--- /dev/null
+#ifndef _IPXE_ENTROPY_H
+#define _IPXE_ENTROPY_H
+
+/** @file
+ *
+ * Entropy source
+ *
+ */
+
+FILE_LICENCE ( GPL2_OR_LATER );
+
+#include <stdint.h>
+#include <string.h>
+#include <assert.h>
+#include <ipxe/api.h>
+#include <ipxe/hash_df.h>
+#include <ipxe/sha256.h>
+#include <config/entropy.h>
+
+/**
+ * Calculate static inline entropy API function name
+ *
+ * @v _prefix Subsystem prefix
+ * @v _api_func API function
+ * @ret _subsys_func Subsystem API function
+ */
+#define ENTROPY_INLINE( _subsys, _api_func ) \
+ SINGLE_API_INLINE ( ENTROPY_PREFIX_ ## _subsys, _api_func )
+
+/**
+ * Provide a entropy API implementation
+ *
+ * @v _prefix Subsystem prefix
+ * @v _api_func API function
+ * @v _func Implementing function
+ */
+#define PROVIDE_ENTROPY( _subsys, _api_func, _func ) \
+ PROVIDE_SINGLE_API ( ENTROPY_PREFIX_ ## _subsys, _api_func, _func )
+
+/**
+ * Provide a static inline entropy API implementation
+ *
+ * @v _prefix Subsystem prefix
+ * @v _api_func API function
+ */
+#define PROVIDE_ENTROPY_INLINE( _subsys, _api_func ) \
+ PROVIDE_SINGLE_API_INLINE ( ENTROPY_PREFIX_ ## _subsys, _api_func )
+
+/** A noise sample */
+typedef uint8_t noise_sample_t;
+
+/** An entropy sample */
+typedef uint8_t entropy_sample_t;
+
+/* Include all architecture-independent entropy API headers */
+#include <ipxe/null_entropy.h>
+#include <ipxe/linux/linux_entropy.h>
+
+/* Include all architecture-dependent entropy API headers */
+#include <bits/entropy.h>
+
+/**
+ * Enable entropy gathering
+ *
+ * @ret rc Return status code
+ */
+int entropy_enable ( void );
+
+/**
+ * Disable entropy gathering
+ *
+ */
+void entropy_disable ( void );
+
+/**
+ * min-entropy per sample
+ *
+ * @ret min_entropy min-entropy of each sample
+ *
+ * min-entropy is defined in ANS X9.82 Part 1-2006 Section 8.3 and in
+ * NIST SP 800-90 Appendix C.3 as
+ *
+ * H_min = -log2 ( p_max )
+ *
+ * where p_max is the probability of the most likely sample value.
+ *
+ * This must be a compile-time constant.
+ */
+double min_entropy_per_sample ( void );
+
+/**
+ * Get noise sample
+ *
+ * @ret noise Noise sample
+ * @ret rc Return status code
+ *
+ * This is the GetNoise function defined in ANS X9.82 Part 2
+ * (October 2011 Draft) Section 6.5.2.
+ */
+int get_noise ( noise_sample_t *noise );
+
+extern int get_entropy_input_tmp ( unsigned int num_samples,
+ uint8_t *tmp, size_t tmp_len );
+
+/** Use SHA-256 as the underlying hash algorithm for Hash_df
+ *
+ * Hash_df using SHA-256 is an Approved algorithm in ANS X9.82.
+ */
+#define entropy_hash_df_algorithm sha256_algorithm
+
+/** Underlying hash algorithm output length (in bytes) */
+#define ENTROPY_HASH_DF_OUTLEN_BYTES SHA256_DIGEST_SIZE
+
+/**
+ * Obtain entropy input
+ *
+ * @v min_entropy_bits Minimum amount of entropy, in bits
+ * @v data Data buffer
+ * @v min_len Minimum length of entropy input, in bytes
+ * @v max_len Maximum length of entropy input, in bytes
+ * @ret len Length of entropy input, in bytes, or negative error
+ *
+ * This is the implementation of the Get_entropy_input function (using
+ * an entropy source as the source of entropy input and condensing
+ * each entropy source output after each GetEntropy call) as defined
+ * in ANS X9.82 Part 4 (April 2011 Draft) Section 13.3.4.2.
+ *
+ * To minimise code size, the number of samples required is calculated
+ * at compilation time.
