
            "encryption dictionary",
            this->m->file->getLastOffset(),
            "Unsupported /R or /V in encryption dictionary; R = " +
                std::to_string(R) + " (max 6), V = " + std::to_string(V) +
                " (max 5)");
    }

    this->m->encp->encryption_V = V;
    this->m->encp->encryption_R = R;

    // OE, UE, and Perms are only present if V >= 5.
    std::string OE;
    std::string UE;
    std::string Perms;

    if (V < 5) {
        // These must be exactly the right number of bytes.
        pad_short_parameter(O, key_bytes);
        pad_short_parameter(U, key_bytes);
        if (!((O.length() == key_bytes) && (U.length() == key_bytes))) {
            throw damagedPDF(
                "encryption dictionary",
                "incorrect length for /O and/or /U in encryption dictionary");
        }
    } else {
        if (!(encryption_dict.getKey("/OE").isString() &&
              encryption_dict.getKey("/UE").isString() &&
              encryption_dict.getKey("/Perms").isString())) {
            throw damagedPDF(
                "encryption dictionary",
                "some V=5 encryption dictionary parameters are missing or the "
                "wrong type");
        }
        OE = encryption_dict.getKey("/OE").getStringValue();
        UE = encryption_dict.getKey("/UE").getStringValue();
        Perms = encryption_dict.getKey("/Perms").getStringValue();

        // These may be longer than the minimum number of bytes.
        pad_short_parameter(O, OU_key_bytes_V5);
        pad_short_parameter(U, OU_key_bytes_V5);
        pad_short_parameter(OE, OUE_key_bytes_V5);
        pad_short_parameter(UE, OUE_key_bytes_V5);
        pad_short_parameter(Perms, Perms_key_bytes_V5);
    }

    int Length = 0;
    if (V <= 1) {
        Length = 40;
    } else if (V == 4) {
        Length = 128;
    } else if (V == 5) {
        Length = 256;
    } else {
        if (encryption_dict.getKey("/Length").isInteger()) {
            Length = encryption_dict.getKey("/Length").getIntValueAsInt();
            if ((Length % 8) || (Length < 40) || (Length > 128)) {
                Length = 0;
            }
        }
    }
    if (Length == 0) {
        // Still no Length? Just take a guess.
        Length = 128;
    }

    this->m->encp->encrypt_metadata = true;
    if ((V >= 4) && (encryption_dict.getKey("/EncryptMetadata").isBool())) {
        this->m->encp->encrypt_metadata =
            encryption_dict.getKey("/EncryptMetadata").getBoolValue();
    }

    if ((V == 4) || (V == 5)) {
        QPDFObjectHandle CF = encryption_dict.getKey("/CF");
        for (auto const& filter: CF.getKeys()) {
            QPDFObjectHandle cdict = CF.getKey(filter);
            if (cdict.isDictionary()) {
                encryption_method_e method = e_none;
                if (cdict.getKey("/CFM").isName()) {
                    std::string method_name = cdict.getKey("/CFM").getName();
                    if (method_name == "/V2") {
                        QTC::TC("qpdf", "QPDF_encryption CFM V2");
                        method = e_rc4;
                    } else if (method_name == "/AESV2") {
                        QTC::TC("qpdf", "QPDF_encryption CFM AESV2");
                        method = e_aes;
                    } else if (method_name == "/AESV3") {
                        QTC::TC("qpdf", "QPDF_encryption CFM AESV3");
                        method = e_aesv3;
                    } else {
                        // Don't complain now -- maybe we won't need
                        // to reference this type.
                        method = e_unknown;
                    }
                }
                this->m->encp->crypt_filters[filter] = method;
            }
        }

        QPDFObjectHandle StmF = encryption_dict.getKey("/StmF");
        QPDFObjectHandle StrF = encryption_dict.getKey("/StrF");
        QPDFObjectHandle EFF = encryption_dict.getKey("/EFF");
        this->m->encp->cf_stream = interpretCF(this->m->encp, StmF);
        this->m->encp->cf_string = interpretCF(this->m->encp, StrF);
        if (EFF.isName()) {
            // qpdf does not use this for anything other than
            // informational purposes. This is intended to instruct
            // conforming writers on which crypt filter should be used
            // when new file attachments are added to a PDF file, but
            // qpdf never generates encrypted files with non-default
            // crypt filters. Prior to 10.2, I was under the mistaken
            // impression that this was supposed to be used for
            // decrypting attachments, but the code was wrong in a way
            // that turns out not to have mattered because no writers
            // were generating files the way I was imagining. Still,
            // providing this information could be useful when looking
            // at a file generated by something else, such as Acrobat
            // when specifying that only attachments should be
            // encrypted.
            this->m->encp->cf_file = interpretCF(this->m->encp, EFF);
        } else {
            this->m->encp->cf_file = this->m->encp->cf_stream;
        }
    }

    EncryptionData data(
        V,
        R,
        Length / 8,
        P,
        O,
        U,
        OE,
        UE,
        Perms,
        id1,
        this->m->encp->encrypt_metadata);
    if (this->m->provided_password_is_hex_key) {
        // ignore passwords in file
    } else {
        this->m->encp->owner_password_matched = check_owner_password(
            this->m->encp->user_password,
            this->m->encp->provided_password,
            data);
        if (this->m->encp->owner_password_matched && (V < 5)) {
            // password supplied was owner password; user_password has
            // been initialized for V < 5
            if (getTrimmedUserPassword() == this->m->encp->provided_password) {
                this->m->encp->user_password_matched = true;
                QTC::TC("qpdf", "QPDF_encryption user matches owner V < 5");
            }
        } else {
            this->m->encp->user_password_matched =
                check_user_password(this->m->encp->provided_password, data);
            if (this->m->encp->user_password_matched) {
                this->m->encp->user_password = this->m->encp->provided_password;
            }
        }
        if (this->m->encp->user_password_matched &&
            this->m->encp->owner_password_matched) {
            QTC::TC("qpdf", "QPDF_encryption same password", (V < 5) ? 0 : 1);
        }
        if (!(this->m->encp->owner_password_matched ||
              this->m->encp->user_password_matched)) {
            throw QPDFExc(
                qpdf_e_password,
                this->m->file->getName(),
                "",
                0,
                "invalid password");
        }
    }

    if (this->m->provided_password_is_hex_key) {
        this->m->encp->encryption_key =
            QUtil::hex_decode(this->m->encp->provided_password);
    } else if (V < 5) {
        // For V < 5, the user password is encrypted with the owner
        // password, and the user password is always used for
        // computing the encryption key.
        this->m->encp->encryption_key =
            compute_encryption_key(this->m->encp->user_password, data);
    } else {
        // For V >= 5, either password can be used independently to
        // compute the encryption key, and neither password can be
        // used to recover the other.
        bool perms_valid;
        this->m->encp->encryption_key = recover_encryption_key_with_password(
            this->m->encp->provided_password, data, perms_valid);
        if (!perms_valid) {
            warn(damagedPDF(
                "encryption dictionary",
                "/Perms field in encryption dictionary doesn't match expected "
                "value"));
        }
    }
}

std::string
QPDF::getKeyForObject(
    std::shared_ptr<EncryptionParameters> encp,
    QPDFObjGen const& og,
    bool use_aes)
{
    if (!encp->encrypted) {
        throw std::logic_error(
            "request for encryption key in non-encrypted PDF");
    }

    if (og != encp->cached_key_og) {
        encp->cached_object_encryption_key = compute_data_key(
            encp->encryption_key,
            og.getObj(),
            og.getGen(),
            use_aes,
            encp->encryption_V,
            encp->encryption_R);
        encp->cached_key_og = og;
    }

    return encp->cached_object_encryption_key;
}

void
QPDF::decryptString(std::string& str, QPDFObjGen const& og)
{
    if (!og.isIndirect()) {
        return;
    }
    bool use_aes = false;
    if (this->m->encp->encryption_V >= 4) {
        switch (this->m->encp->cf_string) {
        case e_none:
            return;

        case e_aes:
            use_aes = true;
            break;

        case e_aesv3:
            use_aes = true;
            break;

        case e_rc4:
            break;

        default:
            warn(damagedPDF(
                "unknown encryption filter for strings (check /StrF in "
                "/Encrypt dictionary); strings may be decrypted improperly"));
            // To avoid repeated warnings, reset cf_string.  Assume
            // we'd want to use AES if V == 4.
            this->m->encp->cf_string = e_aes;
            use_aes = true;
            break;
        }
    }

    std::string key = getKeyForObject(this->m->encp, og, use_aes);
    try {
        if (use_aes) {
            QTC::TC("qpdf", "QPDF_encryption aes decode string");
            Pl_Buffer bufpl("decrypted string");
            Pl_AES_PDF pl(
                "aes decrypt string",
                &bufpl,
                false,
                QUtil::unsigned_char_pointer(key),
                key.length());
            pl.writeString(str);
            pl.finish();
            auto buf = bufpl.getBufferSharedPointer();
            str = std::string(
                reinterpret_cast<char*>(buf->getBuffer()), buf->getSize());
        } else {
            QTC::TC("qpdf", "QPDF_encryption rc4 decode string");
            size_t vlen = str.length();
            // Using std::shared_ptr guarantees that tmp will
            // be freed even if rc4.process throws an exception.
            auto tmp = QUtil::make_unique_cstr(str);
            RC4 rc4(QUtil::unsigned_char_pointer(key), toI(key.length()));
            auto data = QUtil::unsigned_char_pointer(tmp.get());
            rc4.process(data, vlen, data);
            str = std::string(tmp.get(), vlen);
        }
    } catch (QPDFExc&) {
        throw;
    } catch (std::runtime_error& e) {
        throw damagedPDF(
            "error decrypting string for object " + og.unparse() + ": " +
            e.what());
    }
}

void
QPDF::decryptStream(
    std::shared_ptr<EncryptionParameters> encp,
    std::shared_ptr<InputSource> file,
    QPDF& qpdf_for_warning,
    Pipeline*& pipeline,
    QPDFObjGen const& og,
    QPDFObjectHandle& stream_dict,
    std::vector<std::shared_ptr<Pipeline>>& heap)
{
    std::string type;
    if (stream_dict.getKey("/Type").isName()) {
        type = stream_dict.getKey("/Type").getName();
    }
    if (type == "/XRef") {
        QTC::TC("qpdf", "QPDF_encryption xref stream from encrypted file");
        return;
    }
    bool use_aes = false;
    if (encp->encryption_V >= 4) {
        encryption_method_e method = e_unknown;
        std::string method_source = "/StmF from /Encrypt dictionary";

        if (stream_dict.getKey("/Filter").isOrHasName("/Crypt")) {
            if (stream_dict.getKey("/DecodeParms").isDictionary()) {
                QPDFObjectHandle decode_parms =
                    stream_dict.getKey("/DecodeParms");
                if (decode_parms.isDictionaryOfType(
                        "/CryptFilterDecodeParms")) {
                    QTC::TC("qpdf", "QPDF_encryption stream crypt filter");
                    method = interpretCF(encp, decode_parms.getKey("/Name"));
                    method_source = "stream's Crypt decode parameters";
                }
            } else if (
                stream_dict.getKey("/DecodeParms").isArray() &&
                stream_dict.getKey("/Filter").isArray()) {
                QPDFObjectHandle filter = stream_dict.getKey("/Filter");
                QPDFObjectHandle decode = stream_dict.getKey("/DecodeParms");
                if (filter.getArrayNItems() == decode.getArrayNItems()) {
                    for (int i = 0; i < filter.getArrayNItems(); ++i) {
                        if (filter.getArrayItem(i).isNameAndEquals("/Crypt")) {
                            QPDFObjectHandle crypt_params =
                                decode.getArrayItem(i);
                            if (crypt_params.isDictionary() &&
                                crypt_params.getKey("/Name").isName()) {
                                QTC::TC("qpdf", "QPDF_encrypt crypt array");
                                method = interpretCF(
                                    encp, crypt_params.getKey("/Name"));
                                method_source = "stream's Crypt "
                                                "decode parameters (array)";
                            }
                        }
                    }
                }
            }
        }

        if (method == e_unknown) {
            if ((!encp->encrypt_metadata) && (type == "/Metadata")) {
                QTC::TC("qpdf", "QPDF_encryption cleartext metadata");
                method = e_none;
            } else {
                method = encp->cf_stream;
            }
        }
        use_aes = false;
        switch (method) {
        case e_none:
            return;
            break;

        case e_aes:
            use_aes = true;
            break;

        case e_aesv3:
            use_aes = true;
            break;

        case e_rc4:
            break;

        default:
            // filter local to this stream.
            qpdf_for_warning.warn(QPDFExc(
                qpdf_e_damaged_pdf,
                file->getName(),
                "",
                file->getLastOffset(),
                "unknown encryption filter for streams (check " +
                    method_source + "); streams may be decrypted improperly"));
            // To avoid repeated warnings, reset cf_stream.  Assume
            // we'd want to use AES if V == 4.
            encp->cf_stream = e_aes;
            use_aes = true;
            break;
        }
    }
    std::string key = getKeyForObject(encp, og, use_aes);
    std::shared_ptr<Pipeline> new_pipeline;
    if (use_aes) {
        QTC::TC("qpdf", "QPDF_encryption aes decode stream");
        new_pipeline = std::make_shared<Pl_AES_PDF>(
            "AES stream decryption",
            pipeline,
            false,
            QUtil::unsigned_char_pointer(key),
            key.length());
    } else {
        QTC::TC("qpdf", "QPDF_encryption rc4 decode stream");
        new_pipeline = std::make_shared<Pl_RC4>(
            "RC4 stream decryption",
            pipeline,
            QUtil::unsigned_char_pointer(key),
            toI(key.length()));
    }
    pipeline = new_pipeline.get();
    heap.push_back(new_pipeline);
}

void
QPDF::compute_encryption_O_U(
    char const* user_password,
    char const* owner_password,
    int V,
    int R,
    int key_len,
    int P,
    bool encrypt_metadata,
    std::string const& id1,
    std::string& O,
    std::string& U)
{
    if (V >= 5) {
        throw std::logic_error(
            "compute_encryption_O_U called for file with V >= 5");
    }
    EncryptionData data(
        V, R, key_len, P, "", "", "", "", "", id1, encrypt_metadata);
    data.setO(compute_O_value(user_password, owner_password, data));
    O = data.getO();
    data.setU(compute_U_value(user_password, data));
    U = data.getU();
}

void
QPDF::compute_encryption_parameters_V5(
    char const* user_password,
    char const* owner_password,
    int V,
    int R,
    int key_len,
    int P,
    bool encrypt_metadata,
    std::string const& id1,
    std::string& encryption_key,
    std::string& O,
    std::string& U,
    std::string& OE,
    std::string& UE,
    std::string& Perms)
{
    EncryptionData data(
        V, R, key_len, P, "", "", "", "", "", id1, encrypt_metadata);
    unsigned char k[key_bytes];
    QUtil::initializeWithRandomBytes(k, key_bytes);
    encryption_key = std::string(reinterpret_cast<char*>(k), key_bytes);
    compute_U_UE_value_V5(user_password, encryption_key, data, U, UE);
    compute_O_OE_value_V5(owner_password, encryption_key, data, U, O, OE);
    Perms = compute_Perms_value_V5(encryption_key, data);
    data.setV5EncryptionParameters(O, OE, U, UE, Perms);
}

std::string const&
QPDF::getPaddedUserPassword() const
{
    return this->m->encp->user_password;
}

std::string
QPDF::getTrimmedUserPassword() const
{
    std::string result = this->m->encp->user_password;
    trim_user_password(result);
    return result;
}

std::string
QPDF::getEncryptionKey() const
{
    return this->m->encp->encryption_key;
}

bool
QPDF::isEncrypted() const
{
    return this->m->encp->encrypted;
}

bool
QPDF::isEncrypted(int& R, int& P)
{
    int V;
    encryption_method_e stream, string, file;
    return isEncrypted(R, P, V, stream, string, file);
}

bool
QPDF::isEncrypted(
    int& R,
    int& P,
    int& V,
    encryption_method_e& stream_method,
    encryption_method_e& string_method,
    encryption_method_e& file_method)
{
    if (this->m->encp->encrypted) {
        QPDFObjectHandle trailer = getTrailer();
        QPDFObjectHandle encrypt = trailer.getKey("/Encrypt");
        QPDFObjectHandle Pkey = encrypt.getKey("/P");
        QPDFObjectHandle Rkey = encrypt.getKey("/R");
        QPDFObjectHandle Vkey = encrypt.getKey("/V");
        P = static_cast<int>(Pkey.getIntValue());
        R = Rkey.getIntValueAsInt();
        V = Vkey.getIntValueAsInt();
        stream_method = this->m->encp->cf_stream;
        string_method = this->m->encp->cf_string;
        file_method = this->m->encp->cf_file;
        return true;
    } else {
        return false;
    }
}

bool
QPDF::ownerPasswordMatched() const
{
    return this->m->encp->owner_password_matched;
}

bool
QPDF::userPasswordMatched() const
{
    return this->m->encp->user_password_matched;
}

static bool
is_bit_set(int P, int bit)
{
    // Bits in P are numbered from 1 in the spec
    return ((P & (1 << (bit - 1))) != 0);
}

bool
QPDF::allowAccessibility()
{
    int R = 0;
    int P = 0;
    bool status = true;
    if (isEncrypted(R, P)) {
        if (R < 3) {
            status = is_bit_set(P, 5);
        } else {
            status = is_bit_set(P, 10);
        }
    }
    return status;
}

bool
QPDF::allowExtractAll()
{
    int R = 0;
    int P = 0;
    bool status = true;
    if (isEncrypted(R, P)) {
        status = is_bit_set(P, 5);
    }
    return status;
}

bool
QPDF::allowPrintLowRes()
{
    int R = 0;
    int P = 0;
    bool status = true;
    if (isEncrypted(R, P)) {
        status = is_bit_set(P, 3);
    }
    return status;
}

bool
QPDF::allowPrintHighRes()
{
    int R = 0;
    int P = 0;
    bool status = true;
    if (isEncrypted(R, P)) {
        status = is_bit_set(P, 3);
        if ((R >= 3) && (!is_bit_set(P, 12))) {
            status = false;
        }
    }
    return status;
}

bool
QPDF::allowModifyAssembly()
{
    int R = 0;
    int P = 0;
    bool status = true;
    if (isEncrypted(R, P)) {
        if (R < 3) {
            status = is_bit_set(P, 4);
        } else {
            status = is_bit_set(P, 11);
        }
    }
    return status;
}

bool
QPDF::allowModifyForm()
{
    int R = 0;
    int P = 0;
    bool status = true;
    if (isEncrypted(R, P)) {
        if (R < 3) {
            status = is_bit_set(P, 6);
        } else {
            status = is_bit_set(P, 9);
        }
    }
    return status;
}

bool
QPDF::allowModifyAnnotation()
{
    int R = 0;
    int P = 0;
    bool status = true;
    if (isEncrypted(R, P)) {
        status = is_bit_set(P, 6);
    }
    return status;
}

bool
QPDF::allowModifyOther()
{
    int R = 0;
    int P = 0;
    bool status = true;
    if (isEncrypted(R, P)) {
        status = is_bit_set(P, 4);
    }
    return status;
}

bool
QPDF::allowModifyAll()
{
    int R = 0;
    int P = 0;
    bool status = true;
    if (isEncrypted(R, P)) {
        status = (is_bit_set(P, 4) && is_bit_set(P, 6));
        if (R >= 3) {
            status = status && (is_bit_set(P, 9) && is_bit_set(P, 11));
        }
    }
    return status;
}