Side Channels
=========================

Many cryptographic systems can be broken by side channels. This document notes
side channel protections which are currently implemented, as well as areas of
the code which are known to be vulnerable to side channels. The latter are
obviously all open for future improvement.

The following text assumes the reader is already familiar with cryptographic
implementations, side channel attacks, and common countermeasures.

RSA
----------------------

Blinding is always used to protect private key operations (there is no way to
turn it off). As an optimization, instead of choosing a new random mask and
inverse with each decryption, both the mask and its inverse are simply squared
to choose the next blinding factor. This is much faster than computing a fresh
value each time, and the additional relation is thought to provide only minimal
useful information for an attacker. Every BOTAN_BLINDING_REINIT_INTERVAL
(default 32) operations, a new starting point is chosen.

RSA signing uses the CRT optimization, which is much faster but vulnerable to
trivial fault attacks [RsaFault] which can result in the key being entirely
compromised. To protect against this (or any other computational error which
would have the same effect as a fault attack in this case), after every private
key operation the result is checked for consistency with the public key. This
introduces only slight additional overhead and blocks most fault attacks; it is
possible to use a second fault attack to bypass this verification, but such a
double fault attack requires significantly more control on the part of an
attacker than a BellCore style attack, which is possible if any error at all
occurs during either modular exponentiation involved in the RSA signature
operation.

See blinding.cpp and rsa.cpp.

If the OpenSSL provider is enabled, then no explicit blinding is done; we assume
OpenSSL handles this. See openssl_rsa.cpp.

Decryption of PKCS #1 v1.5 Ciphertexts
----------------------------------------

This padding scheme is used with RSA, and is very vulnerable to errors. In a
scenario where an attacker can repeatedly present RSA ciphertexts, and a
legitimate key holder will attempt to decrypt each ciphertext and simply
indicates to the attacker if the PKCS padding was valid or not (without
revealing any additional information), the attacker can use this behavior as an
oracle to perform iterative decryption of arbitrary RSA ciphertexts encrypted
under that key. This is the famous million message attack [MillionMsg].  A side
channel such as a difference in time taken to handle valid and invalid RSA
ciphertexts is enough to mount the attack [MillionMsgTiming].

Preventing this issue in full requires some application level changes. In
protocols which know the expected length of the encrypted key, PK_Decryptor
provides the function `decrypt_or_random` which first generates a random fake
key, then decrypts the presented ciphertext, then in constant time either copies
out the random key or the decrypted plaintext depending on if the ciphertext was
valid or not (valid padding and expected plaintext length). Then in the case of
an attack, the protocol will carry on with a randomly chosen key, which will
presumably cause total failure in a way that does not allow an attacker to
distinguish (via any timing or other side channel, nor any error messages
specific to the one situation vs the other) if the RSA padding was valid or
invalid.

One very important user of PKCS #1 v1.5 encryption is the TLS protocol. In TLS,
some extra versioning information is embedded in the plaintext message, along
with the key. It turns out that this version information must be treated in an
identical (constant-time) way with the PKCS padding, or again the system is
broken. [VersionOracle]. This is supported by a special version of
PK_Decryptor::decrypt_or_random that additionally allows verifying one or more
content bytes, in addition to the PKCS padding.

See eme_pkcs.cpp and pubkey.cpp.

Verification of PKCS #1 v1.5 Signatures
----------------------------------------

One way of verifying PKCS #1 v1.5 signature padding is to decode it with an
ASN.1 BER parser. However such a design commonly leads to accepting signatures
besides the (single) valid RSA PKCS #1 v1.5 signature for any given message,
because often the BER parser accepts variations of the encoding which are
actually invalid. It also needlessly exposes the BER parser to untrusted inputs.

It is safer and simpler to instead re-encode the hash value we are expecting
using the PKCS #1 v1.5 encoding rules, and const time compare our expected
encoding with the output of the RSA operation. So that is what Botan does.

See emsa_pkcs.cpp.

OAEP
----------------------

RSA OAEP is (PKCS#1 v2) is the recommended version of RSA encoding standard,
because it is not directly vulnerable to Bleichenbacher attack. However, if
implemented incorrectly, a side channel can be presented to an attacker and
create an oracle for decrypting RSA ciphertexts [OaepTiming].

