Botan  1.11.15
Public Member Functions | Static Public Member Functions | List of all members
Botan::HMAC_RNG Class Reference

#include <hmac_rng.h>

Inheritance diagram for Botan::HMAC_RNG:
Botan::RandomNumberGenerator

Public Member Functions

void add_entropy (const byte[], size_t)
 
void clear ()
 
 HMAC_RNG (MessageAuthenticationCode *extractor, MessageAuthenticationCode *prf)
 
bool is_seeded () const
 
std::string name () const
 
byte next_byte ()
 
virtual secure_vector< byterandom_vec (size_t bytes)
 
void randomize (byte buf[], size_t len)
 
void reseed (size_t poll_bits)
 

Static Public Member Functions

static RandomNumberGeneratormake_rng ()
 

Detailed Description

HMAC_RNG - based on the design described in "On Extract-then-Expand Key Derivation Functions and an HMAC-based KDF" by Hugo Krawczyk (henceforce, 'E-t-E')

However it actually can be parameterized with any two MAC functions, not restricted to HMAC (this variation is also described in Krawczyk's paper), for instance one could use HMAC(SHA-512) as the extractor and CMAC(AES-256) as the PRF.

Definition at line 27 of file hmac_rng.h.

Constructor & Destructor Documentation

Botan::HMAC_RNG::HMAC_RNG ( MessageAuthenticationCode extractor,
MessageAuthenticationCode prf 
)
Parameters
extractora MAC used for extracting the entropy
prfa MAC used as a PRF using HKDF construction

Definition at line 42 of file hmac_rng.cpp.

References Botan::Buffered_Computation::process().

43  :
44  m_extractor(extractor), m_prf(prf)
45  {
46  if(!m_prf->valid_keylength(m_extractor->output_length()) ||
47  !m_extractor->valid_keylength(m_prf->output_length()))
48  throw Invalid_Argument("HMAC_RNG: Bad algo combination " +
49  m_extractor->name() + " and " +
50  m_prf->name());
51 
52  // First PRF inputs are all zero, as specified in section 2
53  m_K.resize(m_prf->output_length());
54 
55  /*
56  Normally we want to feedback PRF outputs to the extractor function
57  to ensure a single bad poll does not reduce entropy. Thus in reseed
58  we'll want to invoke the PRF before we reset the PRF key, but until
59  the first reseed the PRF is unkeyed. Rather than trying to keep
60  track of this, just set the initial PRF key to constant zero.
61  Since all PRF inputs in the first reseed are constants, this
62  amounts to suffixing the seed in the first poll with a fixed
63  constant string.
64 
65  The PRF key will not be used to generate outputs until after reseed
66  sets m_seeded to true.
67  */
68  secure_vector<byte> prf_key(m_extractor->output_length());
69  m_prf->set_key(prf_key);
70 
71  /*
72  Use PRF("Botan HMAC_RNG XTS") as the intitial XTS key.
73 
74  This will be used during the first extraction sequence; XTS values
75  after this one are generated using the PRF.
76 
77  If I understand the E-t-E paper correctly (specifically Section 4),
78  using this fixed extractor key is safe to do.
79  */
80  m_extractor->set_key(prf->process("Botan HMAC_RNG XTS"));
81  }
std::invalid_argument Invalid_Argument
Definition: exceptn.h:20

Member Function Documentation

void Botan::HMAC_RNG::add_entropy ( const byte  in[],
size_t  length 
)
virtual

Add entropy to this RNG.

Parameters
ina byte array containg the entropy to be added
lengththe length of the byte array in

Implements Botan::RandomNumberGenerator.

Definition at line 186 of file hmac_rng.cpp.

References reseed().

187  {
188  m_extractor->update(input, length);
189  reseed(BOTAN_RNG_RESEED_POLL_BITS);
190  }
void reseed(size_t poll_bits)
Definition: hmac_rng.cpp:120
void Botan::HMAC_RNG::clear ( )
virtual

Clear all internally held values of this RNG.

Implements Botan::RandomNumberGenerator.

Definition at line 195 of file hmac_rng.cpp.

References Botan::zeroise().

196  {
197  m_collected_entropy_estimate = 0;
198  m_extractor->clear();
199  m_prf->clear();
200  zeroise(m_K);
201  m_counter = 0;
202  }
void zeroise(std::vector< T, Alloc > &vec)
Definition: secmem.h:168
bool Botan::HMAC_RNG::is_seeded ( ) const
virtual

Check whether this RNG is seeded.

Returns
true if this RNG was already seeded, false otherwise.

Implements Botan::RandomNumberGenerator.

Definition at line 178 of file hmac_rng.cpp.

Referenced by randomize().

179  {
180  return (m_collected_entropy_estimate >= 256);
181  }
RandomNumberGenerator * Botan::RandomNumberGenerator::make_rng ( )
staticinherited

Create a seeded and active RNG object for general application use Added in 1.8.0

Definition at line 14 of file rng.cpp.

15  {
16  std::unique_ptr<RandomNumberGenerator> rng(
17  new HMAC_RNG(make_a<MessageAuthenticationCode>("HMAC(SHA-512)"),
18  make_a<MessageAuthenticationCode>("HMAC(SHA-256)"))
19  );
20 
21  rng->reseed(256);
22 
23  return rng.release();
24  }
std::string Botan::HMAC_RNG::name ( ) const
virtual

Return the name of this object

Implements Botan::RandomNumberGenerator.

Definition at line 207 of file hmac_rng.cpp.

Referenced by randomize().

208  {
209  return "HMAC_RNG(" + m_extractor->name() + "," + m_prf->name() + ")";
210  }
byte Botan::RandomNumberGenerator::next_byte ( )
inlineinherited

Return a random byte

Returns
random byte

Definition at line 53 of file rng.h.

