DCMTK: randseed.txt file

DCMTK Version 3.7.0 OFFIS DICOM Toolkit

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randseed.txt file

    ==========================================
    RANDOM SEEDS FOR TLS TRANSMISSION IN DCMTK
    ==========================================

    The Transport Layer Security (TLS) protocol relies on a pseudo-random number
    generator (PRNG) for the creation of session keys.  The ability of an
    attacker (e.g. eavesdropper) to predict the sequence of random numbers
    issued by the PRNG would facilitate cryptographic attacks against the TLS
    protected communication. For this reason it is important that the internal
    state of the PRNG is initialized with true random data which cannot be
    predicted by an attacker. The OpenSSL library on which DCMTK relies for its
    TLS functions initially tests whether the PRNG has been seeded with
    sufficient random data. If this test fails, the a warning is issued and no
    secure TLS connection can be negotiated:

        Warning: PRNG for TLS not seeded with sufficient random data.

    For this reason, all TLS-aware DCMTK applications allow to initialize the
    PRNG with a random seed which is read from file. Optionally the (modified)
    state of the PRNG can be written back to file upon termination of the
    application, providing a new random seed for the next application run. The
    command line options that control this behaviour for the storescu and
    storescp applications are:

        +rs   --seed              [f]ilename: string
                                  seed random generator with contents of [f]
        +ws   --write-seed        write back modified seed (only with --seed)
        +wf   --write-seed-file   [f]ilename: string (only with --seed)
                                  write modified seed to file [f]

    This document describes sources from which an initial random seed file
    containing physical random data can be obtained.

    1. Kernel Random Number Source Device

    Some modern Unix operating systems (in particular Linux) have a kernel
    module that gathers environmental noise from device drivers and other
    sources and feeds them into an entropy pool.  Random data can be read from
    this entropy pool using the /dev/random and /dev/urandom devices (see
    random(4) manual page.) For example, the following command creates a 1024
    byte file "randseed.bin" that can be used to seed the OpenSSL PRNG:

        dd if=/dev/urandom of=randseed.bin count=2

    2. GUI content

    The content of the screen on systems with graphical user interface is hardly
    predictable for an attacker (unless an application is run as a service in
    the background with a well-predictable blank or login screen.)  For this
    reason, the Win32 version of OpenSSL always seeds the PRNG with the content
    of the current screen. This feature is built-in and does not require
    additional user interaction.

    3. Entropy Gathering Demon (EGD)

    Entropy Gathering Demon is a user space application (Perl script) that can
    be run as a daemon on Unix systems that do not provide a kernel random
    number source device. EGD is available from http://www.lothar.com/tech/crypto/

    On systems where EGD is installed, the "openssl" tool can be used to
    generate a random seeed for the PRNG from EGD. If the EGD socket is
    installed as /etc/entropy, the following command creates a 1024 byte
    file "randseed.bin" that can be used to seed the OpenSSL PRNG:

        openssl rand -rand /etc/entropy -out randseed.bin 1024

    4. Pseudo Random Number Generator Daemon (PRNGD)

    Pseudo Random Number Generator Daemon is a user space application that
    "offers an EGD compatible interface to obtain random data and is intended to
    be used as an entropy source to feed other software, especially software
    based on OpenSSL. Like EGD it calls system programs to collect entropy.
    Unlike EGD it does not generate a pool of random bits that can be called
    from other software. Rather more it feeds the bits gathered into the OpenSSL
    PRNG from which the "random bits" are obtained when requested. This way,
    PRNGD is never drained and can never block (unlike EGD), so it is also
    suitable to seed inetd-started programs. It also features a seed-save file,
    so that it is immediately usable after system start." PRNGD is available
    from http://www.aet.tu-cottbus.de/personen/jaenicke/postfix_tls/prngd.html

    5. Pretty Good Privacy (PGP)

    Pretty Good Privacy contains a random data generator that relies on the time
    intervals between keystrokes.  The following Unix shell commands would cause
    PGP 2.6.3i to create a file /tmp/random_data/randseed.bin with about 1000
    bits of random data, which can be used to seed the OpenSSL PRNG:

        mkdir /tmp/random_data
        chmod 700 /tmp/random_data
        setenv PGPPATH /tmp/random_data
        pgp -kg 1024 -u random

    6. Medical Images

    Medical image modalities such as CT, MR, Ultrasound or CR produce lots of
    unpredictable random noise in the background that is perfectly suited to
    initialize a PRNG, given that the image is never made available to a third
    party.  The following OpenSSL command line would read a DICOM file
    "image.dcm" and create a 1024 byte file "randseed.bin" that can be used to
    seed the OpenSSL PRNG:

        openssl rand -rand image.dcm -out randseed.bin 1024

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