Secure Science Corporation Advisory CSA-056
http://www.securescience.net
e-response@securescience.net
877-570-0455

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LibTomCrypt version <=1.02 contained weak signature scheme used
with ECC keys, allowing trivial signature forgeries.

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Vulnerability Classification: Cryptography - arbitrary message signing

Discovery Date: May 1, 2005
Vendor Reported Advisory
Discovery Published: May 2, 2005

Abstract:
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LibTomCrypt is a fairly comprehensive, modular and portable 
cryptographic toolkit that provides developers with a vast array of well 
known published block ciphers, one-way hash functions, chaining modes, 
pseudo-random number generators, public key cryptography and a plethora 
of other routines. A vulnerability was found by the author within the 
signature scheme used with the Elliptic Curve Cryptosystem routines that 
will allow arbitrary signatures to be created by an attacker.

Description:
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During recent cryptographic review by the author, a mathematical flaw 
was found within the implementation of the El Gamal signature algorithm 
used in LibTomCrypt versions <=1.02 An attacker can create a valid 
random signature by selecting a random value for a, and then computing 
(a^-1)C (where the inverse is modulo the order of the curve), 
essentially allowing an attacker to sign arbitrary messages without the 
private key.


Affected Vendors:
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All vendors using LTC <=1.02 that apply ECC with signatures.

Vendor and Patch Information:
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Secure Science Corporation is distributing this advisory on behalf of 
the author. The author has stated that LibTomCrypt 1.03 will be released 
May 7, 2005 with X9.62 ECDSA implemented, a FIPS 180-2 standard.

Solution:
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X9.62 ECDSA implementation.


Credits:
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Secure Science Corporation - Tom St Denis

Disclaimer:
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the information we provide on this website and/or through our security 
advisories. Our advisories are a service to our customers intended to 
promote secure installation and use of Secure Science Corporation products.