+ */
+static inline __attribute__ (( always_inline )) int
+get_entropy_input ( unsigned int min_entropy_bits, void *data, size_t min_len,
+ size_t max_len ) {
+ size_t tmp_len = ( ( ( min_entropy_bits * 2 ) + 7 ) / 8 );
+ uint8_t tmp_buf[ tmp_len ];
+ uint8_t *tmp = ( ( tmp_len > max_len ) ? tmp_buf : data );
+ double min_samples;
+ unsigned int num_samples;
+ unsigned int n;
+ int rc;
+
+ /* Sanity checks */
+ linker_assert ( ( min_entropy_per_sample() <=
+ ( 8 * sizeof ( noise_sample_t ) ) ),
+ min_entropy_per_sample_is_impossibly_high );
+ linker_assert ( ( min_entropy_bits <= ( 8 * max_len ) ),
+ entropy_buffer_too_small );
+
+ /* Round up minimum entropy to an integral number of bytes */
+ min_entropy_bits = ( ( min_entropy_bits + 7 ) & ~7 );
+
+ /* Calculate number of samples required to contain sufficient entropy */
+ min_samples = ( ( min_entropy_bits * 1.0 ) / min_entropy_per_sample() );
+
+ /* Round up to a whole number of samples. We don't have the
+ * ceil() function available, so do the rounding by hand.
+ */
+ num_samples = min_samples;
+ if ( num_samples < min_samples )
+ num_samples++;
+ linker_assert ( ( num_samples >= min_samples ), rounding_error );
+
+ /* Floating-point operations are not allowed in iPXE since we
+ * never set up a suitable environment. Abort the build
+ * unless the calculated number of samples is a compile-time
+ * constant.
+ */
+ linker_assert ( __builtin_constant_p ( num_samples ),
+ num_samples_not_constant );
+
+ /* (Unnumbered). The output length of the hash function shall
+ * meet or exceed the security strength indicated by the
+ * min_entropy parameter.
+ */
+ linker_assert ( ( ( 8 * ENTROPY_HASH_DF_OUTLEN_BYTES ) >=
+ min_entropy_bits ), hash_df_algorithm_too_weak );
+
+ /* 1. If ( min_length > max_length ), then return ( FAILURE, Null ) */
+ linker_assert ( ( min_len <= max_len ), min_len_greater_than_max_len );
+
+ /* 2. n = 2 * min_entropy */
+ n = ( 2 * min_entropy_bits );
+
+ /* 3. entropy_total = 0
+ * 4. tmp = a fixed n-bit value, such as 0^n
+ * 5. While ( entropy_total < min_entropy )
+ * 5.1. ( status, entropy_bitstring, assessed_entropy )
+ * = GetEntropy()
+ * 5.2. If status indicates an error, return ( status, Null )
+ * 5.3. nonce = MakeNextNonce()
+ * 5.4. tmp = tmp XOR df ( ( nonce || entropy_bitstring ), n )
+ * 5.5. entropy_total = entropy_total + assessed_entropy
+ *
+ * (The implementation of these steps is inside the function
+ * get_entropy_input_tmp().)
+ */
+ linker_assert ( __builtin_constant_p ( tmp_len ),
+ tmp_len_not_constant );
+ linker_assert ( ( n == ( 8 * tmp_len ) ), tmp_len_mismatch );
+ if ( ( rc = get_entropy_input_tmp ( num_samples, tmp, tmp_len ) ) != 0 )
+ return rc;
+
+ /* 6. If ( n < min_length ), then tmp = tmp || 0^(min_length-n)
+ * 7. If ( n > max_length ), then tmp = df ( tmp, max_length )
+ * 8. Return ( SUCCESS, tmp )
+ */
+ if ( tmp_len < min_len ) {
+ /* (Data is already in-place.) */
+ linker_assert ( ( data == tmp ), data_not_inplace );
+ memset ( ( data + tmp_len ), 0, ( min_len - tmp_len ) );
+ return min_len;
+ } else if ( tmp_len > max_len ) {
+ linker_assert ( ( tmp == tmp_buf ), data_inplace );
+ hash_df ( &entropy_hash_df_algorithm, tmp, tmp_len,
+ data, max_len );
+ return max_len;
+ } else {
+ /* (Data is already in-place.) */
+ linker_assert ( ( data == tmp ), data_not_inplace );
+ return tmp_len;
+ }
+}
+
+#endif /* _IPXE_ENTROPY_H */
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