This attack is avoided in Botan by making the OAEP decoding operation run
without any conditional jumps or indexes, with the only variance in runtime
coming from the length of the RSA key (which is public information).

See eme_oaep.cpp.

Modular Exponentiation
------------------------

Modular exponentiation uses a fixed window algorithm with Montgomery
representation. A side channel silent table lookup is used to access the
precomputed powers. See powm_mnt.cpp.

The Karatsuba multiplication algorithm has some conditional branches that
probably expose information through the branch predictor, but probably? does not
expose a timing channel since the same amount of work is done on both sides of
the conditional. There is certainly room for improvement here. See mp_karat.cpp
for details.

The Montgomery reduction is written (and tested) to run in constant time. See
mp_monty.cpp.

ECC point decoding
----------------------

The API function OS2ECP, which is used to convert byte strings to ECC points,
verifies that all points satisfy the ECC curve equation. Points that do not
satisfy the equation are invalid, and can sometimes be used to break
protocols ([InvalidCurve] [InvalidCurveTLS]). See point_gfp.cpp.

ECC scalar multiply
----------------------

There are two implementations of scalar multiply, PointGFp::operator* and the
class Blinded_Point_Multiply. The default scalar multiply uses the Montgomery
ladder. However it currently leaks the size of the scalar, because the loop
iterations are bounded by the scalar size.

Blinded_Point_Multiply (used by ECDH, ECDSA, etc) applies several additional
side channel countermeasures. The scalar is masked by a small multiple of the
group order (this is commonly called Coron's first countermeasure [CoronDpa]),
the size of the scalar mask is currently controlled by build.h value
BOTAN_POINTGFP_SCALAR_BLINDING_BITS which defaults to 20 bits.

Botan stores all ECC points in Jacobian representation. This form allows faster
computation by representing points (x,y) as (X,Y,Z) where x=X/Z^2 and
y=Y/Z^3. As the representation is redundant, for any randomly chosen r,
(X*r^2,Y*r^3,Z*r) is an equivalent point. Changing the point values prevents an
attacker from mounting attacks based on the input point remaining unchanged over
multiple executions. This is commonly called Coron's third countermeasure, see
again [CoronDpa].

Currently Blinded_Point_Multiply uses one of two different algorithms, depending
on a build-time flag. If BOTAN_POINTGFP_BLINDED_MULTIPLY_USE_MONTGOMERY_LADDER
is set in build.h (default is for it *not* to be set), then a randomized
Montgomery ladder algorithm from [RandomMonty] is used. Otherwise, a simple
fixed window exponentiation is used; the current version leaks exponent bits
through memory index values. We rely on scalar blinding to reduce this
leakage. It would obviously be better for Blinded_Point_Multiply to converge on
a single side channel silent algorithm.

See point_gfp.cpp.

ECDH
----------------------

ECDH verifies (through its use of OS2ECP) that all input points
received from the other party satisfy the curve equation. This
prevents twist attacks. The same check is performed on the output
point, which helps prevent fault attacks.

ECDSA
----------------------

Inversion of the ECDSA nonce k must be done in constant time, as any
leak of even a single bit of the nonce can be sufficient to allow
recovering the private key. In Botan all inverses modulo an odd number
are performed using a constant time algorithm due to Niels Möller.

x25519
----------------------

The x25519 code is independent of the main Weiserstrass form ECC code, instead
based on curve25519-donna-c64.c by Adam Langley. The code seems immune to cache
based side channels. It does make use of integer multiplications; on some old
CPUs these multiplications take variable time and might allow a side channel
attack. This is not considered a problem on modern processors.

TLS CBC ciphersuites
----------------------

The original TLS v1.0 CBC Mac-then-Encrypt mode is vulnerable to an oracle
attack.  If an attacker can distinguish padding errors through different error
messages [TlsCbcOracle] or via a side channel attack like [Lucky13], they can
abuse the server as a decryption oracle.

