From f807ad17f327c40d2ed89739f7ed037ea9a80ee5 Mon Sep 17 00:00:00 2001 From: Matt Caswell Date: Mon, 3 Dec 2018 17:01:07 +0000 Subject: [PATCH] Disallow Ed448 signature malleability Check that s is less than the order before attempting to verify the signature as per RFC8032 5.2.7 Fixes #7706 Reviewed-by: Kurt Roeckx (Merged from https://github.com/openssl/openssl/pull/7748) (cherry picked from commit 08afd2f37a4465c90b9b9e2081c9e8df4726db89) --- crypto/ec/curve448/eddsa.c | 30 ++++++++++++++++++++++++++++-- 1 file changed, 28 insertions(+), 2 deletions(-) diff --git a/crypto/ec/curve448/eddsa.c b/crypto/ec/curve448/eddsa.c index 909413a535..b28f7dff91 100644 --- a/crypto/ec/curve448/eddsa.c +++ b/crypto/ec/curve448/eddsa.c @@ -246,10 +246,36 @@ c448_error_t c448_ed448_verify( uint8_t context_len) { curve448_point_t pk_point, r_point; - c448_error_t error = - curve448_point_decode_like_eddsa_and_mul_by_ratio(pk_point, pubkey); + c448_error_t error; curve448_scalar_t challenge_scalar; curve448_scalar_t response_scalar; + /* Order in little endian format */ + static const uint8_t order[] = { + 0xF3, 0x44, 0x58, 0xAB, 0x92, 0xC2, 0x78, 0x23, 0x55, 0x8F, 0xC5, 0x8D, + 0x72, 0xC2, 0x6C, 0x21, 0x90, 0x36, 0xD6, 0xAE, 0x49, 0xDB, 0x4E, 0xC4, + 0xE9, 0x23, 0xCA, 0x7C, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, + 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, + 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0x3F, 0x00 + }; + int i; + + /* + * Check that s (second 57 bytes of the sig) is less than the order. Both + * s and the order are in little-endian format. This can be done in + * variable time, since if this is not the case the signature if publicly + * invalid. + */ + for (i = EDDSA_448_PUBLIC_BYTES - 1; i >= 0; i--) { + if (signature[i + EDDSA_448_PUBLIC_BYTES] > order[i]) + return C448_FAILURE; + if (signature[i + EDDSA_448_PUBLIC_BYTES] < order[i]) + break; + } + if (i < 0) + return C448_FAILURE; + + error = + curve448_point_decode_like_eddsa_and_mul_by_ratio(pk_point, pubkey); if (C448_SUCCESS != error) return error; -- 2.25.1