# Tag Info

9

In addition to the performance problems poncho already mentioned when using RSA signatures without hashing I just want to add on the security warning of poncho: Reordering If you have a message $m>N$ with $N$ being the RSA modulus, then you have to perform at least 2 RSA signatures as $m$ does not longer fit into $Z_N$. Let us assume that it requires ...

9

This is standard mathematical notation and not specific to cryptography. The $\Pi$ symbol means Product in much the same sense $\Sigma$ means Sum. For instance, $\Pi_{i=0}^2{u_i^{m_i}} = u_0^{m_0}u_1^{m_1}u_2^{m_2}$

8

Yes. Modern cryptosystems are designed and analysed under the assumption that the key is never used for anything else. If you use your encryption keys for digital signatures, you are violating that assumption, and it is very easy to construct schemes where this violation will compromise security. It is possible to construct schemes that can use the same ...

7

I hope I got your point and try to answer your question. Actually, if I understand you right, then what you call an attack actually means an adversary acting in a specific attack model. To clarify this, we need to review the security models for digital signature schemes and when we have discussed this we can clarify issues. Basically, we have to discuss ...

6

OpenPGP as defined by RFC 4880 knows two different encodings. Binary encoding Obviously, there is no reasonable limitation to an (ASCII) character subset in binary encoding. Radix 64 Radix 64 is also often entitled ASCII armored. In the end, it is a base64 encoding with a checksum. The content may consist of [a-zA-Y0-0+/=]. ASCII-armored OpenPGP ...

6

If you search on "timestamp", "timestamping", and "notary" on Crypto.SE and Security.SE, you'll find lots of references. I've collected a number of timestamping services that were mentioned in one of those places; this should provide a number of companies and online services you can check out: http://www.proofofexistence.com/ https://www.btproof.com/ ...

6

Well, one reason to hash the data before signing it is because RSA can handle only so much data; we might want to sign messages longer than that. For example, suppose we are using a 2k RSA key; that means that the RSA operation can handle messages up to 2047 bits; or 255 bytes. We often want to sign messages longer than 255 bytes. By hashing the message ...

6

No, signing the hash of the public key cannot introduce a weakness on a secure signature scheme. When we have a signature scheme, we assume that it is secure in an chosen text model, where the attacker has access to the public key, and can ask any text of his choosing to be signed. We can see that any such scheme (such as ECCDSA, or so we believe) cannot ...

6

Because the RFC says so. Signing and verifying using this key format is done according to the Digital Signature Standard [FIPS-186-2] using the SHA-1 hash [FIPS-180-2]. It says the same for RSA half a page down. Apparently the signature algorithm is a defined part of the public key method's specification, rather than being negotiated ...

5

I think you don't quite understand how RSA signatures work (and why they are the size they are). When generating an RSA signature, we follow a two-step process: We take that hash of the message we're signing, and convert (and pad) it into an integer $M$ which is between 0 and $N$ (where $N$ is a large integer that specified by the RSA key) We use the RSA ...

5

In a nutshell there are two main uses cases for signing an existing signature: validation: the signature of another person (ex: a superior) is required to give effect to a primary signature. The second signature covers the content, the first signature and potentially additional data added by the second signer. witness / notary: a second person signs only ...

5

Authentication can either mean entity authentication or data authentication. Data authentication is a means to demonstrate that some specific data originates from a specific source and has not been modified during transmission and/or upon storage. It can be achieved by the use of digital signatures in a public key, i.e., asymmetric, setting or message ...

4

In common cryptographic protocols there is only a need to generate RSA asymmetric keys now and then, say once a year. The key pair generation does not have to take place in the same environment either; e.g. you could use openssl command line if that is available. Note that RSA key pair generation time depends on finding large primes; the runtime is not ...

4

It mainly depends on how the algorithm was selected. If it was selected by a public competition like for AES, then it is likely to be secure. If it was forced in by the NSA such as Dual-EC random number generator, then you may have some doubts. Other questions you may want to ask yourself are: Is this an "original" algorithm or was the problem that it ...

4

An RSA signature is a sequence of bytes of the same size of the modulus. If the key uses a 1024-bit modulus $n$, then the signature value is, numerically, an integer in the $1..n-1$ range, and the PKCS#1 standard specifies that this integer should be encoded as a sequence of bytes of the same length as would be needed to encode the modulus, i.e. 128 bytes ...