The side channel protection for Lucky13 follows the approach proposed in the
Lucky13 paper. It is not perfectly constant time, but does hide the padding
oracle in practice. Tools to test TLS CBC decoding are included in the timing
tests. See https://github.com/randombit/botan/pull/675 for more information.

The Encrypt-then-MAC extension, which completely avoids the side channel, is
implemented and used by default for CBC ciphersuites.

CBC mode padding
----------------------

In theory, any good protocol protects CBC ciphertexts with a MAC. But in
practice, some protocols are not good and cannot be fixed immediately. To avoid
making a bad problem worse, the code to handle decoding CBC ciphertext padding
bytes runs in constant time, depending only on the block size of the cipher.

AES
----------------------

On x86 processors which support it, AES-NI instruction set is used, as it is
fast and (presumed) side channel silent. There is no support at the moment for
the similar ARMv8 or POWER AES instructions; patches would be welcome.

On x86 processors without AES-NI but with SSSE3 (which includes older Intel
Atoms and Core2 Duos, and even now some embedded or low power x86 chips), a
version of AES using pshufb is used which is both fast and side channel silent.
It is based on code by Mike Hamburg [VectorAes], see aes_ssse3.cpp. This same
technique could be applied with NEON or AltiVec, and the paper suggests some
optimizations for the AltiVec shuffle.

On all other processors, a class 4K table lookup version based on the original
Rijndael code is used. This approach relatively fast, but now known to be very
vulnerable to side channels. The implementation does make modifications in the
first and last rounds to reduce the cache signature, but these merely increase
the number of observations required. See [AesCacheColl] for one paper which
analyzes a number of implementations including Botan. Botan already follows both
of their suggested countermeasures, which increased the number of samples
required from 2**13 to the only slightly less pitiful 2**19 samples.

The Botan block cipher API already supports bitslicing implementations, so a
const time 8x bitsliced AES could be integrated fairly easily.

GCM
---------------------

On platforms that support a carryless multiply instruction (recent x86 and ARM),
GCM is fast and constant time.

On all other platforms, GCM is slow and constant time. It uses a simple bit at
at time loop. It would be much faster using a table lookup, but we wish to avoid
side channels. One improvement here would be the option of using a 2K or 4K
table, but using a side-channel silent (masked) table lookup.

OCB
-----------------------

It is straightforward to implement OCB mode in a efficient way that does not
depend on any secret branches or lookups. See ocb.cpp for the implementation.

Poly1305
----------------------

The Poly1305 implementation does not have any secret lookups or conditionals.
The code is based on the public domain version by Andrew Moon.

DES/3DES
----------------------

The DES implementation uses table lookups, and is likely vulnerable to side
channel attacks. DES or 3DES should be avoided in new systems. The proper fix
would be a scalar bitsliced implementation, this is not seen as worth the
engineering investment given these algorithms end of life status.

Twofish
------------------------

This algorithm uses table lookups with secret sboxes. No cache-based side
channel attack on Twofish has ever been published, but it is possible nobody
sufficiently skilled has ever tried.

ChaCha20, Serpent, Threefish, ...
-----------------------------------

Some algorithms including ChaCha, Salsa, Serpent and Threefish are 'naturally'
silent to cache and timing side channels on all recent processors.

IDEA
---------------

IDEA encryption, decryption, and key schedule are implemented to take constant
time regardless of their inputs.

Hash Functions
-------------------------

Most hash functions included in Botan such as MD5, SHA-1, SHA-2, SHA-3, Skein,
and BLAKE2 do not require any input-dependent memory lookups, and so seem to not be
affected by common CPU side channels.

Memory comparisons
----------------------

The function same_mem in header mem_ops.h provides a constant-time comparison
function. It is used when comparing MACs or other secret values. It is also
exposed for application use.

Memory zeroizing
----------------------

There is no way in portable C/C++ to zero out an array before freeing it, in
such a way that it is guaranteed that the compiler will not elide the
'additional' (seemingly unnecessary) writes to zero out the memory.

The function secure_scrub_memory (in mem_ops.cpp) uses some system specific
trick to zero out an array. On Windows it uses the directly supported API
function RtlSecureZeroMemory.