Referenced by Botan::random_prime().

54  {
55  byte out;
56  this->randomize(&out, 1);
57  return out;
58  }
virtual void randomize(byte output[], size_t length)=0
uint8_t byte
Definition: types.h:31
virtual secure_vector<byte> Botan::RandomNumberGenerator::random_vec ( size_t  bytes)
inlinevirtualinherited

Return a random vector

Parameters
bytesnumber of bytes in the result
Returns
randomized vector of length bytes

Definition at line 42 of file rng.h.

Referenced by Botan::TLS::Client_Key_Exchange::Client_Key_Exchange(), Botan::Curve25519_PrivateKey::Curve25519_PrivateKey(), Botan::TLS::Session::encrypt(), Botan::KeyPair::encryption_consistency_check(), Botan::generate_bcrypt(), Botan::mceies_encrypt(), Botan::OctetString::OctetString(), Botan::pbes2_encrypt(), Botan::BigInt::randomize(), Botan::TLS::Session_Manager_SQL::Session_Manager_SQL(), and Botan::KeyPair::signature_consistency_check().

43  {
44  secure_vector<byte> output(bytes);
45  randomize(&output[0], output.size());
46  return output;
47  }
virtual void randomize(byte output[], size_t length)=0
void Botan::HMAC_RNG::randomize ( byte  output[],
size_t  length 
)
virtual

Randomize a byte array.

Parameters
outputthe byte array to hold the random output.
lengththe length of the byte array output.

Implements Botan::RandomNumberGenerator.

Definition at line 86 of file hmac_rng.cpp.

References Botan::copy_mem(), is_seeded(), name(), and reseed().

87  {
88  if(!is_seeded())
89  {
90  reseed(256);
91  if(!is_seeded())
92  throw PRNG_Unseeded(name());
93  }
94 
95  const size_t max_per_prf_iter = m_prf->output_length() / 2;
96 
97  m_output_since_reseed += length;
98 
99  if(m_output_since_reseed >= BOTAN_RNG_MAX_OUTPUT_BEFORE_RESEED)
100  reseed(BOTAN_RNG_RESEED_POLL_BITS);
101 
102  /*
103  HMAC KDF as described in E-t-E, using a CTXinfo of "rng"
104  */
105  while(length)
106  {
107  hmac_prf(*m_prf, m_K, m_counter, "rng");
108 
109  const size_t copied = std::min<size_t>(length, max_per_prf_iter);
110 
111  copy_mem(out, &m_K[0], copied);
112  out += copied;
113  length -= copied;
114  }
115  }
std::string name() const
Definition: hmac_rng.cpp:207
void reseed(size_t poll_bits)
Definition: hmac_rng.cpp:120
bool is_seeded() const
Definition: hmac_rng.cpp:178
void copy_mem(T *out, const T *in, size_t n)
Definition: mem_ops.h:39
void Botan::HMAC_RNG::reseed ( size_t  bits_to_collect)
virtual

Seed this RNG using the entropy sources it contains.

Parameters
bits_to_collectis the number of bits of entropy to attempt to gather from the entropy sources

Implements Botan::RandomNumberGenerator.

Definition at line 120 of file hmac_rng.cpp.

References Botan::EntropySource::poll_available_sources(), and Botan::zeroise().

Referenced by add_entropy(), and randomize().

121  {
122  /*
123  Using the terminology of E-t-E, XTR is the MAC function (normally
124  HMAC) seeded with XTS (below) and we form SKM, the key material, by
125  polling as many sources as we think needed to reach our polling
126  goal. We then also include feedback of the current PRK so that
127  a bad poll doesn't wipe us out.
128  */
129 
130  double bits_collected = 0;
131 
132  Entropy_Accumulator accum(
133  [&](const byte in[], size_t in_len, double entropy_estimate)
134  {
135  m_extractor->update(in, in_len);
136  bits_collected += entropy_estimate;
137  return (bits_collected >= poll_bits);
138  });
139 
141 
142  /*
143  * It is necessary to feed forward poll data. Otherwise, a good poll
144  * (collecting a large amount of conditional entropy) followed by a
145  * bad one (collecting little) would be unsafe. Do this by
146  * generating new PRF outputs using the previous key and feeding
147  * them into the extractor function.
148  *
149  * Cycle the RNG once (CTXinfo="rng"), then generate a new PRF
150  * output using the CTXinfo "reseed". Provide these values as input
151  * to the extractor function.
152  */
153  hmac_prf(*m_prf, m_K, m_counter, "rng");
154  m_extractor->update(m_K); // K is the CTXinfo=rng PRF output
155 
156  hmac_prf(*m_prf, m_K, m_counter, "reseed");
157  m_extractor->update(m_K); // K is the CTXinfo=reseed PRF output
158 
159  /* Now derive the new PRK using everything that has been fed into
160  the extractor, and set the PRF key to that */
161  m_prf->set_key(m_extractor->final());
162 
163  // Now generate a new PRF output to use as the XTS extractor salt
164  hmac_prf(*m_prf, m_K, m_counter, "xts");
165  m_extractor->set_key(m_K);
166 
167  // Reset state
168  zeroise(m_K);
169  m_counter = 0;
170 
171  m_collected_entropy_estimate =
172  std::min<size_t>(m_collected_entropy_estimate + bits_collected,
173  m_extractor->output_length() * 8);
174 
175  m_output_since_reseed = 0;
176  }
static void poll_available_sources(class Entropy_Accumulator &accum)
uint8_t byte
Definition: types.h:31
void zeroise(std::vector< T, Alloc > &vec)
Definition: secmem.h:168

The documentation for this class was generated from the following files: