WIP - entropyentropy

15 files changed, 476 insertions, 619 deletions

diff --git a/src/arch/arm/include/bits/entropy.h b/src/arch/arm/include/bits/entropy.h
deleted file mode 100644
index 75fdc90..0000000
--- a/src/arch/arm/include/bits/entropy.h
+++ /dev/null
@@ -1,12 +0,0 @@
-#ifndef _BITS_ENTROPY_H
-#define _BITS_ENTROPY_H
-
-/** @file
- *
- * ARM-specific entropy API implementations
- *
- */
-
-FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
-
-#endif /* _BITS_ENTROPY_H */
diff --git a/src/arch/loong64/include/bits/entropy.h b/src/arch/loong64/include/bits/entropy.h
deleted file mode 100644
index 8d37269..0000000
--- a/src/arch/loong64/include/bits/entropy.h
+++ /dev/null
@@ -1,12 +0,0 @@
-#ifndef _BITS_ENTROPY_H
-#define _BITS_ENTROPY_H
-
-/** @file
- *
- * LoongArch64-specific entropy API implementations
- *
- */
-
-FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
-
-#endif /* _BITS_ENTROPY_H */
diff --git a/src/arch/x86/core/rdrand.c b/src/arch/x86/core/rdrand.c
index 29605ab..850ab1f 100644
--- a/src/arch/x86/core/rdrand.c
+++ b/src/arch/x86/core/rdrand.c
@@ -32,12 +32,15 @@ FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
 #include <errno.h>
 #include <ipxe/cpuid.h>
 #include <ipxe/entropy.h>
+#include <ipxe/drbg.h>
+
+struct entropy_source rdrand_entropy __entropy_source ( ENTROPY_PREFERRED );
 
 /** Number of times to retry RDRAND instruction */
 #define RDRAND_RETRY_COUNT 16
 
 /** Colour for debug messages */
-#define colour CPUID_FEATURES_INTEL_ECX_RDRAND
+#define colour &rdrand_entropy
 
 /**
  * Enable entropy gathering
@@ -54,16 +57,15 @@ static int rdrand_entropy_enable ( void ) {
 		return -ENOTSUP;
 	}
 
-	return 0;
-}
-
-/**
- * Disable entropy gathering
- *
- */
-static void rdrand_entropy_disable ( void ) {
+	/* Data returned by RDRAND is theoretically full entropy, up
+	 * to a security strength of 128 bits, so assume that each
+	 * sample contains exactly 8 bits of entropy.
+	 */
+	if ( DRBG_SECURITY_STRENGTH > 128 )
+		return -ENOTSUP;
+	entropy_init ( &rdrand_entropy, MIN_ENTROPY ( 8.0 ) );
 
-	/* Nothing to do */
+	return 0;
 }
 
 /**
@@ -93,7 +95,9 @@ static int rdrand_get_noise ( noise_sample_t *noise ) {
 	return 0;
 }
 
-PROVIDE_ENTROPY_INLINE ( rdrand, min_entropy_per_sample );
-PROVIDE_ENTROPY ( rdrand, entropy_enable, rdrand_entropy_enable );
-PROVIDE_ENTROPY ( rdrand, entropy_disable, rdrand_entropy_disable );
-PROVIDE_ENTROPY ( rdrand, get_noise, rdrand_get_noise );
+/** Hardware random number generator entropy source */
+struct entropy_source rdrand_entropy __entropy_source ( ENTROPY_PREFERRED ) = {
+	.name = "rdrand",
+	.enable = rdrand_entropy_enable,
+	.get_noise = rdrand_get_noise,
+};
diff --git a/src/arch/x86/include/bits/entropy.h b/src/arch/x86/include/bits/entropy.h
deleted file mode 100644
index 7accea3..0000000
--- a/src/arch/x86/include/bits/entropy.h
+++ /dev/null
@@ -1,15 +0,0 @@
-#ifndef _BITS_ENTROPY_H
-#define _BITS_ENTROPY_H
-
-/** @file
- *
- * x86-specific entropy API implementations
- *
- */
-
-FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
-
-#include <ipxe/rtc_entropy.h>
-#include <ipxe/rdrand.h>
-
-#endif /* _BITS_ENTROPY_H */
diff --git a/src/arch/x86/include/ipxe/rdrand.h b/src/arch/x86/include/ipxe/rdrand.h
deleted file mode 100644
index c9c170f..0000000
--- a/src/arch/x86/include/ipxe/rdrand.h
+++ /dev/null
@@ -1,37 +0,0 @@
-#ifndef _IPXE_RDRAND_H
-#define _IPXE_RDRAND_H
-
-/** @file
- *
- * Hardware random number generator
- *
- */
-
-FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
-
-#include <stdint.h>
-#include <ipxe/drbg.h>
-
-#ifdef ENTROPY_RDRAND
-#define ENTROPY_PREFIX_rdrand
-#else
-#define ENTROPY_PREFIX_rdrand __rdrand_
-#endif
-
-/**
- * min-entropy per sample
- *
- * @ret min_entropy	min-entropy of each sample
- */
-static inline __always_inline min_entropy_t
-ENTROPY_INLINE ( rdrand, min_entropy_per_sample ) ( void ) {
-
-	/* Data returned by RDRAND is theoretically full entropy, up
-	 * to a security strength of 128 bits.
-	 */
-	if ( DRBG_SECURITY_STRENGTH > 128 )
-		return 0;
-	return MIN_ENTROPY ( 8 * sizeof ( noise_sample_t ) );
-}
-
-#endif /* _IPXE_RDRAND_H */
diff --git a/src/arch/x86/include/ipxe/rtc_entropy.h b/src/arch/x86/include/ipxe/rtc_entropy.h
deleted file mode 100644
index 581abcd..0000000
--- a/src/arch/x86/include/ipxe/rtc_entropy.h
+++ /dev/null
@@ -1,62 +0,0 @@
-#ifndef _IPXE_RTC_ENTROPY_H
-#define _IPXE_RTC_ENTROPY_H
-
-/** @file
- *
- * RTC-based entropy source
- *
- */
-
-FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
-
-#include <stdint.h>
-
-#ifdef ENTROPY_RTC
-#define ENTROPY_PREFIX_rtc
-#else
-#define ENTROPY_PREFIX_rtc __rtc_
-#endif
-
-/**
- * min-entropy per sample
- *
- * @ret min_entropy	min-entropy of each sample
- */
-static inline __always_inline min_entropy_t
-ENTROPY_INLINE ( rtc, min_entropy_per_sample ) ( void ) {
-
-	/* The min-entropy has been measured on several platforms
-	 * using the entropy_sample test code.  Modelling the samples
-	 * as independent, and using a confidence level of 99.99%, the
-	 * measurements were as follows:
-	 *
-	 *    qemu-kvm		: 7.38 bits
-	 *    VMware		: 7.46 bits
-	 *    Physical hardware	: 2.67 bits
-	 *
-	 * We choose the lowest of these (2.67 bits) and apply a 50%
-	 * safety margin to allow for some potential non-independence
-	 * of samples.
-	 */
-	return MIN_ENTROPY ( 1.3 );
-}
-
-extern uint8_t rtc_sample ( void );
-
-/**
- * Get noise sample
- *
- * @ret noise		Noise sample
- * @ret rc		Return status code
- */
-static inline __always_inline int
-ENTROPY_INLINE ( rtc, get_noise ) ( noise_sample_t *noise ) {
-
-	/* Get sample */
-	*noise = rtc_sample();
-
-	/* Always successful */
-	return 0;
-}
-
-#endif /* _IPXE_RTC_ENTROPY_H */
diff --git a/src/arch/x86/interface/pcbios/rtc_entropy.c b/src/arch/x86/interface/pcbios/rtc_entropy.c
index c400d8a..8f47ff6 100644
--- a/src/arch/x86/interface/pcbios/rtc_entropy.c
+++ b/src/arch/x86/interface/pcbios/rtc_entropy.c
@@ -39,6 +39,8 @@ FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
 #include <ipxe/cpuid.h>
 #include <ipxe/entropy.h>
 
+struct entropy_source rtc_entropy __entropy_source ( ENTROPY_NORMAL );
+
 /** Maximum time to wait for an RTC interrupt, in milliseconds */
 #define RTC_MAX_WAIT_MS 100
 
@@ -203,6 +205,21 @@ static int rtc_entropy_enable ( void ) {
 	if ( ( rc = rtc_entropy_check() ) != 0 )
 		goto err_check;
 
+	/* The min-entropy has been measured on several platforms
+	 * using the entropy_sample test code.  Modelling the samples
+	 * as independent, and using a confidence level of 99.99%, the
+	 * measurements were as follows:
+	 *
+	 *    qemu-kvm		: 7.38 bits
+	 *    VMware		: 7.46 bits
+	 *    Physical hardware	: 2.67 bits
+	 *
+	 * We choose the lowest of these (2.67 bits) and apply a 50%
+	 * safety margin to allow for some potential non-independence
+	 * of samples.
+	 */
+	entropy_init ( &rtc_entropy, MIN_ENTROPY ( 1.3 ) );
+
 	return 0;
 
  err_check:
@@ -226,11 +243,12 @@ static void rtc_entropy_disable ( void ) {
 }
 
 /**
- * Measure a single RTC tick
+ * Get noise sample
  *
- * @ret delta		Length of RTC tick (in TSC units)
+ * @ret noise		Noise sample
+ * @ret rc		Return status code
  */
-uint8_t rtc_sample ( void ) {
+static int rtc_get_noise ( noise_sample_t *noise ) {
 	uint32_t before;
 	uint32_t after;
 	uint32_t temp;
@@ -265,10 +283,14 @@ uint8_t rtc_sample ( void ) {
 		: "=a" ( after ), "=d" ( before ), "=Q" ( temp )
 		: "2" ( 0 ) );
 
-	return ( after - before );
+	*noise = ( after - before );
+	return 0;
 }
 
-PROVIDE_ENTROPY_INLINE ( rtc, min_entropy_per_sample );
-PROVIDE_ENTROPY ( rtc, entropy_enable, rtc_entropy_enable );
-PROVIDE_ENTROPY ( rtc, entropy_disable, rtc_entropy_disable );
-PROVIDE_ENTROPY_INLINE ( rtc, get_noise );
+/** RTC entropy source */
+struct entropy_source rtc_entropy __entropy_source ( ENTROPY_NORMAL ) = {
+	.name = "rtc",
+	.enable = rtc_entropy_enable,
+	.disable = rtc_entropy_disable,
+	.get_noise = rtc_get_noise,
+};
diff --git a/src/crypto/entropy.c b/src/crypto/entropy.c
index ced6fd9..6490c8e 100644
--- a/src/crypto/entropy.c
+++ b/src/crypto/entropy.c
@@ -50,46 +50,61 @@ FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
 #define EINFO_EPIPE_ADAPTIVE_PROPORTION_TEST \
 	__einfo_uniqify ( EINFO_EPIPE, 0x02, "Adaptive proportion test failed" )
 
+/** Current entropy source */
+static struct entropy_source *entropy_source;
+
 /**
- * Calculate cutoff value for the repetition count test
- *
- * @ret cutoff		Cutoff value
+ * Enable entropy gathering
  *
- * This is the cutoff value for the Repetition Count Test defined in
- * ANS X9.82 Part 2 (October 2011 Draft) Section 8.5.2.1.2.
+ * @ret rc		Return status code
  */
-static inline __attribute__ (( always_inline )) unsigned int
-repetition_count_cutoff ( void ) {
-	double max_repetitions;
-	unsigned int cutoff;
+int entropy_enable ( void ) {
+	int rc;
 
-	/* The cutoff formula for the repetition test is:
-	 *
-	 *   C = ( 1 + ( -log2(W) / H_min ) )
-	 *
-	 * where W is set at 2^(-30) (in ANS X9.82 Part 2 (October
-	 * 2011 Draft) Section 8.5.2.1.3.1).
-	 */
-	max_repetitions = ( 1 + ( MIN_ENTROPY ( 30 ) /
-				  min_entropy_per_sample() ) );
+	/* Enable selected source, if applicable */
+	if ( entropy_source ) {
 
-	/* Round up to a whole number of repetitions.  We don't have
-	 * the ceil() function available, so do the rounding by hand.
-	 */
-	cutoff = max_repetitions;
-	if ( cutoff < max_repetitions )
-		cutoff++;
-	linker_assert ( ( cutoff >= max_repetitions ), 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 repetitions is a
-	 * compile-time constant.
-	 */
-	linker_assert ( __builtin_constant_p ( cutoff ),
-			repetition_count_cutoff_not_constant );
+		/* Enable entropy source */
+		if ( ( rc = entropy_source->enable() ) != 0 ) {
+			DBGC ( &entropy_source, "ENTROPY could not enable "
+			       "source %s: %s\n", entropy_source->name,
+			       strerror ( rc ) );
+			return rc;
+		}
 
-	return cutoff;
+		/* Sanity checks */
+		assert ( entropy_source->min_entropy_per_sample > 0 );
+		assert ( entropy_source->repetition_count_cutoff > 0 );
+		assert ( entropy_source->adaptive_proportion_cutoff > 0 );
+		assert ( entropy_source->startup_test_count > 0 );
+
+		return 0;
+	}
+
+	/* Find the first working source */
+	rc = -ENOENT;
+	for_each_table_entry ( entropy_source, ENTROPY_SOURCES ) {
+		if ( ( rc = entropy_enable() ) == 0 ) {
+			DBGC ( &entropy_source, "ENTROPY using source %s\n",
+			       entropy_source->name );
+			break;
+		}
+	}
+	return rc;
+}
+
+/**
+ * Disable entropy gathering
+ *
+ */
+void entropy_disable ( void ) {
+
+	/* Sanity check */
+	assert ( entropy_source != NULL );
+
+	/* Disable entropy gathering, if applicable */
+	if ( entropy_source->disable )
+		entropy_source->disable();
 }
 
 /**
@@ -104,6 +119,8 @@ repetition_count_cutoff ( void ) {
 static int repetition_count_test ( noise_sample_t sample ) {
 	static noise_sample_t most_recent_sample;
 	static unsigned int repetition_count = 0;
+	unsigned int repetition_count_cutoff =
+		entropy_source->repetition_count_cutoff;
 
 	/* A = the most recently seen sample value
 	 * B = the number of times that value A has been seen in a row
@@ -124,7 +141,7 @@ static int repetition_count_test ( noise_sample_t sample ) {
 		/*    i.  If B >= C, then an error condition is raised
 		 *        due to a failure of the test
 		 */
-		if ( repetition_count >= repetition_count_cutoff() )
+		if ( repetition_count >= repetition_count_cutoff )
 			return -EPIPE_REPETITION_COUNT_TEST;
 
 	} else {
@@ -142,117 +159,6 @@ static int repetition_count_test ( noise_sample_t sample ) {
 }
 
 /**
- * Window size for the adaptive proportion test
- *
- * ANS X9.82 Part 2 (October 2011 Draft) Section 8.5.2.1.3.1.1 allows
- * five possible window sizes: 16, 64, 256, 4096 and 65536.
- *
- * We expect to generate relatively few (<256) entropy samples during
- * a typical iPXE run; the use of a large window size would mean that
- * the test would never complete a single cycle.  We use a window size
- * of 64, which is the smallest window size that permits values of
- * H_min down to one bit per sample.
- */
-#define ADAPTIVE_PROPORTION_WINDOW_SIZE 64
-
-/**
- * Combine adaptive proportion test window size and min-entropy
- *
- * @v n			N (window size)
- * @v h			H (min-entropy)
- * @ret n_h		(N,H) combined value
- */
-#define APC_N_H( n, h ) ( ( (n) << 8 ) | (h) )
-
-/**
- * Define a row of the adaptive proportion cutoff table
- *
- * @v h			H (min-entropy)
- * @v c16		Cutoff for N=16
- * @v c64		Cutoff for N=64
- * @v c256		Cutoff for N=256
- * @v c4096		Cutoff for N=4096
- * @v c65536		Cutoff for N=65536
- */
-#define APC_TABLE_ROW( h, c16, c64, c256, c4096, c65536)	   \
-	case APC_N_H ( 16, h ) :	return c16;		   \
-	case APC_N_H ( 64, h ) :	return c64;   		   \
-	case APC_N_H ( 256, h ) :	return c256;		   \
-	case APC_N_H ( 4096, h ) :	return c4096;		   \
-	case APC_N_H ( 65536, h ) :	return c65536;
-
-/** Value used to represent "N/A" in adaptive proportion cutoff table */
-#define APC_NA 0
-
-/**
- * Look up value in adaptive proportion test cutoff table
- *
- * @v n			N (window size)
- * @v h			H (min-entropy)
- * @ret cutoff		Cutoff
- *
- * This is the table of cutoff values defined in ANS X9.82 Part 2
- * (October 2011 Draft) Section 8.5.2.1.3.1.2.
- */
-static inline __attribute__ (( always_inline )) unsigned int
-adaptive_proportion_cutoff_lookup ( unsigned int n, unsigned int h ) {
-	switch ( APC_N_H ( n, h ) ) {
-		APC_TABLE_ROW (  1, APC_NA,     51,    168,   2240,  33537 );
-		APC_TABLE_ROW (  2, APC_NA,     35,    100,   1193,  17053 );
-		APC_TABLE_ROW (  3,     10,     24,     61,    643,   8705 );
-		APC_TABLE_ROW (  4,      8,     16,     38,    354,   4473 );
-		APC_TABLE_ROW (  5,      6,     12,     25,    200,   2321 );
-		APC_TABLE_ROW (  6,      5,      9,     17,    117,   1220 );
-		APC_TABLE_ROW (  7,      4,      7,     15,     71,    653 );
-		APC_TABLE_ROW (  8,      4,      5,      9,     45,    358 );
-		APC_TABLE_ROW (  9,      3,      4,      7,     30,    202 );
-		APC_TABLE_ROW ( 10,      3,      4,      5,     21,    118 );
-		APC_TABLE_ROW ( 11,      2,      3,      4,     15,     71 );
-		APC_TABLE_ROW ( 12,      2,      3,      4,     11,     45 );
-		APC_TABLE_ROW ( 13,      2,      2,      3,      9,     30 );
-		APC_TABLE_ROW ( 14,      2,      2,      3,      7,     21 );
-		APC_TABLE_ROW ( 15,      1,      2,      2,      6,     15 );
-		APC_TABLE_ROW ( 16,      1,      2,      2,      5,     11 );
-		APC_TABLE_ROW ( 17,      1,      1,      2,      4,      9 );
-		APC_TABLE_ROW ( 18,      1,      1,      2,      4,      7 );
-		APC_TABLE_ROW ( 19,      1,      1,      1,      3,      6 );
-		APC_TABLE_ROW ( 20,      1,      1,      1,      3,      5 );
-	default:
-		return APC_NA;
-	}
-}
-
-/**
- * Calculate cutoff value for the adaptive proportion test
- *
- * @ret cutoff		Cutoff value
- *
- * This is the cutoff value for the Adaptive Proportion Test defined
- * in ANS X9.82 Part 2 (October 2011 Draft) Section 8.5.2.1.3.1.2.
- */
-static inline __attribute__ (( always_inline )) unsigned int
-adaptive_proportion_cutoff ( void ) {
-	unsigned int h;
-	unsigned int n;
-	unsigned int cutoff;
-
-	/* Look up cutoff value in cutoff table */
-	n = ADAPTIVE_PROPORTION_WINDOW_SIZE;
-	h = ( min_entropy_per_sample() / MIN_ENTROPY_SCALE );
-	cutoff = adaptive_proportion_cutoff_lookup ( n, h );
-
-	/* Fail unless cutoff value is a build-time constant */
-	linker_assert ( __builtin_constant_p ( cutoff ),
-			adaptive_proportion_cutoff_not_constant );
-
-	/* Fail if cutoff value is N/A */
-	linker_assert ( ( cutoff != APC_NA ),
-			adaptive_proportion_cutoff_not_applicable );
-
-	return cutoff;
-}
-
-/**
  * Perform adaptive proportion test
  *
  * @v sample		Noise sample
@@ -265,6 +171,8 @@ static int adaptive_proportion_test ( noise_sample_t sample ) {
 	static noise_sample_t current_counted_sample;
 	static unsigned int sample_count = ADAPTIVE_PROPORTION_WINDOW_SIZE;
 	static unsigned int repetition_count;
+	unsigned int adaptive_proportion_cutoff =
+		entropy_source->adaptive_proportion_cutoff;
 
 	/* A = the sample value currently being counted
 	 * B = the number of samples examined in this run of the test so far
@@ -312,7 +220,7 @@ static int adaptive_proportion_test ( noise_sample_t sample ) {
 			 *      condition, because the test has
 			 *      detected a failure
 			 */
-			if ( repetition_count > adaptive_proportion_cutoff() )
+			if ( repetition_count > adaptive_proportion_cutoff )
 				return -EPIPE_ADAPTIVE_PROPORTION_TEST;
 
 		}
@@ -322,6 +230,23 @@ static int adaptive_proportion_test ( noise_sample_t sample ) {
 }
 
 /**
+ * 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 ) {
+
+	/* Sanity check */
+	assert ( entropy_source != NULL );
+
+	return entropy_source->get_noise ( noise );
+}
+
+/**
  * Get entropy sample
  *
  * @ret entropy		Entropy sample
@@ -334,6 +259,9 @@ static int get_entropy ( entropy_sample_t *entropy ) {
 	static int rc = 0;
 	noise_sample_t noise;
 
+	/* Sanity check */
+	assert ( entropy_source != NULL );
+
 	/* Any failure is permanent */
 	if ( rc != 0 )
 		return rc;
@@ -358,31 +286,6 @@ static int get_entropy ( entropy_sample_t *entropy ) {
 }
 
 /**
- * Calculate number of samples required for startup tests
- *
- * @ret num_samples	Number of samples required
- *
- * ANS X9.82 Part 2 (October 2011 Draft) Section 8.5.2.1.5 requires
- * that at least one full cycle of the continuous tests must be
- * performed at start-up.
- */
-static inline __attribute__ (( always_inline )) unsigned int
-startup_test_count ( void ) {
-	unsigned int num_samples;
-
-	/* At least max(N,C) samples shall be generated by the noise
-	 * source for start-up testing.
-	 */
-	num_samples = repetition_count_cutoff();
-	if ( num_samples < adaptive_proportion_cutoff() )
-		num_samples = adaptive_proportion_cutoff();
-	linker_assert ( __builtin_constant_p ( num_samples ),
-			startup_test_count_not_constant );
-
-	return num_samples;
-}
-
-/**
  * Create next nonce value
  *
  * @ret nonce		Nonce
@@ -402,7 +305,7 @@ static uint32_t make_next_nonce ( void ) {
 /**
  * Obtain entropy input temporary buffer
  *
- * @v num_samples	Number of entropy samples
+ * @v min_entropy	Min-entropy required
  * @v tmp		Temporary buffer
  * @v tmp_len		Length of temporary buffer
  * @ret rc		Return status code
@@ -412,11 +315,8 @@ static uint32_t make_next_nonce ( void ) {
  * 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.
  */
-int get_entropy_input_tmp ( unsigned int num_samples, uint8_t *tmp,
+int get_entropy_input_tmp ( min_entropy_t min_entropy, uint8_t *tmp,
 			    size_t tmp_len ) {
 	static unsigned int startup_tested = 0;
 	struct {
@@ -424,6 +324,7 @@ int get_entropy_input_tmp ( unsigned int num_samples, uint8_t *tmp,
 		entropy_sample_t sample;
 	} __attribute__ (( packed )) data;;
 	uint8_t df_buf[tmp_len];
+	min_entropy_t entropy_total;
 	unsigned int i;
 	int rc;
 
@@ -432,22 +333,20 @@ int get_entropy_input_tmp ( unsigned int num_samples, uint8_t *tmp,
 		return rc;
 
 	/* Perform mandatory startup tests, if not yet performed */
-	for ( ; startup_tested < startup_test_count() ; startup_tested++ ) {
+	for ( ; startup_tested < entropy_source->startup_test_count ;
+	      startup_tested++ ) {
 		if ( ( rc = get_entropy ( &data.sample ) ) != 0 )
 			goto err_get_entropy;
 	}
 
-	/* 3.  entropy_total = 0
-	 *
-	 * (Nothing to do; the number of entropy samples required has
-	 * already been precalculated.)
-	 */
+	/* 3.  entropy_total = 0 */
+	entropy_total = MIN_ENTROPY ( 0 );
 
 	/* 4.  tmp = a fixed n-bit value, such as 0^n */
 	memset ( tmp, 0, tmp_len );
 
 	/* 5.  While ( entropy_total < min_entropy ) */
-	while ( num_samples-- ) {
+	while ( entropy_total < min_entropy ) {
 		/* 5.1.  ( status, entropy_bitstring, assessed_entropy )
 		 *       = GetEntropy()
 		 * 5.2.  If status indicates an error, return ( status, Null )
@@ -466,11 +365,8 @@ int get_entropy_input_tmp ( unsigned int num_samples, uint8_t *tmp,
 		for ( i = 0 ; i < tmp_len ; i++ )
 			tmp[i] ^= df_buf[i];
 
-		/* 5.5.  entropy_total = entropy_total + assessed_entropy
-		 *
-		 * (Nothing to do; the number of entropy samples
-		 * required has already been precalculated.)
-		 */
+		/* 5.5.  entropy_total = entropy_total + assessed_entropy */
+		entropy_total += entropy_source->min_entropy_per_sample;
 	}
 
 	/* Disable entropy gathering */
diff --git a/src/crypto/null_entropy.c b/src/crypto/null_entropy.c
deleted file mode 100644
index d1e1a6f..0000000
--- a/src/crypto/null_entropy.c
+++ /dev/null
@@ -1,40 +0,0 @@
-/*
- * Copyright (C) 2012 Michael Brown <mbrown@fensystems.co.uk>.
- *
- * This program is free software; you can redistribute it and/or
- * modify it under the terms of the GNU General Public License as
- * published by the Free Software Foundation; either version 2 of the
- * License, or any later version.
- *
- * This program is distributed in the hope that it will be useful, but
- * WITHOUT ANY WARRANTY; without even the implied warranty of
- * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
- * General Public License for more details.
- *
- * You should have received a copy of the GNU General Public License
- * along with this program; if not, write to the Free Software
- * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
- * 02110-1301, USA.
- *
- * You can also choose to distribute this program under the terms of
- * the Unmodified Binary Distribution Licence (as given in the file
- * COPYING.UBDL), provided that you have satisfied its requirements.
- */
-
-FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
-
-/** @file
- *
- * Nonexistent entropy source
- *
- *
- * This source provides no entropy and must NOT be used in a
- * security-sensitive environment.
- */
-
-#include <ipxe/entropy.h>
-
-PROVIDE_ENTROPY_INLINE ( null, min_entropy_per_sample );
-PROVIDE_ENTROPY_INLINE ( null, entropy_enable );
-PROVIDE_ENTROPY_INLINE ( null, entropy_disable );
-PROVIDE_ENTROPY_INLINE ( null, get_noise );
diff --git a/src/include/ipxe/efi/efi_entropy.h b/src/include/ipxe/efi/efi_entropy.h
deleted file mode 100644
index 5b16fd7..0000000
--- a/src/include/ipxe/efi/efi_entropy.h
+++ /dev/null
@@ -1,35 +0,0 @@
-#ifndef _IPXE_EFI_ENTROPY_H
-#define _IPXE_EFI_ENTROPY_H
-
-/** @file
- *
- * EFI entropy source
- *
- */
-
-FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
-
-#include <stdint.h>
-
-#ifdef ENTROPY_EFI
-#define ENTROPY_PREFIX_efi
-#else
-#define ENTROPY_PREFIX_efi __efi_
-#endif
-
-/**
- * min-entropy per sample
- *
- * @ret min_entropy	min-entropy of each sample
- */
-static inline __always_inline min_entropy_t
-ENTROPY_INLINE ( efi, min_entropy_per_sample ) ( void ) {
-
-	/* We use essentially the same mechanism as for the BIOS
-	 * RTC-based entropy source, and so assume the same
-	 * min-entropy per sample.
-	 */
-	return MIN_ENTROPY ( 1.3 );
-}
-
-#endif /* _IPXE_EFI_ENTROPY_H */
diff --git a/src/include/ipxe/entropy.h b/src/include/ipxe/entropy.h
index d2e3ce5..b2343ac 100644
--- a/src/include/ipxe/entropy.h
+++ b/src/include/ipxe/entropy.h
@@ -12,40 +12,11 @@ FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
 #include <stdint.h>
 #include <string.h>
 #include <assert.h>
-#include <ipxe/api.h>
 #include <ipxe/hash_df.h>
 #include <ipxe/sha256.h>
+#include <ipxe/tables.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;
 
@@ -71,56 +42,93 @@ typedef unsigned int min_entropy_t;
 #define MIN_ENTROPY( bits ) \
 	( ( min_entropy_t ) ( (bits) * MIN_ENTROPY_SCALE ) )
 
-/* Include all architecture-independent entropy API headers */
-#include <ipxe/null_entropy.h>
-#include <ipxe/efi/efi_entropy.h>
-#include <ipxe/linux/linux_entropy.h>
+/** An entropy source */
+struct entropy_source {
+	/** Name */
+	const char *name;
+	/**
+	 * min-entropy per 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.
+	 *
+	 * Filled in by entropy_init().
+	 */
+	min_entropy_t min_entropy_per_sample;
+	/**
+	 * Repetition count test cutoff value
+	 *
+	 * This is the cutoff value for the Repetition Count Test
+	 * defined in ANS X9.82 Part 2 (October 2011 Draft) Section
+	 * 8.5.2.1.2.
+	 *
+	 * Filled in by entropy_init().
+	 */
+	unsigned int repetition_count_cutoff;
+	/**
+	 * Adaptive proportion test cutoff value
+	 *
+	 * This is the cutoff value for the Adaptive Proportion Test
+	 * defined in ANS X9.82 Part 2 (October 2011 Draft) Section
+	 * 8.5.2.1.3.1.2.
+	 *
+	 * Filled in by entropy_init().
+	 */
+	unsigned int adaptive_proportion_cutoff;
+	/**
+	 * Startup test count
+	 *
+	 * ANS X9.82 Part 2 (October 2011 Draft) Section 8.5.2.1.5
+	 * requires that at least one full cycle of the continuous
+	 * tests must be performed at start-up.
+	 */
+	unsigned int startup_test_count;
+	/**
+	 * Enable entropy gathering
+	 *
+	 * @ret rc		Return status code
+	 */
+	int ( * enable ) ( void );
+	/**
+	 * Disable entropy gathering
+	 *
+	 */
+	void ( * disable ) ( 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 );
+};
 
-/* Include all architecture-dependent entropy API headers */
-#include <bits/entropy.h>
+/** Entropy source table */
+#define ENTROPY_SOURCES __table ( struct entropy_source, "entropy_sources" )
 
-/**
- * Enable entropy gathering
- *
- * @ret rc		Return status code
- */
-int entropy_enable ( void );
+/** Declare an entropy source */
+#define __entropy_source( order ) __table_entry ( ENTROPY_SOURCES, order )
 
-/**
- * Disable entropy gathering
+/** @defgroup entropy_source_order Entropy source order
  *
+ * @{
  */
-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.
- */
-min_entropy_t min_entropy_per_sample ( void );
+#define ENTROPY_PREFERRED	01	/**< Preferred entropy source */
+#define ENTROPY_NORMAL		02	/**< Normal entropy source */
+#define ENTROPY_FALLBACK	03	/**< Fallback entropy source */
 
-/**
- * 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 );
+extern int get_entropy_input_tmp ( min_entropy_t min_entropy, uint8_t *tmp,
+				   size_t tmp_len );
 
 /** Use SHA-256 as the underlying hash algorithm for Hash_df
  *
@@ -145,8 +153,9 @@ extern int get_entropy_input_tmp ( unsigned int num_samples,
  * 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.
+ * This function is inlined to avoid unnecessarily linking in
+ * hash_df(), since the length inputs are always compile-time
+ * constants.
  */
 static inline __attribute__ (( always_inline )) int
 get_entropy_input ( unsigned int min_entropy_bits, void *data, size_t min_len,
@@ -154,41 +163,16 @@ get_entropy_input ( unsigned int min_entropy_bits, void *data, size_t min_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() <=
-			  MIN_ENTROPY ( 8 * sizeof ( noise_sample_t ) ) ),
-			min_entropy_per_sample_is_impossibly_high );
+	/* Sanity check */
 	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 ( min_entropy_bits ) /
-			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.
@@ -218,8 +202,10 @@ get_entropy_input ( unsigned int min_entropy_bits, void *data, size_t min_len,
 	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 )
+	if ( ( rc = get_entropy_input_tmp ( MIN_ENTROPY ( min_entropy_bits ),
+					    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 )
@@ -242,4 +228,229 @@ get_entropy_input ( unsigned int min_entropy_bits, void *data, size_t min_len,
 	}
 }
 
+/**
+ * Calculate cutoff value for the repetition count test
+ *
+ * @v min_entropy_per_sample	Min-entropy per sample
+ * @ret cutoff			Cutoff value
+ *
+ * This is the cutoff value for the Repetition Count Test defined in
+ * ANS X9.82 Part 2 (October 2011 Draft) Section 8.5.2.1.2.
+ */
+static inline __attribute__ (( always_inline )) unsigned int
+entropy_repetition_count_cutoff ( min_entropy_t min_entropy_per_sample ) {
+	double max_repetitions;
+	unsigned int cutoff;
+
+	/* The cutoff formula for the repetition test is:
+	 *
+	 *   C = ( 1 + ( -log2(W) / H_min ) )
+	 *
+	 * where W is set at 2^(-30) (in ANS X9.82 Part 2 (October
+	 * 2011 Draft) Section 8.5.2.1.3.1).
+	 */
+	max_repetitions = ( 1 + ( MIN_ENTROPY ( 30 ) /
+				  min_entropy_per_sample ) );
+
+	/* Round up to a whole number of repetitions.  We don't have
+	 * the ceil() function available, so do the rounding by hand.
+	 */
+	cutoff = max_repetitions;
+	if ( cutoff < max_repetitions )
+		cutoff++;
+	linker_assert ( ( cutoff >= max_repetitions ), 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 repetitions is a
+	 * compile-time constant.
+	 */
+	linker_assert ( __builtin_constant_p ( cutoff ),
+			repetition_count_cutoff_not_constant );
+
+	return cutoff;
+}
+
+/**
+ * Window size for the adaptive proportion test
+ *
+ * ANS X9.82 Part 2 (October 2011 Draft) Section 8.5.2.1.3.1.1 allows
+ * five possible window sizes: 16, 64, 256, 4096 and 65536.
+ *
+ * We expect to generate relatively few (<256) entropy samples during
+ * a typical iPXE run; the use of a large window size would mean that
+ * the test would never complete a single cycle.  We use a window size
+ * of 64, which is the smallest window size that permits values of
+ * H_min down to one bit per sample.
+ */
+#define ADAPTIVE_PROPORTION_WINDOW_SIZE 64
+
+/**
+ * Combine adaptive proportion test window size and min-entropy
+ *
+ * @v n			N (window size)
+ * @v h			H (min-entropy)
+ * @ret n_h		(N,H) combined value
+ */
+#define APC_N_H( n, h ) ( ( (n) << 8 ) | (h) )
+
+/**
+ * Define a row of the adaptive proportion cutoff table
+ *
+ * @v h			H (min-entropy)
+ * @v c16		Cutoff for N=16
+ * @v c64		Cutoff for N=64
+ * @v c256		Cutoff for N=256
+ * @v c4096		Cutoff for N=4096
+ * @v c65536		Cutoff for N=65536
+ */
+#define APC_TABLE_ROW( h, c16, c64, c256, c4096, c65536)	   \
+	case APC_N_H ( 16, h ) :	return c16;		   \
+	case APC_N_H ( 64, h ) :	return c64;   		   \
+	case APC_N_H ( 256, h ) :	return c256;		   \
+	case APC_N_H ( 4096, h ) :	return c4096;		   \
+	case APC_N_H ( 65536, h ) :	return c65536;
+
+/** Value used to represent "N/A" in adaptive proportion cutoff table */
+#define APC_NA 0
+
+/**
+ * Look up value in adaptive proportion test cutoff table
+ *
+ * @v n			N (window size)
+ * @v h			H (min-entropy)
+ * @ret cutoff		Cutoff
+ *
+ * This is the table of cutoff values defined in ANS X9.82 Part 2
+ * (October 2011 Draft) Section 8.5.2.1.3.1.2.
+ */
+static inline __attribute__ (( always_inline )) unsigned int
+entropy_adaptive_proportion_cutoff_lookup ( unsigned int n, unsigned int h ) {
+	switch ( APC_N_H ( n, h ) ) {
+		APC_TABLE_ROW (  1, APC_NA,     51,    168,   2240,  33537 );
+		APC_TABLE_ROW (  2, APC_NA,     35,    100,   1193,  17053 );
+		APC_TABLE_ROW (  3,     10,     24,     61,    643,   8705 );
+		APC_TABLE_ROW (  4,      8,     16,     38,    354,   4473 );
+		APC_TABLE_ROW (  5,      6,     12,     25,    200,   2321 );
+		APC_TABLE_ROW (  6,      5,      9,     17,    117,   1220 );
+		APC_TABLE_ROW (  7,      4,      7,     15,     71,    653 );
+		APC_TABLE_ROW (  8,      4,      5,      9,     45,    358 );
+		APC_TABLE_ROW (  9,      3,      4,      7,     30,    202 );
+		APC_TABLE_ROW ( 10,      3,      4,      5,     21,    118 );
+		APC_TABLE_ROW ( 11,      2,      3,      4,     15,     71 );
+		APC_TABLE_ROW ( 12,      2,      3,      4,     11,     45 );
+		APC_TABLE_ROW ( 13,      2,      2,      3,      9,     30 );
+		APC_TABLE_ROW ( 14,      2,      2,      3,      7,     21 );
+		APC_TABLE_ROW ( 15,      1,      2,      2,      6,     15 );
+		APC_TABLE_ROW ( 16,      1,      2,      2,      5,     11 );
+		APC_TABLE_ROW ( 17,      1,      1,      2,      4,      9 );
+		APC_TABLE_ROW ( 18,      1,      1,      2,      4,      7 );
+		APC_TABLE_ROW ( 19,      1,      1,      1,      3,      6 );
+		APC_TABLE_ROW ( 20,      1,      1,      1,      3,      5 );
+	default:
+		return APC_NA;
+	}
+}
+
+/**
+ * Calculate cutoff value for the adaptive proportion test
+ *
+ * @v min_entropy_per_sample	Min-entropy per sample
+ * @ret cutoff			Cutoff value
+ *
+ * This is the cutoff value for the Adaptive Proportion Test defined
+ * in ANS X9.82 Part 2 (October 2011 Draft) Section 8.5.2.1.3.1.2.
+ */
+static inline __attribute__ (( always_inline )) unsigned int
+entropy_adaptive_proportion_cutoff ( min_entropy_t min_entropy_per_sample ) {
+	unsigned int h;
+	unsigned int n;
+	unsigned int cutoff;
+
+	/* Look up cutoff value in cutoff table */
+	n = ADAPTIVE_PROPORTION_WINDOW_SIZE;
+	h = ( min_entropy_per_sample / MIN_ENTROPY_SCALE );
+	cutoff = entropy_adaptive_proportion_cutoff_lookup ( n, h );
+
+	/* Fail unless cutoff value is a compile-time constant */
+	linker_assert ( __builtin_constant_p ( cutoff ),
+			adaptive_proportion_cutoff_not_constant );
+
+	/* Fail if cutoff value is N/A */
+	linker_assert ( ( cutoff != APC_NA ),
+			adaptive_proportion_cutoff_not_applicable );
+
+	return cutoff;
+}
+
+/**
+ * Calculate number of samples required for startup tests
+ *
+ * @v repetition_count_cutoff	Repetition count test cutoff value
+ * @v adaptive_proportion_cutoff Adaptive proportion test cutoff value
+ * @ret num_samples		Number of samples required
+ *
+ * ANS X9.82 Part 2 (October 2011 Draft) Section 8.5.2.1.5 requires
+ * that at least one full cycle of the continuous tests must be
+ * performed at start-up.
+ */
+static inline __attribute__ (( always_inline )) unsigned int
+entropy_startup_test_count ( unsigned int repetition_count_cutoff,
+			     unsigned int adaptive_proportion_cutoff ) {
+	unsigned int num_samples;
+
+	/* At least max(N,C) samples shall be generated by the noise
+	 * source for start-up testing.
+	 */
+	num_samples = repetition_count_cutoff;
+	if ( num_samples < adaptive_proportion_cutoff )
+		num_samples = adaptive_proportion_cutoff;
+	linker_assert ( __builtin_constant_p ( num_samples ),
+			startup_test_count_not_constant );
+
+	return num_samples;
+}
+
+/**
+ * Initialise entropy source
+ *
+ * @v source			Entropy source
+ * @v min_entropy_per_sample	Min-entropy per sample
+ *
+ * The cutoff value calculations for the repetition count test and the
+ * adaptive proportion test are provided as static inline functions
+ * since the results will always be compile-time constants.
+ */
+static inline __attribute__ (( always_inline )) void
+entropy_init ( struct entropy_source *source,
+	       min_entropy_t min_entropy_per_sample ) {
+	unsigned int repetition_count_cutoff;
+	unsigned int adaptive_proportion_cutoff;
+	unsigned int startup_test_count;
+
+	/* Sanity check */
+	linker_assert ( ( min_entropy_per_sample <=
+			  MIN_ENTROPY ( 8 * sizeof ( noise_sample_t ) ) ),
+			min_entropy_per_sample_is_impossibly_high );
+
+	/* Calculate test cutoff values */
+	repetition_count_cutoff =
+		entropy_repetition_count_cutoff ( min_entropy_per_sample );
+	adaptive_proportion_cutoff =
+		entropy_adaptive_proportion_cutoff ( min_entropy_per_sample );
+	startup_test_count =
+		entropy_startup_test_count ( repetition_count_cutoff,
+					     adaptive_proportion_cutoff );
+
+	/* Record min-entropy per sample and test cutoff values */
+	source->min_entropy_per_sample = min_entropy_per_sample;
+	source->repetition_count_cutoff = repetition_count_cutoff;
+	source->adaptive_proportion_cutoff = adaptive_proportion_cutoff;
+	source->startup_test_count = startup_test_count;
+}
+
+extern int entropy_enable ( void );
+extern void entropy_disable ( void );
+extern int get_noise ( noise_sample_t *noise );
+
 #endif /* _IPXE_ENTROPY_H */
diff --git a/src/include/ipxe/linux/linux_entropy.h b/src/include/ipxe/linux/linux_entropy.h
deleted file mode 100644
index ea8c1f1..0000000
--- a/src/include/ipxe/linux/linux_entropy.h
+++ /dev/null
@@ -1,34 +0,0 @@
-#ifndef _IPXE_LINUX_ENTROPY_H
-#define _IPXE_LINUX_ENTROPY_H
-
-/** @file
- *
- * /dev/random-based entropy source
- *
- */
-
-FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
-
-#ifdef ENTROPY_LINUX
-#define ENTROPY_PREFIX_linux
-#else
-#define ENTROPY_PREFIX_linux __linux_
-#endif
-
-/**
- * min-entropy per sample
- *
- * @ret min_entropy	min-entropy of each sample
- */
-static inline __always_inline min_entropy_t
-ENTROPY_INLINE ( linux, min_entropy_per_sample ) ( void ) {
-
-	/* linux_get_noise() reads a single byte from /dev/random,
-	 * which is supposed to block until a sufficient amount of
-	 * entropy is available.  We therefore assume that each sample
-	 * contains exactly 8 bits of entropy.
-	 */
-	return MIN_ENTROPY ( 8.0 );
-}
-
-#endif /* _IPXE_LINUX_ENTROPY_H */
diff --git a/src/include/ipxe/null_entropy.h b/src/include/ipxe/null_entropy.h
deleted file mode 100644
index 5a6bb62..0000000
--- a/src/include/ipxe/null_entropy.h
+++ /dev/null
@@ -1,52 +0,0 @@
-#ifndef _IPXE_NULL_ENTROPY_H
-#define _IPXE_NULL_ENTROPY_H
-
-/** @file
- *
- * Nonexistent entropy source
- *
- * This source provides no entropy and must NOT be used in a
- * security-sensitive environment.
- */
-
-FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
-
-#include <stdint.h>
-
-#ifdef ENTROPY_NULL
-#define ENTROPY_PREFIX_null
-#else
-#define ENTROPY_PREFIX_null __null_
-#endif
-
-static inline __always_inline int
-ENTROPY_INLINE ( null, entropy_enable ) ( void ) {
-	/* Do nothing */
-	return 0;
-}
-
-static inline __always_inline void
-ENTROPY_INLINE ( null, entropy_disable ) ( void ) {
-	/* Do nothing */
-}
-
-static inline __always_inline min_entropy_t
-ENTROPY_INLINE ( null, min_entropy_per_sample ) ( void ) {
-	/* Actual amount of min-entropy is zero.  To avoid
-	 * division-by-zero errors and to allow compilation of
-	 * entropy-consuming code, pretend to have 1 bit of entropy in
-	 * each sample.
-	 */
-	return MIN_ENTROPY ( 1.0 );
-}
-
-static inline __always_inline int
-ENTROPY_INLINE ( null, get_noise ) ( noise_sample_t *noise ) {
-
-	/* All sample values are constant */
-	*noise = 0x01;
-
-	return 0;
-}
-
-#endif /* _IPXE_NULL_ENTROPY_H */
diff --git a/src/interface/efi/efi_entropy.c b/src/interface/efi/efi_entropy.c
index 1e8ddfb..e5c3935 100644
--- a/src/interface/efi/efi_entropy.c
+++ b/src/interface/efi/efi_entropy.c
@@ -36,6 +36,8 @@ FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
  *
  */
 