4

Why the CFS signature is affected Let us review the structure of the CFS signature, which is strongly related to the Niederreiter PKE scheme. In the Niederreiter PKE scheme, a public key is $H \in \mathbb{F}^{n \times k}$, which is a scrambled parity-check matrix of the Goppa codes. A plaintext is a decodable error; for example, we set $S = \{\vec{e} \in ... 4 Short Answer: NO, it is not safe, do NOT do this. Longer Answer: You are true that you can use your RSA keypair for both operations. This approach is used in many applications and scenarios. There are Web Services or Single Sign-On implementations, which enforce you to use the same key pair for both operations. X.509 certificates do not allow you (by ... 4 No, the signer is per definition in possession of the secret signing key and thus can always produce signatures for any message of his choice. Consequently, a notion of unforgeability is not meaningful with respect to the signer. For a signature scheme one requires unforgeability for parties who are not in possession of the secret signing key but only the ... 4 Yes, there is an attack that has a fair chance of success, like$2^{-8}$per$Sign$(and perhaps$Sm$) submitted; or even more depending on exactly how the encryption padding is removed/verified. The attack does not require submitting a message for signature to a party knowing the signature private key. I consider the attacker successful whenever she manages ... 4 You appear to be testing a DER-encoded integer. DER-encoded integers are defined so that they can encode both positive and negative values (in cryptography, we don't need to encode negative integers all that often; however ASN.1 isn't specific to cryptography). DER-encoded integers are defined this way: Positive values are represented by the minimal ... 4 No. Cryptography alone cannot solve this problem. Solving this problem requires a combination of technical (e.g., cryptography, systems security) and non-technical (e.g., legal, regulatory, contractual) solutions. Even the technical part is not solely a cryptography question; it as much about systems security. 4 Yes. An independent witness to the signing may vouch for the initial signing, and do so by signing the whole document. E.g. this could indicate that the signing was done by an officer of the company and not a rogue employee. 4 First of all I do not know your implementation, but it seems that you have some basic misunderstandings. Signature: ECDSA(sha256(Data) ) ECDSA is typically implemented in a way that you do not explicitly hash the data prior to passing it to the signing algorithm (but as this might be your own implementation and signing may still work correctly). ... 4 Guillou and Quisquater (link) present a zero-knowledge proof of an RSA signature. Basically, the scheme is as follows: Public knowledge: RSA modulus$n$, public RSA exponent$v$, preimage$X$. Secret knowledge for prover:$A$, such that$A^v = X \mod n$. $$\begin{matrix} \mathcal{P} & & \mathcal{V} \\ r \xleftarrow{\} \mathbb{Z}_n^* ... 4 Informally, a signature scheme with message recovery is one where some or all of the message is embedded in the signature, allowing to conserve bandwidth when transmitting a signed message, compared to a signature scheme with appendix. Total message recovery A signature scheme with total message recovery [some sources make total implicit, e.g. the HAC ... 4 You can use multi-signatures. One example is the BN06 scheme described in the paper: Bellare, Neven - Multi-signatures in the plain public-Key model and a general forking lemma 3 I'll give a simple example with (textbook) RSA signing. I'm going to assume you understand RSA. First key gen: p\gets 7,q\gets 13,n\gets pq=91, e\gets 5, d\gets 29 Thus your public key is (e,n) and your private key is d. Say we want to sign the message m=35, we compute s=m^d\bmod{n} which is s\gets 42\equiv 35^{29}\bmod{n}. The message and ... 3 Your parameters are incorrect. I don't know how you got them, so I don't know which one is in error; however at least one of them is wrong. You have n = 149 \times 191. The decryption exponent satisfies:$$ed \equiv 1 \pmod{\operatorname{lcm}(p-1, q-1)},$$that is,$7d \equiv 1 \pmod{14060}$; the minimal$d$that satisfies this is$d = 10043$. Now, if ... 3 If you want$N$serial numbers, your serial numbers will have to use$n$bits for uniqueness, where$n = \log_2 N$. So if you have 100 bits to use for the serial, you could use 20 to get about a million serials and have 80 bits to use for a cryptographic MAC or signature. Now there are two approaches, the symmetric and the asymmetric. In the symmetric ... 3 If you look at exact security, the height matters. The reason is that it defines the number of OTS key pairs and hence the possible number of one time signatures per MSS key pair. To forge a MSS signature, it is enough to generate a forgery for 1 out of$2^h$OTS signatures. Hence you get a reduction in the bit security of$h\$ bits.

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