International Association for Cryptologic Research

International Association
for Cryptologic Research

CryptoDB

Tomáš Rosa

Publications

Year
Venue
Title
2007
EPRINT
A Note on the Relay Attacks on e-passports: The Case of Czech e-passports
Martin Hlavac Tomas Rosa
The threat of relay attacks on authentication protocols is often well recognized, especially for contactless applications like RFID chips. It is, therefore, a bit surprising to meet an implementation that actually encourages rather than eliminates these attacks. We present our experimental observations concerning Czech e-passports. These show clearly an inherent weakness rooted in lower layers of ISO 14443. As the behavior is unavoidable, it induces a question on whether the e-passport should not have used a different communication protocol or authentication scheme.
2005
EPRINT
One-Time HNP or Attacks on a Flawed El Gamal Revisited
Tomas Rosa
We present a modification of the well-known hidden number problem (HNP) which we refer to as a one-time HNP (OT-HNP). We also present an algorithm for solving such a problem together with its formal analysis. We show then that carefully designed instances of OT-HNP can be used to break certain flawed implementations of public key schemes efficiently. We work, for instance, with Nguyen?s attack on El Gamal?s signature scheme in the GNU Privacy Guard of version 1.2.3. The technique employed there was not based on HNP, since it was supposed that more than one signature would be necessary, which seemed to be a wastage. We will see, however, that by using OT-HNP one signature is still far enough, while retaining certain elegance of the HNP approach. We also present an experimental confirmation of this result.
2003
CHES
2003
EPRINT
Attacking RSA-based Sessions in SSL/TLS
In this paper we present a practically feasible attack on RSA-based sessions in SSL/TLS protocols. These protocols incorporate the PKCS#1 (v. 1.5) encoding method for the RSA encryption of a premaster-secret value. The premaster-secret is the only secret value that is used for deriving all the particular session keys. Therefore, an attacker who can recover the premaster-secret can decrypt the whole captured SSL/TLS session. We show that incorporating a version number check over PKCS#1 plaintext used in the SSL/TLS creates a side channel that allows the attacker to invert the RSA encryption. The attacker can then either recover the premaster-secret or sign a message on behalf of the server. Practical tests showed that two thirds of randomly chosen Internet SSL/TLS servers were vulnerable. The attack is an extension of Bleichenbacher?s attack on PKCS#1 (v. 1.5). We introduce the concept of a bad-version oracle (BVO) that covers the side channel leakage, and present several methods that speed up the original algorithm. Our attack was successfully tested in practice and the results of complexity measurements are presented in the paper. Plugging a testing server (2x Pentium III/1.4 GHz, 1 GB RAM, 100 Mb/s Ethernet, OS RedHat 7.2, Apache 1.3.27), it was possible to achieve a speed of 67.7 BVO calls per second for a 1024 bits RSA key. The median time for a whole attack on the premaster-secret could be then estimated as 54 hours and 42 minutes. We also propose and discuss countermeasures, which are both cryptographically acceptable and practically feasible.
2003
EPRINT
Side Channel Attacks on CBC Encrypted Messages in the PKCS#7 Format
Vlastimil Klima Tomas Rosa
Vaudenay has shown in [5] that a CBC encryption mode ([2], [9]) combined with the PKCS#5 padding [3] scheme allows an attacker to invert the underlying block cipher, provided she has access to a valid-padding oracle which for each input ciphertext tells her whether the corresponding plaintext has a valid padding or not. Having on mind the countermeasures against this attack, different padding schemes have been studied in [1]. The best one is referred to as the ABYT-PAD. It is designed for byte-oriented messages. It removes the valid-padding oracle, thereby defeating Vaudenay's attack, since all deciphered plaintexts are valid in this padding scheme. In this paper, we try to combine the well-known cryptographic message syntax standard PKCS#7 [8] with the use of ABYT-PAD instead of PKCS#5. Let us assume that we have access to a PKCS#7CONF oracle that tells us for a given ciphertext (encapsulated in the PKCS#7 structure) whether the deciphered plaintext is correct or not according to the PKCS#7 (v1.6) syntax. This is probably a very natural assumption, because applications usually have to reflect this situation in its behavior. It could be a message for the user, an API error message, an entry in the log file, different timing behavior, etc. We show that access to such an oracle again enables an attacker to invert the underlying block cipher. The attack requires single captured ciphertext and approximately 128 oracle calls per one ciphertext byte. It shows that we cannot hope to fully solve problems with side channel attacks on the CBC encryption mode by using a ?magic? padding method or an obscure message-encoding format. Strong cryptographic integrity checks of ciphertexts should be incorporated instead.