+struct entropy_source efi_entropy __entropy_source ( ENTROPY_NORMAL );
+
 /** Random number generator protocol */
 static EFI_RNG_PROTOCOL *efirng;
 EFI_REQUEST_PROTOCOL ( EFI_RNG_PROTOCOL, &efirng );
@@ -91,6 +93,12 @@ static int efi_entropy_enable ( void ) {
 		return rc;
 	}
 
+	/* We use essentially the same mechanism as for the BIOS
+	 * RTC-based entropy source, and so assume the same
+	 * min-entropy per sample.
+	 */
+	entropy_init ( &efi_entropy, MIN_ENTROPY ( 1.3 ) );
+
 	return 0;
 }
 
@@ -235,7 +243,10 @@ static int efi_get_noise ( noise_sample_t *noise ) {
 	return 0;
 }
 
-PROVIDE_ENTROPY_INLINE ( efi, min_entropy_per_sample );
-PROVIDE_ENTROPY ( efi, entropy_enable, efi_entropy_enable );
-PROVIDE_ENTROPY ( efi, entropy_disable, efi_entropy_disable );
-PROVIDE_ENTROPY ( efi, get_noise, efi_get_noise );
+/** EFI entropy source */
+struct entropy_source efi_entropy __entropy_source ( ENTROPY_NORMAL ) = {
+	.name = "efi",
+	.enable = efi_entropy_enable,
+	.disable = efi_entropy_disable,
+	.get_noise = efi_get_noise,
+};
diff --git a/src/interface/linux/linux_entropy.c b/src/interface/linux/linux_entropy.c
index 257e993..f249697 100644
--- a/src/interface/linux/linux_entropy.c
+++ b/src/interface/linux/linux_entropy.c
@@ -34,6 +34,8 @@ FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
 #include <ipxe/linux_api.h>
 #include <ipxe/entropy.h>
 
+struct entropy_source linux_entropy __entropy_source ( ENTROPY_NORMAL );
+
 /** Entropy source filename */
 static const char entropy_filename[] = "/dev/random";
 
@@ -55,6 +57,13 @@ static int linux_entropy_enable ( void ) {
 		return entropy_fd;
 	}
 
+	/* linux_get_noise() reads a single byte from /dev/random,
+	 * which is supposed to block until a sufficient amount of
+	 * entropy is available.  We therefore assume that each sample
+	 * contains exactly 8 bits of entropy.
+	 */
+	entropy_init ( &linux_entropy, MIN_ENTROPY ( 8.0 ) );
+
 	return 0;
 }
 
@@ -95,7 +104,10 @@ static int linux_get_noise ( noise_sample_t *noise ) {
 	return 0;
 }
 
-PROVIDE_ENTROPY_INLINE ( linux, min_entropy_per_sample );
-PROVIDE_ENTROPY ( linux, entropy_enable, linux_entropy_enable );
-PROVIDE_ENTROPY ( linux, entropy_disable, linux_entropy_disable );
-PROVIDE_ENTROPY ( linux, get_noise, linux_get_noise );
+/** Linux entropy source */
+struct entropy_source linux_entropy __entropy_source ( ENTROPY_NORMAL ) = {
+	.name = "linux",
+	.enable = linux_entropy_enable,
+	.disable = linux_entropy_disable,
+	.get_noise = linux_get_noise,
+};