On other platforms, by default the trick of referencing memset through a
volatile function pointer is used. This approach is not guaranteed to work on
all platforms, and currently there is no systematic check of the resulting
binary function that it is compiled as expected. But, it is the best approach
currently known and has been verified to work as expected on common platforms.

If BOTAN_USE_VOLATILE_MEMSET_FOR_ZERO is set to 0 in build.h (not the default) a
byte at a time loop through a volatile pointer is used to overwrite the array.

Memory allocation
----------------------

Botan's secure_vector type is a std::vector with a custom allocator. The
allocator calls secure_scrub_memory before freeing memory.

Some operating systems support an API call to lock a range of pages
into memory, such that they will never be swapped out (mlock on POSIX,
VirtualLock on Windows). On many POSIX systems mlock is only usable by
root, but on Linux, FreeBSD and possibly other systems a small amount
of memory can be mlock'ed by processes without extra credentials.

If available, Botan uses such a region for storing key material. It is
created in anonymous mapped memory (not disk backed), locked in
memory, and scrubbed on free. This memory pool is used by
secure_vector when available. It can be disabled at runtime setting
the environment variable BOTAN_MLOCK_POOL_SIZE to 0.

Automated Analysis
---------------------

Currently the main tool used by the Botan developers for testing for side
channels at runtime is valgrind; valgrind's runtime API is used to taint memory
values, and any jumps or indexes using data derived from these values will cause
a valgrind warning. This technique was first used by Adam Langley in ctgrind.
See header ct_utils.h.

To check, install valgrind, configure the build with --with-valgrind, and run
the tests.

References
---------------

[AesCacheColl] Bonneau, Mironov "Cache-Collision Timing Attacks Against AES"
(http://www.jbonneau.com/doc/BM06-CHES-aes_cache_timing.pdf)

[CoronDpa] Coron,
"Resistance against Differential Power Analysis for Elliptic Curve Cryptosystems"
(https://citeseer.ist.psu.edu/viewdoc/summary?doi=10.1.1.1.5695)

[InvalidCurve] Biehl, Meyer, Müller: Differential fault attacks on
elliptic curve cryptosystems
(https://www.iacr.org/archive/crypto2000/18800131/18800131.pdf)

[InvalidCurveTLS] Jager, Schwenk, Somorovsky: Practical Invalid Curve
Attacks on TLS-ECDH
(https://www.nds.rub.de/research/publications/ESORICS15/)

[SafeCurves] Bernstein, Lange: SafeCurves: choosing safe curves for
elliptic-curve cryptography. (https://safecurves.cr.yp.to)

[Lucky13] AlFardan, Paterson "Lucky Thirteen: Breaking the TLS and DTLS Record Protocols"
(http://www.isg.rhul.ac.uk/tls/TLStiming.pdf)

[MillionMsg] Bleichenbacher "Chosen Ciphertext Attacks Against Protocols Based
on the RSA Encryption Standard PKCS1"
(https://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.19.8543)

[MillionMsgTiming] Meyer, Somorovsky, Weiss, Schwenk, Schinzel, Tews: Revisiting
SSL/TLS Implementations: New Bleichenbacher Side Channels and Attacks
(https://www.nds.rub.de/research/publications/mswsst2014-bleichenbacher-usenix14/)

[OaepTiming] Manger, "A Chosen Ciphertext Attack on RSA Optimal Asymmetric
Encryption Padding (OAEP) as Standardized in PKCS #1 v2.0"
(http://archiv.infsec.ethz.ch/education/fs08/secsem/Manger01.pdf)

[RsaFault] Boneh, Demillo, Lipton
"On the importance of checking cryptographic protocols for faults"
(https://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.48.9764)

[RandomMonty] Le, Tan, Tunstall "Randomizing the Montgomery Powering Ladder"
(https://eprint.iacr.org/2015/657)

[VectorAes] Hamburg, "Accelerating AES with Vector Permute Instructions"
https://shiftleft.org/papers/vector_aes/vector_aes.pdf

[VersionOracle] Klíma, Pokorný, Rosa "Attacking RSA-based Sessions in SSL/TLS"
(https://eprint.iacr.org/2003/052)