2002
CHES
2002
EPRINT
Strengthened Encryption in the CBC Mode
Vlastimil Klima Tomas Rosa
Vaudenay [1] has presented an attack on the CBC mode of block ciphers, which uses padding according to the PKCS#5 standard. One of the countermeasures, which he has assumed, consisted of the encryption of the message M?= M || padding || hash(M || padding) instead of the original M. This can increase the length of the message by several blocks compared with the present padding. Moreover, Wagner [1] showed a security weakness in this proposal. The next correction, which Vaudenay proposed ("A Fix Which May Work") has a general character and doesn't solve practical problems with the real cryptographic interfaces used in contemporary applications. In this article we propose three variants of the CBC mode. From the external point of view they behave the same as the present CBC mode with the PKCS#5 padding, but they prevent Vaudenay's attack.
2002
EPRINT
Further Results and Considerations on Side Channel Attacks on RSA
Vlastimil Klima Tomas Rosa
This paper contains three parts. In the first part we present a new side channel attack on plaintext encrypted by EME-OAEP PKCS#1 v.2.1. In contrast with Manger?s attack, we attack that part of the plaintext, which is shielded by the OAEP method. In the second part we show that Bleichenbacher?s and Manger?s attack on the RSA encryption scheme PKCS#1 v.1.5 and EME-OAEP PKCS#1 v.2.1 can be converted to an attack on the RSA signature scheme with any message encoding (not only PKCS). This is a new threat for those implementations of PKI, in which the roles of signature and encryption keys are not strictly separated. This situation is often encountered in the SSL protocol used to secure access to web servers. In the third part we deploy a general idea of fault-based attacks on the RSA-KEM scheme and present two particular attacks as the examples. The result is the private key instead of the plaintext as with attacks on PKCS#1 v.1.5 and v.2.1. These attacks should highlight the fact that the RSA-KEM scheme is not an entirely universal solution to problems of RSAES-OAEP implementation and that even here the manner of implementation is significant.
2002
EPRINT
Attack on Private Signature Keys of the OpenPGP Format, PGP(TM) Programs and Other Applications Compatible with OpenPGP
Vlastimil Klima Tomas Rosa
The article describes an attack on OpenPGP format, which leads to disclosure of the private signature keys of the DSA and RSA algorithms. The OpenPGP format is used in a number of applications including PGP, GNU Privacy Guard and other programs specified on the list of products compatible with OpenPGP, which is available at http://www.pgpi.org/products. Therefore all these applications must undergo the same revision as the actual program PGP. The success of the attack was practically verified and demonstrated on the PGP program, version 7.0.3 with a combination of AES and DH/DSS algorithms. As the private signature key is the basic information of the whole system which is kept secret, it is encrypted using the strong cipher. However, it shows that this protection is illusory, as the attacker has neither to attack this cipher nor user?s secret passphrase. A modification of the private key file in a certain manner and subsequent capturing of one signed message is sufficient for successful attack. Insufficient protection of the integrity of the public as well as private parts of signature keys in the OpenPGP format is analyzed in DSA and RSA algorithms and on the basis of this, a procedure of attacks is shown on both private signature keys. The attacks apply to all lengths of parameters (modules, keys) of RSA and DSA. In the end the cryptographic measures for correction of the OpenPGP format as well as PGP format are proposed.
2002
EPRINT
Key-collisions in (EC)DSA: Attacking Non-repudiation
Tomas Rosa
A new kind of attack on the non-repudiation property of digital signature schemes is presented. We introduce a notion of key-collisions, which may allow an attacker to claim that the message (presented to a judge) has been signed by someone else. We show how to compute key-collisions for the DSA and ECDSA signature schemes effectively. The main idea of these attacks has been inspired by the well-known notion of message-collisions, where an attacker claims that the signature presented at the court belongs to a different message. Both of these collision-based attacks significantly weaken the non-repudiation property of signature schemes. Moreover, they weaken the non-repudiation of protocols based on these schemes. It is shown that key-collision resistance of the (EC)DSA schemes requires the incorporation of a mechanism ensuring honest generation of (EC)DSA instances. The usage of such a mechanism shall be verifiable by an independent third party without revealing any secret information. We propose and discuss basic general countermeasures against key-collision attacks on the (EC)DSA schemes.