From 200f9b26fe0a2f235a2af8b30c4be9f12f6bc9cb Mon Sep 17 00:00:00 2001 From: Tim Ruffing Date: Wed, 27 Jul 2022 14:39:13 +0200 Subject: [PATCH] bip340: Allow variable-length messages --- bip-0340.mediawiki | 31 ++++++++++++++++++++++++++++--- bip-0340/reference.py | 4 ---- bip-0340/test-vectors.csv | 4 ++++ bip-0340/test-vectors.py | 20 +++++++++++++++++++- 4 files changed, 51 insertions(+), 8 deletions(-) diff --git a/bip-0340.mediawiki b/bip-0340.mediawiki index e3f61befc2..3b61c578ce 100644 --- a/bip-0340.mediawiki +++ b/bip-0340.mediawiki @@ -138,7 +138,7 @@ As an alternative to generating keys randomly, it is also possible and safe to r Input: * The secret key ''sk'': a 32-byte array -* The message ''m'': a 32-byte array +* The message ''m'': a byte array * Auxiliary random data ''a'': a 32-byte array The algorithm ''Sign(sk, m)'' is defined as: @@ -174,7 +174,7 @@ It should be noted that various alternative signing algorithms can be used to pr Input: * The public key ''pk'': a 32-byte array -* The message ''m'': a 32-byte array +* The message ''m'': a byte array * A signature ''sig'': a 64-byte array The algorithm ''Verify(pk, m, sig)'' is defined as: @@ -197,7 +197,7 @@ Note that the correctness of verification relies on the fact that ''lift_x'' alw Input: * The number ''u'' of signatures * The public keys ''pk1..u'': ''u'' 32-byte arrays -* The messages ''m1..u'': ''u'' 32-byte arrays +* The messages ''m1..u'': ''u'' byte arrays * The signatures ''sig1..u'': ''u'' 64-byte arrays The algorithm ''BatchVerify(pk1..u, m1..u, sig1..u)'' is defined as: @@ -213,6 +213,30 @@ The algorithm ''BatchVerify(pk1..u, m1..u, sig1..uIn theory, the message size is restricted due to the fact that SHA256 accepts byte strings only up to size of 2^61-1 bytes. +It is understood that implementations may reject messages which are too large in their environment or application context, +e.g., messages which exceed predefined buffers or would otherwise cause resource exhaustion. + +Earlier revisions of this BIP required messages to be exactly 32 bytes. +This restriction puts a burden on callers +who typically need to perform pre-hashing of the actual input message by feeding it through SHA256 (or another collision-resistant cryptographic hash function) +to create a 32-byte digest which can be passed to signing or verification +(as for example done in [[bip-0341.mediawiki|BIP341]].) + +Since pre-hashing may not always be desirable, +e.g., when actual messages are shorter than 32 bytes,Another reason to omit pre-hashing is to protect against certain types of cryptanalytic advances against the hash function used for pre-hashing: If pre-hashing is used, an attacker that can find collisions in the pre-hashing function can necessarily forge signatures under chosen-message attacks. If pre-hashing is not used, an attacker that can find collisions in SHA256 (as used inside the signature scheme) may not be able to forge signatures. However, this seeming advantage is mostly irrelevant in the context of Bitcoin, which already relies on collision resistance of SHA256 in other places, e.g., for transaction hashes. +the restriction to 32-byte messages has been lifted. +We note that pre-hashing is recommended for performance reasons in applications that deal with large messages. +If large messages are not pre-hashed, +the algorithms of the signature scheme will perform more hashing internally. +In particular, the signing algorithm needs two sequential hashing passes over the message, +which means that the full message must necessarily be kept in memory during signing, +and large messages entail a runtime penalty.Typically, messages of 56 bytes or longer enjoy a performance benefit from pre-hashing, assuming the speed of SHA256 inside the signing algorithm matches that of the pre-hashing done by the calling application. + == Applications == There are several interesting applications beyond simple signatures. @@ -248,6 +272,7 @@ The reference implementation is for demonstration purposes only and not to be us To help implementors understand updates to this BIP, we keep a list of substantial changes. * 2022-08: Fix function signature of lift_x in reference code +* 2023-04: Allow messages of arbitrary size == Footnotes == diff --git a/bip-0340/reference.py b/bip-0340/reference.py index 162bb88d99..b327e0a228 100644 --- a/bip-0340/reference.py +++ b/bip-0340/reference.py @@ -96,8 +96,6 @@ def pubkey_gen(seckey: bytes) -> bytes: return bytes_from_point(P) def schnorr_sign(msg: bytes, seckey: bytes, aux_rand: bytes) -> bytes: - if len(msg) != 32: - raise ValueError('The message must be a 32-byte array.') d0 = int_from_bytes(seckey) if not (1 <= d0 <= n - 1): raise ValueError('The secret key must be an integer in the range 1..n-1.') @@ -121,8 +119,6 @@ def schnorr_sign(msg: bytes, seckey: bytes, aux_rand: bytes) -> bytes: return sig def schnorr_verify(msg: bytes, pubkey: bytes, sig: bytes) -> bool: - if len(msg) != 32: - raise ValueError('The message must be a 32-byte array.') if len(pubkey) != 32: raise ValueError('The public key must be a 32-byte array.') if len(sig) != 64: diff --git a/bip-0340/test-vectors.csv b/bip-0340/test-vectors.csv index a1a63e1283..672339129a 100644 --- a/bip-0340/test-vectors.csv +++ b/bip-0340/test-vectors.csv @@ -14,3 +14,7 @@ index,secret key,public key,aux_rand,message,signature,verification result,comme 12,,DFF1D77F2A671C5F36183726DB2341BE58FEAE1DA2DECED843240F7B502BA659,,243F6A8885A308D313198A2E03707344A4093822299F31D0082EFA98EC4E6C89,FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFEFFFFFC2F69E89B4C5564D00349106B8497785DD7D1D713A8AE82B32FA79D5F7FC407D39B,FALSE,sig[0:32] is equal to field size 13,,DFF1D77F2A671C5F36183726DB2341BE58FEAE1DA2DECED843240F7B502BA659,,243F6A8885A308D313198A2E03707344A4093822299F31D0082EFA98EC4E6C89,6CFF5C3BA86C69EA4B7376F31A9BCB4F74C1976089B2D9963DA2E5543E177769FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFEBAAEDCE6AF48A03BBFD25E8CD0364141,FALSE,sig[32:64] is equal to curve order 14,,FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFEFFFFFC30,,243F6A8885A308D313198A2E03707344A4093822299F31D0082EFA98EC4E6C89,6CFF5C3BA86C69EA4B7376F31A9BCB4F74C1976089B2D9963DA2E5543E17776969E89B4C5564D00349106B8497785DD7D1D713A8AE82B32FA79D5F7FC407D39B,FALSE,public key is not a valid X coordinate because it exceeds the field size +15,0340034003400340034003400340034003400340034003400340034003400340,778CAA53B4393AC467774D09497A87224BF9FAB6F6E68B23086497324D6FD117,0000000000000000000000000000000000000000000000000000000000000000,,71535DB165ECD9FBBC046E5FFAEA61186BB6AD436732FCCC25291A55895464CF6069CE26BF03466228F19A3A62DB8A649F2D560FAC652827D1AF0574E427AB63,TRUE,message of size 0 (added 2022-12) +16,0340034003400340034003400340034003400340034003400340034003400340,778CAA53B4393AC467774D09497A87224BF9FAB6F6E68B23086497324D6FD117,0000000000000000000000000000000000000000000000000000000000000000,11,08A20A0AFEF64124649232E0693C583AB1B9934AE63B4C3511F3AE1134C6A303EA3173BFEA6683BD101FA5AA5DBC1996FE7CACFC5A577D33EC14564CEC2BACBF,TRUE,message of size 1 (added 2022-12) +17,0340034003400340034003400340034003400340034003400340034003400340,778CAA53B4393AC467774D09497A87224BF9FAB6F6E68B23086497324D6FD117,0000000000000000000000000000000000000000000000000000000000000000,0102030405060708090A0B0C0D0E0F1011,5130F39A4059B43BC7CAC09A19ECE52B5D8699D1A71E3C52DA9AFDB6B50AC370C4A482B77BF960F8681540E25B6771ECE1E5A37FD80E5A51897C5566A97EA5A5,TRUE,message of size 17 (added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message of size 100 (added 2022-12) diff --git a/bip-0340/test-vectors.py b/bip-0340/test-vectors.py index d1bf6b28bc..317f2ece69 100644 --- a/bip-0340/test-vectors.py +++ b/bip-0340/test-vectors.py @@ -249,6 +249,20 @@ def vector14(): return (None, pubkey, None, msg, sig, "FALSE", "public key is not a valid X coordinate because it exceeds the field size") +def varlen_vector(msg_int): + seckey = bytes_from_int(int(16 * "0340", 16)) + pubkey = pubkey_gen(seckey) + aux_rand = bytes_from_int(0) + msg = msg_int.to_bytes((msg_int.bit_length() + 7) // 8, "big") + sig = schnorr_sign(msg, seckey, aux_rand) + comment = "message of size %d (added 2022-12)" + return (seckey, pubkey, aux_rand, msg, sig, "TRUE", comment % len(msg)) + +vector15 = lambda : varlen_vector(0) +vector16 = lambda : varlen_vector(0x11) +vector17 = lambda : varlen_vector(0x0102030405060708090A0B0C0D0E0F1011) +vector18 = lambda : varlen_vector(int(100 * "99", 16)) + vectors = [ vector0(), vector1(), @@ -264,7 +278,11 @@ def vector14(): vector11(), vector12(), vector13(), - vector14() + vector14(), + vector15(), + vector16(), + vector17(), + vector18(), ] # Converts the byte strings of a test vector into hex strings