mbedtls_ecp_gen_privkey_mx: make bit manipulations unconditional
Don't calculate the bit-size of the initially generated random number.
This is not necessary to reach the desired distribution of private
keys, and creates a (tiny) side channel opportunity.
This changes the way the result is derived from the random number, but
does not affect the resulting distribution.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/tests/suites/test_suite_ecp.data b/tests/suites/test_suite_ecp.data
index 21a7192..5f92ca4 100644
--- a/tests/suites/test_suite_ecp.data
+++ b/tests/suites/test_suite_ecp.data
@@ -277,17 +277,13 @@
mbedtls_ecp_gen_key:MBEDTLS_ECP_DP_SECP192R1
ECP generate Montgomery key: Curve25519, random in range
-genkey_mx_known_answer:254:"4f0102030405060708090a0b0c0d0e0f101112131415161718191a1b1c1d1ef8":"4f0102030405060708090a0b0c0d0e0f101112131415161718191a1b1c1d1ef8"
-
-ECP generate Montgomery key: Curve25519, set high bit
-genkey_mx_known_answer:254:"0f0102030405060708090a0b0c0d0e0f101112131415161718191a1b1c1d1ef8":"4f0102030405060708090a0b0c0d0e0f101112131415161718191a1b1c1d1ef8"
+genkey_mx_known_answer:254:"9e020406080a0c0e10121416181a1c1e20222426282a2c2e30323436383a3df0":"4f0102030405060708090a0b0c0d0e0f101112131415161718191a1b1c1d1ef8"
ECP generate Montgomery key: Curve25519, clear higher bit
-## If the bit 255 is set, the library shifts the random number right.
genkey_mx_known_answer:254:"ff0102030405060708090a0b0c0d0e0f101112131415161718191a1b1c1d1ef8":"7f808101820283038404850586068707880889098a0a8b0b8c0c8d0d8e0e8f78"
ECP generate Montgomery key: Curve25519, clear low bits
-genkey_mx_known_answer:254:"4f0102030405060708090a0b0c0d0e0f101112131415161718191a1b1c1d1eff":"4f0102030405060708090a0b0c0d0e0f101112131415161718191a1b1c1d1ef8"
+genkey_mx_known_answer:254:"9e020406080a0c0e10121416181a1c1e20222426282a2c2e30323436383a3dff":"4f0102030405060708090a0b0c0d0e0f101112131415161718191a1b1c1d1ef8"
ECP generate Montgomery key: Curve25519, random = all-bits-zero
genkey_mx_known_answer:254:"0000000000000000000000000000000000000000000000000000000000000000":"4000000000000000000000000000000000000000000000000000000000000000"